VAZQUEZ, MARÍA DELMAR, C. BEJARANO, ROSARIO AZCÓN & JOSÉ-MIGUEL BAREA.
Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental delZaidín, CSIC, Prof. Albareda 1, 18008 Granada, SPAIN.
The effect of two Rhizobium meliloti strains,the wild type (WT) and its genetically modified (GM) derivate was comparedwith regard to formation and physiological performance of the arbuscularmycorrhizal association between Glomus mosseae and alfalfa (Medicagosativa L.) roots. A microcosm time-course experiment wasdesigned to evaluate fungal metabolic activity using histochemical staining methods as succinate dehidrogenase(SDH) and alkaline phosphatase (ALP) enzyme markers. For each harvest time,there were no significant differences in growth response to any of therhizobial WT and GM treatments on AM-plants. This was a unexpected result because the GM strain ussually behavesas an improved rhizobial inoculant, which was demonstrated to benefit AMformation and function in several situations. Nodulation followed awell-defined pattern i.e. the GM strainproduced less, but bigger, nodules than the WT strain. In spite of thelack of an improved growth response to the GM strain in the test soil, thisinoculation treatment (GM Rhizobium), in some way, benefited the development of the AM symbiosis(Glomus mosseae - Medicagosativa). This is important concerning risk assessment of genetically modifiedorganisms to be released in the rhizosphere. In fact, at harvest, about the80% of the AM mycelium in plants inoculated with the GM Rhizobium was alive (SDH activity) thoughout the experiment while only a 10 to 20%of the intraradical mycelium remained alive in plant inoculated with the WTstrain.