A novel PGPR strain Kocuria rhizophila Y1 enhances salt stress tolerance in maize by regulating phytohormone levels, nutrient acquisition, redox potential, ion homeostasis, photosynthetic capacity and stress-responsive genes expression

High salinity is harmful for crop physiology and yield. Accumulating evidences indicate that plant growth promoting rhizobacteria (PGPR) can enhance crop growth and reduce the negative impacts of salt stress through regulation of some molecular, biochemical and physiological features. In the present study, a novel halo-tolerant strain Y1 was isolated from maize rhizosphere soil. Sequence analysis of 16S rRNA gene confirmed its identity as Kocuria rhizophila. This strain tolerated up to 10 % NaCl and showed two growth promoting traits like phosphate solubilization and IAA production. The impacts of K. rhizophila Y1 on maize (Zea mays L.) growth and development with salt treatment (0, 100 and 200 mM NaCl) were further examined. The results showed that inoculation with K. rhizophila Y1 strain could protect maize from salt stress by regulating plant hormones (IAA and ABA) levels and improving nutrient acquisition. In detail, inoculation with K. rhizophila Y1 significantly improved growth performance, biomass production, seed germination rate, photosynthetic capacity, antioxidant levels, relative water content and chlorophyll accumulation in maize under saline conditions in comparison with non-inoculation treatment. Moreover, strain Y1 inoculated maize showed lower levels of Na+ and electrolyte leakage under salt treatment compared to non-inoculated ones. Inoculated maize also showed higher transcript levels of genes encoding antioxidants (ZmGR1 and ZmAPX1) and genes involved in salt tolerance (ZmNHX1, ZmNHX2, ZmNHX3, ZmWRKY58 and ZmDREB2A) than in non-inoculated plants. In summary, this study indicated the important contribution of K. rhizophila Y1 in mitigating the deleterious effects of salinity on maize growth and development by regulating plant hormones and nutrient acquisition, and thereby maintaining ion homeostasis, improving photosynthetic capacity, enhancing redox potential and stress-responsive genes expression.