AZOARCUS SPP. AND THEIR INTERACTIONS WITH GRASS-ROOTS

The current knowledge on the divergence within the genus Azoarcus and about interactions with grasses is summarized. Grass-associated member s of this genus of diazotrophs have only been isolated from a salt-and flood-tolerant pioneer plant in Pakistan, Kallar grass (Leptochloa fu sca (L.) Kunth). Members of these bacteria belong to the beta subclass of the Proteobacteria, most closely related to purple bacteria such a s Rhodocyclus purpureus. The isolates from one single host plant showe d a surprising divergence, consisting of five groups of Azoarcus disti nct at the species level. Molecular diagnostic tests, which are based on 16S ribosomal RNA sequences, allowed preliminary assignment of isol ates to Azoarcus by PCR amplification and sequencing of PCR products. Moreover, the moleculer tests enabled us to detect an unculturable str ain in Kallar grass roots, stressing that classical cultivation techni ques at times fail to detect some groups of the microbial population. Using similar techniques, sequences rooting in the Azoarcus clade were also detected in field-grown rice, indicating that the natural host r ange might extend to rice. In gnotobiotic laboratory cultures, a membe r of Azoarcus is able to colonize rice roots endophytically: bacteria invade the roots in the zone of elongation and differentiation, coloni ze the cortex intra- and inter-cellularly, and penetrate deeply into t he vascular system, entering xylem vessels, allowing systemic spreadin g into the rice shoot. Recently, we detected expression of nitrogenase of Azoarcus cells inside roots of rice seedlings, a result encouragin g us to analyze interactions with rice in detail.