Jasmonates mediate plant defense responses to Spodoptera exigua herbivory in tomato and maize foliage

Abstract

Plants evolve diverse strategies to cope with herbivorous insects, in which the lipid-derived phytohormone jasmonic acid (JA) plays a crucial role. This study was conducted to investigate the differential responses of tomato and maize plants to Spodoptera exiguaherbivory and to clarify the role played by JA, methyl-jasmonate (MeJA) and jasmonoyl-L-isoleucine (JA-L-Ile) in their defense responses. JA, MeJA and JA-L-Ile were quantified using HPLC-MS/MS. The results showed that maize plant was more tolerant toS. exiguaherbivory than tomato. Spodopteraexigua attack induced JA, MeJA, and JA-L-Ile to high levels after 2 h of infestation in both test plants. Then, all studied JAsconcentration decreased gradually by increasing infestation time up to 1 week. JA concentration in infested maize was much higher than that in infested tomato leaves. However, MeJA concentration in infested tomato leaves was higher than that in maize. In control plants, JA was not recorded, while MeJA was recorded in comparable values both in tomato and maize. Our results showed that JA plays the main role in increasing defense responses to S. exigua infestation in the studied plants as a direct signaling molecule; however, MeJA could play an indirect role by inducing JA accumulation. JA-L-Ile indicated a less efficient role in defense responses to S. exigua attack in both test plants where its level is much lower than JA and MeJA.

Keywords: HPLC-MS/MS; Solanum lycopersicum; Zea mays; beet armyworm; jasmonic acid (JA); jasmonoyl-L-isoleucine(JA-L-Ile); methyl-jasmonate (MeJA).

Figure 1.

Chlorophyll a/b ratio of tomato (i) and maize (II) as affected by Spodoptera exigua larvae infestation after different periods of time. Each point is a mean of three replicates ± standard error. The different letters represent the statistical significance between different time periods of uninfested (a–c) and infested (a–c) plants at P ≤ 0.05. Asterisks show significant differences between uninfested and infested plants at each time period (*p < .05; **p < .01; ***p < .001).

Figure 2.

Soluble carbohydrate concentration (mg/g DW) of tomato (i) and maize (II) as affected by Spodoptera exigua larvae infestation after different periods of time. Each point is a mean of three replicates ± standard error. The different letters represent the statistical significance between different time periods of uninfested (a–c) and infested (a–c) plants at P ≤ 0.05. Asterisks show significant differences between uninfested and infested plants at each time period (*p < .05; **p < .01; ***p < .001).

Figure 3.

Soluble protein concentration (mg/g DW) of tomato (i) and maize (II) as affected by Spodoptera exigua larvae infestation after different periods of time. Each point is a mean of three replicates ± standard error. The different letters represent the statistical significance between different time periods of uninfested (a–c) and infested (a–c) plants at P ≤ 0.05. Asterisks show significant differences between uninfested and infested plants at each time period (*p < .05; **p < .01; ***p < .001).

Figure 4.

Free amino acid concentration (μg/g DW) of tomato (i) and maize (II) as affected by Spodoptera exigua larvae infestation after different periods of time. Each point is a mean of three replicates ± standard error. The different letters represent the statistical significance between different time periods of uninfested (a–c) and infested (a–c) plants at P ≤ 0.05. Asterisks show significant differences between uninfested and infested plants at each time period (*p < .05; **p < .01; ***p < .001).

Figure 5.

HPLC chromatogram analysis jasmonates. Peak No. (a), jasmonic acid; (b), methyl-jasmonate; (c), jasmonoyl-L-isoleucine.