Muestra

Natural growth inhibitors are present in plants and affect bud opening, seed germination and development of dormancy. One such substance, abscisic acid [3-methyl-5-(1-hydroxy-4-oxo-2,6,6-trimethyl-2-cyclo- hexane-1-y1)-cis,trans-2,4-pentadienoic acid] was isolated and char- acterized in 1965; it has also been isolated from the fungus Cenospora

rosicola.

The structural similarity of abscisic acid (ABA) to the carotenoids prompted research on the relationship of these two groups of com- pounds and it has now been demonstrated that some xanthophylls, particularly violaxanthin, produce a germination inhibitor on exposure to light. Evidence has now accumulated in support of an indirect ‘apo- carotenoid’ pathway for ABA biosynthesis, the most likely precleav- age precursors being 9′-cis-neoxanthin and 9-cis-violaxanthin which fracture across the 11,12 (11′,12′) double bond to produce xanthoxin which in plant tissues is readily transformed into ABA (see A. D. Parry and R. Horgan, Phytochem., 1991, 30, 815). However, it is also possible that ABA arises from farnesol at the C15 sesquiterpenoid level of the

MVA pathway (q.v.). The theoretical postulation involves a number of steps in which, as with the apo-carotenoid pathway, cis-Δ2_{′-xanthoxin}

could also be involved. In accord with isoprenoid biosynthesis, mevalonic acid (MVA) has been demonstrated to be stereochemically incorporated into ABA in higher plants, and likewise labelled acetate in Cenospora rosicola. Other substances related to abscisic acid have N _H Kinetin N N CH2 O NH N C CH CH CH Zeatin N _HN N N CH2 CH3 CH2OH CH NH• _C Me O cis-∆2_-Xanthoxin HO CHO Me OH Me Me

Abscisic acid (ABA) O H H C C C CH COOH

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been isolated from plants and include vomifoliol (several sources), which lacks the 2,4-pentadiene side-chain; it has the same activity as abscisic acid in stomatal closure tests. Little or no work appears to have been reported on the effects of abscisic acid on the production of pharmacognostically interesting substances.

A number of synthetic growth inhibitors have been studied; the first to be described was maleic hydrazide in 1949. N-Dimethyl- aminosuccinamic acid can be considered as a hydrazine derivative and acts as a shoot-elongation inhibitor by suppressing the oxidation of tryptamine to IAA. Sciuchetti and colleagues showed that this com- pound sprayed on to Datura stramonium and D. innoxia plants reduced the eventual height of the plants and lowered, overall, their total alka- loid content; however, significant increases were noted in the concen- trations of stem alkaloid (56% increase in the second week and 90% in the fourth week compared with the controls). The inhibitor, tributyl 2,4-dichlorobenzylphosphonium chloride (phosphon) produced simi- lar results with D. ferox. Trigonelline, an alkaloid of fenugreek seeds, promotes cell arrest in G2 (a specific period preceding mitotic division

of the nucleus) in various legumes.

ETHYLENE

It has been known for many years that ethylene induces growth responses in plants, and in 1932 it was demonstrated that the ethylene evolved by stored apples inhibited the growth of potato shoots enclosed with them; it has a role in fruit ripening. Current thought maintains that this simple compound should be included among the natural plant hor- mones. Ethylene is synthesized in the plant from S-adenosylmethionine via the intermediate 1-aminocyclopropane-1-carboxylic acid (ACC). The gene for ACC synthase has been cloned from tomato squash (H. Klee and M. Estelle in Annu. Rev. Plant Physiol., 1991, 42, 529). One biochemical action of ethylene is the stimulation of the de novo synthesis and secretion of cell-wall dissolving enzymes such as cel- lulase during leaf abscission and fruit ripening. A compound that gives rise to a typical ethylene response in plants is (2-chloroethyl)phospho- nic acid (ethephon) applied in aqueous solution in concentrations of

the order of 100–5000 p.p.m. In the cell sap, at pH values above 4.0 it is broken down to ethylene and phosphate. It is marketed as Ethrel.

At low concentration ethylene has been shown to increase the sen- noside concentration in Cassia angustifolia, and applied to tobacco leaves it stimulates production of the stress compounds phytuberin and phytuberol (these are compounds produced in response to tobacco mosaic virus); with Digitalis lanata tissue cultures, cardenolide accu- mulation is decreased. Ethephon is now increasingly used as stan- dard practice for enhancing the flow of rubber latex. Sprayed on to the scraped bark (tapping groove) of the rubber tree it increases latex yields by from 36 to 130%.

Other growth regulators

In addition to the well-known plant growth substances discussed above, a very large number of other compounds have been isolated from natural sources which in some way influence plant growth. Some are widely distributed and others are of restricted occurrence. Generally they have a less specific action than the regulators already mentioned above. They have no common chemical structure and only a few recurrent functional groups (e.g. phenolic hydroxy groups and α-methylene-γ-butyrolactone moieties). This implies that these substances may be acting at many different sites along the growth regulatory process. Substances involved include aliphatic and aro- matic carboxylic acids, phenolic and neutral compounds, salicylate, polyamines, S- and N-heterocyclic compounds, including alkaloids and terpenes. Acorus calamus produces a number of sesquiterpenes having the skeletal structures of cadinane, acorane and eudesmane which inhibit the germination of lettuce seeds (K. Nawamaki and M. Kuroyanagi, Phytochemistry, 1996, 43, 1175). A new class of plant growth regulators known as brassinosteroids is found in the seeds, pollens, galls, leaves, flower-buds and shoots of a considerable range of plants. Some 40 of these compounds are known; they stimulate cell enlargement and cell division and influence gene expression and nucleic acid metabolism at the molecular level, see V. A. Khripach

et al. (1999), Brassinosteroids, a New Class of Plant Hormone, San Diego: Academic Press.

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