Nod factor hydrolysis in Medicago truncatula : signal inactivation or formation of secondary signals?

Nod factors (NFs) of Sinorhizobium meliloti are hydrolyzed by the host plant Medicago truncatula. The NF-cleaving enzyme MtNFH1 (M. truncatula Nod factor hydrolase 1) is a key enzyme in this process. MtNFH1 is a chitinase-like enzyme that rapidly cleaves S. meliloti NFs into lipo-disaccharides and sulfated oligosaccharides. In this chapter, we summarize our current knowledge on NF hydrolysis by M. truncatula. Mutant analysis indicated that MtNFH1 plays a role in infection of root hairs and in formed nodules. Remarkably, nodules of MtNFH1 mutant plants were often clustered together and showed abnormal branching. A similar nodule branching phenotype was observed for transformed wild-type plants expressing MtNFH1 under the control of a tandem CaMV 35S promoter and for wild-type plants inoculated with a NF-overproducing S. meliloti strain. These findings suggest that MtNFH1 inactivates NFs when they reach too high levels. However, it is also possible that NF cleavage products retain certain NF activity or possess properties different from NFs.