Priming of Immune System in Tomato by Treatment with Low Concentration of L-Methionine

Abstract

Various metabolites, including phytohormones, phytoalexins, and amino acids, take part in the plant immune system. Herein, we analyzed the effects of L-methionine (Met), a sulfur-containing amino acid, on the plant immune system in tomato. Treatment with low concentrations of Met enhanced the resistance of tomato to a broad range of diseases caused by the hemi-biotrophic bacterial pathogen Pseudomonas syringae pv. tomato (Pst) and the necrotrophic fungal pathogen Botrytis cinerea (Bc), although it did not induce the production of any antimicrobial substances against these pathogens in tomato leaf tissues. Analyses of gene expression and phytohormone accumulation indicated that Met treatment alone did not activate the defense signals mediated by salicylic acid, jasmonic acid, and ethylene. However, the salicylic acid-responsive defense gene and the jasmonic acid-responsive gene were induced more rapidly in Met-treated plants after infection with Pst and Bc, respectively. These findings suggest that low concentrations of Met have a priming effect on the phytohormone-mediated immune system in tomato.

Keywords: disease resistance; methionine; phytohormone; priming; tomato.

Figure 1

Induction of resistance to tomato leaf speck disease and gray mold disease by L-methionine treatment. (A) The growth of Pst in tomato leaf tissues. CFUs, colony forming units. Values are shown as the means (±SEs) (n = 10). The asterisk indicates a statistically significant difference between data from the control and Met-treated plants (*, p < 0.05). (B) The area of spreading lesions caused by Bc. Values are shown as the means (±SEs) (n = 25). Asterisks indicate statistically significant differences between data from the control and Met-treated plants (**, p < 0.01).

Figure 2

Effects of a low concentration of L-methionine on defense gene expression and phytohormone levels in tomato plants. (A) Expression of defense-related genes in L-methionine-treated tomato plants. Values presented are the means (±SEs) (n = 8). No significant differences at the p < 0.05 level were detected in gene expression between the control and Met-treated plants. (B) Phytohormone levels in L-methionine-treated tomato plants. Values presented are the means (±SEs) (n = 4). No significant differences at the p < 0.05 level were detected in phytohormone levels between the control and Met-treated plants.

Figure 3

Expression of defense-related genes after infection with pathogens. (A) Expression of the PR1b gene after infection with Pst. (B) Expression of the PI2 gene after infection with the Bc. Values presented are the means (±SEs) (n = 8). Open circles, water-treated control plants; closed circles, Met-treated plants. Asterisks indicate statistically significant differences between data from the water-treated and Met-treated plants at each time point (*, p < 0.05).