Study of the Physiological and Biochemical Characteristics of Cupriavidus metallidurans YX16 and Growth-promoting on Maize Seedlings under Salt Stress

Enhancing the saline alkali tolerance of crops via microorganisms has become a focal point in the reclamation and utilization of saline-alkali lands. In this study, a salt-tolerant plant growth-promoting bacterium, Cupriavidus metallidurans strain YX16, was isolated fromt the red soil of Yunnan Province in China. It is characterized by salt tolerance, nitrogen fixation, inorganic phosphorus solubilization, and indole-3-acetic acid (IAA) production (83.75 ± 1.14 µg/mL), but lacks the ability to solubilize potassium and organic phosphorus, and it does not produce siderophores or 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Under salt stress, inoculation (YX16 suspension) increased the germination rate, radicle length, and index of maize seeds by 23.08%, 33.33%, and 32.62%, respectively. Furthermore, the primary root length, plant height, root dry weight, shoot fresh weight, and shoot dry weight elevated by 23.95%, 52.45%, 28.38%, 51.46%, and 107.69%, respectively, whereas the total root length and root surface area increased by 44.08% and 32.48%, respectively. Inoculation (YX16 suspension) increased the root soluble protein and proline contents by 29.24% and 19.09%, respectively, and the leaf soluble sugar, soluble protein, and proline contents by 16.16%, 18.70%, and 18.71%, respectively, thereby activating the antioxidant system. Specifically, the root superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities elevated by 22.60%, 75.67%, and 15.50%, respectively, while the leaf SOD and POD activities enhanced by 14.11% and 40.27%, respectively; however, the leaf CAT activity and chlorophyll content decreased by 25.80% and 20.81%, respectively. Moreover, the malondialdehyde (MDA) in both the roots and leaves decreased by 27.50% and 49.57%, respectively. Compared with the uninoculated control, the bacterial treatment decreased the root soluble sugar content by 20.64% and the leaf CAT activity by 13.08%, but elevated the root proline content by 29.16%, the root dry weight by 18.75%, and the leaf chlorophyll content by 12.74%; however, other antioxidant enzyme (SOD, POD) activities and root dehydrogenase activity (DHA) were not significantly affected. These findings indicate that Cupriavidus metallidurans YX16 is a salt-tolerant plant growth-promoting rhizobacterium(PGPR) that effectively alleviates salt-induced damage and promotes maize growth, providing a basis for the development of microbial agents for the amelioration of saline-alkali soils.