Interorganismal signaling in suboptimum environments: The legume-rhizobia symbiosis

Abstract

During the Rhizobium -legume interactions that lead to the establishment of the nitrogen fixing symbiosis, an exchange of molecular signals regulates the expression of genes essential for infection, nodule development, and function. All stages of nodule formation and function are impacted by stressful environmental conditions and by soil nitrogen levels. Researchers have concluded that these conditions decrease N 2 fixation activity by directly inhibiting the activity of the nitrogenase complex and by suppressing and/or delaying root infection and nodulation. Infection and early nodule development processes in the soybean- Bradyrhizobium japonicum association are sensitive to suboptimal temperatures. The nodABC genes of Sinorhizobium meliloti and Rhizobium leguminosarum are not expressed under acid conditions. Low pHalso reduced growth and multiplication of rhizobia in soil and increased numbers of ineffective rhizobia. High temperatures were found to increase the release of nod gene inducers from seeds during the first 24 h but decrease the nod gene-inducing activity from bean and soybean roots. Combined nitrogen (NO 3 - , NH 4 + , and urea)has been demonstrated to influence symbiotic N 2 fixation from the initial bidirectional signal exchange between symbionts through to nodule senescence. Nitrate affects a broad range of infection events, including decreases in symbiotic signal exchange, root hair deformation, the binding of rhizobia to root hairs, the number of infection threads formed, and an increase in the number of aborted infection events. Water supply has a major effect on nodulation and N 2 fixation. The relationship between soil moisture and nodulation has been recognized in that nodulation increases with soil water content until waterlogging occurs. Water stresses (deficit and excess) decrease the number of infection threads formed and inhibit nodulation. Under high-salinity conditions, bacterial colonization and root hair curling of plants grown at 100 mol m −3 NaCl are both reduced when compared to those of plants grown at 50 mol m −3 and the proportion of root hairs containing infection threads is reduced by about 30%. All of the negative effects of these environmental factors on legume nodulation and N 2 fixation are known to act entirely or partially through early nod gene induction and nodule infection. Specific compounds have been used to preactivate bacterial nod genes or rhizobia prior to their use as inoculants. Soybean inoculated with preactivated B. japonicum increases nodule numbers and weights (about 30%), seasonal levels of N 2 fixation (35%), and yields (10–40%) when compared to conventional inoculants and when soil temperatures were low at seeding time. Inoculation of soybean with preactivated B. japonicum also accelerated root hair infection and nodule development and increased seasonal N 2 fixation and yield of soybean when soils were saline, acidic, or high in available mineral N. Inoculation of pea with preactivated R. leguminosarum almost doubled nodule number and nodule dry matter and enhanced the final grain yield by 10%.