Abscisic Acid Suppression of Phenylalanine Ammonia- Lyase Activity and mRNA, and Resistance of Soybeans to Phytophthora megasperma f.sp. glycineal

Etiolated hypocotyls of the resistant soybean (Glycine max [L.] Merr.) cultivar Harosoy 63 became susceptible to Phytophthora megasperma (Drechs.) f.sp. glycinea (Hildeb.) Kuan and Erwin race 1 after treatment with abscisic acid. Susceptibility was expressed by increases in lesion size and a major decrease in accumulation of the isoflavonoid phytoalexin, glyceollin. In untreated hypocotyls, activity of phenylalanine ammonia-lyase and accumulation of mRNA for this enzyme increased rapidly after infection, but these increases were suppressed in abscisic acidtreated hypocotyls. The results suggest the possibility that biosynthesis of glyceollin in the resistance response of soybeans may be controlled at the transcriptional level by changes in abscisic acid concentrations caused by infection. The accumulation of the phytoalexin, glyceollin (a mixture of three isomers), is believed to play a major role in resistance ofsoybean (Glycine max [L.] Merr.) cultivars to incompatible races of the fungal pathogen Phytophthora megasperma (Drechs.) f.sp. glycinea (Hildeb.) Kuan and Erwin. Glyceollin production can be promoted also by a variety of stimuli that include polysaccharide and glycoprotein elicitors from microbial or plant sources ( 1, 9, 1 1 ), salts of heavy metals ( 1 9, 23), and ultraviolet light (4). Perhaps the simplest explanation for this similarity in response to such diverse agents is that they indirectly exert their influence through a common control system. Plant growth hormones play a major, if not fully defined, role in the control of plant metabolic processes, and there appears to be no reason a priori why this should not include metabolic responses to infection. In soybeans, it has been proposed that the plant growth hormone, ethylene, does not act as a messenger in the induction ofglyceollin biosynthesis (16) but may influence the flow of carbon into the biosynthetic pathway (12). In beans (Phaseolus vulgaris L.), production of isoflavonoid phytoalexins in response to mercuric chloride or to cell wall material from Rhizoctonia solani was influenced by ABA treatment either positively or negatively, depending on the plant organ and ' D. M. C. was supported by a grant for basic research from the CIBA-GEIGY Corporation. ABA concentration (8). Henfling et al. (10) reported that ABA treatment suppressed resistance and terpenoid phytoalexin accumulation in potato tubers infected with P. infestans or Cladosporium cucumerinum. Similarly, Salt et al. (18) found that ABA treatments increased susceptibility of tobacco to Peronospora tabacina. In this paper, the effect of exogenously sl;.pplied ABA on the expression of resistance and the accumulation of the phytoalexin, glyceollin, in an incompatible interaction of soybeans with Pmg is described. Effects on activity of the enzyme PAL2 (EC 4.3.1.5), which plays a key role in the synthesis of glyceollin and other phenylpropanoids, and on the production ofmRNA for PAL are examined also. MATERIALS AND METHODS