Horizons: Rhizobium, the clever burglar

Abstract

It is not known how leguminous plants recognize an invading Rhizobium bacterium as a symbiont rather than a pathogen. Data suggest that the legume host can elicit a defense response to the invading Rhizobium, which may control the number/location of infections. In a successful infection, the invading Rhizobium either does not trigger a defense response or has some mechanism to suppress the host defenses. Martinez-Abarca et al. report that inoculation of alfalfa with a Rhizobium meliloti nodC mutant, but not the wild-type strain, stimulates the production of salicylic acid (SA) in roots. The nodC gene encodes the chitin synthase involved in the synthesis of the lipochitin nodulation signals required for nodule formation; thus, nodC mutants cannot form nodules. SA is thought to be a key signal molecule in the elicitation of the plant defense response. Indeed, plants unable to synthesize SA cannot withstand pathogen invasion. Martinez-Abarca et al. report that the addition of 25μm SA to alfalfa roots prior to R. meliloti inoculation delays nodule appearance, reduces the number of nodules formed and reduces the number of primordia formed by addition of the Nod signal. These results are consistent with their hypothesis that Nod signals may act, at least in part, by suppressing the normal plant defense response. If further substantiated, this hypothesis would bring us significantly closer to a more complete understanding of symbiont–host interaction.