RESUMO
This study aimed to evaluate the effectiveness of exopolysaccharides (EPS) produced by Lactobacillus plantarum in controlling bacterial spot and eliciting defense mechanisms, and verify alterations in the tomato plant physiological responses. Tomato plants with five definitive leaves were sprayed with EPS, acibenzolar-S-methyl (ASM, positive control), or distilled water (negative control) and inoculated with Xanthomonas gardneri after 3 days. Local and systemic protection was evaluated 7, 15, and 21 days after inoculation. Biochemical defense mechanisms (peroxidases [POX], polyphenoloxidases [PPO], catalase [CAT], superoxide dismutase [SOD], accumulation of hydrogen peroxide [H2O2], and physiological changes) were quantified. In addition, histochemical analyses were examined through light and fluorescence microscopy in treated tomato plants, inoculated or not with X. gardneri. On average, EPS and ASM provided 55 and 96% reduction, respectively, of the bacteriosis severity in treated leaves, compared to the control plants. Increased activities of PPO, CAT, and SOD were found in the EPS-treated plants after being challenged with the pathogen. ASM caused increases in all enzymes analyzed, especially in inoculated plants. The rate of photosynthesis was three times higher, while stomatal conductance and transpiration were 36% lower in the plants sprayed with EPS and challenged with the pathogen, compared to the inoculated control plants. The photochemical efficiency and Soil Plant Analysis Development (SPAD) index did not change. Under light and fluorescence microscopy, it could be seen accumulation of cellulosic compounds in the epidermis of the leaves treated with EPS. Therefore, application of EPS can be considered an alternative for the control of bacterial spot in tomato plants. The study discusses how biochemical and physiological alterations caused by the polysaccharide in the plants contributed to reduce the severity of the disease.
