ROOT COLONIZATION AND SYSTEMIC SPREADING OF AZOARCUS SP STRAIN-BH72 IN GRASSES

The invasive properties of Azoarcus sp. strain BH72, an endorhizospher ic isolate of Kallar grass, on gnotobiotically grown seedlings of Oryz a sativa IR36 and Leptochloa fusca (L.) Kunth were studied. Additional ly, Azoarcus spp. were localized in roots of field-grown Kallar grass. To facilitate localization and to assure identity of bacteria, geneti cally engineered microorganisms expressing beta-glucuronidase were als o used as inocula. beta-Glucuronidase staining indicated that the apic al region of the root behind the meristem was the most intensively col onized. Light and electron microscopy showed that strain BH72 penetrat ed the rhizoplane preferentially in the zones of elongation and differ entiation and colonized the root interior inter- and intracellularly. In addition to the root cortex, stelar tissue was also colonized; bact eria were found in the xylem. No evidence was obtained that Azoarcus s pp. could reside in living plant cells; rather, plant cells were appar ently destroyed after bacteria had penetrated the cell wall. A common pathogenicity test on tobacco leaves provided no evidence that represe ntative strains of Azoarcus spp. are phytopathogenic. Compared with th e control, inoculation with strain BH72 significantly promoted growth of rice seedlings. This effect was reversed when the plant medium was supplemented with malate (0.2 g/liter). N2 fixation was apparently not involved, because the same response was obtained with a nifK mutant o f strain BH72, which has a Nif- phenotype. Also, Western blot (immunob lot) analysis of protein extracts from rice seedlings gave no indicati on that nitrogenase was present. PCR and Western immunoblotting, using primers specific for eubacteria and antibodies recognizing type-speci fic antigens, respectively, indicated that strain BH72 could colonize rice plants systemically, probably mediated by longitudinal spreading through vessels.