Minisymposium 11: Plant-symbiont interactions
Abs #
24005: Identification of a naturally occurring supernodulation mutant in Medicago truncatula
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 analysis of the plant side of the symbiosis, but with the development of legume model systems such as L. japonicus and M. truncatula, 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. We have isolated a new supernodulation mutant of Medicago truncatula (temporarily designated sn-1). The mutant was identified as a naturally occurring mutation in the Jemalong cultivar. The sn-1 mutation reverts at a rate of 0.5%, suggesting the lesion may be due to a mobile genetic element. The phenotype of sn-1 is practically indistinguishable from the phenotype of sunn, a previously characterized supernodulator resulting from a mutation in a LRR-receptor kinase, except for a difference in root length evident only when the plants are inoculated and grown on plates versus in an aeroponic chamber. However sequencing of the 3 kb SUNN gene in sn-1 plants revealed no lesion in SUNN, and we can find no lesion in the first 150 bases of promoter sequence or 150 bases past the poly A site. Additionally, the expression of the SUNN gene product as determined by RT-PCR appears normal in sn-1 plants. Shoot grafting experiments show that supernodulation in sn-1 is a shoot-controlled phenotype, as in sunn. Interestingly, in grafting experiments, sn-1 shoots do not rescue sunn roots and vice versa. F1 analysis of progeny from sn-1 crossed with sunn demonstrated that the two mutations do not complement each other and the combined double heterozygous mutant phenotype is indistinguishable from either mutation alone under the conditions we are using for nodulation and growth. Analysis of the F2 from the cross as well as mapping of the gene through a cross to the polymorphic ecotype A20 will be presented.
