Differential impacts of various plant growth-promoting and osmotic tolerant bacterial strains on proline and sugar accumulation to enhance stress adaptations in tea plants.

Abstract

Drought stress poses a severe threat to tea plantations globally, leading to a significant reduction in yields. Use of plant growth-promoting bacteria (PGPB) has emerged as a promising strategy to alleviate the detrimental effects of water stress. This study investigates nine distinct bacterial strains, isolated from a drought-prone region in North-East India, for their plant growth-promoting (PGP) traits and their ability to mitigate osmotic stress. These strains were identified based on morphological characteristics and 16S rRNA molecular analysis. Among them, the strains-Chryseobacterium bernardetii (S₄), Cytobacillus gottheilii (S₅), Kitasatospora aureofaciens (S₇), Kocuria palustris (A), and Brachybacterium rhamnosum (B)-exhibited higher PGP activities under osmotic stress conditions (- 0.19 MPa and - 0.93 MPa induced by PEG-6000). Additionally, K. palustris (A) and B. rhamnosum (B) demonstrate effective adaptation to oxidative stress by reducing proline accumulation and were also found to be catalase (CAT) positive. The effect of these osmotolerant PGPBs was further evaluated on tea seedlings under drought stress. Pot experiments in nursery were conducted with three treatments: a positive control (plants watered frequently), a negative control (no treatment), and eight treatments (T1-T8: bacterial inoculations). When comparing the efficacy of bacterial isolates and delivery methods-bioencapsulation and soil drenching. Treatment T6 (comprising strains S₄, S₅, S₇, A, and B) inoculation via soil drenching method improved drought tolerance by effectively modulating osmolyte concentrations, as evidenced by a reduction in total soluble sugars compared to the negative control, highlighting their potential role as bioformulation enhancing osmotolerance and alleviating drought stress in tea plants.