Prostaglandins and related substances in plants.

II. Introduction

Prostaglandins were discovered in plants about 20 years ago, but their presence in plants is not widely known; this review is an attempt to remedy that. Two mini-reviews (Saniewski, 1979; Bosisio, 1988) and a review mostly on chemical aspects of prostaglandins in plants have appeared in recent years (Panossian, 1987). The present review covers the entire field of prostaglandin and related substances in plants and emphasizes the physiology of the compounds mentioned.

Long before prostaglandins (PGs) were detected in plants they had been discovered in mammals (humans). They were detected as early as 1930 by two American gynecologists, R. Kurzrok and C. C. Lieb, when they reported that the human uterus, on contact with fresh human semen, was provoked into either strong contraction or relaxation. Both U. S. von Euler, in Sweden, and M. W. Goldblatt, in England, subsequently discovered marked stimulation of smooth muscle by seminal plasma. Von Euler called the active agent "prostaglandin" because he thought it came from the prostate gland. It was later shown that the PG was actually produced by the seminal vesicles. However, it was subsequently discovered that PGs also occurred in many other tissues or organs of mammalian systems, such as lung, iris, brain, thymus, pancreas, kidney, and so forth. Prostaglandins possess a variety of physiological and pharmacological properties in animal systems, and they are synthesized in the cell micro-somes from essential fatty acids. Although discovered in mammalian systems, PGs are now known to occur widely throughout the animal kingdom.

Prostaglandins are 20-carbon carboxylic fatty acids consisting of two side-chains joined by a cyclo-pentane ring structure. Prostaglandins are structural derivatives of prostanoic acid and are classified into four main groups - PGA, PGB, PGE, and PGF - which differ in the ring substituents and the number of double bonds in the molecule. However, there are other PGs not mentioned here.

A major precursor for prostaglandin biosynthesis is cell membrane phospholipid. For instance, [PGE.sub.2] and [PGF.sub.2[Alpha]] production depends on the hydrolysis of esterified arachidonic acid (from membrane phospholipid) which yields unesterified arachidonic acid. Arachidonic acid is present largely in the 2-position of the phospholipid molecule and the activity of the enzyme responsible for the cleavage, phospholipase [A.sub.2], may control a critical regulatory step in many PG-generating systems. Prostaglandins may also play an important role in the regulation of membrane functions. The conversion of arachidonic acid (20:4) to cyclic endoperoxide ([PGG.sub.2]) is catalyzed by the enzyme cyclo-oxygenase. [PGG.sub.2] is converted to [PGE.sub.2] and [PGF.sub.2[Alpha]]. Arachidonic acid may also be converted to other pharmacologically active C20 compounds, e.g., leukotrienes, prostacyclin, and thromboxanes. All the above-mentioned compounds, including PGs, are known collectively as "eicosanoids." Another enzyme, lipoxygenase, can also convert arachidonic acid to PGs. Besides arachidonic acid there are two other direct precursors of PGs: di-homo-[Gamma]-linolenic acid (20:3) and 5, 8, 11, 14, 17-eicosapentaenoic acid (20:5).

A. ABBREVIATIONS USED FREQUENTLY IN THIS REVIEW

12-oxo-PDA - 12-oxo-cis-10, 15-phytodienoic acid

AMO-1618 - inhibitor of gibberellin synthesis

Cyclic-AMP - cyclic-3[prime] 5[prime] adenosine monophosphate

[GA.sub.3] - gibberellic acid

GC - gas chromatography

GC-MS - gas chromatography-mass spectrometry

HPLC - high performance liquid chromatography

IR - infrared

MS - mass-spectrometry

MR - nuclear magnetic resonance

PG - prostaglandin

[PGA.sub.1] and [PGA.sub.2] - prostaglandins [A.sub.1] and [A.sub.2]

[PGB.sub.1] and [PGB.sub.2] - prostaglandins [B.sub.1] and [B.sub.2]

[PGD.sub.2] - prostaglandin [D.sub.2]

[PGE.sub.1] and [PGE.sub.2] - prostaglandins [E.sub.1] and [E.sub.2]

[PGF.sub.1[Alpha]] and [PGF.sub.2[Alpha]] - prostaglandins [F.sub.1[Alpha]] and [F.sub.2[Alpha]]

[PGG.sub.2] - prostaglandin [G.sub.2]

RIA - radio-immunoassay

SDP - short-day plant

TLC - thin layer chromatography

[TXB.sub.2] - thromboxane [B.sub.2]

UV - ultraviolet

III. Discovery of Prostaglandins in Plants

A. IN THE ONION

The first recorded attempt to extract prostaglandins from a plant source (wheat bran) was that of Albro and Fishbein (1971). They obtained a compound that co-chromatographed with [PGF.sub.2[Alpha]] but was later identified as 5, 8, 12-trihydroxy-trans-9-octadecenoic acid. However, two years later, Attrep et al. (1973) discovered [PGA.sub.1] in Allium cepa (onion). This was the first report of the occurrence of a PG in a plant. The evidence comprised a limited thin layer chromatographic study, infrared spectral analysis, and the fact that the onion extract that had co-chromatographed with [PGA.sub.1] was found to lower the blood pressure of rats. The lowering of blood pressure is a characteristic of certain prostaglandins.

Attrep et al. (1980) reexamined the findings obtained from the 1973 study on the onion with more exact tests. The additional studies included, among others, gas chromatographic-mass spectral analysis (GC-MS). The new investigation proved unequivocally the presence of a prostaglandin ([PGA.sub.1]) in a plant.

Another study of a number of different Allium species was conducted by Pobozsny et al. (1979). They detected different prostaglandins in the various extracts, but they did not make a detailed study of the structures of the different PGs. This work corroborated the study done by Attrep et al. (1973, 1980).

B. IN A RED ALGA

The discovery of a prostaglandin in a plant was followed by the detection of [PGE.sub.2] and [PGF.sub.2[Alpha]] in a lower plant, the red alga Gracilaria lichenoides (Gregson et al., 1979). A fraction of the algal extract displayed potent anti-hypertensive activity when given intravenously to hypersensitive rats. After a sequence of chromatographic separations, of this fraction two separate fractions were obtained, one of which pharmacologically was active and the other inactive. After spectral, optical rotation, and pharmacologic data on the two fractions were examined, it was deduced that they were slightly impure [PGE.sub.2] and [PGF.sub.2[Alpha]], respectively. To facilitate complete characterization, the two fractions were esterified and purified by preparative thin layer chromatography (TLC). It was then proved unequivocally that they were the methyl esters of [PGE.sub.2] and [PGF.sub.2[Alpha]] by comparing them with nuclear magnetic resonance (NMR), TLC, and mass-spectrometric (MS) data from the literature and with authentic sam-pies. It was determined that [PGE.sub.2] and [PGF.sub.2[Alpha]] constitute 0.05-0.07% and 0.07-0.1% of the dry weight, respectively.

C. IN POPLAR AND LARCH

Meanwhile, in Siberia, a group of Russian scientists were working on the prostaglandins that occur in the buds and cambial zone of poplar (Populus balsamifera) and the Siberian larch (Larix sibirica). They published a series of papers in Russian which culminated in a report in Phytochemistry (Levin et al., 1988). They detected [PGA.sub.2], [PGB.sub.2], [PGE.sub.2], and [PGF.sub.2[Alpha]] and isolated the different PGs in pure form. Besides buds and the cambial zone, extracts were made also of leaves, needles, heartwood, and sapwood of young and mature trees. Prostaglandins were detected only in the buds and cambial zone of both the poplar and the larch. The evidence for the identification of the different PGs was produced by means of TLC, HPLC, UV and IR spectra, NMR, and GC-MS analyses. Prostaglandins were also detected by means of a bioassay (isolated rat's uterine horn). In addition to identifying the prostaglandins, it was important to determine how the levels of these compounds in the tissues change in the course of the annual cycle. This was followed using GC over a period of two years. The PG content of buds varied during the annual cycle and had no obvious connection with the surrounding air temperature; the content depended, apparently, on ongoing biochemical processes in living tissue which are connected with the different phases of the annual cycle. The PG content in buds of P. balsamifera was the highest during April (spring) and the lowest during February (winter). The PG concentrations during these two periods were 70.0 and 5.5 [[micro]gram] [g.sup.-1] dry weight tissue, respectively. Similar results were obtained with the buds of L. sibirica. The authors concluded that the possible explanation of these annual changes in PG content could be linked to the fact that PGs are bioregulators of reproductive processes and also of those biochemical processes that occur in winter and allow the living tissues to survive in low temperatures for long periods. The authors proposed that PGs are the hypothetical flowering hormone called "florigen." They came to the above conclusion partly through this study of the buds of the poplar and the larch and also through previous results obtained by Groenewald and Visser (1978) and Janistyn (1982a).

D. IN PROKARYOTIC ORGANISMS

Prostaglandins were also detected in certain prokaryotic organisms (Kruger et al., 1990), including three different cyanobacteria (blue-green algae): Microcystis aeruginosa, Anabaena variabilis, and Anacystis nidulans. A prostaglandin was also detected in a bacterium (Pseudomonas species).

The prepared extracts were differentially extracted with different solvents and finally separated by TLC. The band that co-chromatographed with authentic [PGF.sub.2[Alpha]] was scraped off the thin layer plate and was resolved by radio-immunoassay (RIA), using an RIA kit based on monoclonal antibodies specific for [PGF.sub.2[Alpha]]. The assay procedure included the preparation of a standard curve in which known amounts of PG (radioactive and non-radioactive) were used to compete for a fixed amount of PG antibody. The standard curve was then used to determine the PG content of the assay samples from the binding obtained with each sample. The reason why the researchers looked specifically for [PGF.sub.2[Alpha]] is that this specific PG is thus far the most abundant one occurring in plants (Bild et al., 1978; Gregson et al., 1979; Groenewald et al., 1983; Janistyn, 1982a). [PGF.sub.2[Alpha]] was detected in all three cyanobacteria and the Pseudomonas species. The [PGF.sub.2[Alpha]] concentrations ranged from 93.7 to 225.5 pg [g.sup.-1] dry mass.

E. IN YEASTS

Recently appearing was a report concerning evidence for pharmacologically active prostaglandins in yeasts (Kock et al., 1991). Using RIA, [PGF.sub.2[Alpha]] was detected in various genera, species, and strains of the family Lipomycetaceae, and [PGF.sub.2[Alpha]] was also shown to occur in Saccharomyces cerevisiae.

It was also demonstrated that extracts from the yeast Dipodascopsis uninucleata inhibit blood platelet aggregation. This result indicated that eicosanoid metabolites may be present. When arachidonic acid was incubated with cultures of D. uninucleata, the formation of aspirin-sensitive compounds was observed by GC-MS. Two of these compounds were identified as isomers of [Alpha]-pentanol [PGF.sub.2[Alpha]]-[Gamma]-lactone, a metabolite of prostacyclin, the potent platelet-aggregation inhibitor. All the yeasts produced significant quantities (456 to 4200 pg [g.sup.-1]) of [PGF.sub.2[Alpha]]. Results of RIA should, however, be treated with caution, as cross-reactions of structurally related compounds are possible. Such reactions may result in overestimation of the amount of PG present in a sample.

More recently, Botha et al. (1993) obtained evidence of PG production during ascosporogenesis of the yeast Dipodascopsis tothii. According to TLC analyses, [PGE.sub.2] and [PGF.sub.2[Alpha]] are produced during ascosporogenesis. Botha et al. (1992) had earlier found that specific inhibitors of the cyclo-oxygenase enzyme (i.e., aspirin and indomethacin) seriously repressed ascosporogenesis in the genus Dipodascopsis. This implied that PG biosynthesis occurs during asci formation.

F. IN OENOTHERA STRICTA

In 1994, Groenewald et al. detected [PGF.sub.2[Alpha]] in seeds (0.607 ng [g.sup.-1]) and flowers (0.641 ng [g.sup.-1]) of Oenothera stricta by using a radio-immunoassay kit (RIA). Only trace amounts of [PGF.sub.2[Alpha]] could be detected in leaves and stems. Gas-chromatographic analyses of fatty acids revealed, among other fatty acids, [Gamma]-linolenic acid, which also occurs in the evening primrose (Oenothera biennis) and which is an indirect precursor of [PGF.sub.2[Alpha]]. The direct precursor of [PGF.sub.2[Alpha]], archidonic acid, could not be detected in O. stricta.

G. IN BRYOPHYTES

Prostaglandin [F.sub.2[Alpha]] was determined using RIA in two South African mosses and a liverwort. The mosses were Amblystechium serpens and Brachythecium implicatum; the liverwort was Marchantia parviloba. The concentration of PG varied from 615 to 1566 pg [g.sup.-1] dry mass (Groenewald et el., 1990).

H. IN PHARBITIS NIL AND KALANCHOE BLOSSFELDIANA

Prostaglandin [F.sub.2[Alpha]] has been detected in the short-day plants Pharbitis nil (Groenewald et al., 1983) and Kalanchoe blossfeldiana (Janistyn, 1982a).

IV. Enzymes Responsible for Synthesis of Prostaglandins

A. IN SOYBEAN AND ALLIUM SPECIES

Enzymes responsible for the synthesis of prostaglandins in plants were also studied. Bild et el. (1978), for instance, extracted from soybean an enzyme (lipoxygenase-2) that catalyzed the formation of [PGF.sub.2[Alpha]] from arachidonic acid. Ali et al. (1990) made a comparative study of the in vitro synthesis of prostaglandins and thromboxanes in plants belonging to the Liliaceae family: garlic (Allium sativum), onions (Allium cepa), and Allium porum. These plants were studied because they have been used in herbal medicine since antiquity and also because PGs have been shown to occur in the onion (Attrep et al., 1973, 1980; Pobozsny et al., 1979). Homogenates of garlic, onion, and Allium porum were incubated in vitro with [14C]-arachidonic acid. Separation of labeled prostaglandins and thromboxanes were accomplished by TLC, and the Rf-values were compared with those of authentic standards. The prostaglandins and thromboxane identified were 6-keto-[PGF.sub.1[Alpha]], [PGF.sub.2[Alpha]], [TXB.sub.2] (thromboxane), [PGE.sub.2], and [PGD.sub.2]. To the best of our knowledge, this is the first report of the occurrence of a thromboxane in a plant extract. [PGE.sub.2] and [PGD.sub.2] were the major metabolites of arachidonic acid studied among the three Allium species. Garlic was found to have the highest capacity to metabolize the radioactive arachidonic acid. The synthesis of prostaglandins and thromboxane was inhibited by pre-incubation of homogenates with indomethacin or was completely destroyed by boiling the plant extract prior to incubation with arachidonic acid. Since indomethacin is an inhibitor of the enzyme cyclo-oxygenase, the authors suggest that this inhibition confirms the presence of the enzyme in the plant extracts.

B. IN ALOE VERA

The same group of researchers that worked on the occurrence of PGs in the Allium species extracts studied another member of the Liliaceae, Aloe vera (Afzal et al., 1991). The reason why A. vera extracts were studied was because of the well-known healing properties of this plant. Reports of the medicinal virtues of A. vera date back to the times of Hippocrates and Alexander the Great. Homogenates were prepared from fresh plant tissue. Extracts were pre-incubated with or without indomethacin (inhibitor of cyclo-oxygenase). Synthesis of PGs was initiated by the addition of [14C]-arachidonic acid. Labeled products were separated from the labeled precursor using mini silicic acid columns. The fractions containing prostaglandins and thromboxanes were resolved on TLC. The following compounds were detected in the extracts: 6-keto-[PGF.sub.1[Alpha]], [PGF.sub.2[Alpha]], [TXB.sub.2], [PGE.sub.2], and [PGD.sub.2]. Incubation of the [14C]-arachidonic acid with homogenized boiled plant extract resulted in complete inhibition of the synthesis of labeled products. Similarly, incubation of [14C]-arachidonic acid with a plant extract in the presence of indomethacin did not produce any significant amount of prostaglandins and thromboxane. According to the authors, the inhibition by indomethacin is a confirmation of the presence of cyclo-oxygenase. Since, as far as is known, indomethacin is not an inhibitor of lipoxygenase, it is highly possible that cyclo-oxygenase was involved in the above-mentioned conversion of arachidonic acid to prostaglandins and a thromboxane.

C. IN CORN LEAF HOMOGENATES

A group of Hungarian scientists (Forster et al., 1984) claimed that they showed the occurrence of [PGF.sub.2[Alpha]] and [PGE.sub.2] in corn leaf homogenates that were incubated with 3H-labeled arachidonic acid. They used a selective extraction procedure and TLC to resolve the prostaglandins. No physicochemical characterization of the prostaglandins was performed. The researchers also claimed to have shown the involvement of cyclo-oxygenase, although they had not studied the effect of cyclo-oxygenase inhibitors on the corn leaf homogenates.

V. Occurrence of Prostaglandin Precursors in Plants

A. IN ALGAE

It is important to know whether the direct precursors of prostaglandins [arachidonic acid (20:4), di-homo-[Gamma]-linolenic acid (20:3), and eicosapentaenoic acid (20:5)] occur in plants. It has been known for a long time that certain marine algae contain all three of the above-mentioned acids. For instance, Hitchcock and Nichols (1971) listed red algae, brown algae, green algae, and saltwater diatoms that contain all three direct precursors of prostaglandins. Recently a research group (Gerwick et al., 1990) at Oregon State University reported on the discovery of novel eicosanoids from the Rhodophyta (red algae). The compounds are arachidonic acid derivatives, and inherent in these seaweed natural-product structures is evidence of a highly evolved lipoxygenase-type metabolism that matches or exceeds the complexity of comparable metabolic routes in mammalian systems. Also, some of these compounds are very expensive chemicals. Burgess et al. (1991) also reported on a new eicosapentaenoic acid which was present in an aqueous extract of the red alga Bossiella orbigniana. The unsaturated fatty acid was endogenously present and was also produced enzymatically when radioactive arachidonic acid was added to an aqueous extract of the alga. The compound was named bosseopentaenoic acid and is new to science. Jiang and Gerwick (1991) extracted new eicosanoids and other hydroxylated fatty acids from the marine red alga Gracilariopsis lemaneiformis.

Stratmann et al. (1992) discovered a new branch of eicosanoid metabolism in a marine brown alga (Ectocarpus siliculosus). Labeled arachidonic acid applied to an extract of the alga produced a pheromone, dictyotene.

It was found that the red microalga Porphyridium cruentum (Cohen, 1990) is a new source of eicosapentaenoic acid and arachidonic acid. The conditions leading to a high content of either fatty acid were investigated. By imposing nitrogen starvation, it was possible to obtain a lipid mixture that may be separated in arachidonic acid and eicosapentaenoic acid-rich fractions.

Marine algae belonging to the Chlorophycophyta, Rhodophycophyta, and Phaeophycophyta were subjected to blue, white, and red light (Radwan et al., 1988) and the effect of the light quality on the production of arachidonic acid was ascertained. It was found that the light quality had a definite effect on the proportion of arachidonic acid in the lipids of certain algae.

Some freshwater algae - namely, Nitella and Navicula pelliculosa (diatom) - also produce arachidonic acid (Hitchcock & Nichols, 1971).

B. IN BRYOPHYTES

It has been known for a long time that certain mosses (bryophytes) and ferns contain direct precursors of prostaglandins, namely, di-homo-[Gamma]-linolenic acid, arachidonic acid, and eicosapentaenoic acid (Hitchcock & Nichols, 1971; Gellerman et al., 1975; Huneck, 1983). The fact that these direct precursors occurred in certain mosses encouraged Groenewald et al. (1990) to search for arachidonic acid in two South African mosses and a liverwort: Amblystechium serpens, Brachythecium implicatum, and Marchantia parviloba, respectively. Arachidonic acid was detected in all three plants, and the relative content ranged from 3.5% to 19.8% of total lipids.

C. IN WHEAT GERM OIL AND POPLAR

It has long been thought that higher plants (angiosperms) do not possess arachidonic acid as a constituent of their lipid fraction. However, Janistyn (1982b) has shown that arachidonic acid occurs in wheat germ oil. The arachidonic acid was characterized by GC and GC-MS. The concentration of arachidonic acid was 12.560 pmol [g.sup.-1]. This study shows the unequivocal presence of arachidonic acid in a lipid fraction of a higher plant. Levin et al. (1990) detected arachidonic acid in the buds of Populus balsamifera (angiosperm). They studied the interrelationships between the contents of arachidonic acid and prostaglandins in the course of the annual plant growth cycle. The PGs also occurred in the buds. Neither arachidonic acid nor PGs were detected in the leaves. There was a close positive linear relationship between the changes in the content of arachidonic acid and PGs within the various periods of the annual growth cycle. The correlation coefficient was found to be 0.95. The lowest content of arachidonic acid and PGs occurred in February and March, when all the physiological functions are at a minimum. In April all the reproductive functions regenerate and there is a sharp increase in the content of biologically active substances (among others, arachidonic acid and PGs) because of the growth requirements. The arachidonic acid and PG content in March was 2.9 and 9.5 [[micro]gram] [g.sup.-1] dry weight respectively and that in April was 52.3 and 71.0 [[micro]gram] [g.sup.-1] dry weight respectively. It was suggested that arachidonic acid is the substrate for synthesis of prostaglandins in plants as well as in animals. The arachidonic acid in the buds of P. balsamifera was characterized mainly by TLC, GC, and NMR.

D. IN GARLIC

Afzal et al. (1985) detected arachidonic acid and eicosapentaenoic acid in extracts of garlic (Allium sativum). They were resolved by means of TLC and GC. The arachidonic acid and eicosapentaenoic acid contents were 0.3% and 0.56% of the lipid fraction, respectively.

E. IN ALOE VERA

Afzal et al. (1991) reported the detection of arachidonic acid in extracts of Aloe vera (Liliaceae). The lipid fraction of the extracts was resolved with TLC, and the methyl esters of the fatty acids were separated by GC. Fatty acids ranging from 16:0 to 24:0 were identified including 20:4 (arachidonic acid). The arachidonic acid content was 3.1% of the lipid fraction.

F. IN YEAST

Van Dyk et al. (1991) isolated a novel arachidonic acid metabolite from the yeast Dipodascopsis uninucleata. A culture of the yeast was fed with exogenous arachidonic acid and the new compound that was synthesized was resolved by TLC, HPLC, and electron impact-mass spectrometry. It was found to be 3-hydroxy-5, 8, 11, 14-eicosatetraenoic acid. This compound is now chemically synthesized and shows promise in the treatment of cancer (J. L. F. Kock, pers. comm.).

It is possible that if one should look carefully at the plant kingdom, arachidonic acid may be detected in many more different plants, especially higher plants. It thus seems clear that one or more of the direct precursors of PGs do occur in a wide spectrum of plants, i.e., from lower to higher plants.

VI. Unsaturated Polyhydroxy Fatty Acids with Prostaglandin-Like Activity

A. IN ROOTS OF BRYONIA ALBA

There have appeared in the literature a few reports on the detection of unsaturated polyhydroxy fatty acids (C18-acids) with prostaglandin-like activity. Panossian et al. (1983) isolated eight major components from the fraction of unsaturated polyhydroxy fatty acids from the roots of Bryonia alba L. (Cucurbitaceae). The isolated components were identified as four diastereoisomeric pairs of 9, 12, 13-trihydroxy-trans- 10, cis-15-octadecadienoic acid; 12, 15, 16-trihydroxy-cis-9, trans-13-octa-decadienoic acid; 9, 10, 13-trihydroxy-trans-11, cis-15-octadeca-dienoic acid; and 12, 13, 16-trihydroxy-cis-9, trans-14-octadeca- dienoic acid. The compounds were resolved by TLC and GC-MS. The unsaturated fatty acids showed PG-like activity in in vitro bioassay tests.

B. IN THE ONION

In another paper, Claeys et el. (1986) reported on the discovery of two trihydroxy unsaturated fatty acids (C18-acids) in a different plant belonging to the Liliaceae, namely Allium cepa (onion). The two fatty acids, resolved by TLC and GC-MS, were characterized as being 9, 10, 13-trihydroxy-11-octadecenoic acid and 9, 12, 13-trihydroxy-10-octadecenoic acid. These acids showed PG-like activity in different in vitro bioassays, and it is clear that the different compounds mentioned by Panossian et el. (1983) and Claeys et el. (1986) are of a non-prostanoid nature, although, as mentioned before, they showed PG-like activity. It thus appears that the presence of a five-membered ring structure is not an absolute prerequisite for PG-like activity.

VII. Prostaglandin-Like Compounds

A. IN FLAX SEED EXTRACT

There are a number of reports of prostaglandin-like compounds that have been detected in plants. Probably the first report is that of Zimmerman and Feng (1978). They characterized a PG-like metabolite of linolenic acid which was catalyzed by a flax seed (Linum usitatissimum L. var. Linott) extract. The compound was found to be 8-[2(cis-pent-2[prime]-enyl)-3-oxo-ciscyclopent-4-enyl] octanoic acid. Characterization of the compound was done by TLC, GC-MS, and NMR. The compound contains a cyclopentenone ring structure and it is analogous to that of the A-type prostaglandins. The cumbersome full chemical name of the compound was shortened to 12-oxo-cis-10,15-phytodienoic acid, or 12-oxo-PDA. Prostaglandins are 20-carbon compounds and 12-oxo-PDA is an 18-carbon compound. Various papers on different aspects of 12-oxo-PDA were published by Vick and Zimmerman (1979, 1984). The biosynthesis of 12-oxo-PDA was elucidated. Linolenic acid is the substrate for lipoxygenase with the formation of 13-L-hydroperoxy-cis-9, 15-trans-11-octadecatrienoic acid (hydroperoxy unsaturated fatty acid). The hydroperoxy fatty acid is converted to 12-oxo-PDA by the enzyme hydroperoxide cyclase. Vick and Zimmerman (1984) also demonstrated that jasmonic acid was indirectly synthesized from 12-oxo-PDA. It is now known that jasmonic acid and its methyl ester possess plant growth-regulating properties (Sembdner & Parthier, 1993).

B. IN CHROMOLAENA MORII

Bohlmann et al. (1981) extracted a PG-like fatty acid derivative from the aerial parts of Chromolaena morii (Compositae). The chemical structure of the compound was elucidated by TLC, NMR, and GC-MS. It is an 18-carbon compound with a cyclopentenone ring and two alkyl side chains and a carboxylic group, and is very similar to 12-oxo-PDA. It is also, probably, synthesized from linolenic acid. The compound was named chromomoric acid. One year later, Bohlmann et el. (1982) published paper on the discovery of another seven PG-like compounds from, again, Chromolaena morii. All the compounds have a cyclopentenone ring and different side chains. It was proposed that they were formed from linolenic acid.

C. IN AQUATIC SEDGE

Soon after the above-mentioned reports, a study appeared concerning the detection of another PG-like compound (van Aller et al., 1983). A C20 cyclic trihydroxy unsaturated fatty acid was isolated and characterized from the aquatic sedge Eleocharis microcarpa Torr. (Cyperaceae), using TLC, HPLC, NMR, and GC-MS. The compound was given the chemical name 11-hydroxy-14-(3, 5-di-hydroxy-2-methylcyclo-pentyl)-tetradec-9-ene-12-yneoic acid. As can be seen from the name of the compound, it has an acetylenic bond at carbon-12. This is the first known PG-like compound with such a bond. The compound was found to have allelopathic properties against certain cyanobacteria.

As far as can be ascertained, no known physiological role for the PG-like compounds has been found. The fact is that these compounds have not been studied in detail, except for 12-oxo-PDA, which has been studied in some detail and which, as mentioned earlier, is an indirect precursor of jasmonic acid.

VIII. Possible Physiological Roles of Prostaglandins

A. FLOWERING OF PHARBITIS NIL

Of the many papers published concerning the physiological effects of prostaglandins in plants, the most extensive was the study of the possible involvement of PGs in the flowering of the short-day plant (SDP) Pharbitis nil (Groenewald & Visser, 1974). They are the first recorded researchers on the physiological effects of PG in plants. Their paper appeared shortly after the Attrep et al.'s (1973) paper on the discovery of [PGA.sub.1] in the onion (Allium cepa). In the first half of 1973, when Groenewald and Visser did their experiments on Pharbitis nil, it was not yet known whether PG occurred naturally in plants.

1. Effect of Inhibitors on Flowering of Intact Plants

Groenewald and Visser (1974) studied the effect of four different inhibitors of prostaglandin biosynthesis, in mammalian systems, on the flowering of Pharbitis nil (SDP). The inhibitors used were aspirin (0-acetylsalicylic acid), phenylbutazone (4-butyl-3, 5-dioxo-1, 2-diphenyl pyrazolidine), indomethacin (1-[p-chloro-benzoyl]-2-methyl-5-methoxy-indoleacetic acid) and niflumic acid (2-[3[prime]-trifluoromethylanilino]-nicotinic acid). At least two of the inhibitors used - namely, aspirin and indomethacin - are known inhibitors of the enzyme cyclo-oxygenase. This enzyme catalyzes the conversion of arachidonic acid to [PGG.sub.2], which is an intermediate metabolite and is converted to prostaglandin ([PGF.sub.2[Alpha]] and [PGE.sub.2]).

Pharbitis nil seeds were germinated in sand and the seedlings were exposed to a regime of 16 h light and 8 h darkness (long days) for 9 days. The seedlings were subsequently treated with the three inhibitors, which were dissolved in 60% ethanol (200 mg 100 [ml.sup.-1]). The cotyledons of the seedlings were dipped once in the test solutions. The treated seedling were kept in light for 6 h and were then subjected to 16 h darkness. (A single 16 h dark period is necessary for flower induction). At the end of the dark period, the seedlings were placed under long day conditions (16 h light and 8 h darkness) for 29 days when all the controls (60% ethanol) flowered. It was found that the applied inhibitors of PG biosynthesis inhibited flowering to a greater or lesser extent. Aspirin, phenylbutazone, indomethacin, and niflumic acid inhibited flowering by 90%, 50%, 30%, and 20%, respectively. Controls, which did not receive the short-day induction period and which were kept under long days for the entire duration of the experiment, failed to flower.

Aspirin (0-acetylsalicylic acid) is a derivative of salicylic acid (2-hydroxybenzoic acid) and salicylic acid which occurs naturally in the cotyledons, together with other low molecular weight phenolic acids, of Pharbitis nil (Groenewald, 1981). There is evidence that there is a greater amount of salicylic acid present in P. nil plants kept under long days than in plants kept under short days (Groenewald, 1981). Studies with labeled aspirin showed that applied aspirin to cotyledons is converted to salicylic acid and gentisic acid (2, 5-dihydroxybenzoic acid) (Groenewald, 1981). It is possible, therefore, that salicylic acid is a natural inhibitor of flowering in P. nil during long days. In this regard it is known that salicylic acid is an inhibitor of PG biosynthesis, together with gentisic acid and a few other phenolic acids that occur naturally in plants (Flower, 1974). In another experiment studying the effect of gentisic acid, salicylic acid, and gallic acid (3, 4, 5-trihydroxybenzoic acid) on the flowering of P. nil, it was found that the above-mentioned phenolic acids inhibited flowering by 70%, 40%, and 40%, respectively (Groenewald, 1981). The fact that flowering was more effectively inhibited by aspirin (90%) than by salicylic acid (40%) may be due to poor penetration by the latter compound into the cotyledons.

It was found by Pryce (1972) that another phenolic acid, gallic acid, is a natural inhibitor of flowering in the short-day plant Kalanchoe blossfeldiana. It is not known whether gallic acid is an inhibitor of PG biosynthesis, but, as mentioned above, it inhibited flowering of P. nil by 40%.

It must be mentioned that, in contrast, that salicylic acid, which was excreted by aphids feeding on vegetative and reproductive shoots of the short-day plant Xanthium strumarium, was found to induce flowering in the long-day plant Lemna gibba strain [G.sub.3] (Cleland & Ajami, 1974). The stimulatory effects of salicylic acid on flowering of other species of Lemna were later demonstrated (Raskin, 1992). However, the possibility that salicylic acid functions as an endogenous regulator of flowering in X. strumarium and Lemnaceae is diminished by the fact that salicylic acid did not induce flowering in X. strumarium. Furthermore, no differences could be detected in the levels of salicylic acid in vegetative and flowering X. strumarium and Lemna. Moreover, the effect of salicylic acid on Lemna is not specific, since a large variety of benzoic acids, nonphenolic compounds including chelating agents, ferricyanide, nicotinic acid, and cytokinins could induce flowering (Raskin, 1992). The induction of flowering in other plant families apart from the Lemnaceae are few and far between (Raskin, 1992). It seems, however, that salicylic acid is not a universal inducer of flowering in many different photoperiodic-sensitive plants and is most probably not an endogenous inducer or promoter of flowering in general.

2. Effect of Prostaglandins, Arachidonic Acid, and Inhibitors on Excised Apices

In another paper, Groenewald and Visser (1978) reported the effect of various prostaglandins, arachidonic acid, and inhibitors of PG biosynthesis on the flowering of excised shoot apices of P. nil. The excised apices were grown aseptically in agar in test tubes. The various test compounds used (arachidonic acid) were dissolved in the agar in which the apices were placed. In one experiment where arachidonic acid, [PGE.sub.1], gentisic acid, acetylsalicylic acid, salicylic acid, and oleic acid were applied to excised shoot apices under inductive conditions (short days), it was found that arachidonic acid and [PGE.sub.1] hastened flower formation by 29 days and 28 days, respectively, as compared with controls. (The plantlets that had developed from the apices produced terminal flowers in the test tubes). The benzoic acid derivatives (gentisic acid, acetylsalicylic acid, and salicylic acid) and oleic acid inhibited flowering completely. All three benzoic acid derivatives and oleic acid are potent inhibitors of PG biosynthesis (Flower, 1974; Pace-Asciak & Wolfe, 1968). The above-mentioned compounds were used at concentrations of [10.sup.-4] M.

The effect of arachidonic acid and eight different PGs were studied on the flowering of apices subjected to non-inductive (long day) conditions. However, the plantlets could not be induced to flower. Since the plantlets produced leaves, it is possible that production of natural inhibitors of flowering could have been produced, thus preventing flowering. Some of the natural inhibitors of flowering could be low-molecular-weight phenolic acids. In this regard, it is interesting to note that naturally occurring phenolic acids inhibited membrane-regulated uptake of potassium ions in barley (Glass, 1974). Since PGs are presumed to have their function in membranes, it is conceivable that phenolic acids, produced during long days, could have interfered with the activity of applied PG and thus prevented flowering. In order to avoid natural inhibitor formation, de Fossard (1974) advocates the growing of specimens under inductive conditions.

3. Hypothesis for the Control of Flowering

One hypothesis about the control of flowering in SDPs is that flower-promoting substances exist along with flower-inhibiting substances, and the two interact to give a certain response. It is well documented that the enzyme phenylalanine ammonia-lyase (PAL), which catalyzes the deamination of phenylalanine to cinnamic acid (a key reaction in the biosynthesis of a wide range of secondary plant products such as phenolics, flavonoids, lignins, etc.), is subject to photocontrol in a wide variety of species (Smith, 1975). It is thus possible that the inhibitory effect of non-inductive photoperiods (long days) on the SDP Pharbitis nil may be due to a relatively high rate of phenolic acid production, which in turn inhibits PG biosynthesis. Under short days, the possible formation of prostaglandin could take place from precursors (arachidonic acid, etc.) without the interference of PG-biosynthesis inhibitors, with the result that flowering is promoted.

4. Prostaglandin in Flowering Pharbitis nil and Kalanchoe blossfeldiana

Supporting evidence for the above-mentioned hypothesis comes from Groenewald et al. (1983), who found that the concentration of a prostaglandin ([PGF.sub.2[Alpha]]) is about 20 times higher in Pharbitis nil plants kept under short days than in plants kept under long days. The prostaglandin was determined by RIA. Other support comes from Janistyn (1982a), who could identify [PGF.sub.2[Alpha]] only in flowering plants of the SDP Kalanchoe blossfeldiana. The PG was characterized by GC-MS analysis. It must be remembered that a phenolic acid (gallic acid) was reported to be a natural inhibitor of flowering in this same plant (Pryce, 1972), and applied gallic acid inhibited flowering of Pharbitis nil as well (Groenewald, 1981).

Levin et al. (1988, 1990) reported the detection of prostaglandins and arachidonic acid in buds of Populus balsamifera. These compounds could be detected only in buds and not in leaves. There is evidence that arachidonic acid is the precursor of the PGs identified in the buds. Levin and coworkers suggest that the results obtained in their two reports on P. balsamifera, together with the results obtained by Groenewald and Visser (1974, 1978) and Groenewald et al. (1983) on Pharbitis nil, could indicate that PGs are the hypothetical flowering hormone (florigen).

Groenewald et al. (1994) detected PG mainly in the flowers and seeds of Oenothera stricta. Prostaglandin was detected in leaves and stems only in traces.

B. ASCOSPOROGENESIS OF YEAST IS REPRESSED BY PROSTAGLANDIN INHIBITORS

All of the above-mentioned papers support the hypothesis that prostaglandin could be involved in flowering. There are, however, interesting reports that ascosporogenesis (sexual stage) of the yeast Dipodascopsis is seriously repressed by cyclo-oxygenase inhibitors (aspirin and indomethacin). The result implies that PG biosynthesis occurs during asci formation. There is also evidence that two prostaglandins ([PGE.sub.2] and [PGF.sub.2[Alpha]]) are produced during ascosporogenesis in Dipodascopsis tothii (Botha et al., 1992, 1993). All this implies that PGs may be involved in sexual reproduction of both certain higher and lower organisms (higher plants and fungi).

C. GIBBERELLIC ACID-CONTROLLED RESPONSES

Curry and Galsky (1975) studied the action of prostaglandins on gibberellic acid-con-trolled responses in barley endosperm. It was found that [PGE.sub.1] and [PGE.sub.2] were responsible for the induction of acid phosphatase activity in barley half-seeds lacking embryos. These compounds were found to raise the total amount of enzyme synthesized, as well as the amount released into the medium, to nearly the same levels obtainable when the seeds are incubated in gibberellic acid ([GA.sub.3]) or cyclic-3[prime]5[prime]-adenosine monophosphate (cyclic-AMP). The enzyme induction by PG was inhibited by acetylsalicylic acid, cycloheximide, absicisic acid and 6-methyl purine. The enzyme induction by [GA.sub.3] and cyclic-AMP was also inhibited by the above-mentioned inhibitors. According to the authors, the re-suits suggest that the biochemical processes stimulated by these compounds that ultimately lead to the synthesis of acid phosphatase must be similar. It is interesting to note that one of the inhibitors used (acetylsalicylic acid) is also a potent inhibitor of PG biosynthesis. One way to interpret the results is that the PGs may lead to the synthesis of cyclic-AMP or [GA.sub.3]. [It is known that cyclic-AMP synthesis is stimulated by [PGE.sub.1] and [PGE.sub.2] in a variety of animal systems (Kuehl, 1974)]. It was found that an inhibitor of gibberellin biosynthesis (AMO-1618) has no effect on the ability of [PGE.sub.1] and [PGE.sub.2] to stimulate acid phosphatase activity, and it may mean that neither of the two PGs stimulate gibberellin biosynthesis. There is, however, the possibility that their effect may be on the synthesis of cyclic-AMP.

D. CYCLIC-AMP RELATED RESPONSE

Galsky's group published another paper on prostaglandin and cyclic-AMP related responses (Favus et al., 1977). The inhibition of crown-gall tumor formation on potato discs by cyclic-AMP and [PGE.sub.1] and [PGE.sub.2] were studied. Cyclic-AMP was found to inhibit the induction of crown-gall tumors by Agrobacterium tumefaciens on potato discs by as much as 60%. Similar results were obtained with [PGE.sub.1] and [PGE.sub.2]. Results obtained showed that cyclic-AMP and prostaglandins do not physically block the bacteria from inducing tumors; rather, the results strongly suggest that these compounds are involved in one of the early steps in the transformation of normal plant cells into tumor cells. It was also found that these compounds have no effect on the growth of the bacteria or the viable cell count.

E. PHOTOSYNTHESIS

1. Effect on Isolated Chloroplasts

Barr and Crane (1977) studied the effect of four different prostaglandins ([PGA.sub.2], [PGB.sub.2], [PGE.sub.2], and [PGF.sub.2[Alpha]]) on isolated spinach chloroplasts. It is known that certain fatty acids, especially unsaturated ones such as linoleic and linolenic acids, inhibit the Hill reaction in isolated chloroplasts. The purpose of this study was to ascertain the effect of other lipids, namely, prostaglandins, on the activity of chloroplasts. Oxygen evolution or uptake involving photo-systems I and II (PS-I and PS-II) was measured with a Clark-type electrode. It was found that the effect of [PGE.sub.2] and [PGF.sub.2[Alpha]] was to inhibit the successive PS-I reactions from plastoquinone through P700 to the methylviologen acceptance site in the electron transport chain. On the other hand, [PGA.sub.2] and [PGB.sub.2] stimulated the reactions tested. The prostaglandin effect on PS-II activity was as follows. Here again, as in PS-I, [PGF.sub.2[Alpha]] and [PGB.sub.2] stimulate the overall. [H.sub.2]0 [approaches] methylviologen reaction over its combined PS-II and PS-I pathway. Among other reactions not mentioned, dimethylbenzoquinone reduction by PS-II is inhibited (0.33-1.65 [[micro]gram] [ml.sup.-1]).

The conclusion is that prostaglandins may provide evidence that the rate of photosynthesis is regulated by internal chloroplast control mechanisms.

2. Effect on Corn Leaf Segments

In another study on the effect of a prostaglandin ([PGF.sub.2[Alpha]]) on photosynthesis, Forster et al. (1984) showed that corn leaf segments that were floated on [PGF.sub.2[Alpha]] solutions for a day caused a reduction of about 50% in the incorporation of 14[CO.sub.2], 3H-thymine, and 14C-glycine. The results show that prostaglandin has a significant effect on photosynthesis, nucleic acid metabolism, and protein synthesis.

F. PLANT BIOASSAY SYSTEMS

1. Effect on Three Tissue Elongation Tests and a Stomatal Aperture Test

Studies on the effect of prostaglandins on four plant bioassay systems were performed by Larque-Saavedra (1979). [PGE.sub.1] and [PGF.sub.1[Alpha]] were tested for activity on the following plant bioassay systems: wheat (Triticum vulgare L.) coleoptile growth test, cress (Lepidium sativum L.) root elongation test, wheat root elongation test, and Commelina communis L. stomatal aperture test. In the coleoptile and root bioassay systems, prostaglandins were found to have a negligible effect on growth. Results on stomata show that prostaglandins close stomata when they are treated with or without [CO.sub.2]-free air. [PGF.sub.1[Alpha]] at 2 x [10.sup.-4] M reduced stomatal aperture down to 47% ([PGE.sub.1], 66%) as compared with controls in [CO.sub.2]-free air. In normal air, [10.sup.-4] M [PGE.sub.1] closed stomata down to 28% compared with control, and [10.sup.-6] M [PGF.sub.1[Alpha]] closed them by 51%. Dilute concentrations of prostaglandins were the most effective. Since PGs are presumed to have their action in membranes, and also since it is known that applied PG increased membrane permeability to potassium ions in Nitella flexilis (J. Freimanis, pers. comm.), it is possible that PG regulated the outflow of potassium ions from the guard cells of Commelina communis L., thus causing stomates to close. It is known that potassium ions are involved in, among other things, the opening and closure of stomates (Salisbury & Ross, 1992).

2. Effect on Two Tissue Elongation Tests and a Seed Germination Test

In somewhat similar experiments, Saniewski et al. (1979) found with wheat coleoptile and lettuce hypocotyl elongation tests that prostaglandins had no significant effects. However, the germination of Amaranthus caudatus seeds was stimulated in the light by [PGA.sub.1], [PGA.sub.2], and [PGE.sub.1]. No effect was observed in the dark. The effect of prostaglandins on acid phosphatase activity in A. caudatus seeds was also determined. At a temperature of 29 [degrees] C in the dark treatment with [PGE.sub.1] and [PGF.sub.2[Alpha]] stimulated the activity of the enzyme, whereas [PGE.sub.2] inhibited it. No significant effects were observed with seeds exposed to light at 25 [degrees] C.

These effects of PGs can be summarized as follows: It seems that the PGs tested do not possess auxin-like activity. It is known that in seeds of Amaranthus caudatus white light inhibits germination and that gibberellins or cytokinins reverse the inhibition. As already mentioned, certain PGs stimulated the germination of the seeds, and it is possible that the PGs show a gibberellin-like effect in this case. In this regard, Curry and Galsky (1975) found that PGs had an effect on gibberellin-controlled responses in barley endosperm. This corroborates the work of Saniewski et al. (1979) with A. caudatus seeds. See Gawienowski and Chatterjee (1980) on [GA.sub.3]-controlled action of PG.

G. MEMBRANE PERMEABILITY

1. Effect on Leaflet Movements

It is well known that prostaglandins play a key role in, among other things, the regulation of cell membrane permeability in the animal kingdom (Hinman, 1972). Roblin and Bonmort (1984) carried out experiments bearing in mind the essential property of PG to alter cell membrane permeability. They studied the effects of [PGE.sub.1], [PGE.sub.2], precursors (di-homo-[Gamma]-linolenic acid and arachidonic acid), and inhibitors (indomethacin and phenylbutazone) of PG biosynthesis on dark- and light-induced leaflet movements in Cassia fasciculata Michx. The leaflet movements are brought about by motor organs (or "pulvini") localized at the leaflet bases and are induced by transferring leaves from light to darkness (scotonasty) or from darkness to light (photonasty). The kinetics and mechanism of the movements are well documented (Fondeville et al., 1966; Gaillochet & Gavaudan, 1968; Satter et al., 1970; Roblin, 1982). They are the results of turgor variations in the parenchyma cells of the motor organs. Another report (Satter & Galston, 1981) suggests that migrations of [K.sup.+] and [Cl.sup.-] take place across membranes, resulting in the escape of water from, or uptake of water by, these cells.

[PGE.sub.1] at concentrations of [10.sup.-4] to [10.sup.-6] M increased the rate of both scotonastic and photonastic leaflet movements. [PGE.sub.2] gave the same general results. The precursor of [PGE.sub.1] (di-homo-[Gamma]-linolenic acid) acted on the scotonastic movements in a biphasic way according to the applied concentration. At [10.sup.-5] and [10.sup.-4] M the movements were more rapid, and at [10.sup.-3] M they were partly retarded. The photonastic movements by this precursor were enhanced in the range of [10.sup.-5]-[10.sup.-3] M. Arachidonic acid, the precursor of [PGE.sub.2] induced the same modifications as di-homo-[Gamma]-linolenic acid on the kinetics of the scotonastic and photonastic movements in the range of concentrations used.

The two inhibitors used (indomethacin and phenylbutazone) inhibited the scotonastic movement in a dose-dependent manner. However, both inhibitors promoted the light-induced opening movements. Saeedi et al. (1984) tested the effects of two other inhibitors of PG biosynthesis - namely, salicylic and acetylsalicylic acids - on the scotonastic and photonastic leaflet movements of Cassia fasciculata, with similar results to those obtained with the previously tested inhibitors indomethacin and phenylbutazone. The scotonastic movements were inhibited in a dose-dependent manner, and, again, the unexpected result of promotion of the photonastic movements was obtained with both inhibitors.

The results indicate that the compounds tested (prostaglandins and precursors) can act directly on cell membrane permeability in view of their effects on the increased rate of the scotonastic and photonastic leaflet movements. These effects are consistent with the interpretation that ion transport is involved in both types of movement. There is evidence that the metabolic chain may be physiologically active in the plant since the precursors (arachidonic and di-homo-[Gamma]-linolenic acids) and the prostaglandins all acted in the same manner on the leaflet movements.

The reason why the inhibitors promoted the photonastic movements of the leaflets is not clear.

2. Effect of Electrofusion of Mesophyll Protoplasts

The effect of prostaglandins on cell membranes is further illustrated by the paper by Christov and Vaklinova (1987) on the effects of [PGE.sub.2] and [PGF.sub.2[Alpha]] on the electrofusion of pea (Pisum sativum cv. Ran 1) mesophyll protoplasts. The prostaglandins influenced electrofusion by lowering the threshold voltage necessary for fusion of dielectrophoretically arranged pairs of protoplasts. The direct current voltage threshold also decreased with increasing [Ca.sup.2+] concentration up to 0.1 mM Ca[Cl.sub.2], and the effects of the prostaglandins were more pronounced when Ca[Cl.sub.2] was present in the medium. It was suggested that [PGE.sub.2] and [PGF.sub.2[Alpha]] facilitate the electrofusion of pea mesophyll protoplasts by changing the fluidity of the cell membrane (plasmalemma).

It is possible that calcium ions may play an important role in the enhancement of prostaglandin action on cell membranes, for it is known (J. Freimanis, pers. comm.) that prostaglandin increased membrane permeability to potassium ions in Nitella flexilis only in the presence of calcium ions.

H. STEROIDS

1. Effect of Prostaglandin Inhibitors and Cortisol on Plant Growth

A few years ago, Geuns (1977) reported the stimulatory effect of cortisol (steroid), among other corticosteroids, on the root growth (100% stimulation) and the hypocotyl growth (50% stimulation) of etiolated mung bean seedlings (Phaseolus aureus Roxb.).

It is known in mammalian systems that cortisol is associated with modulation of the arachidonic acid cascade in the formation of prostaglandins, but it has not been studied in plants. From a mammalian study on the inhibition of the plant growth regulator gibberellic acid ([GA.sub.3]) by indomethacin (PG inhibitor), Gawienowski and Chatterjee (1980) hypothesized that PGs were involved in gibberellin action. [It was reported in barley endosperm by Curry and Galsky (1975) that prostaglandins are possibly involved in [GA.sub.3]-controlled responses.] To study the effect of certain PG inhibitors and cortisol on plant growth and development, mung bean seedlings (Vigna radiata L. Wilzek cv. Jumbo) were also used by Gawienowski et al. (1985). The PG inhibitors used by them were aspirin (acetylsalicylic acid), indomethacin, and benoxaprofen. The results were the following: Cortisol at 4.1 x [10.sup.-5] M significantly increased both radicle length and the number of secondary roots. At a concentration of 8.3 x [10.sup.-5] M, cortisol significantly increased the length of the radicle but had no significant effect on secondary root development. Neither of the tested cortisol treatments significantly affected hypocotyl length. Except for indomethacin, none of the tested PG inhibitors had any significant effect on radicle length, number of secondary roots, or hypocotyl length. Indomethacin at 5 x [10.sup.-7] M significantly increased the number of secondary roots as compared with untreated controls. An increase in development of secondary roots by seedlings treated with cortisol plus aspirin as compared with seedlings treated only with cortisol was also observed. Cortisol plus the other two inhibitors did not show similar results.

Several conclusions were drawn from the results obtained. The general lack of any measurable growth or development response to the three inhibitors (aspirin, indomethacin, and benoxaprofen) could be due to a lack of PG biosynthesis in developing mung bean seedlings or to a reserve of PG in the seed with the possible result that no new synthesis of PG occurred over the short duration of the experiments. Another possibility is that the drug effect on the PG synthetase in the plant may be different from that in mammalian systems. The significant increase in secondary roots following treatment with 5 x [10.sup.-7] M indomethacin may be related to a possible metabolism of this compound into a metabolite with physiological action similar to the plant growth regulator indole-3-acetic acid, since indomethacin is a derivative of indole-3-acetic acid.

The increase in development of secondary roots by seedlings treated with cortisol plus aspirin as compared with seedlings treated only with cortisol would be consistent with a prostaglandin system in the mung bean seedlings. In this case, the PG may be inhibitory and the inhibition of PG synthesis by the PG biosynthesis inhibitor (aspirin) may be the cause of the root development. [Cortisol is known to stimulate lipomodulin action which inhibits mammalian phospholipase [A.sub.2] activity, with the result that arachidonic acid is not liberated from the phospholipid. On the other hand, aspirin is an inhibitor of cyclo-oxygenase action. This enzyme is responsible for the transformation of arachidonic acid to prostaglandins ([PGE.sub.2] and [PGF.sub.2[Alpha]]) and other eicosanoids]. It is suggested that the results indicate a possible PG-like system in the mung beans which is related to the plant growth process.

IX. Conclusion

It is clear that prostaglandins occur in many different plants, ranging from lower to higher plants. It is possible that they play a role or different roles in the biochemistry of plants analogous to the role they play in animals. In mammals they are active in reproduction, and in certain plants they are possibly involved in flowering. In mammals they regulate the permeability of cell membranes - also the case in plants. It is therefore possible that there are some areas where the action of prostaglandins in animals and in plants is probably similar. Is it possible that a plant hormone ([GA.sub.3]) can interact with a prostaglandin in the cell membrane with the result that adenylate cyclase is activated with the formation of cyclic-AMP from ATP with the further result that a cascade of reactions is started in the cytosol?

X. Acknowledgments

We are grateful to the FRD and the University of the Orange Free State for financial assistance.

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Title Annotation:	Interpreting Botanical Progress
Author:	Groenewald, E.G.
Publication:	The Botanical Review
Date:	Jul 1, 1997
Words:	10563
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Topics:	Growth (Plants) Analysis Plant biochemistry Plant growth Analysis Plants Chemical analysis Prostaglandin synthesis Prostaglandins Synthesis