Minisymposium 9: Plant Pathogen/Symbiont Interactions
Abs #
21004: A leucine-rich repeat receptor-like kinase is mutated in the Medicago truncatula hypernodulating sunn mutant
The symbiotic interaction between legumes and rhizobia that allows legumes to utilize atmospheric nitrogen by hosting the rhizobia in root structures called nodules involves complex molecular interactions between the two organisms. Until recently the molecular intractability of most crop legumes has limited molecular genetic analysis of the plant side of the symbiosis. With the development of legume model systems this impasse has been eliminated, and several genes involved in nodulation have been recently cloned. Non-nodulating mutants appear to affect rhizobial recognition and response steps of the symbiosis, while hypernodulation mutants affect genes controlling the extent of symbiotic development. Nodule number is controlled through an autoregulatory mechanism in which nodulating roots induce a shoot response that signals back to the roots inhibiting further nodule initiation and development. The EMS-induced sunn mutant of Medicago truncatula has a defect in this autoregulatory system developing up to 10 times more nodules than wild-type plants despite a similar number of infection events. As in most legume hypernodulation mutants, this root phenotype is determined by the genotype of the shoot suggesting that SUNN is involved in the shoot control of nodulation. The sunn locus maps to the short arm of chromosome 4 and is tightly linked to the marker DNABP. A 400 kb BAC contig spanning the region between flanking recombination events was assembled and the sequence of ~260 kb of this region was screened for candidate genes. A single base change was detected in a leucine-rich repeat receptor-like kinase (LRR-RLK) gene with a high degree of similarity to Arabidopsis CLAVATA1 and to LRR-RLK genes recently discovered to be involved in nodulation regulation in Lotus japonicus (HAR1) and soybean (NARK). The resulting Arg to Lys change is at a position in the kinase domain that is invariant in all characterized serine/threonine and tyrosine protein kinases. We speculate that SUNN may be involved in responding to the root-derived signal via the LRR domain and triggering the shoot response via the interaction of the kinase domain with other proteins. The downstream effects of the sunn mutation on gene expression in roots are being explored. Work supported by USDA grant #2002-02656.
