New Types of Plant Growth Regulators of Microbial Origin: The Likelihood of Practical Use

Abstract

With the exception of the brassinosteriods, all of the ‘major’ plant growth regulators have been isolated from microorganisms: IAA from Rhizopus suinis [38] and Neurospora crassa [39]; GA3 from Gibberella fujikuori (Fusarium moniliforme) [3] and other GA’s from Sphaceloma and Elsinoe species [29]; ABA from Cercospora cruenta [15, 27, 28], Botrytis cinerea [19], Rhizoctonia solani, Fusarium oxysporum f. sp. lycopersici, Ceratocystis coerulescens, and C. fimbriata [8]; ethylene from myriad fungi [16]; trans-zeatin, trans-zeatin riboside, and 6-(4-hydroxy-1,3-dimethyl-but-trans-2-enylamino)-9-β-D-ribofuranosylpurine from Pseudomonas syringae pv. savastanoi [31]. However, there exist many microbial metabolites that have either plant growth regulatory or phytotoxic activity, e.g., the cytocholasins are a potent group of compounds. Cytochalasin H inhibits flowering in tobacco [43] while its congener, pyrichalasin H, inhibits root and shoot growth in rice [25]. Hexylitaconic acid significantly promotes growth in rice seedlings [17]. Neovasinone stimulates lettuce root growth 70% at 100 ppm while neovasinin inhibits root growth 50% at 300 ppm, yet the structures differ by a carbonyl function [24]. Herbicidins are potent, selective toxins that kill weeds [2, 12, 34, 35]. With respect to synthetic modifications, deoxyabscisic acid has been made with a view to regulating plant growth [32].