Nodulation signal specificity and perception in legumes

The protein products of the rhizobial nodulation genes synthesize lipo-chitin oligosaccharides (LCO) signal molecules (Nod signals) that induce root hair deformations and nodule primordia on legume roots. To determine structure-function relationships, a collection of synthetic and natural product LCOs was assayed on Glycine soja roots. Structure-function studies indicated that there was interdependence on backbone length and the presence of substitutions on the LCO reducing end. Pentameric LCOs were active only if modified with a 2-O-methylfucosyl moiety on the reducing end. However, tetrameric LCOs were only active if they lacked this modification. Thus, G. soja recognized LCO without reducing end substitutions, despite the reported importance of these modifications for host range.; Structure function studies were carried out on another tropical legume, Vigna umbellata. In contrast to the studies with G. soja , a tetrameric LCO without any reducing end modification were not active on V. umbellata. A Bradyrhizobium japonicum nodZ mutant, which produces only LCO without 2-O-methylfucose at the reducing end, was able to induce nodule structures on both G. soja and V. umbellata. Combinations of LCOs produced by the nodZ mutant acted cooperatively to produce nodule-like structures on V. umbellata roots.; A recent study reported that a Nod signal binding protein from Dolichos biflorus is a candidate for a Nod signal receptor. This protein, LNP, was found to be an apyrase. To determine if apyrases might play a role in the early events in nodulation, putative orthologs of LNP were analyzed from the model legume Medicago truncatula. Four putative apyrase genes were identified from M. truncatula. Two of the genes identified from M. truncatula, Mtapy1 and Mtapy4 are expressed in roots, and are inducible within three hours after inoculation with Sinorhizobium meliloti. Mtapy1, Mtapy3, and Mtapy4 are present on a single BAC clone, indicating that these apyrases are clustered on the genome. Screening of nodulation deficient mutant lines of M. truncatula revealed that two such mutant lines do not express apyrases to any detectable level. The data suggest a role for apyrases early in the nodulation response, before the involvement of root cortical cell division leading to nodule development.