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 [HO], 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.

Lactic acid bacteria produce several exopolysaccharides (EPS) that may have antimicrobial action and/or induce defense responses in plants. This work aims to evaluate the potential of EPS produced by Lactobacillus plantarum in the protection of tomato plants against bacterial spot caused by Xanthomonas gardneri, as well as to predict the possible mechanisms of action. The EPS were characterized through FTIR and applied at 0; 0.5; 1.5 and 3.0 mg mL -1 in tomato plants with five expanded leaves, followed by the pathogen inoculation after 3 or 7 days. Antimicrobial activity of the biopolymer (1.5 or 10.0 mg mL -1 ) was evaluated in bioassay when EPS was incorporated into culture medium or embedded in antibiogram disk. The defense mechanisms i.e., total phenolic compounds and flavonoids content, phenylalanine ammonia-lyase (PAL), glutathione reductase (GR) and lipoxygenase (LOX) activities, were measured in tomato plants treated with EPS (1.5 mg mL -1 ), inoculated or not with X. gardneri. EPS reduced bacterial spot symptoms by up to 72.0% compared to the control. There were no direct effects of EPS on the in vitro growth of X. gardneri. The spectrophotometric profile, ascorbic and ellagic acid concentrations were change in tomato plants after EPS application, in plants challenged with the pathogen. Increases in PAL, GR and LOX activities were observed in plants treated with EPS. Thus, the application of L. plantarum exopolysaccharides can be considered as an effective alternative for controlling bacterial spot in tomato plants. This paper also discusses how these exopolysaccharides reduced the severity of the disease.

No abstract