Physiological and histological aspects of innate and shiitake-induced resistance against bacterial spot on tomatoes

This study aimed to evaluate the protective effects conferred by polysaccharides obtained from basidiocarps discarded from the commercial production of Lentinula edodes (shiitake mushroom) in tomato cultivars against bacterial spot disease and to study their mode of action. Among 13 cultivars analyzed for resistance to Xanthomonas gardneri cvs. Kada, Natália and Sena were selected and sprayed with polysaccharides from shiitake (PSHII – 1.5 mg/mL) or distilled water 4 days before inoculation with the bacteria. PSHII promoted a 37.7% reduction in disease severity and its mode of action on plants was evaluated according to changes in photosynthetic parameters, and histopathological aspects. Leaves of plants treated with PSHII showed lower stomatal conductance and transpiration rate compared to the control, at the time of inoculation. In addition, in plants of the most susceptible cvs. (Kada and Natalia) treated with PSHII, phenols accumulated in cells close to the epidermis. We observed that the plants of the most resistant cultivar, Sena, presented higher stomatal density, mesophyll thickness and size of palisade cells. In Sena, we also observed intense cell proliferation near the X. gardneri infection site and deposition of phenolic compounds inside the mesophyll. Control of bacterial spot in different tomato cultivars through induced resistance and the mechanisms presented by the plants to resist the bacteriosis are discussed.