MOLECULAR PLANT PATHOLOGY (2006) 7 (3), 197–207 DOI: 10.1111/J.1364-3703.2006.00328.X © 2006 BLACKWELL PUBLISHING LTD 197 Blackwell Publishing Ltd Review Unravelling the molecular basis for symbiotic signal transduction in legumes BRENDAN K. RIELY 1 , JEONG-HWAN MUN 1 AND JEAN-MICHEL ANÉ 2 * 1 Department of Plant Pathology, University of California, One Shields Ave., Davis, CA 95616, USA 2 Department of Agronomy, University of Wisconsin, 1575 Linden Drive, Madison, WI 53706, USA INTRODUCTION Under nitrogen-limiting conditions, legumes develop a nitrogen- fixing symbiosis with soil bacteria known as rhizobia, a symbiosis which culminates from an exchange of molecular signals between the two organisms. Legumes secrete an array of flavonoid and isoflavonoid compounds from their roots, which activate the expression of bacterial nodulation (nod) genes. Nod genes encode proteins involved in the synthesis and secretion of Nod factors, β- 1,4-linked N-acetylglucosamine tetramers or pentamers harbouring various substituents along their backbone. The nature of these substituents varies with bacterial species and confers the often high level of host specificity observed in these interactions. Compatible legumes recognize rhizobial Nod factors and initiate a series of responses to facilitate bacterial infection and nodule development. Actively growing root hairs from compatible hosts reorientate their growth in the direction of Nod factor perception, express ‘early nodulin’ genes (ENOD) and trigger oscillations in intracellular calcium concentrations originating from the nucleus termed ‘calcium spiking’. Concurrent with these events, cortical cells re-enter the cell cycle, giving rise to the primordium of a novel organ, termed the ‘nodule’ (Fig. 1). Responding root hairs curl around the bacteria, which subsequently traverse a plant-derived, tubular infection thread through the epidermis to the nodule meristem (Fig. 2). It is within the dividing cells that rhizobia are released from the infection thread into the plant cell cytoplasm as membrane-bound droplets or ‘symbiosomes’. These organelles and the plant cells that house them are factories where atmospheric nitrogen is converted to ammonia with energy supplied by host photosynthates. Based on the nature of the responses and the range of Nod factor structures that elicit them, the molecular and cellular events preceding nodule development have been modelled as two distinct pathways, namely ‘signalling’ and ‘entry’ (Ardourel et al., 1994). Signalling events occur in response to low concentrations of Nod factors and are not contingent on a specific Nod factor structure. A concentration of 10 -12 M of compatible Nod factors is sufficient to induce calcium spiking, ENOD gene expression and cortical cell divisions that are believed to make the host receptive to infection (Ardourel et al., 1994). These events are also stimulated by alternative Nod factor structures, although they require higher concentrations than host-specific Nod factors to achieve similar effects (Oldroyd et al., 2001b; Shaw and Long, 2003). By contrast, infection thread formation and bacterial entry require specific Nod factor decorations (Ardourel et al., 1994; Geurts et al., 1997; Limpens et al., 2003), a phenomenon postulated to guard against entry of non-symbiotic bacteria (Ardourel et al., 1994). Root hairs exhibit a rapid, tip-focused calcium influx and extracellular alka- lization but only in response to high concentrations (e.g. 10 -9 M) of host-specific Nod factors (Felle et al., 1998; Shaw and Long, 2003). Ion fluxes may also participate in infection thread formation and bacterial entry. Genetic screens in the model legumes Medicago truncatula and Lotus japonicus have identified symbiotic mutants that are blocked at different points in the legume–rhizobium interaction. These ‘Nod–’ mutants are incapable of forming root nodules (Table 1). By contrast, ‘Nod++’ mutants form up to ten times the number of functional root nodules as wild-type plants, with the corresponding genes proposed to regulate either infection or nodule development. Some of these genes in both categories have recently been cloned, generating models of the biochemical mechanisms regulating signalling, infection and nodule develop- ment. We review these recent reports and discuss the hypotheses arising from the molecular characterization of the gene products. PUTATIVE NOD FACTOR RECEPTORS Nod– mutants have been identified in pea (sym10), in M. truncatula (nfp) and in L. japonicus (nfr1 and nfr5) (Ben Amor et al., 2003; Madsen et al., 2003; Radutoiu et al., 2003) that do not undergo root hair deformations, ENOD gene expression or calcium spiking, the earliest responses to Nod factors. This phenotype suggests *Correspondence: Tel.: +1 608 262 6457; Fax: +1 608 262 5217; E-mail: jane@wisc.edu