Infection of Intact Roots of Kallar Grass and Rice Seedlings by Azoarcus

Abstract

A new group of nitrogen-fixing bacteria (proposed name Azoarcus) occuring in high numbers in the root interior of field-grown Kallar grass were monitored in roots of insitu-grown and reinoculated plants. Bacteria were followed making use of the reporter gene gusA introduced into Azoarcus BH72 and of antibodies specific for this bacterium and dinitrogenase reductase from Rhodospirillum rubrum. High spatial resolution was obtained by light- and electron microscopy. Bacteria were shown to colonize rice and Kallar grass roots inter- and intracellularly. Colonization was sometimes accompanied by appearance of an extracellular matrix of at least partial bacterial origin. Possible sites for N₂-fixation are shown.