BIOLOGY OF AZOSPIRILLUM-SUGARCANE-ASSOCIATION .2. ULTRASTRUCTURE

Tissue cultures of sugarcane support abundant growth of Azospirillum brasilense (SP 7). Visible after 1-2 weeks as a white or pink slime, this growth reaches 2×108 bacteria/mm2 on the surface of callus. Growth of the bacterium is strictly extracellular in viable callus, and instances of intracellular growth result from rupture of the cell wall during senescence of callus tissue. A significant proportion of the bacterial population on callus is pleomorphic. Varying the nitrogen source in the nutrient medium caused no obvious effect on callus cell structure. The presence of the bacterium caused structural alterations in callus cells which did not inhibit overall growth of the bacterium. Growth of callus as tight groups of cells lacking intercellular spaces may be important for the establishment of a long-term association with Azospirillum. The interface of bacteria and live callus tissue is at the surface of tight cell groups. Browning of the surface cell layers of these groups in the presence of Azospirillum is not of the rapid nature known for hypersensitivity reactions. Rather, this production of phenolics appears to be due to the accumulation of extracellular bacterial metabolites. The ultrastructure of this and other callus reactions is described. As evidenced by organogenesis, the associated cultures have remained viable for at least 18-20 months. © 1979 Springer-Verlag.