Patterns of Nodule Development and Nodulin Gene Expression in Alfalfa and Afghanistan Pea

Abstract

Several investigators have shown a correlation between the morphological events observed during nodule development and the patterns of nodulin gene expression in nodules arrested at different stages of development. We have studied the expression of the early nodulins ENOD2 and ENOD12, and the late nodulin leghemo globin in nitrogen-fixing alfalfa and Afghanistan pea nodules and in Rhizobium meliloti mutant-induced alfalfa nodules. Our goal is to use nodulins as molecular markers to track the stages of nodule development. We have also examined the responses of alfalfa and Afghanistan pea to treatment with N-1-(naphthyl)phthalamic acid (NPA), a known auxin transport inhibitor. NPA-induced alfalfa nodules differ from wild type R. meliloti-induced nodules in their morphology and pattern of gene expression. Only MsENOD2 and Nms30 transcripts are detected; MsENOD12 and leghemoglobin are not expressed. However, the NPA-induced nodule-like structures developed on Afghanistan pea roots contain transcripts for both PsENOD2 and PsENOD12; late nodulin genes are not expressed. We have also determined that certain flavonoids induce alfalfa roots to develop nodule-like structures, which contain transcripts for ENOD2. These flavonoid-induced nodules more closely resemble wild type R. meliloti nodules in structure than the nodules induced by R. meliloti exo mutants or by NPA.