Plant-growth-regulating compounds, groups

There are five groups of plant-growth-regulating compounds auxin, gibberellin, cytokinin, ethylene, and abscisic acid. For the most part, each group contains both naturally occurring hormones and synthetic substances. 

Sesquiterpenes are the most numerous of all terpenoid compounds. The approximately 5000 known compounds of this type mostly can be grouped into about 30 major skeletal types, but at least 200 less common skeletal types are known (Fig. 21.1). The distribution of sesquiterpenes in plants is essentially the same as monoterpenes. Sesquiterpene hydrocarbons are common essential oil components. Although only a few fungi accumulate monoterpenes, many accumulate sesquiterpenes. Abscisic acid, a plant growth-regulating compound, is synthesized as a sesquiterpene in fungi. Despite the fact that this compound often is considered a sesquiterpene, in higher plants abscisic acid is derived from the breakdown of xanthophylls and should be considered as a tetraterpene derivative see Chapter 26). 

This paper traces in some detail the path which led to the discovery of a new class of herbicides, the 2-(5-oxo-2-imidazolin-2-yl)arylcarboxylates. The journey started when it was found that a phthalimide, a-isopropyl-a-methyl-l,3-dioxo-2-isoindoline-acetamide, had sufficient herbicidal activity to warrant further synthesis effort. This work led to a series of analogs essentially devoid of herbicidal activity yet possessing interesting plant growth regulating effects. Further chemical modifications resulted in the synthesis of two new groups of compounds, imidazoisoindolediones and dihydro-imidazoisoindolediones, and the return of herbicidal activity. The imidazoisoindolediones were in turn transformed into o-(5-oxo-imidazo-lin-2-yl)benzoates, the first members of a very interesting new class of herbicides. 

These requirements are valid for the groups of the compounds investigated, they are not valid, however, in the case of benzoic acid, naphthenic acids and aliphatic acid derivatives (e.g., S-carboxymethyl dithiocarbamate), compounds with plant growth-regulating action discovered later (Veldstra, 1952 Van der Kerk et al.,... 

Several carotenoid metabolites have important functions. The most important of these to consider is vitamin A, which is a metabolite of /8-carotene. This compound plays a key role in vision and in other biological reactions. The role of vitamin A in animals has been reviewed by Pitt (1971). Trisporic acid, also a metabolite of j8-carotene, is important in sexual reproduction in Mucorales, a group of fungi (Bu Lock et al., 1976). Sporopol-lenin, found in the outer layer (exine) of both spores and pollen, is considered a carotenoid polymer (Krinsky, 1971). It has been proposed that abscisic acid, an important plant growth regulator, may be a carotenoid metabolite, but a direct biosynthetic pathway from GGPP seems more probable (Burden and Taylor, 1976). 

The molecular manipulation of nicotinic acid designed to obtain a compound with prolonged hypoHpidemic activity has led to a substance in which the carboxyl group of nicotinic acid has been exchanged for the tetrazole moiety [70]. Such an approach had been taken previously in the plant-hormone series, where the carboxyl functions of 3-indolylacetic acid and 2,4-dichlorophenoxyacetic acids had been replaced with the tetrazole group [71]. Parenthetically, the plant-growth regulator, 2,4-dichlorophenoxyacetic acid, has also been reported to possess hypocholesterolemic activity in experimental animals [72]. 

Another group of compounds, which are exemplified by 1-naphthaleneacetic acid and 1-naphthaleneacetamide, are auxin mimics and find use as plant growth regulators. They stimulate root formation in cuttings and transplants. 

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