Defence signalling marker gene responses to hormonal elicitation differ between roots and shoots

Abstract

Phytohormones such as jasmonic acid (JA), salicylic acid (SA), ethylene (ET) and abscisic acid (ABA) play a key role in regulation of plant immune responses to different attackers. Extensive research over recent years has led to the identification of molecular markers for specific hormonal-regulated defence pathways. However, most of our current knowledge on the regulation of plant immunity derives from studies focused on above-ground organs, mainly on the model plant Arabidopsis thaliana. Therefore, it is unclear whether the paradigms based on experiments on above-ground organs are entirely transferable to roots. Here, we used the non-model plant Brassica rapa to study the regulation dynamics of hormonal-related marker genes in both roots and shoots. These markers were identified in Arabidopsis shoots after elicitation of the JA-, SA-, ET- or ABA-signalling pathways, and are commonly used to study induced responses. We assessed whether the regulation of those genes by hormonal elicitation differs between roots and shoots. To discern whether the differences in marker gene expression between roots and shoots are related to differences in hormone production or to differential responsiveness, we also measured actual hormone content in the treated tissue after elicitation. Our results show that some of the widely used markers did not show specific responsiveness to single hormone applications in B. rapa. We further found that hormonal elicitation led to different response patterns of the molecular markers in shoots and roots. Our results suggest that the regulation of some hormonal-related marker genes in B. rapa is organ specific and differs from the Arabidopsis-derived paradigms.

Keywords: Brassica; hormonal signalling; marker genes; phytohormones; plant defences.

Figure 1.

Relative expression of the hormonal-related marker genes in Brassica rapa shoots (left column, panels A–D) and roots (right column, panels E–H) in response to hormonal application. Expression levels of (A, E) VSP2, (B, F) PR1, (C, G) ERF1 and (D, H) BrLEA4 were measured at 4, 8, 24 and 48 h after local MeJA, ABA, SA or ethephon application. Data were normalized over the housekeeping gene TIP41, and are represented as mean log₂ fold changes (log₂ FC + SE) in relation to the respective control. In each hormonal treatment, asterisks over the horizontal line represent the overall significant treatment main effect and those over individual bars indicate significant differences between the treatment group and their respective control plants, according to two-way ANOVA (n = 3–4 per treatment and harvest time) *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

Figure 2.

Phytohormone levels in Brassica rapa shoots (left column, panels A–C) and roots (right column, panels D–F) in response to hormonal application. The levels of (A, D) JA, (B, E) SA and (C, F) ABA were measured at 4, 8, 24 and 48 h after local MeJA, ABA, SA or ethephon application. Bars represent log₂ fold changes (log₂ FC + SE) of concentrations in relation to the respective control. In each hormonal treatment, asterisks over the horizontal line represent the overall significant treatment main effect and those over individual bars indicate significant differences between the treatment group and their respective control plants, according to two-way ANOVA (n = 3–4 per treatment and harvest time, except for ABA-treated roots at 24 h where n = 2) *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

Figure 3.

Summarizing scheme of the changes in the phytohormone levels and hormonal-related marker genes in Brassica rapa shoots and roots after local hormonal elicitation. Light green indicates reduction/down-regulation and dark green indicates a strong reduction/down-regulation of phytohormone/gene expression levels measured in the same treated organ. Orange indicates increase/up-regulation of phytohormone/gene expression levels and a strong increase/up-regulation is indicated in red. Yellow indicates no changes compared to the respective control group.