The effect of a genetically modified Rhizobium meliloti inoculant on fungal alkaline phosphatase and succinate dehydrogenase activities in mycorrhizal alfalfa plants as affected by the water status in soil

A time-course pot experiment was designed to compare the effect of two Rhiz obium meliloti strains, the wild type (WT) and its genetically modified (GM ) derivate, on the physiological activity of the arbuscular mycorrhizal (AM ) fungus Glomus mosseae during the colonization of alfalfa (Medicago sativa L.) roots as affected by the water status in the growing medium using hist ochemical staining methods as succinate dehydrogenase (SDH) and alkaline ph osphatase (ALP) enzyme markers. At each harvest time, and for each water le vel, there were no significant differences in plant growth response between the effect of the two rhizobial WT and GM treatments on AM-plants. This is an unexpected result because the GM strain usually behaves as an improved rhizobial inoculant. Nodulation followed already described patterns, i.e. t he GM strain produced less, but bigger, nodules than the WT strain. In spit e of the lack of an improved growth response to the GM strain, this did not adversely affect the development of the AM symbiosis (Glomus mosseae-Medic ago sativa). It was also found that, under well-watered conditions, about 8 0% of the AM mycelium in plants inoculated with the GM Rhizobium was alive (SDH activity) throughout the experiment, while only 10-20% of the intrarad icalmycelium remained alive in plants inoculated with the WT strain. Both r hizobial strains behaved similarly under water-limiting conditions in regar d to AM development.