Efficient colonization of the endophytes Herbaspirillum huttiense RCA24 and Enterobacter cloacae RCA25 influences the physiological parameters of Oryza sativa L. cv. Baldo rice.

The application of bacterial inoculants is a promising technology to support sustainable crop production systems. Several important bacterial characteristics, such as biological nitrogen fixation, phosphate solubilisation, ACC (1-aminocyclopropane-1-carboxylate) deaminase activity, and production of siderophores and phytohormones, can be assessed as plant growth promotion (PGP) traits. Our aim was to evaluate the effects of nitrogen-fixing and indole-3-acetic acid (IAA) producing endophytes in two Oryza sativa cultivars (Baldo and Vialone Nano). Three bacteria, Herbaspirillum huttiense RCA24, Enterobacter asburiae RCA23 and Staphylococcus sp. 377, producing different IAA levels, were tested for their ability to enhance nifH gene expression and nitrogenase activity in Enterobacter cloacae RCA25. Results showed that H. huttiense RCA24 performed best. The co-inoculation of 'Baldo' rice with strains RCA25 and RCA24 in a 10:1 ratio led to significant improvements of nitrogen-fixation. Furthermore, changes in physiological parameters such as chlorophyll, nitrogen content and shoot dry weight were observed in co-inoculated plants. Based on confocal laser scanning microscopy analysis, strain RCA24 was the best colonizer of the root interior and the only IAA producer located in the same root niche occupied by RCA25 cells. This work shows that the choice of a bio-inoculum having the right composition is one of the key aspects to be considered when a microbial consortium is used for the inoculation of a specific host plant cultivar.