Recent Advances in Legume-Microbe Interactions: Recognition, Defense Response, and Symbiosis from a Genomic Perspective

Abstract

The ability of legumes to form symbiotic mutualistic relationships with certain bacteria in the Rhizobiales (collectively called rhizobia) and harness the ability of the bacteria to fix atmospheric N 2 into ammonia has a tremendous impact on natural and agricultural ecosystems. The interaction enables legumes to produce protein-rich seeds and foliage that are critical to many human and animal diets. Past research has illuminated many of the facets of plant-bacterium recognition, nodule formation, nitrogen fixation, and ammonia assimilation. Less well understood are the mechanisms that allow bacterial colonization without triggering plant defense responses. Specifically, how do legumes recognize friend from foe? Recent genomic research probing legume-pathogen and legume-rhizobial interactions are providing clues to help answer this question. Of particular interest are the roles of flavonoid compounds in legume-rhizobial and legume-pathogen interactions. Legumes are a rich source of flavonoids, notably the isoflavones and isoflavanones, which are not found in Arabidopsis (Arabidopsis thaliana). Legume nodules are also rich sources of Cys cluster proteins (CCPs), some of which have been shown to have antimicrobial activity and may play a role in protecting nodules from pathogens. This Update will summarize recent information on molecular characterization of legume disease resistance (R) genes, R-genemediated interactions with pathogens, and parallels in legume-rhizobial interactions. ROLE OF R GENES AND RECEPTORS IN LEGUME-MICROBE INTERACTIONSWhile over 40 disease R genes have been isolated from plants (Martin et al., 2003), only two have been isolated from a legume. Isolation of R genes from legumes has been slow due to lack of detailed genetic maps, appropriate mapping populations, and chromosome walking tools. In addition, the polyploid genomes of many crop legumes makes obtaining mutants difficult, and high frequency transformation systems for legumes are limited. With the active development of genetic and genomic tools for model and crop legumes, isolation of more legume R genes should be forthcoming. Numerous R gene homologs have been identified in legume species by sequence identity of conserved motifs with known R genes (Kanazin et al., 1996;Yu et al., 1996;Zhu et al., 2002). The challenge is to identify the specific genes conferring resistance to a particular pathogen.The Rpg1-b gene from soybean (Glycine max) confers resistance to Pseudomonas syringae pv glycinea (causing bacterial blight) carrying the avrB gene in a classic genefor-gene specific manner (Ashfield et al., 2004). The Rpg1-b gene is a member of the coiled-coil nucleotidebinding Leu-rich repeat (CC-NB-LRR) class of R genes. Interestingly, it shares only limited sequence similarity with the Arabidopsis RPM1 gene, which also confers resistance to P. syringae expressing avrB. Phylogenetic analysis demonstrated that Rpg1-b and RPM1 are not orthologous, suggesting that R genes with avrB specificity have arisen at least twice during plant evolution...

Phenolic acids act as signaling molecules in plant-microbe symbioses

“…145 Recent advances in genomic research provide vital clues to the enigma of legume-rhizobia recognition by antimicrolbial molecules. 146 effect through the synthesis of nod signal molecules. Theunis et al 117 have demonstrated that flavonoids secreted by host plants, activate expression of the nod gene locus, y4wEFG.…”

Medicago truncatula as a model for understanding plant interactions with other organisms, plant development and stress biology: past, present and future

“…Medicago truncatula is also being increasingly used as a model to study pathogen resistance mechanisms (Ellwood et al 2006b;Tivoli et al 2006;Foster-Hartnett et al 2007;Samac and Graham 2007). The value and basis of M. truncatula as a model for nectrotrophic pathogens has been reviewed by Tivoli et al (2006).…”

Molecular Determinants of a Symbiotic Chronic Infection

“…In fact, many rhizobial genes required for either host invasion or chronic persistence have orthologs in closely related alphaproteobacterial pathogens, such as Agrobacterium and Brucella, and often these orthologs have an effect on virulence (11,140,156). Moreover, plants can detect phytopathogens using receptors that are evolutionarily related to those employed by legumes to detect symbiotic rhizobia (144,180). Thus, the broad importance of this symbiosis is ever growing.…”