Recent Trends in Microbial Inoculants in Agriculture

Soil microbes have great potential for agriculture. A wide variety of microbes have been utilized to enhance crop production. In the current issue, enhanced growth of rice plants is reported with the inoculation of a nitrogen (N)-fixing bacterium, Azospirillum sp. B510 (2), which possesses a disease-suppressive property against the rice blast fungus (Magnaporthe oryzae) and the bacterial leaf blight pathogen Xanthomonas oryzae (25). The importance of biological nitrogen (N) fixation has been recalled just recently in the N acquisition in tropical rain forest development (5). The development of mass production and inoculation techniques makes culturable microbes much more attractive research targets among numerous and diverse soil microbes. Beneficial microbial inoculants in agriculture are mainly plant growth-promoting bacteria and fungi, and they function through different mechanisms, e.g., the supply of nutrients, the production of plant hormones, and the suppression of various crop pests. N is the most limiting nutrient in crop production, on many occasions, and, thus, there are increasing applications of symbiotic or free-living N-fixing bacteria in sustainable agricultural systems. Biomass production of sugarcane and oil palm was increased by the inoculation of Enterobacter spp. (12) and Bacillus sphaericus (26) strains, respectively, both of which were isolated from the rhizosphere and possessed N-fixing ability. N fixation by the inoculants contributed to increased N nutrition in the host plants. An energetic screening revealed that diverse rhizobacteria possessing N-fixing ability had a plant growth-promoting effect on rice and were a source of biofertilizers (8). Masunaka et al. (14) reported an example in which the solubilization of minerals in the soil is the main effect of a plant growth-promoting fungus, Trichoderma koningi. The PGPF does not produce plant hormones (9) and behaves like mycorrhizal fungi in the establishment of symbiotic associations rather than fungal parasites. A lower production of an isoflavonoid, phytoalexin vesitol, a major defensive response of leguminous plants, was involved in their symbiotic associations. Quorum sensing is a population density-dependent regulation mechanism used by bacteria to regulate gene expression ; many Gram-negative plant pathogens control the expression of virulence factors with their quorum-sensing systems (22). N-acyl-L-homoserine lactones (AHLs) are signal compounds involved in quorum sensing, and AHL-degrading bacteria have been utilized in the biocontrol of plant diseases (21). Chryseobacterium spp. (16) and Micro-bacterium spp. (23) have been isolated from potato roots and leaves, respectively, as AHL-degrading bacteria, and their application as biocontrol agents is expected. Suppression of plant diseases is an …