Abstract
Besides genistein and daidzein, which are active inducers of the nodYABCSUIJ operon in Bradyrhizobium japonicum, soybean seeds also excrete compounds that are not inducers of the nodYABCSUIJ genes but enhance induction of this operon in the presence of a suboptimal genistein concentration. This synergism was studied in detail, and specific compounds were identified in seed exudate which specifically induce the nodD1 gene but not the nodYABCSUIJ operon. Therefore, our current hypothesis is that the observed synergism is caused by a specific induction of nodD1. The specific nodD1 inducers from soybean seed extract have been purified and characterized chemically. They appear to be derivatives of genistein, glycitein, and daidzein with glucose, malonyl, and acetyl groups attached. Both root and seed exudate appear to contain these compounds, with the seed being the major source. No hydrolysis of these compounds to their aglycone forms was detected in the presence of B. japonicum. A model for nod gene induction in B. japonicum is discussed.
Highlights
NodD1.The specific nodDl inducers from soybean seed of the right inducer molecules, which were identified as speextract have been purified and characterized chemi- cific flavonoids
NodD responsibleforflavonoid activationand recognition points to a direct interaction between the two
Bradyrhizobium japonicum is the symbiont of Glycine species
Summary
R. leguminosarumbv. trifolii nodulates Trifolium species, and Rhizobium meliloti nodulates species of Medicago and Melilotus. B. japonicum has two nodD genes (Gottfert et al, 1989),of which the nodDl gene is themost intensively studied andwas found to be essential for flavonoid induction of the nod genes (Banfalvi et al, 1988). The main inducers of the nodYABC genes were isolated from soybean, the common host plant of this bacterium,and identified as the isoflavonesgenistein and daidzein (Banfalvi et al, 1988; Gottfert et al, 1988; Kosslak et al, 1987). In contrast to most other rhizobia, the nodDl gene in B. japonicum is alsoinducible, and genistein and daidzein appeared tobe good inducers (Banfalviet al., 1988). Able to induce the nodDl gene but not the nodYABCSUIJ genes These inducerswere purified from soybean seeds and characterized chemically. § Present address: Dept. of Plant Molecular Biology, Botanical Laboratory, Leiden University, Nonnensteeg 3,2311 VJ Leiden, The Netherlands
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