Ethylene plant hormone function

Ethylene (C2H4) is produced in essentially every part of every seed plant and affects a number of metabolic functions in very small concentrations. It is therefore considered a plant hormone [38]. Cultured plant cells are also known to produce C2H4. 

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. 

Extensive studies on ethylene production in higher plants have firmly established the hormonal function of the gas during plant development and in responses to environmental stimuli [1,2], However, the biological significance of microbial ethylene production is not known. 

Of course, the most obvious agonists of regulatory functions in plants are the five well-known classes of plant hormone, lAA, GAs, CKs, ABA and ethylene. In addition, there are several newer candidates including the brassinosteroids, jas-monate and the polyamines. The hormonal status of these newer entries is still somewhat uncertain. 

It s amazing, but the simplest of all alkenes, ethylene, is an important plant hormone. Among other functions, ethylene acts to promote ripening of fruit. Moreover, production of ethylene is autocatalytic that is, a little ethylene induces the formation of more from the amino acid methionine, and its effects are magnified. Accordingly, fruits such as tomatoes and bananas are now typically shipped green in well-ventilated containers so they will arrive unspoiled. Ripening can then be started by exposure to ethylene. 

Phytohormones are substances that, at low concentration, function to coordinate plant growth and development. The compormds that have been considered as plant hormones include indole-3-acetic acid (auxin), cytokinins, gibberelhns (GA), ethylene and abscisic acid (ABA). In addition, brassinosteroids, jasmonic acid (JA) and saUcyhc acid (SA) have been shown to display important growth regulating activities and are also considered to function as phytohormones. 

The stmctural features that make it possible to classify compoimds into families are called functional groups. A functional group is a group of atoms within a molecule that has a characteristic chemical behavior. Chemically, a given functional group behaves in nearly the same way in every molecule it s a part of. For example, compare ethylene, a plant hormone that causes fmit to ripen, with menthene, a much more complicated molecule found in peppermint oil. Both substances contain a carbon-carbon double-bond functional group, and both therefore react with Br2 in the same way to give a product in which a Br atom... 

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. 

Ethylene occurs naturally in petroleum and natural gas, but only to a very small percentage. It also occurs naturally in plants where it functions as a hormone and has a number of important effects on the growth and development of plants. These effects have been used for thousands of years, although the chemical mechanism involved was not understood. For example, the ancient Chinese are said to have burned incense in closed containers in order to facilitate the ripening of pears. Although they were certainly not... 

While the dynamics of hormone receptors in plants is poorly understood, there are several rather hopeful experiments that have been reported. With regard to the possible correlations between abundance of receptors and hormonal responsiveness, some reports are appearing in the literature indicating that a correlation of this sort exists [11, 28]. In view of the abundance of receptors in different fractions of tissues, one may entertain the uneasy possibility that each of the various receptors might be involved in distinctive regulatory functions of the hormone. The first relevant evidence has recently appeared, and this indicates that a mutant which is deficient in an ethylene receptor loses each of the characteristic responses to ethylene, thus suggesting that a single receptor mediates the broad array of responses to this hormone [2]. 

It is also possible that the five known major hormone classes can be subdivided. Particularly, the plethora of different gibberellins now known may in the future be grouped into different subsets functioning in the same plant. Furthermore, it is probable that indoleacetic acid, zeatin, abscisic acid and ethylene are not the only active hormones in their classes. 

This chapter is intended to provide an overview of the functions of the five major plant growth hormones in the intact plant auxin (lAA), gibberellins (GA), cytokinins (K), ethylene, and abscisic acid (ABA). The effects of these hormones have been described in detail on numerous occasions over the last 10 years (see general references at the end of this chapter). This review utilizes these previous reviews as well as including a survey of the literature published between January 1977 and January 1980. The most recent and surely the most detailed treatment of the subject is to be found in Phytohormones and Related Compounds A Comprehensive Treatise. D.S. Letham et al., Volumes 1 and 2, 1978. The intent of the present review is to provide a more general overview and to consider overall patterns of regulatory controls by hormones in the whole plant. 

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