Plant hormone, ethylene production

Pinosylvin and pinosylvin monomethyl ether are formed in the reaction zone in Pinus spp. as a response to fungus infections (57, 61). There is also an interesting correlation between the stilbene production in Pinus radiata and the presence of ethylene (plant hormone) in the wood (105, 106). The processes involved in heartwood formation and the formation of phenolic compounds in reaction zones have been extensively investigated and reviewed by Hillis (59). 

Ethylene is now considered to be one of the main plant-hormones involved in fruit development. Many responses formerly believed to result from the presence of auxins are now ascribed to induced ethylene production.425 The biosynthetic pathway for formation of ethylene from methionine, in a wide variety of plant tissues, including shoots of mung bean,426 tomato,427 and pea427 carrot427 and tomato428 roots and the fruits of apple,429,430 tomato,427 and avocado,427 has been elucidated, and is as follows. 

Cyclization. A second kind of reaction is represented by the conversion of S-adenosylmethionine to aminocyclopropanecarboxylic acid, a precursor to the plant hormone ethylene (see Chapter 24).159 The quinonoid intermediate cyclizes with elimination of methylthioadenosine to give a Schiff base of the product (Eq. 14-27).160-161a The cyclization step appears to be a simple SN2-like reaction.162... 

The simplest unsaturated carbon compound, ethylene, exerts a major influence on many if not all aspects of plant growth and development. Although ethylene is a gas at physiological temperatures and pressures, it is now recognized as a plant hormone because it is a natural product of metabolism, acts in trace amounts and is neither a substrate nor cofactor in reactions which are associated with major developmental plant processes. Whether or not ethylene meets all the standard criteria established for hormones, there is no question that this gas is a powerful natural regulating substance in plant metabolism, and that it acts and interacts with other recognized plant hormones. With the advent of gas chromatography, ethylene has become the simplest plant hormone to assay since it is evolved from the tissues and requires no extraction or purification prior to analysis. 

Plants under water stress are known to produce increased amounts of ethylene, show a rise in ABA and a decline in endogenous cytokinins (52,53). Other plant hormones are also probably involved in the response to water stress and other stress and wounding actions. The surge of ethylene production upon stress may therefore represent a response to a disturbance of the hormonal balance in tissues. The dampened oscillation curve for wound ethylene production may reflect the dynamic return of the disturbed hormonal system to a proper hormonal balance under the new tissue conditions, and thus may also reflect a healing phenomenon. 

But chrysanthemic acid derivatives are by far not the only examples of cyclopropane-containing structures in nature. In fact, the highly strained three-membered carbocycle is virtually ubiquitous. It occurs, for example, in every green plant in the form of 1-aminocyclopropanecarboxylic acid (ACC) 2, a direct precursor to the plant hormone ethylene [3]. In addition, the cyclopropane unit is found in a variety of other natural products, inter alia in terpenes and in various cyclopropanated fatty acids [4]. The biochemical precursors of the latter are unsaturated fatty acids, and in view of the existence of polyunsaturated fatty... 

Feedback Relationship Between Ethylene and Other Plant Hormones If ethylene production in ripening fruit is an index of aging and senescence, then its suppression should result in retardation, or antagonism to ripening, aging, and senescence. 

Production and Inhibition of Ethylene. Now I would like to illustrate how knowledge about a plant hormone can be used to control and regulate its action. Methionine is the precursor of ethylene in plant tissues (30). Therefore, any compound which blocks methionine metabolism might be expected to inhibit ethylene biosynthesis. Rhizobitoxine was recognized as an inhibitor of methionine biosynthesis (31) as were its analogues shown in Figure 6 (32). 9... 

Among the most noticeable findings from the research on plant hormones, the identification of receptors and receptor genes must be emphasized. In the last decade, since the publication of the previous edition of the book Comprehensive Natural Products Chemistry,5 the receptors and the receptor genes for ethylene, BRs, cytokinins, auxins, GAs, and abscisic acid have been identified, as described in the section for each plant hormone. 

Most of the compounds cited in this introductory section are produced in metabolic processes where the cyclopropane-containing metabolite appears to be the stable end product or secondary product with as yet unobvious metabolic function. However, this is not the case in at least two types of systems, in which cyclopropyl species are key and necessary intermediate structures in high flux metabolic pathways. The first example is the squalene (76) and phytoene (88) biosynthesis where presqualene pyrophosphate (77) and prephytoene pyrophosphate (89) are obligate cyclopropanoid intermediates in the net head-to-head condensations of two farnesyl pyrophosphate (73) or two geranylgeranyl pyrophosphate (66) molecules respectively. The second example is in plant hormone metabolism where C(3) and C(4) of the amino acid methionine are excised as the simple hormone ethylene via intermediacy of 1-aminocyclopropane-l-carboxylic acid (9). Both examples will be discussed in detail in the Section II. 

Ethylene formation is fastidiously controlled in plants (as are other hormone productions) and the oxidative conversion of sp -hybridized C(3) and C(4) of methionine to sp -hybridized ethylene (139) via the (now you see it, now you don t) three-membered ring mechanism is a pretty act of metabolic functional group conjuring. 

These variations in behavior indicate that harvesting melons at different stages of maturity causes subsequent biochemical events involved in amino acid accumulation to follow markedly different pathways. Recent work shows that melon fhiit harvested up to ten days before commercial maturity exhibits climacteric behavior with respect to ethylene production showing that at least this aspect of ripening is not completely inhibited by premature separation from the plant(P). However, the amount of ethylene produced is dependent on maturity at harvest and fruit harvested five days prematurely generated only about half of the amount of ethylene produced by fruit harvested two days before maturity. Also the lag time required to initiate ethylene production after harvest depended on maturity and was longer for prematurely harvested fruit. Changes in the content of the phytohormone abscisic acid were also correlated with that of ethylene. However whether the different maturity related metabolic responses observed above result from the action of these or other plant hormones awaits further study. 

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