Transcriptional Feedback in Plant Growth and Defense by PIFs, BZR1, HY5, and MYC Transcription Factors

Growth of Arabidopsis is controlled by the activity of a set of bHLH and bZIP transcription factors of which phytochrome interacting factor4 (PIF4), BRASSINAZOLE-RESISTANT 1 (BZR1), and elongated hypocotyl 5 (HY5) have been most extensively studied. Defense responses are controlled by a set of MYC transcription factors of which MYC2 is best characterized. Moreover, hundreds of additional proteins (here named co-factors) have been identified which (in)directly may affect the expression or activity of these TFs. Thus, regulation of expression of genes encoding these co-factors becomes an integral part of understanding the molecular control of growth and defense. Here, we review RNA-seq data related to PIF, BZR1, HY5, or MYC activity, which indicate that 125 co-factor genes affecting PIFs, HY5, BZR1, or MYCs are themselves under transcriptional control by these TFs, thus revealing potential feedback regulation in growth and defense. The transcriptional feedback on co-factor genes related to PIF4, BZR1, and MYC2 by PIFs, BZR1, or MYCs, mostly results in negative feedback on PIF4, BZR1, or MYC2 activity. In contrast, transcription feedback on co-factor genes for HY5 by HY5 mostly results in positive feedback on HY5 activity. PIF4 and BZR1 exert a balanced regulating of photoreceptor-gene expression, whose products directly or indirectly affect PIF4, HY5, and MYC2 protein stability as a function of light. Growth itself is balanced by both multiple positive and multiple negative feedback on PIF4 and BZR1 activity. The balance between growth and defense is mostly through direct cross-regulation between HY5 and MYC2 as previously described, but also through potential transcriptional feedback on co-factor genes for MYC2 by PIF4, BZR1, and HY5 and through transcriptional feedback of co-factors for PIF4 and BZR1 by MYC2. The interlocking feed-forward and feed-backward transcriptional regulation of PIF4, BZR1, HY5, and MYC2 co-factors is a signature of robust and temporal control of signaling related to growth and defense.