Bimodal retrograde signaling disrupts a suppressor network and activates a key transcriptional activator to direct stress responses

Plastid-to-nucleus communication, crucial for regulating stress-responsive gene expression, has long intrigued researchers. This study reveals how the plastidial metabolite 2-C-methyl-D-erythritol-2,4-cyclopyrophosphate (MEcPP) orchestrates transcriptional reprogramming by modulating the rapid stress response element (RSRE), a conserved regulatory hub in the plant general stress response network. Yeast one-hybrid assays identified HAT1, a class II HD-Zip protein, as a negative regulator of RSRE. Genetic analyses, including HAT1 overexpression and knockdowns, confirmed its role in suppressing RSRE activity. Interaction assays uncovered a suppression network involving HAT1, the co-repressor TOPLESS (TPL), and the nuclear importin IMPα-9. Furthermore, HAT1 interacts with calmodulin-binding transcription activator 3 (CAMTA3), a calcium/calmodulin-binding transcription factor known to activate RSRE. AlphaFold modeling provided insights into the architecture of the HAT1-RSRE complex and HAT-CAMTA3 interaction, supported by conserved domains across plant species. Under stress condition, MEcPP accumulation promotes the 26S proteasomal degradation of TPL and IMPα-9 while reduces auxin-dependent HAT1 expression. Additionally, MEcPP enhances Ca²⁺ influx, activating CAMTA3 and enabling it to bind RSRE, thereby initiating the transcription of stress response genes. This dual mechanism—dismantling suppressors (HAT1, TPL, and IMPα-9) and activating CAMTA3—underscores MEcPP's central role in plastid-to-nucleus signaling. These findings emphasize MEcPP's pivotal function in dynamically regulating gene expression to maintain cellular homeostasis under environmental stress.