Indole-3-acetic acid (IAA) and its relation to stress-tolerance

Several environmental conditions affect the Rhizobium-legume symbiosis. Arid or acidic soils, too high and low temperatures, and poor water-holding capacities are all factors decreasing the efficiency of plant-bacteria interaction. Moreover, legume root nodule nitrogen-fixing activity is severely affected by salt stress. Some authors hypothesized that, among the metabolites with biocontrol functions, the auxin IAA might be involved in defence plant reactions. Recently, we have shown in E. coli that IAA is able, by inducing different cellular defence systems, to enhance protection against adverse conditions such as UV, high salt and low pH. To investigate if these IAA effects were more widespread, we introduced in S. meliloti 1021 a new pathway for the biosynthesis of IAA (RD64 strain), under the control of a stationary phase-activated promoter active both in free-living bacteria and bacteroids. We found that RD64 showed an increased tolerance to several stress conditions (55°C, 4°C, UV-irradiation, 0.5 M NaCl and pH 3). We have also investigated the response of nodulated alfalfa plants to salt stress, focusing on proline accumulation, known to induce salt stress tolerance. We found that plants exposed to 0.3 M NaCl showed a 2-fold proline increase only if nodulated by the RD64 IAA-overproducing strain.