ArabidopsisBasic Helix-Loop-Helix Transcription Factors MYC2, MYC3, and MYC4 Regulate Glucosinolate Biosynthesis, Insect Performance, and Feeding Behavior  

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.

Interaction between MYC2 and ETHYLENE INSENSITIVE3 Modulates Antagonism between Jasmonate and Ethylene Signaling inArabidopsis

“…Consistent with previous studies (Lorenzo et al, 2004;Fernández-Calvo et al, 2011;Schweizer et al, 2013), our results showed that MYC2 upregulated JA-induced expression of the woundresponsive genes VEGETATIVE STORAGE PROTEIN1 (VSP1), VSP2, and TYROSINE AMINOTRANSFERASE3 (TAT3) ( Figure 9A) and the herbivore-inducible genes CYP79B3, BRANCHED-CHAIN AMINOTRANSFERASE4 (BCAT4), and BILE ACID TRANS-PORTER5 (BAT5) (Figure 9B), which are required for the biosynthesis of the secondary metabolites glucosinolates (Zhao et al, 2002;Kliebenstein et al, 2005;Schweizer et al, 2013). Interestingly, the double mutant ein3 eil1 exhibited upregulated expression of these wound-responsive genes (VSP1, VSP2, and TAT3) as well as herbivory-inducible genes (CYP79B3, BCAT4, and BAT5) when treated with (or even without) JA compared with the wild type ( Figures 9A and 9B).…”

“…MYC2 functions as a key transcription factor to positively regulate diverse JA responses (Kazan and Manners, 2013), including root growth (Boter et al, 2004;Lorenzo et al, 2004), secondary metabolism (Dombrecht et al, 2007;Hong et al, 2012;Schweizer et al, 2013), wound response, and plant defense against insect attack (Zhang and Turner, 2008;Fernández-Calvo et al, 2011;Schweizer et al, 2013). Surprisingly, MYC2 also acts as a negative regulator to repress JA-mediated plant resistance to necrotrophic fungi and pathogenesis-related gene expression (e.g., PDF1.2) (Anderson et al, 2004;Lorenzo et al, 2004;Zhai et al, 2013).…”

“…wound-responsive and herbivory-inducible genes ( Figures 9A and 9B), the myc2 myc3 myc4 triple mutant, which was almost completely devoid of glucosinolates (Schweizer et al, 2013), exhibited susceptibility to the generalist herbivores Spodoptera littoralis (Schweizer et al, 2013) and Spodoptera exigua ( Figures 9C and 9D), while plant defense against these generalist herbivores was enhanced in the ET-signaling mutants ein3 eil1 ( Figures 9C and 9D), etr1, and ein2 (Stotz et al, 2000;Mewis et al, 2005Mewis et al, , 2006Bodenhausen and Reymond, 2007). These results demonstrate that the abolishment of EIN3 and EIL1 derepresses MYC2, which enhances the expression of wound-responsive and herbivore-inducible genes and elevates plant defenses against generalist herbivores.…”

“…JA represses apical hook formation (Turner et al, 2002) and positively regulates plant defense against insect attack (Fernández-Calvo et al, 2011;Schweizer et al, 2013), while ET functions oppositely (Guzmán and Ecker, 1990;Mewis et al, 2005Mewis et al, , 2006Bodenhausen and Reymond, 2007). However, the molecular mechanism for such antagonism between JA and ET signaling remains unclear.…”

“…Upon perception of JA signal (Fonseca et al, 2009;Yan et al, 2009;Sheard et al, 2010), the F-box protein CORO-NATINE INSENSITIVE1 (COI1) (Xie et al, 1998;Yan et al, 2009) recruits the JASMONATE ZIM-DOMAIN (JAZ) proteins (Chini et al, 2007;Thines et al, 2007;Yan et al, 2007) for degradation, which leads to the release of various downstream factors, including MYC2/JASMONATE INSENSITIVE1 (JIN1), MYC3, and MYC4 (Cheng et al, 2011;Fernández-Calvo et al, 2011;Niu et al, 2011), as well as WD-repeat/bHLH/MYB complex , MYB21, MYB24, and MYB57 (Mandaokar et al, 2006;Song et al, 2011) and the IIId bHLH factors (Nakata et al, 2013;Song et al, 2013b), which regulate diverse JA-mediated functions. These functions include root growth (Dathe et al, 1981;Chen et al, 2011), apical hook formation (Turner et al, 2002), flowering (Robson et al, 2010), stamen development (McConn and Browse, 1996;Song et al, 2011Song et al, , 2013a, leaf senescence (Ueda and Kato, 1980;Shan et al, 2011), secondary metabolism (De Geyter et al, 2012;Schweizer et al, 2013), drought responses (Seo et al, 2011), wounding responses (Mason and Mullet, 1990;Acosta et al, 2013;Mousavi et al, 2013), and defense against pathogen infection Vijayan et al, 1998;Melotto et al, 2006;Rowe et al, 2010;Yang et al, 2012;Zheng et al, 2012) and insect attack (McConn et al, 1997;…”