Intercellular and intracellular colonization of Azorhizobium caulinodans in Arabidopsis thaliana under nitrogen deficient conditions

Abstract

Azorhizobium caulinodans ORS571 is a microsymbiont of the tropical legume Sesbania rostrata. This bacterium induces nodules on the stems and roots of host plants, and fixes nitrogen in the host cells within such nodules. It is also an endophytic bacterium of some non-leguminous plants, such as Arabidopsis thaliana, rice, wheat, and tomato. It is known that this bacterium colonizes in the intercellular spaces and xylems of the roots of A. thaliana plants, but these observations were not conducted under completely nitrogen-free conditions. In this study, in which A. thaliana plants were inoculated with A. caulinodans on nitrogen-free medium, it was found that A. caulinodans promoted the growth of the A. thaliana plants, however, the promotion was not caused by nitrogen fixation. Detailed observations revealed that A. caulinodans intercellularly and intracellularly infected not only the roots but also almost the whole bodies of the A. thaliana plants, except for the stems. In the leaves, infection was frequently observed not only in the stomata and intercellular spaces of the mesophyll cells, but also in the guard cells and mesophyll cells. Interestingly, chloroplasts were also infected with A. caulinodans. In the flowers, A. caulinodans was detected in the pollen cells, in the stigma and the center of the style, and in the space around the ovule in the ovary. This suggests that A. caulinodans colonizes the inside of ovaries via pollen tubes. Taken together, it can be said that this bacterium is more likely to infect A. thaliana plants in nitrogen deficient conditions than in nitrogen sufficient conditions. The findings herein are thought to be the basis for the use of A. caulinodans inoculation for the enhancement of plant growth in oligotrophic conditions.