Endophytic bacteria in rice seeds inhibit early colonization of roots by Azospirillum brasilense

From the rhizoplane of Oryza sativa, vars. Morelos A-88 and Apatzingan, rice plantlets, we isolated two bacterial strains: Corynebacterium flavescens and Bacillus pumilus. By scanning electron microscopy, endophytic bacteria were frequently identified at the base of secondary roots, between the epidermis and the mucilaginous layer. Endophytes were also identified in the intercellular spaces when the mucilaginous layer was disrupted. These endophytic bacteria were not pathogenic when assayed on tobacco leaves. Plantlets from the rice varieties cultured gnotobiotically under hydroponic conditions were inoculated with Azospirillum brasilense, 6-81 or UAP-154 strains. Control experiments were performed using non-inoculated plantlets or plantlets previously treated with nalidixic acid. Comparison of the length of inoculated or nalidixic acid-treated plantlets, with non-inoculated plantlets revealed a significant (p<0.05) promotion of the growth of the shoots at 15 days of culture in plantlets colonized exclusively by endophytes. A. brasilense seems to be excluded from the rhizoplane by the endophytic bacteria, suggesting that endophytes compete with Azospirillum, and also that A. brasilense inhibits growth of rice. Our results indicate that endophytic bacteria could participate in the growth and development of rice plants.