MLKs kinases phosphorylate the ESCRT component FREE1 to suppress abscisic acid sensitivity of seedling establishment

ABI5-FLZ13 module transcriptionally represses growth-related genes to delay seed germination in response to ABA

The bZIP transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5) is a master regulator of seed germination and post-germinative growth in response to abscisic acid (ABA), but the detailed molecular mechanism by which it represses plant growth remains unclear. In this study, we used proximity labeling to map the neighboring proteome of ABI5 and identified FCS-LIKE ZINC FINGER PROTEIN 13 (FLZ13) as a novel ABI5 interaction partner. Phenotypic analysis of flz13 mutants and FLZ13-overexpressing lines demonstrated that FLZ13 acts as a positive regulator of ABA signaling. Transcriptomic analysis revealed that both FLZ13 and ABI5 downregulate the expression of ABA-repressed and growth-related genes involved in chlorophyll biosynthesis, photosynthesis, and cell wall organization, thereby repressing seed germination and seedling establishment in response to ABA. Further genetic analysis showed that FLZ13 and ABI5 function together to regulate seed germination. Collectively, our findings reveal a previously uncharacterized transcriptional regulatory mechanism by which ABA mediates inhibition of seed germination and seedling establishment.

The plant FYVE domain-containing protein FREE1 associates with microprocessor components to repress miRNA biogenesis

FYVE domain protein required for endosomal sorting 1 (FREE1), originally identified as a plant-specific component of the endosomal sorting complex required for transport (ESCRT) machinery, plays diverse roles either in endosomal sorting in the cytoplasm or in transcriptional regulation of abscisic acid signaling in the nucleus. However, to date, a role for FREE1 or other ESCRT components in the regulation of plant miRNA biology has not been discovered. Here, we demonstrate a nuclear function of FREE1 as a cofactor in miRNA biogenesis in plants. FREE1 directly interacts with the plant core microprocessor component CPL1 in nuclear bodies and disturbs the association between HYL1, SE and CPL1. Inactivation of FREE1 in the nucleus increases the binding affinity between HYL1, SE, and CPL1 and causes a transition of HYL1 from the inactive hyperphosphorylated version to the active hypophosphorylated form, thereby promoting miRNA biogenesis. Our results suggest that FREE1 has evolved as a negative regulator of miRNA biogenesis and provides evidence for a link between FYVE domain-containing proteins and miRNA biogenesis in plants.