Effect of disabling bacteroid proline catabolism on the response of soybeans to repeated drought stress

The aim of this study is to evaluate the contribution of bacteroid proline catabolism as an adaptation to drought stress in soybean plants. To accomplish this, soybeans (Glycine max L. Merr.) were inoculated with either a parental strain of Bradyrhizobium japonicum which was able to catabolize proline, or a mutant strain unable to catabolize proline. A large strain-dependent difference in nodule number and size was observed. In order to separate inoculant-dependent effects on nodulation from effects on bacteroid proline catabolism, plants inoculated with each strain were only compared to other plants inoculated with the same strain, thus removing the observed inoculant-dependent differences in nodulation as a bar to interpretation of the results. This experimental design allowed a comparison of the drought penalty on yield for plants with parental bacteroids and for plants with mutant bacteroids. The two results were then compared to each other in order to evaluate the impact of the ability of bacteroids to catabolize proline on the response to drought stress. When water stress was mild, soybean plants inoculated with bacteria unable to catabolize proline suffered twice the percentage decrease in seed yield as did plants inoculated with bacteria able to catabolize proline. However, when stress was severe there was no significant effect of the ability of bacteroids to catabolize proline on drought imposed decrease in seed yield. These results suggest that increasing the oxidative flux of proline in bacteroids might provide an agronomically significant yield advantage when stress is modest, but that severe drought stress would probably overwhelm this yield benefit.