Expression of a Pseudomonas aeruginosa citrate synthase gene in tobacco isnot associated with either enhanced citrate accumulation or efflux

Aluminum (Al) toxicity and poor phosphorus (P) availability are factors tha t limit plant growth on many agricultural soils. Previous work reported tha t expression of a Pseudomonas aeruginosa citrate synthase gene in tobacco ( Nicotiana tabacum; CSb lines) resulted in improved Al tolerance (J.M. de la Fuente, V. Ramirez-Rodriguez, J.L. Cabrera-Ponce, L. Herrera-Estrella [199 7] Science 275: 1566-1568) and an enhanced ability to acquire P from alkali ne soils (J. L6pez-Bucio, O. Martinet de la Vega, A. Guevara-Garcia, L. Her rera-Estrella [2000] Nat Biotechnol 18: 450-453). These effects were attrib uted to the P. aeruginosa citrate synthase increasing the biosynthesis and efflux of citrate from roots. To verify these findings we: (a) characterize d citrate efflux from roots of wild-type tobacco; (b) generated tobacco lin es expressing the citrate synthase gene from P, aeruginosa; and (c) analyze d selected CSb lines described above. Al stimulated citrate efflux from int act roots of wild-type tobacco and root apices were found to be responsible for most of the efflux. Despite generating transgenic tobacco lines that e xpressed the citrate synthase protein at up to a 100-fold greater level tha n the previously described CSb lines, these lines did not show increased ac cumulation of citrate in roots or increased Al-activated efflux of citrate from roots. Selected CSb lines, similarly, failed to show differences compa red with controls in either citrate accumulation or efflux. We conclude tha t expression of the P. aeruginosa citrate synthase gene in plants is unlike ly to be a robust and easily reproducible strategy for enhancing the Al tol erance and P-nutrition of crop and pasture species.