Effect of rhizobial 1-aminocyclopropane‐1‐carboxylate (ACC) deaminase on soybean drought tolerance

Abstract

Although soybean (Glycine max L. Merr.) obtains nitrogen through symbiotic nitrogen fixation, its grain yield is constrained by drought stress. The production of the plant hormone ethylene tends to increase under stress conditions. Some rhizobia produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, an enzyme that breaks down ACC, the precursor of ethylene, potentially reducing its inhibitory effects and enhancing plant growth during stress. Due to soybean's greater sensitivity to drought stress than other legumes, rhizobia with high ACC deaminase activity may provide a significant advantage under drought conditions. However, not all rhizobia effectively nodulate soybean and support efficient N2 fixation, making strain selection crucial for improving plant tolerance in water-limited environments. This study aimed to (1) assess the cross-nodulation capability of rhizobia isolated from peanut nodules and (2) determine whether high ACC deaminase activity in rhizobia enhances soybean drought tolerance. A nodulation assay revealed that while peanut rhizobia led to nodulation of soybean roots, they failed to support healthy soybean growth, indicating ineffective nitrogen fixation. Further selection showed that ACC deaminase activities for 49 rhizobial isolates obtained from soybean nodules ranged from 0.38 to 3.45 μmol α-ketobutyrate/mg protein/h. The top five isolates with the highest ACC deaminase activities and the isolate with the lowest activity were evaluated for their nodulation and nitrogen fixation potential in a nodulation assay. Based on the assay results, the isolate with the highest ACC deaminase activity (Bradyrhizobium sp. strain 10) and the strain with the lowest activity (Bradyrhizobium sp. strain 14) were selected for a greenhouse experiment to assess their role in soybean drought tolerance. The experiment consisted of a split-plot design with a randomized complete block design within, the main factors being water regime (irrigated vs. drought) and rhizobial inoculation. Drought stress was applied 70 days after planting and lasted 27 days. The results showed that drought stress significantly reduced plant performance by decreasing biomass, photosynthesis, nodule formation, and nitrogen fixation of soybean plants. However, the rhizobial strain with the highest ACC deaminase activity (strain 10) alleviated drought stress by preserving greater chlorophyll levels, enhancing root and nodule dry weights, and promoting higher carotenoid and proline accumulation (p < 0.001) compared with the strain with the lowest ACC deaminase activity (strain 14). This study emphasizes the importance of selecting host-compatible rhizobial strains to ensure effective nodulation and enhance nitrogen fixation efficiency. Furthermore, the results highlight the potential of rhizobia with high ACC deaminase activity to enhance soybean drought tolerance by alleviating stress effects and supporting plant growth under water-limited conditions.