Diverse microorganisms are living as endophytes in plant tissues and as epiphytes on plant surfaces in nature. Questions about driving forces shaping the microbial community associated with plants remain unanswered. Because legumes developed systems to attain endosymbioses with rhizobia as well as mycorrhizae during their evolution, the above questions can be addressed using legume mutants relevant to genes for symbiosis. Analytical methods for the microbial community have recently been advanced by enrichment procedures of plant-associated microbes and culture-independent analyses targeting the small subunit of rRNA in microbial ecology. In this review, we first deal with interdisciplinary works on the global diversity of bacteria associated with field-grown soybeans with different nodulation genotypes and nitrogen application. A subpopulation of Proteobacteria in aerial parts of soybean shoots was likely to be regulated through both the autoregulation system for plant-rhizobium symbiosis and the nitrogen signaling pathway, suggesting that legumes accommodate a taxonomically characteristic microbial community through unknown plant-microbe communications. In addition to the community views, we then show multiphasic analysis of a beneficial rice endophyte for comparative bacterial genomics and plant responses. The significance and perspectives of community- and genome-based approaches are discussed to achieve a better understanding of plant-microbe interactions.