Azospirillum amazonense inoculation:: effects on growth, yield and N2 fixation of rice (Oryza sativa L.)

Bacteria of the genus Azospirillum are considered to be plant-growth promoting bacteria (PGPR) and stimulate plant growth directly either by synthesising phyto-hormones or by promoting nutrition by the process of biological nitrogen fixation (BNF). Although this genus extensively studied, the effects of inoculation and the possible BNF contribution of the Azospirillum amazonense specie are very scarce. The aim of this study was to isolate, characterise and evaluate auxin production and nitrogenase activity of this species and to select, by inoculation of rice plants, promising isolates based on their ability to improve plant growth, yield and the BNF contribution. One hundred and ten isolates obtained from rice were characterised and grouped according to colony features. Forty-two isolates, confirmed as A. amazonense by the fluorescent in situ hybridization (FISH) technique, were tested for auxin production and nitrogenase activity in vitro. Subsequently plant growth promotion related to plant nutrition effect was evaluated, in vitro and in greenhouse experiments. The BNF contribution to plant growth was evaluated using the N-15 isotope dilution technique. All A. amazonense strains tested produced indoles, but only 10% of them showed high production, above 1.33 mu M mg protein(-1). The nitrogenase activity also was variable and only 9% of isolates showed high nitrogenase activity and the majority (54%) exhibited a low potential. The inoculation of selected strains in rice under gnotobiotic conditions reduced the growth of root and aerial part when compared to the control, showing the negative effects of excess of phytohormone accumulation in the medium. However, in the greenhouse experiment, inoculation of strains of A. amazonense increased grain dry matter accumulation (7 to 11.6%), the number of panicles (3 to 18.6%) and nitrogen accumulation at grain maturation (3.5 to 18.5%). BNF contributions up to 27% were observed for A. amazonense Y2 (wild type strain). The data presented here is the first report that the PGPR effect of A. amazonense for rice plants grown under greenhouse conditions is mainly due the BNF contribution as measured by N-15 isotope dilution technique, in contrast to the hormonal effect observed with other Azospirillum species studied.