Sodium Selenate: An Environmental-Friendly Means to Control Tomato Bacterial Speck Disease

Selenium (Se) is a micronutrient which plays a beneficial role in plants. Its adsorption at low doses can stimulate plant growth and enhance the plant’s capacity to respond to abiotic stresses, such as salinity, drought, cold and hot temperature, probably due to its antioxidant properties. Here, the effect of selenium supplied in soil-drench treatments as sodium selenate (Na2SeO4) at the dose of 4 mg L1 (21.17 M) per plant was studied on tomato (Solanum lycopersicum L.; cv. Rio Grande) against Pseudomonas syringae pv. tomato (Pst), the causal agent of tomato bacterial speck. Sodium selenate treated tomato plants challenged with Pst showed a reduction in disease severity expressed as percentage of diseased area and number of lesions per leaf. Furthermore, Pst bacterial cells were unable to proliferate in treated tomato plants. The effect of sodium selenate against Pst was also assessed in vitro, demonstrating that the growth of the bacterium was affected in a dose-dependent manner (EC50 = 42 ppm). It is notable that in tomato plants treated with sodium selenate at the above reported dose, a marked callose deposition was observed as well as the expression of the salicylicacid- responsive tomato ‘pathogenesis-related protein 1b1’ (PR1b1) but not of the jasmonate-mediated ‘proteinase inhibitor 2’ (PIN2) genes. Induced defence responses and direct antimicrobial activity protect treated tomato plants against Pst attacks, suggesting the potential of sodium selanate as an environmentally friendly and effective bacterial control means. Moreover, the increased Se content in treated tomatoes offers an effective approach to reduce Se deficiency problems in human diets.