A MUTATION IN VICIA-FABA RESULTS IN INEFFECTIVE NODULES WITH IMPAIRED BACTEROID DIFFERENTIATION AND REDUCED SYNTHESIS OF LATE NODULINS

Although numerous reports have documented the effect of bacterially-induced ineffectiveness on root nodule structure, function, and plant gene expression, few studies have detailed the effect of the plant genome on similar parameters. In this report effective (N2-fixing) broadbean (Vicia faba L.) and plant-controlled ineffective (non-N2-fixing) broadbean recessive for the sym-1 gene were compared for nodule structure, developmental expression of nodule enzyme activities, enzyme proteins, and mRNAs involved in N assimilation, leghemoglobin (Lb) synthesis, and acetylene reduction activity (ARA). During development of effective wild-type nodules, glutamine synthetase (GS), aspartate aminotransferase (AAT), phosphoenolpyruvate carboxylase (PEPC) and NADH-glutamate synthase (GOGAT) activities and enzyme proteins increased coincident with nodule ARA. The increases in GS, AAT, and PEPC were associated with increased synthesis of mRNAs for these proteins. Synthesis of Lb polypeptides and mRNAs during development of effective nodules was similar to that of GS, AAT, and PEPC. By contrast, ineffective sym-1 nodules displayed little or no ARA and had neither the increases in enzyme activities nor enzyme proteins and mRNAs as seen for effective nodules. The effect of the sym-1 gene appeared to occur late in nodule development at either the stage of bacterial release from infection threads or differentiation of bacteria into bacteroids. High in vitro enzyme activities, enzyme polypeptides, and mRNA levels in parental effective nodules were dependent upon a signal associated with effective bacteroids that was lacking in, sym-1 nodules. Nodule organogenesis did not appear to be a signal for the induction of GS, PEPC, AAT, and Lb expression in sym-1 nodules.