Abstract
Arabidopsis thaliana plants fend off insect attack by constitutive and inducible production of toxic metabolites, such as glucosinolates (GSs). A triple mutant lacking MYC2, MYC3, and MYC4, three basic helix-loop-helix transcription factors that are known to additively control jasmonate-related defense responses, was shown to have a highly reduced expression of GS biosynthesis genes. The myc2 myc3 myc4 (myc234) triple mutant was almost completely devoid of GS and was extremely susceptible to the generalist herbivore Spodoptera littoralis. On the contrary, the specialist Pieris brassicae was unaffected by the presence of GS and preferred to feed on wild-type plants. In addition, lack of GS in myc234 drastically modified S. littoralis feeding behavior. Surprisingly, the expression of MYB factors known to regulate GS biosynthesis genes was not altered in myc234, suggesting that MYC2/MYC3/MYC4 are necessary for direct transcriptional activation of GS biosynthesis genes. To support this, chromatin immunoprecipitation analysis showed that MYC2 binds directly to the promoter of several GS biosynthesis genes in vivo. Furthermore, yeast two-hybrid and pull-down experiments indicated that MYC2/MYC3/MYC4 interact directly with GS-related MYBs. This specific MYC-MYB interaction plays a crucial role in the regulation of defense secondary metabolite production and underlines the importance of GS in shaping plant interactions with adapted and nonadapted herbivores.
Highlights
Plants resist attack by herbivorous insects with a combination of constitutive and inducible defenses
Among the 50 most differentially expressed genes between Col-0 and myc234, 27 genes are involved in the GS pathway, including most genes responsible for the synthesis of core aliphatic- and indole-GS structures, genes involved in side-chain modification, and genes involved in primary sulfate assimilation (Figure 1; see Supplemental Data Set 1 online)
The basic helix-loop-helix (bHLH) transcription factors (TFs) MYC2, MYC3, and MYC4 control additively subsets of JA-dependent responses, including root growth inhibition, defense against bacterial pathogens, and defense against insect herbivory (Fernández-Calvo et al, 2011)
Summary
Plants resist attack by herbivorous insects with a combination of constitutive and inducible defenses. GSs get mixed with specific b-glucosidases termed myrosinases, releasing an aglucone that spontaneously rearranges into toxic thiocyanates, isothiocyanates, or nitriles (Bones and Rossiter, 2006). GSs are classified into aliphatic, aromatic, and indole GSs depending on the origin of the amino acid–derived side chain (Wittstock and Halkier, 2002). They are present constitutively (Wittstock and Gershenzon, 2002) and accumulate in Different GS classes play a defensive role against generalist herbivores. High levels of aliphatic-GS and insect performance were negatively correlated in different A. thaliana genotypes (Kliebenstein et al, 2005).
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