Pseudomonas sp. UW4 Enhances Drought Resistance in Garlic by Modulating Growth and Physiological Parameters

Drought stress is one of the primary abiotic factors negatively affecting garlic growth, development, and yield formation. The application of plant growth-promoting bacteria (PGPB) could enhance plant tolerance to drought stress. The aim of this study was to explore the regulatory effect of the PGPB Pseudomonas sp. UW4 on growth and physiological indexes of garlic under drought stress. The results revealed that drought stresses significantly reduced total root length, total root surface area, root projection area and total root volume, chlorophyll content, antioxidant enzyme activity and osmolyte content (proline and soluble proteins), and increased relative electrical conductivity and malondialdehyde (MDA) content, all of which could be significantly improved by inoculating the roots with strain UW4. Under drought stress, an increase in total surface area of roots of 87.06% and an increase in root projected area of 40.71% were observed upon inoculation with strain UW4. The a, b, and total content of chlorophyll were increased significantly by 83.63%, 217.33% and 100.02%, respectively. The osmolyte content in leaves significantly increased, and decreased significantly in roots. The content of antioxidants also significantly increased. Moreover, the relative electrical conductivity in leaves and roots was decreased by 23.18% and 41.20%, respectively, upon strain UW4 inoculation. The content of malondialdehyde (MDA) was decreased by 25.23% and 54.08%, respectively, in the presence of strain UW4. The result of principal component analysis (PCA) revealed that the key factors influencing drought tolerance in garlic inoculated with Pseudomonas sp. UW4 could be summarized into two categories: photosynthetic pigments and root growth-related factors, and leaf osmotic adjustment and root antioxidant enzyme-related factors. Based on the result of the Mantel test, it can be inferred that there was a connection between the osmoregulation and antioxidant enzyme systems in the roots and leaves. Based on the D values, the comprehensive evaluation result of drought resistance was that the drought resistance of the garlic inoculated with strain UW4 under drought stress was lower than that of the garlic inoculated with UW4 under normal treatment and higher than that of the garlic under normal treatment. Therefore, Pseudomonas sp. UW4 enhanced the drought resistance of garlic seedlings by improving root phenotype and antioxidant enzyme activity, and increasing the content of shoot chlorophyll.