How to improve legume production under severe environmental stresses

Salinity stress and phosphate (P) deficiency negatively impact agricultural yield throughout the world, affecting plant production whether it is for subsistence or economic gain. The use of plants more tolerant to high salt concentrations and more efficient in the use of P represents the best alternative to achieve a more sustainable agriculture. We show here that a Sinorhizobium meliloti 1021 strain (RD64), engineered to overproduce indole-3- acetic acid (IAA), exhibits long-term cell survival and efficiently overcomes stress conditions as compare to the wild type 1021 strain. Moreover, under P-starved conditions, RD64 is highly effective in mobilizing P from insoluble sources such as phosphate rock when compared to 1021 strain. These abilities were connected to the accumulation of protective osmolytes, the release of higher amounts of P-solubilizing organic acids, the up-regulation of genes coding for the high-affinity P transport system and the induction of acid phosphatase activity. Medicago truncatula plants nodulated by RD64 (Mt-RD64) show higher biomass accumulation and enhanced salt-tolerance as compared to the plants nodulated by 1021 strain (Mt-1021). This effect was positively correlated to the higher internal proline contents and to the enhanced activity of antioxidant enzymes observed for these plants. P-starved Mt-RD64 plants also show a significant increase both in shoot and root fresh weight and release higher levels of organic acids as compared to P-starved Mt-1021 plants. We speculate that the growth promoting effects observed for the model legume Medicago under stressful environmental conditions might be extended to other plant species. Indeed, for legumes such as pea, alfalfa and bean plants, we previously reported an increase in the shoot or seed production for the plants nodulated by IAA-overproducing strains. In addition, for tropical legumes such as soybean and peanut plants, we also have preliminary data indicating the positive effects triggered by the specific IAA-overproducing rhizobia strains.