Root surface colonization of non-cereal crop plants by pleomorphic Azospirillum brasilense Cd

Root surface colonization by Azospirillum brasilense Cd of tomato, pepper and cotton plants under normal growth conditions and soybean plants under normal and water-stress conditions was monitored by scanning electron microscopy and bacterial counts. A. brasilense Cd was capable of efficiently colonizing the elongation and roothair zones of all four plant species tested. In these zones, the bacteria mainly colonized the root surface (tomato, soybean), root-hairs (pepper), or both (cotton), by single cells (tomato, soybean), micro-aggregates (pepper), or a combination of the two (cotton). All inoculated plants demonstrated (i) larger amounts of mucigel-like substance on the root surface than non-inoculated plants and (ii) fibrillar material which anchored the bacterial cells to the root surface and established connections between cells within bacterial aggregates. On non-water-stressed soybean plants, most A. brasilense Cd cells in the rhizosphere occurred as vibroid forms whereas those on water-stressed plants (wilting) were cyst-like. A lower rhizosphere bacterial population was observed on water-stressed plants. When water-stress conditions were eliminated, the bacterial cells reverted to the vibroid form and a concomitant increase in the bacterial population was observed. It is suggested that cyst-like formation is a natural response for A. brasilense Cd in the rhizosphere of water-stressed plants.