Evidence for mutualism between a plant growing in a phosphate-limited desert environment and a mineral phosphate solubilizing (MPS) rhizobacterium

Alkaline desert soils are high in insoluble calcium phosphates but deficien t in soluble orthophosphate (Pi) essential for plant growth. In this extrem e environment, one adaptive strategy could involve specific associations be tween plant roots and mineral phosphate solubilizing (MPS) bacteria. The mo st efficient MPS phenotype in Gram-negative bacteria results from extracell ular oxidation of glucose to gluconic acid via the quinoprotein glucose deh ydrogenase. A unique bacterial population isolated from the roots of Helian thus annus jaegeri growing at the edge of an alkaline dry lake in the Mojav e Desert showed no MPS activity and no gluconic acid production. Addition o f a concentrated solution containing material washed from the roots to thes e bacteria in culture resulted in production of high levels of gluconic aci d. This effect was mimicked by addition of the essential glucose dehydrogen ase redox cofactor 2,7,9-tricarboxyl-1H-pyrrolo[2,3]-quinoline-4,5-dione (P QQ) but the bioactive component was not PQQ DNA hybridization data confirme d that this soil bacterium carried a gene with homology to the Escherichia coli quinoprotein glucose dehydrogenase. These data suggest that expression of the direct oxidation pathway in this bacterium may be regulated by sign aling between the bacteria and the plant root. The resultant acidification of the rhizosphere may play a role in nutrient availability and/or other ec ophysiological parameters essential for the survival of this desert plant. (C) 1999 Federation of European Microbiological Societies. Published by Els evier Science B.V. All rights reserved.