AZCÓN, ROSARIO, CAROLINA GALLEGILLOS, CÉSAR AGUIRRE & JOSÉ-MIGUEL BAREA.
Departamento de Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín, CSIC, Prof. Albareda 1, 18008 Granada, SPAIN.
We have investigated whether Rhizobium meliloti strains have a plant growth promoting (PGPR) effect on, either mycorrhizal or non-mycorrhizal, non-leguminous plant species. SuchPGPR activity was evaluated for both a wild type (WT) strain and its genetically modified (GM) derivative,which has been demonstrated to be an improved inoculant for Medicago sativa. The PGPR effect of these R.meliloti strains was tested in interactions with two arbuscular mycorrhizal (AM) fungi: Glomus moseae or Glomus sp. the test plant, Lactuca sativa L. cv Romana, was grown for 40 days under controlled environmental conditions, as affected by the combination of the microbial treatmentsabove indicated. At harvest, changes in plant biomass accumulation and in root morphology were evaluated. In non-mycorrhizal plants there were no significant effects of none of the rhizobial strains. However, both the WT and the GM rhizobial strains improved very much biomass production, the length of both axis and lateral roots, and the number of lateral roots in mycorrhizal lettuce. The interaction between the (GM) rhizobial strain and G. mosseae produced the highest growth promoting effect corroborating that selectivity in bacteria/mycorrhiza interactions apply also to genetic modifications within a given species. Theunderlying mechanisms involved are being investigated. It is noteworthy that the growth promoting effects on plant development did not correlated with the rhizobial effect on AM formation (tripan blue and SDH staining). The ecological relevance of these resultsin interplant rhizosphere relationships is obvious.