Serratia liquefaciens KM4 Improves Salt Stress Tolerance in Maize by Regulating Redox Potential, Ion Homeostasis, Leaf Gas Exchange and Stress-Related Gene Expression

Abstract

High salinity mitigates crop productivity and quality. Plant growth-promoting soil rhizobacteria (PGPR) improve plant growth and abiotic stress tolerance via mediating various physiological and molecular mechanisms. This study investigated the effects of the PGPR strain Serratia liquefaciens KM4 on the growth and physiological and molecular responsiveness of maize (Zea mays L.) plants under salinity stress (0, 80, and 160 mM NaCl). High salinity significantly reduced plant growth and biomass production, nutrient uptake, leaf relative water content, pigment content, leaf gas exchange attributes, and total flavonoid and phenolic contents in maize. However, osmolyte content (e.g., soluble proteins, proline, and free amino acids), oxidative stress markers, and enzymatic and non-enzymatic antioxidants levels were increased in maize under high salinity. On the other hand, Serratia liquefaciens KM4 inoculation significantly reduced oxidative stress markers, but increased the maize growth and biomass production along with better leaf gas exchange, osmoregulation, antioxidant defense systems, and nutrient uptake under salt stress. Moreover, it was found that all these improvements were accompanied with the upregulation of stress-related genes (APX, CAT, SOD, RBCS, RBCL, H⁺-PPase, HKT1, and NHX1), and downregulation of the key gene in ABA biosynthesis (NCED). Taken together, the results demonstrate the beneficial role of Serratia liquefaciens KM4 in improving plant growth and salt stress tolerance in maize by regulating ion homeostasis, redox potential, leaf gas exchange, and stress-related genes expression.

Keywords: Serratia liquefaciens KM4; antioxidants; gene expression; maize; salt tolerance.

Figure 1

Levels of enzymatic (A) and non-enzymatic (B) antioxidants in maize plants in the absence and presence of Serratia liquefaciens KM4 under different saline concentrations. Data are means ± SE (n = 5). Different letters indicate significant differences among treatments. T1, non-inoculated control plants; T2, Serratia liquefaciens KM4-inoculated plants; T3, 80 mM NaCl-treated plants; T4, plants primed with 80 mM NaCl and Serratia liquefaciens KM4; T5, 160 mM NaCl-treated plants; T6, plants primed with 160 mM NaCl and Serratia liquefaciens KM4.

Figure 1

Levels of enzymatic (A) and non-enzymatic (B) antioxidants in maize plants in the absence and presence of Serratia liquefaciens KM4 under different saline concentrations. Data are means ± SE (n = 5). Different letters indicate significant differences among treatments. T1, non-inoculated control plants; T2, Serratia liquefaciens KM4-inoculated plants; T3, 80 mM NaCl-treated plants; T4, plants primed with 80 mM NaCl and Serratia liquefaciens KM4; T5, 160 mM NaCl-treated plants; T6, plants primed with 160 mM NaCl and Serratia liquefaciens KM4.

Figure 2

Expression levels of antioxidant genes (A) and stress-related genes (B) of maize in absence and presence of Serratia liquefaciens KM4 under different saline concentrations. Data are means ± SE (n = 5). Different letters indicate significant differences among treatments (p ≤ 0.05). T1, non-inoculated control plants; T2, Serratia liquefaciens KM4-inoculated plants; T3, 80 mM NaCl-treated plants; T4, plants primed with 80 mM NaCl and Serratia liquefaciens KM4; T5, 160 mM NaCl-treated plants; T6, plants primed with 160 mM NaCl and Serratia liquefaciens KM4.

Figure 2

Expression levels of antioxidant genes (A) and stress-related genes (B) of maize in absence and presence of Serratia liquefaciens KM4 under different saline concentrations. Data are means ± SE (n = 5). Different letters indicate significant differences among treatments (p ≤ 0.05). T1, non-inoculated control plants; T2, Serratia liquefaciens KM4-inoculated plants; T3, 80 mM NaCl-treated plants; T4, plants primed with 80 mM NaCl and Serratia liquefaciens KM4; T5, 160 mM NaCl-treated plants; T6, plants primed with 160 mM NaCl and Serratia liquefaciens KM4.