Plant Growth-Promoting Bacteria Modulate Biotic and Abiotic Stress Tolerance in Legumes
The role of microorganisms in nitrogen fixation in legumes was discovered in the nineteenth century. The symbiosis of microorganisms in legumes is displayed as formation of nodules on stems or roots. Nitrogen fixation is carried out in nodules where bacteria transformed in bacteroids reside. The first bacterial isolate, namely, Bacillus radicicola from nodule suspension was able to form nodules in Vicia and Pisum, which was later referred to as Rhizobium leguminosarum (Velazquez et al., 2010). Legumes served as the major host for rhizobial species. Plant growth and productivity are directly related to the availability of nitrogenous compounds. The process where microorganisms convert atmospheric nitrogen to plant usable forms of nitrogen is known as biological nitrogen fixation (BNF). The major portion of soil nitrogen is replenished by BNF that decreases the demand for chemical nitrogen fertilizers (Herridge et al., 2008). BNF approximately contributes 50-70 million tons in agricultural systems.
The major bacterial genera in BNF process include Sinorhizobium, Rhizobium, Mesorhizobium, Bradyrhizobium, and Azorhizobium, jointly known as rhizobia (Franche et al., 2009). Recently, a number of new bacterial species with significant input in BNF have been described, e.g., Cupriavidus (Ralstonia) (Chen et al., 2001), Burkholderia (Moulin et al., 2001), Phyllobacterium (Valverde et al., 2005), Ochrobactrum (Zurdo-Pineiro et al., 2007), Herbaspirillum (Valverde et al., 2003), Devosia (Rivas et al., 2002), and Methylobacterium (Sy et al., 2001). Bacteria as bacteroids reside inside the nodules developed on roots and stems of legumes and thereby convert N,to ammonia (Gibson et al., 2008). Legumes get reduced nitrogenous compounds in the exchange of organic acids in the rhizobial symbiotic association. The symbiotic associations of rhizobial bacteria with legumes contribute to nearly 80% of N obtained from BNF (Graham and Vance, 2003). Rhizobia develop symbiosis with nearly 18,000 legume species, with 100 agricultural legume species spanning various geographic regions of the world and thereby contribute to 50% of N from BNF (Graham and Vance, 2003). The N enrichment, sustainability, and productivity of agricultural systems were significantly improved as a result of legume rotation with non-nitrogen-fixing plants. The improvement in readily mineralizable organic nitrogen reserves is mainly derived from legumes sources compared with chemical fertilizers nitrogen input. Thus, BNF serves as renewable N reserve to replace N fertilizer that enriches soil N fertility (Peoples et al., 1995).
Rhizobia may develop nodulation in a wide range of legume hosts. For example, Rhizobium sp. NGR234 nodulate more than 112 hosts (Pueppke and Broughton, 1999). By contrast, R. leguminosarum bv. trifolii nodulates only Trifolium spp., while its close neighbor R. leguminosarum bv. Viciae nodulates sweet pea (Lathyrus spp.), lentil (Lens spp.), vetch (Vicia spp.), and pea (Pisum spp.) (Perret et al., 2000). The symbiotic interaction occurs as a result of molecular signals. For instance, roots exude a variety of compounds such as non-flavonoid inducers, isoflavonoids, and flavonoids in the rhizosphere that attracts rhizobial strains for symbiotic association (Dharmatilake and Bauer, 1992). These chemical compounds mediate expression of nodulation genes and bacterial growth (Hungría and Stacey, 1997). This also enhances the biosynthesis of lipochitin oligosaccharides referred to as nodulation factor (Lerouge et al., 1990). A single rhizobial strain may produce a wide range of metabolites that function as Nod factors (Spaink et al., 1995). Nodulation genes are divided into three groups. First, nodABC, common nodulation genes exist in almost all bacteria except for bradyrhizobia (Giraud et al., 2007). Nod phenotypes arise as a result of mutations in these genes (Debellé et al., 1986). Biosynthesis of hexameric backbone from b-l,4-linked (V-acetyl-D-glucosamin etrimeric is mediated by NodC, while NodA and B carry out the acetylation and deacetylation of glucosamine, respectively (D’Haeze and Holsters, 2002). A second group of genes involves host-specificity nodulation genes (nodPQ, nodG, nodH, nodFE besides several others), which mediate the rate of nodule formation (Schwedock and Long, 1989). The third category of genes belongs to nodD genes (Spaink, 2000). Nod factors expression is induced by flavonoid compounds exuded by plant roots. Plant roots exude a diverse range of sugars, acids, amines, amino acids, and several other low molecular weight compounds including growth regulators, vitamins, alkaloids, steroids, and flavonoids (Bertin et al., 2003), and thus induce biotic and abiotic stress tolerance in plants. Soil bacterial population is significantly affected by these compounds. Some of these exuded compounds—including amino acids, acids, and sugars—serve as energy and C sources for microorganisms (Jaeger et al., 1999). Other compounds involving flavonoids mediate the exchange of signal transduction between antipathogen plant defense system or two symbionts (Phillips and Kapulnik, 1995). Some of the plant root exudates significantly mediate quorum sensing and thereby affects bacterial communication systems (Teplitski et al., 2000). Plants are recognized as soil engineers through the selection of suitable microbes (Simms and Taylor, 2002). The energy sources and C in root exudation can be effectively enhanced by rhizosphere microorganisms that indicate the existence of feedback in plant-rhizobium interactions mediation bacterial nutrition (Phillips et al., 2004).