Physiological and biochemical insights into induced resistance on tomato against septoria leaf spot by a phosphite combined with free amino acids

Septoria leaf spot (SLS), caused by Septoria lycopersici, is a destructive disease in tomato. This study investigated the use of phosphite and potassium combined with L-α-free amino acids (named induced resistance (IR) stimulus), to boost tomato defense responses against S. lycopersici infection. Plants were sprayed with water or with the IR stimulus and challenged or not with S. lycopersici. Mycelial growth of S. lycopersici and conidia germination were not affected by the IR stimulus in vitro. Reduction in SLS severity and less leaf tissue colonization by S. lycopersici (lower TEF-1α expression) occurred for IR stimulus-sprayed plants. The IR stimulus-sprayed plants showed lower concentrations of malondialdehyde, hydrogen peroxide, and superoxide anion radical while the concentrations of sucrose, fructose, and starch and activities of ascorbate peroxidase, catalase, glutathione reductase, and superoxide dismutase were great compared to water-sprayed plants in response to S. lycopersici infection. Less impairment on the photosynthetic apparatus not related to biochemical limitations, abrupt increase in transpiration rate, and higher concentrations of chlorophyll a+b and carotenoids occurred for IR stimulus-sprayed plants facing reduced SLS symptoms. Genes involved in host defense reactions (CHI3, CHI9, GLU, PAL3, POX3, PPOB, and PPOF) and those related to systemic acquired resistance (PAL3 and ICS) or induced systemic resistance ((ethylene production - ACO2, ACO3, ACO5, and ACO4) and jasmonic acid signaling pathway (LOX1.1, LOXB, LOXC, and PDF1.2)) were up-regulated for IR stimulus-sprayed and infected plants. These findings highlight the potential of using this IR stimulus for SLS management.