Ammonium-excreting Azospirillum brasilense C3:gusA inhabiting induced tumors along stem and roots of rice

Rice seedlings when grown in a germination nursery develop tumorous structures (paranodules) along primary roots and at the stembase around their hypocotyls when treated with low concentrations of 2,4-dichlor-phenoxy-acetic-acid (2,4D). Stem tumors are likely to emerge from adventitious root primordia. Introduced mutant NH4+-excreting Azospirillum brasilense (strain C3:gusA) inhabited both stem and root tumors with auxin-affected meristems as attractive infection sites. Evidence is given that introduced A. brasilense C3 follows a crack entry invasion at sites where developing tumors emerged through the root cortex and epidermis. Nitrogenase activity of tumors inhabited by bacteria is significantly increased as compared with untreated control plants with highest rates of nitrogen fixation found at an oxygen tension of 3 kPa O2. When transplanted to a submerged sand, 2,4D-induced root tumors grew to form larger structures and stem tumors elongated to form short thickened roots. Introduced bacteria survived in considerable numbers. Possible ways to selectively induce paranodule tumors along roots are discussed.