Rhizobial nod gene‐inducing activity in pea nodulation mutants: dissociation of nodulation and flavonoid response

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Abstract
The root exudates of 24 pea (Pisum sativum L.) non-nodulating mutants, originating from cv. 'Finale', were tested for Rhizobium nodulation (nod) gene-inducing activity to reveal potential differences from the wild type. The interaction with flavonoid receptor NodD of R. leguminosarum bv. viciae strain 248 was monitored as the activity of the regulated operon nodABC fused to the reporter gene lacZ. In the uninoculated plants, the only decreased exudate activity was found in the line Risnod17 and ascribed to the impaired root growth in this line. Consequently, constitutive deficiency in the flavonoid nod gene inducers is not a cause of the asymbiotic phenotypes in the studied set. On the other hand, line Risnod5 had constitutively higher nod gene-inducing activity which might be responsible for its conditional nodulation and Fix(-) phenotype when the nodules are present. Monitoring of the response of wild-type plants to inoculation revealed an increase in nod gene-inducing activity (Ini response) starting first day after inoculation. The chosen mutant lines Risnod22 (putative locus sym9) and Risnod25 (putatively sym8) showed Ini as well indicating that the flavonoid burst can be mutationally dissociated from nodule initiation. On the other hand, absence of Ini in Risnod20 (assumed sym19) suggests branching of the signal transduction pathway downstream of this mutation and places it upstream of the two former mutations in the transduction of the bacterial Nod factor signal. More intense Ini response observed in the supernodulating mutant RisfixC, included in the study for its contrasting symbiotic phenotype, suggests common regulation of nodulation and Ini response via systemic shoot factor. The time shift in the exudation of isoflavonoid phytoalexin pisatin compared to Ini, its increased level in Risnod25 and lowered level in RisfixC are consistent with the regulatory role of plant defence in nodule initiation, which might be mediated directly by isoflavonoids as negative regulators.

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