Autoregulation of infection in the Sinorhizobium meliloti-Medicago symbiosis
J. BATUT (1), A. GARNERONE (1), F. SORROCHE (1), M. WALCH (1), A. GASTEBOIS (1), C. MASSON-BOIVIN (1) (1) INRA-CNRS, France

Autoregulatory mechanisms are essential to the achievement and maintenance of mutualism in beneficial interactions, such as the widespread nitrogen-fixing rhizobium-legume symbiosis. One of the best documented instances is AON (Autoregulation Of Nodulation) that adjusts root nodule number to endogenous and environmental cues in legumes. In most legumes, nodulation is preceded by the infection of root hairs by compatible rhizobia via specialized structures called Infection threads (ITs) that initiate in the epidermis before invading root cortex. Despite early evidence that the number of ITs is under control (1), the existence of a specific mechanism controlling infection has remained elusive because of the intricacy between the nodulation and infection processes (2). Here we provide experimental evidence for a systemic autoregulation of infection (AOI) pathway in the Sinorhizobium meliloti-Medicago truncatula symbiosis. Contrary to AON, AOI is genetically controlled both by the plant and the bacterium. We provide direct evidence that endosymbiotic bacteria, upon activation of a plant-elicited cAMP-mediated signal transduction cascade (3), down regulate systemically root susceptibility to infection at the pre-infection stage. AOI is thus a new component of the complex regulatory pathway controlling the interaction between rhizobia and legumes.

Abstract Number: P2-16
Session Type: Poster