Evaluation of Two Transgenes for Aluminum Tolerance in Alfalfa

Alfalfa (Medicago sativa L.) production is dramatically reduced in acidic, Al-rich soil. Transgenic plants of several species overexpressing organic acid synthesis and/or organic acid transporter genes have shown enhanced tolerance to Al. The objective of this research was to evaluate the effect of the citrate synthase (CS) and the Daucus carota L. plasma membrane H+-transporting adenosine triphosphatase (H(+)ATPase) (DcPA1) transgenes on Al tolerance in alfalfa. Stem cuttings from a full-sib T-2 population including four isogenic groups (plants with neither transgene, CS only, DcPA1 only, and both transgenes) together with nontransformed check genotypes were evaluated for Al and acid soil tolerance in a greenhouse assay in limed and unlimed soil. Aluminum and acid soil tolerance was assessed by measuring the ratios of root and shoot dry weight in unlimed soil compared to limed soil. The three transgenic populations, CS, DcPA1, and CS plus DcPA1, all showed higher Al and acid soil tolerance and lower levels of Al in shoot tissue than the nontransgenic isogenic population or the nontransgenic parental genotypes. This suggests that an Al-exclusion mechanism could be driving Al and acid soil tolerance in this study. We observed no advantage of combining both transgenes in the same genetic background. These transgenes offer an efficient method to achieve enhanced Al and acid soil tolerant alfalfa cultivars, but more information is needed on their stability across generations and genetic backgrounds and their performance under field conditions.