Stress specific signalling between microbes and plants

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The long-term objective of my research program has been to understand the basics of plant-microbe interactions, and eventually apply these understandings to crop plants. Past Discovery grant funds have allowed my group to demonstrate that microbe to plant signals can enhance nodulation and N2 fixation, and plant growth under conditions stressful to plant growth. Herein I propose to test the hypothesis that stressed Bradyrhizobium japonicum produces an altered suite of compounds that promote the N2-fixing sysmbiosis and plant growth, and that their effects on N2-fixation and plant growth are greater than the effects of the microbial signal compounds produced in the absence of stress. The legume N2-fixation symbiosis begins with exchange of signal compounds between legume and rhizobial partners. We have shown that the rhizobia-to-legume signals (lipo-chitooligosaccharies, LCOs) are also able to directly stimulate plant growth, and that this effect is much larger when plants are stressed. Recent reports have indicated that when Rhizobium tropici is stressed it produces LCOs different from those produced under optimal conditions, suggesting the production of stress-specific LCOs. We have recently conducted proteomic analyses on germinating seeds (Arabidopsis and soybean) treated with LCO and have found that enzymes in pathways related to energy metabolism and stress response are increased, confirming the role of LCOs in improving plant stress tolerance. We will culture a set of characterized rhizobia under stressful and non-stressful conditions and examine the broths in which the strains were grown for ability to enhance plant growth, then isolate and characterize new compounds produced under stressful conditions. These compounds will be compared for effects on soybean and corn under optimal and stressful conditions. We will work with salt stress as it is easy to apply. We will examine the effects of these compounds on: germination, early growth, photosynthetic rates, ontogeny (appearance of leaves, flowering, maturity) and total biomass accumulation. We will conduct subsequent work with the most effective of these compounds. We will also examine the hormone profile, gene expression, proteome and metabolome of stressed and unstressed plants treated with these signal compounds. At a time when increased plant productivity is needed to feed an expanding population and supply biofuels, while crops are challenged by increasing climate variability, understanding phenomena that allow development of low-input technologies that improve crop stress tolerance and productivity is very important. My past discovery grants have also focused on microbe-to-plant signals and this has resulted in basic understandings that allowed the development of technologies now being applied to several 10s of millions ha of agricultural land each year.