Plant growth-promoting bacteria improve leaf antioxidant metabolism of drought-stressed Neotropical trees

Plant growth-promoting bacteria association improved the enzymatic and non-enzymatic antioxidant pathways in Neotropical trees under drought, which led to lower oxidative damage and enhanced drought tolerance in these trees. Water deficit is associated with oxidative stress in plant cells and may, thus, negatively affect the establishment of tree seedlings in reforestation areas. The association with plant growth-promoting bacteria (PGPB) is known to enhance the antioxidant response of crops, but this strategy has not been tested in seedlings of Neotropical trees. We evaluated the effects of inoculation with two PGPB (Azospirillum brasilense and Bacillus sp.) on the antioxidant metabolism of Cecropia pachystachya and Cariniana estrellensis seedlings submitted to drought. We measured the activity of antioxidant enzymes and the content of non-enzymatic antioxidants in leaves, and biometrical parameters of the seedlings. In both tree species, drought decreased the activity of antioxidant enzymes and the content of non-enzymatic antioxidant compounds. For C. pachystachya, the enzymatic and non-enzymatic pathways were mostly influenced by A. brasilense inoculation, which enhanced ascorbate peroxidase (APX) and superoxide dismutase activities and positively affected the level of non-enzymatic antioxidant compounds. In C. estrellensis, A. brasilense inoculation enhanced APX activity. However, A. brasilense and Bacillus sp. inoculation had more influence on the non-enzymatic pathway, as both bacteria induced a greater accumulation of secondary compounds (such as chlorogenic acid, gallic acid, rutin and synapic acid) compared to that in non-inoculated plants under drought. For both species, PGPB improved biometrical parameters related to drought tolerance, as specific leaf area and leaf-area ratio. Our results demonstrate that PGPB induced antioxidant mechanisms in drought-stressed Neotropical trees, increasing drought tolerance. Thus, PGPB inoculation provides a biotechnological alternative to improve the success of reforestation programmes.