EVIDENCE FOR GENETIC CONTROL OF HERBICIDE RESISTANCE IN A RICE-FIELD ISOLATE OF GLOEOCAPSA SP. CAPABLE OF AEROBIC DIAZOTROPHY UNDER PHOTOAUTOTROPHIC CONDITIONS

Abstract

The rice-field isolate of Gloeocapsa sp. grew aerobically at the expense of N₂ as a nitrogen source and showed DCMU-sensitive aerobic nitrogenase activity under photoautotrophic conditions. The two pre-emergence rice- field herbicides Machete and Basalin strongly inhibited growth, photosynthesis, respiration and nitrogenase activity in Nostoc muscorum in a concentration hardly inhibitory to these processes in Gloeocapsa sp. Ethidium bromide treatment of Gloeocapsa sp. resistant to growth inhibition by Machete and Basalin resulted in non-revertible loss of both herbicide resistant phenotypes (Mac^r and Bas^r) without affecting aerobic diazotrophy. The nonrevertible Mac^s Bas^s strain resulting from Ethidium bromide treatment of Mac^r Bas^r Gloeocapsa strain stopped growth, N₂-
fixation and photosynthetic O₂ evolution when inoculated in growth medium containing 10 or 20μg ml^-1 of Machete, Basalin or both. These findings suggest that the rice field isolate of Gloeocapsa sp. is a naturally occurring Machete and Basalin resistant (Mac^r Bas^r) strain dependent on photosynthesis for aerobic diazotrophy and that the genes for the two herbicide-resistant phenotypes (Mac^r Bas^r) are possibly plasmid-borne. The present findings will have a significant bearing on the future of cyanobacterial biotechnology in agriculture.