Synthesis of Nodulins and Nodule-Enhanced Polypeptides by Plant Gene-Controlled Ineffective Alfalfa Nodules

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Abstract
Root nodule development and function are accompanied by synthesis of numerous plant gene-encoded, nodule-specific (nodulin) and nodule-enhanced proteins. While bacterially-induced ineffective nodules have proven important in understanding nodulin synthesis, comparable information is lacking for plant gene-controlled ineffective nodules. Here, we compare synthesis of nodulins and nodule-enhanced proteins, including leghaemoglobin (Lb) and glutamine synthetase (GS), between effective alfalfa (Medicago sativa L. cv. Saranac) and two types of single gene recessive plant mutants (in tSa: early-senescing nodules and in3-3811: tumour-like nodules). Most nodulins were synthesized (in vitro) by nodule mRNA from in1Sa plants but at reduced amounts as compared to effective nodules. By contrast, in3-3811 nodules synthesized primarily a 30 kD nodulin. The proportion of acidic to basic Lbs was higher in young (day 7) non-N2-fixing ‘Saranac’ nodules than in mature (day 38) effective nodules. All Lbs were detected in mature in1Sa nodules but the ratio of acidic to basic Lb in vitro polypeptides resembled day 7 non-N2-fixing ‘Saranac’ nodules. Amounts of nodule-enhanced GS polypeptide were reduced in mature ineffective in1. and in3-3811 nodules as compared to ‘Saranac’. Amounts of Lb and GS mRNAs in both ‘Saranac’ and in1Sa increased during nodule development and decreased upon defoliation. Although in1Sa nodules contained less Lb and GS mRNA than did ‘Saranac’ nodules, differences between mRNA content were not as pronounced as differences in Lb and GS protein. These data suggest that expression of Lb and GS in in1Sa nodules is regulated by both transcriptional and post-transcriptional events.

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