ON THE COVER

Establishment of symbiosis between nitrogen-fixing rhizobia and legume plants depends on lipochitoligosaccharides, called Nod factors. These bacterial nodulation signals are rapidly hydrolyzed by host enzymes. Cai et al. (pages 397–414) have identified a symbiotic function for such a Nod factor cleaving enzyme (MEDICAGO TRUNCATULA NOD FACTOR HYDROLASE1 [MtNFH1]). MtNFH1 plays a role in the fine-tuning of the symbiosis at the stage of rhizobial root infection and in nitrogen-fixing nodules. M. truncatula mutant plants deficient in MtNFH1 display delayed rhizobial root hair infection, nodule hypertrophy, and abnormal nodule branching. Such nodule branching was also observed for wild-type plants inoculated by a Nod factor overproducing Sinorhizobium meliloti strain. The cover image depicts clustered nodules formed on roots of the MtNFH1-deficient mutant nfh1-3 (20 d postinoculation with S. meliloti wild-type bacteria).