Isoflavonoid-inducible resistance to the phytoalexin glyceollin in soybean rhizobia

Abstract

The antibacterial effect of the soybean phytoalexin glyceollin was assayed using a liquid microculture technique. Log-phase cells of Bradyrhizobium japonicum and Sinorhizobium fredii were sensitive to glyceollin.As revealed by growth rates and survival tests, these species were able to tolerate glyceollin after adaptation. (8,9,21,38,42).In a number of plant-pathogen interactions, the induction of phytoalexins, plant-derived antimicrobial low-molecularweight molecules, is believed to play an essential part in host resistance (10, 43). In several legumes, phytoalexins belong to the isoflavonoid type (e.g., pisatin in Pisum sp.; glyceollin in Glycine sp.; medicarpin in Medicago sp.). This is of special interest because flavonoids have been shown to be regulators of rhizobial nod gene activity (see reference 25 for a review). Considering the microsymbionts of soybean, the most active inducing compounds are isoflavonoids (1,14,22). Despite this intriguing bifunctionality of isoflavonoids, i.e., regulatory substances and phytoalexins, only a few data are available concerning the significance of phytoalexins in the interaction between rhizobia and their legume host. Pankhurst and co-workers studied the inhibition of rhizobia by isoflavonoids (28) as well as by flavolans (31). They found an ineffective Rhizobium strain to be more sensitive to the root flavolan of the host plant Lotus corniculatus than its effective counterpart (30). In addition, the ineffective strain induced the synthesis of higher flavolan concentrations in the nodules of this plant (29). Taken together, these results indicate the importance of flavonoids as toxic compounds for rhizobia, which may have consequences for the symbiotic interaction.Glyceollin, a phytoalexin of soybean ( Fig. 1), is strongly induced in response to a number of biological, chemical, and physical stresses, e.g., infection with pathogenic fungi, the presence of heavy metals, or UV radiation (10,19,45

Bradyrhizobium japonicum Mutants Defective in Cyclic β-Glucan Synthesis Show Enhanced Sensitivity to Plant Defense Responses

“…IB). Moreover, none of the B. japonicum strains analyzed in these experiments m etabolized glyceollins, confirming earlier results (Parniske et al, 1991). Why the ab sence of cyclic ß-glucans in B. japonicum AB-14 caused increased sensitivity of the m utant to glyceollins remains to be investigated.…”

“…Bacteria were grown in arabinose-gluconate m e dium (Cole and Elkan, 1973). To m onitor bacterial growth in the presence of phytoalexins, log-phase cells were inoculated into fresh medium in micro cultures of 400-500 |.il according to Parniske et al (1991). Phytoalexins were added from stock solu tions in ethanol.…”

Effect of the plant flavonoid luteolin on Ensifer meliloti 3001 phenotypic responses

“…In other rhizobia, flavonoids can activate efflux pumps involved in resistance to rhizosperic compounds. Isoflavones genistein and daidzein induced resistance to the phytoalexin glyceollin in some soyabean nodulating rhizobia (Parniske et al 1991). In Rhizobium etli, flavonoids induced genes rmeA and rmeB, which encode a MFS pump, that are required both for efficient nodulation in bean (Gonzalez-Pasayo and Martinez-Romero 2000) and for resistance to naringenin, coumaric acid or salicylic acid.…”

Assessment of antibacterial effects of flavonoids by estimation of generation times in liquid bacterial cultures

“…Therefore, it is evident that this method gives quantitative results that can be directly compared with results obtained in various laboratories. In fact, monitoring of growth of microbial cultures was successfully employed in studies on activities of isoflavones previously (Parniske et al 1991;Yazdanyar et al 2001;Ulanowska et al 2006), but those authors used this method for specific purposes of studies on mechanisms of actions of these compounds rather than for screening of their effects on various microorganisms.…”