Minisymposium 9: Legume Biology

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Abs # M0901: Multiple interacting genes regulate nodule number in Medicago truncatula

Presenter:	Frugoli, Julia       Contact Presenter
Authors	Smith, Lucinda  (B)   Schnabel, Elise  (A)   Mukherjee, Arijit  (A)   Long, Sharon  (B)   Frugoli, Julia  (A)
Affiliations:	(A): Clemson University (B): Stanford University

One of the most interesting aspects of legume biology is the symbiotic association between legumes and rhizobia, termed nodulation, which allows legumes to grow in nitrogen-poor soils. Through the study of legume model systems, a molecular understanding of nodulation is emerging. Non-nodulating mutants appear to affect rhizobial recognition and response steps of the symbiosis, while supernodulating mutants affect genes controlling the extent of symbiotic development. We have isolated three independent supernodulating mutants of M. truncatula, two of which (lss and sunn) demonstrate intergenic non-complementation. The phenotype of lss (formerly sn-1) is almost indistinguishable from the phenotype of sunn, a previously characterized supernodulator resulting from a mutation in a leucine-rich repeat receptor kinase. Crossing lss with sunn demonstrated that the two mutations do not complement each other in the F1. However while the lss lesion maps close to SUNN, lss is not a lesion in SUNN: Sequencing of the SUNN gene in lss plants revealed no defect in SUNN and wild type plants segregate in the F2 of the sunn/lss cross. Shoot grafting experiments demonstrate that supernodulation in lss and sunn is a shoot-controlled phenotype, and lss plants have altered expression of SUNN in their shoots but not the roots. A third recently isolated supernodulation mutant, rdn, is distinct from sunn and lss: it complements the sunn mutation in the F1, demonstrates root control of nodule number in grafting experiments, and maps to another chromosome. In addition to these genes with primary effects on nodule number, a fourth gene, rae, was identified as a genetic suppressor of the sunn-1 phenotype. Coupled with the recent discovery that sunn plants are defective in auxin loading in the shoot, our analyses of these four mutants and their interactions provide an initial understanding of the genetic pathway for control of nodule number.