Unravelling the molecular basis for symbiotic signal transduction in legumes

Under nitrogen-limiting conditions, legumes develop a nitrogen-fixing symbiosis with soil bacteria known as rhizobia, a symbiosiswhich culminates from an exchange of molecular signals betweenthe two organisms. Legumes secrete an array of flavonoid andisoflavonoid compounds from their roots, which activate theexpression of bacterial nodulation ( nod ) genes. Nod genes encodeproteins involved in the synthesis and secretion of Nod factors, β -1,4-linked N -acetylglucosamine tetramers or pentamers harbouringvarious substituents along their backbone. The nature of thesesubstituents varies with bacterial species and confers the oftenhigh level of host specificity observed in these interactions.Compatible legumes recognize rhizobial Nod factors and initiatea series of responses to facilitate bacterial infection and noduledevelopment. Actively growing root hairs from compatible hostsreorientate their growth in the direction of Nod factor perception,express ‘early nodulin’ genes ( ENOD ) and trigger oscillations inintracellular calcium concentrations originating from the nucleustermed ‘calcium spiking’. Concurrent with these events, corticalcells re-enter the cell cycle, giving rise to the primordium of a novelorgan, termed the ‘nodule’ (Fig. 1). Responding root hairs curlaround the bacteria, which subsequently traverse a plant-derived,tubular infection thread through the epidermis to the nodulemeristem (Fig. 2). It is within the dividing cells that rhizobia arereleased from the infection thread into the plant cell cytoplasm asmembrane-bound droplets or ‘symbiosomes’. These organelles andthe plant cells that house them are factories where atmosphericnitrogen is converted to ammonia with energy supplied by hostphotosynthates.Based on the nature of the responses and the range of Nodfactor structures that elicit them, the molecular and cellular eventspreceding nodule development have been modelled as twodistinct pathways, namely ‘signalling’ and ‘entry’ (Ardourel et al .,1994). Signalling events occur in response to low concentrationsof Nod factors and are not contingent on a specific Nod factorstructure. A concentration of 10