Jasmonates (JAs) trigger an important transcriptional reprogramming of plant cells to modulate both basal development and stress responses. In spite of the importance of transcriptional regulation, only one transcription factor (TF), the Arabidopsis thaliana basic helix-loop-helix MYC2, has been described so far as a direct target of JAZ repressors. By means of yeast two-hybrid screening and tandem affinity purification strategies, we identified two previously unknown targets of JAZ repressors, the TFs MYC3 and MYC4, phylogenetically closely related to MYC2. We show that MYC3 and MYC4 interact in vitro and in vivo with JAZ repressors and also form homo-and heterodimers with MYC2 and among themselves. They both are nuclear proteins that bind DNA with sequence specificity similar to that of MYC2. Loss-of-function mutations in any of these two TFs impair full responsiveness to JA and enhance the JA insensitivity of myc2 mutants. Moreover, the triple mutant myc2 myc3 myc4 is as impaired as coi1-1 in the activation of several, but not all, JA-mediated responses such as the defense against bacterial pathogens and insect herbivory. Our results show that MYC3 and MYC4 are activators of JAregulated programs that act additively with MYC2 to regulate specifically different subsets of the JA-dependent transcriptional response.
INTRODUCTIONThe plant hormones jasmonates (JAs) are fatty acid-derived oxylipins required for the regulation of multiple physiological aspects of plant growth, development, and defense (Wasternack, 2007;Kazan and Manners, 2008;Browse, 2009;Pauwels et al., 2009). Thus, JAs are widely recognized as regulators of plant responses to environmental stresses such as pathogen and pest attack, wounding, ozone exposure, and water deficit (Devoto et al., 2005;Browse and Howe, 2008). They are also important regulators of growth and developmental programs such as gamete development, the cell cycle, root growth, tendril coiling, and senescence in many plant species (Pauwels et al., 2008;Zhang and Turner, 2008;Reinbothe et al., 2009;Yoshida et al., 2009). JAs are being recognized as important integrators of developmental and stress signals to modulate the allocation of resources to grow or to defend (Moreno et al., 2009;Robson et al., 2010).Transcription is a major regulatory step in the activation of these responses, and JAs trigger an important transcriptional reprogramming of the cells to switch the basal developmental programs into the necessary stress response program (Reymond et al., 2004;Devoto et al., 2005;Mandaokar et al., 2006;Pauwels et al., 2008). The signaling events that lead to transcriptional reprogramming are starting to be elucidated. Upon elicitation by exogenous or endogenous signals, the hormone (+)-7-iso-jasmonoyl-L-isoleucine [also known as (3R,7S)-jasmonoyl-L-isoleucine or JA-Ile] is synthesized by JAR1 (Fonseca et al., 2009b;Suza et al., 2010;Wasternack and Kombrink, 2010). JA-Ile is perceived by a receptor complex formed by the protein COI1 and the JAZ repressors (Xie et al., 1998;Thines et...
