The plastidial retrograde signal methyl erythritol cyclopyrophosphate is a regulator of salicylic acid and jasmonic acid crosstalk

Abstract

The exquisite harmony between hormones and their corresponding signaling pathways is central to prioritizing plant responses to simultaneous and/or successive environmental trepidations. The crosstalk between jasmonic acid (JA) and salicylic acid (SA) is an established effective mechanism that optimizes and tailors plant adaptive responses. However, the underlying regulatory modules of this crosstalk are largely unknown. Global transcriptomic analyses of mutant plants (ceh1) with elevated levels of the stress-induced plastidial retrograde signaling metabolite 2-C-methyl-D-erythritol cyclopyrophosphate (MEcPP) revealed robustly induced JA marker genes, expected to be suppressed by the presence of constitutively high SA levels in the mutant background. Analyses of a range of genotypes with varying SA and MEcPP levels established the selective role of MEcPP-mediated signal(s) in induction of JA-responsive genes in the presence of elevated SA. Metabolic profiling revealed the presence of high levels of the JA precursor 12-oxo-phytodienoic acid (OPDA), but near wild type levels of JA in the ceh1 mutant plants. Analyses of coronatine-insensitive 1 (coi1)/ceh1 double mutant plants confirmed that the MEcPP-mediated induction is JA receptor COI1 dependent, potentially through elevated OPDA. These findings identify MEcPP as a previously unrecognized central regulatory module that induces JA-responsive genes in the presence of high SA, thereby staging a multifaceted plant response within the environmental context.

Keywords: Coronatine-insensitive1 (COI1); MEcPP (2-C-methyl-d-erythritol cyclopyrophosphate); hormonal interplay; jasmonic acid (JA); plastidial retrograde signaling metabolite; salicylic acid (SA); stress responses..

Fig. 1.

Comparative analysis of genes with modulated expression in ceh1 mutant and wild type plants treated with SA or JA. (A) Gene ontology of up-regulated genes within the intersection of ceh1 and SA-treated (left), and ceh1 and JA-treated (right) wild type plants. Only significantly over-represented categories are shown as determined by Classification SuperViewer (http://bar.utoronto.ca/). (B) Venn diagram of up- or down- regulated transcripts (≥2-fold, P<0.05) in ceh1 and wild type treated with SA or JA.

Fig. 2.

High SA does not impede induction of JA-responsive genes in the ceh1 mutant. (A) Schematic representation of SA and JA pathway genes and their interactions. (B) Relative expression levels of SA and JA marker genes in ceh1 compared with parent line (P _HPL ::LUC). Data are means of three biological replicates and three technical replicates ±SEM. Asterisks denote significant differences determined by Student’s t test (P<0.05). (C) Relative expression of genes with altered transcript levels in mock, JA-, SA-, and JA+SA-treated wild type Col-0 plants. Data are means of three biological replicates and three technical replicates ±SEM. Asterisks denote significant differences determined by Student’s t test (P<0.05).

Fig. 3.

Constitutively high SA levels fail to repress levels of JA precursor 12-OPDA in the ceh1 mutant. Analyses of the levels of SA (A), 12-OPDA (B), JA (C) and MEcPP (D) in Col-0, ssi2, mekk1-5, ceh1, ceh1/eds16-1, and eds16-1 genotypes. Data are means of three biological replicates ±SD. Asterisks denote significant differences from Col-0 as determined by Student’s t test (P<0.05). Brackets and above-indicated P value denote significance or the lack of between ceh1 and ceh1/eds16-1 as determined by Student’s t test.

Fig. 4.

MEcPP interference with SA suppression of JA marker genes is COI1 dependent. Relative expression levels of PR1, PDF1.2, and VSP2 in Col-0, ssi2, mekk1-5, ceh1, ceh1/eds16-1, ceh1/coi1, coi1, and eds16-1 genotypes. Data are means of three biological replicates and three technical replicates ±SEM. Asterisks denote significant differences from Col-0 as determined by Student’s t-test (P<0.05). Brackets and above-indicated P value denote significance or the lack of between ceh1 and ceh1/eds16-1 as determined by Student’s t-test.

Fig. 5.

Schematic model of MEcPP calibrating SA–JA antagonism via a COI1-dependent but JA-independent path. Stress specific accumulation of MEcPP either through increased levels of 12-OPDA or directly via a COI1-dependent but JA-independent path induces of VSP2 and PDF1.2, thereby calibrating the SA-mediated suppression of JA-responsive genes.