Ameliorative Effect of Bacillus subtilis on Growth Attributes and Antioxidant Defense Capacity in Tomato (Solanum lycopersicum L.) Under Zinc Toxicity: An Outstanding PGPR to Achieve Heavy Metal Resistance

Abstract The growing problem of heavy metal contamination in agricultural soils poses a serious threat to the world. The accumulation of zinc (Zn) metal in the soil causes Zn stress in plants, inhibiting their growth and productivity. In order to mitigate heavy metal stress, the use of plant growth-promoting rhizobacteria (PGPR) such as Bacillus subtilis is an appealing option due to its favorable effects on plant physiology and biochemistry. This study evaluated the potential of PGPR strain B. subtilis ( Bs ) IMG22, which is used as a beneficial inoculant for enhancing growth and stress tolerance in tomato ( Solanum lycopersicum L.) plants exposed to different levels of Zn toxicity (0.5 and 5 mM). The results revealed that Zn stress adversely influenced the physiological and biochemical parameters in tomato. The Zn-stressed plants showed a decline in growth attributes, leaf relative water content (RWC), and photosynthetic pigment contents. Under Zn stress, the enzymatic activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX); the contents of protein and proline; and the levels of malondialdehyde (MDA) and hydrogen peroxide (H 2 O 2 ) increased. On the other hand, B. subtilis IMG22 inoculation attenuated the levels of oxidative stress indicators and considerably boosted the growth attributes, photosynthetic pigment contents, protein amount, proline accumulation, and antioxidant enzyme activities in Zn-stressed plants compared to uninoculated plants. Therefore, it was concluded that B. subtilis is an ideal candidate for mitigating Zn-induced heavy metal stress, as it promoted tomato growth and enhanced antioxidant enzyme activities and proline content against Zn-induced oxidative stress. Graphical Abstract