Bacterial Flora of Endophytes in the Maturing Seed of Cultivated Rice (Oryza sativa)

Endophytic bacteria in the seeds of rice plants (Oryza sativa, cultivar Nipponbare) were studied during the rice maturation process by comparing them with the bacteria at the surface of rice seeds. The endophytic and surface bacteria were isolated from cultivated rice seeds by using a nutrient broth (NB) and a diluted nutrient broth (DNB) agar medium. The endophytes at the early stages of maturation were relatively diverse, consisting of strains closely related to the genera Bacillus, Sphingomonas and Pantoea. In contrast, the endophytic and surface bacteria at the middle and late stages of the maturation process (39 strains altogether) were all closely related only to the genus Bacillus with the exception of one isolate, and almost all of these strains were motile and spore-forming. It was deduced that these bacteria might have migrated into the rice seeds before they hardened that is, during the early stage (the endophytes at the early stage were also all motile and about half of them were spore-forming), subsisting as endospores during the middle and late stages of the maturation process. Furthermore, most of the isolates (19 of 23) from the surface bacteria and the endophytic bacteria grew better when sucrose (1.2 M) was added to the NB liquid medium. These bacteria may be able to adapt to a high osmotic pressure.