Perturbations in plant energy homeostasis prime lateral root initiation via SnRK1-bZIP63-ARF19 signaling

Plants adjust their energy metabolism to continuous environmental fluctuations, resulting in a tremendous plasticity in their architec- ture. The regulatory circuits involved, however, remain largely unresolved. In Arabidopsis, moderate perturbations in photosyn- thetic activity, administered by short-term low light exposure or unexpected darkness, lead to increased lateral root (LR) initiation. Consistent with expression of low-energy markers, these treat- ments alter energy homeostasis and reduce sugar availability in roots. Here, we demonstrate that the LR response requires the met- abolic stress sensor kinase Snf1-RELATED-KINASE1 (SnRK1), which phosphorylates the transcription factor BASIC LEUCINE ZIPPER63 (bZIP63) that directly binds and activates the promoter of AUXIN RESPONSE FACTOR19 (ARF19), a key regulator of LR initiation. Con- sistently, starvation-induced ARF19 transcription is impaired in bzip63 mutants. This study highlights a positive developmental function of SnRK1. During energy limitation, LRs are initiated and primed for outgrowth upon recovery. Hence, this study provides mechanistic insights into how energy shapes the agronomically im- portant root system.