The effects of the Oryza sativa calcium/calmodulin-dependent protein kinase OsCCaMK genotype (dominant homozygous [D], heterozygous [H], recessive homozygous [R]) on rice root-associated bacteria, including endophytes and epiphytes, were examined by using a Tos17 rice mutant line under paddy and upland field conditions. Roots were sampled at the flowering stage and were subjected to clone library analyses. The relative abundance of Alphaproteobacteria was noticeably decreased in R plants under both paddy and upland conditions (0.8% and 3.0%, respectively) relative to those in D plants (10.3% and 17.4%, respectively). Population shifts of the Sphingomonadales and Rhizobiales were mainly responsible for this low abundance in R plants. The abundance of Anaerolineae (Chloroflexi) and Clostridia (Firmicutes) was increased in R plants under paddy conditions. The abundance of a subpopulation of Actinobacteria (Saccharothrix spp. and unclassified Actinosynnemataceae) was increased in R plants under upland conditions. Principal coordinate analysis revealed unidirectional community shifts in relation to OsCCaMK gene dosage under both conditions. In addition, shoot length, tiller number, and plant weight decreased as the OsCCaMK gene dosage decreased under upland conditions. These results suggest significant impacts of OsCCaMK on both the diversity of root-associated bacteria and rice plant growth under both paddy and upland field conditions.Legumes developed systems to attain mutual symbiosis with rhizobia and mycorrhizae during their evolution. The genetic requirements for rhizobial and arbuscular mycorrhizal (AM) fungal interactions in plants overlap in a common symbiosis pathway (CSP) that leads to successful root nodule (RN) and AM symbioses (21,24,46). Similarly, the negative control of the degree of nodulation and mycorrhization of roots is also regulated through a common signaling system, so-called autoregulation of nodulation (42). These findings raise the question of whether molecular components regulating RN and AM symbioses also affect other symbiotic microbes in the phytosphere.Diverse microorganisms reside in and on plants as endophytes and epiphytes (11,29,35,48). These symbiotic microbes assist plants in the uptake of nutrients (22), scavenge toxic compounds (5), and exert considerable influence upon the overall health of host plants (6). However, many questions remain about the driving forces and ecological rules underlying the relationships between these microbes and plants (12,36).Recently, it was shown that symbiosis-defective mutants of Medicago truncatula (30) and soybean (16, 32) possess bacterial and fungal communities in their roots different from those in wild-type host plants and that certain bacteria preferentially associate with mycorrhizal roots (41). These findings indicate that genetic alteration in RN/AM signaling pathways can also alter the microflora of the rhizosphere. Interestingly, analyses of the rhizosphere of soybeans indicated that the bacterial community structures of nonnodulated...