Proline fed to intact soybean plants influences acetylene reducing activity and content and metabolism of proline in bacteroids

Abstract

Supplying L-proline to the root system of intact soybean (Glycine max [L.] Merr.) plants stimulated acetylene reducing activity to the same extent as did supplying succinate. Feeding L-proline also caused an increase in bacteroid proline dehydrogenase activity that was highly correlated with the increase in acetylene-reducing activity. Twenty-four hours after irrigating with L-proline, endogenous proline content had increased in host cell cytoplasm and bacteroids, about three- and eightfold, respectively. In bacteroids, proline concentration was calculated to be at least 3.5 millimolar. In experiments in which [U-14C]L-proline was supplied to uprooted, intact plants incubated in aerated solution, 14C-labeled products of proline metabolism, as well as [14C]proline itself, accumulated in both host cells and bacteroids. When plants were incubated in aerated solutions containing [5-3H]L-proline, 3H-labeled proline was found in host cells and bacteroids. [3H] Pyrroline-5-carboxylate was found in bacteroids, but not host cells, after a 2-hour incubation in [5-3H]L-proline. When [U-14C]L-proline was supplied for 24 hours, a significant amount of [14C] pyrroline-5-carboxylate was found in the host cells, in contrast with the results from the shorter incubation in [5-3H]proline, although the amount in the host cells was only about half the quantity found in the bacteroids. Taken as a whole, these results indicate that proline crosses both plant and bacterial membranes under the in vivo experimental conditions utilized and are consistent with a significant role for proline as an energy source in support of bacteroid functioning. In spite of the increase in acetylene-reducing activity when proline was supplied to the root system of intact plants, proline application did not rescue stem-girdled plants from loss of acetylene-reducing activity, although succinate application did. This suggests a nonphloem route for succinate, but not proline, from roots to nodules.