METTL4 interacts with histone methylation readers MRG1/2 to regulate alternative splicing and promote thermomorphogenesis in Arabidopsis

Alternative splicing (AS) functions in thermomorphogenesis, but how AS is regulated during this process remains unclear. Here we show that METTL4 physically interacts with MRG1/2 in Arabidopsis thaliana. The mettl4-1, mrg1 mrg2, and mettl4 mrg1 mrg2 mutants exhibit reduced hypocotyl elongation under warm temperature conditions (28°C), indicating that METTL4 and MRG1/2 cooperatively promote thermomorphogenesis. Transcriptome analysis revealed that both METTL4 and MRG1/2 are required for warm temperature-induced reprogramming of gene transcription and AS. Further analysis showed that METTL4 and MRG1/2 modulate the AS of key brassinosteroid (BR)-signaling genes BES1, GSK1, and SBI1 in thermomorphogenesis. In addition, METTL4 promotes MRG2 binding to the chromatin of BES1 and GSK1 loci. METTL4 also promotes hypocotyl elongation and regulates the AS of BR signaling genes independently of its methyltransferase activity. Taken together, our findings demonstrate that METTL4 and MRG1/2 co-regulate AS and BR signaling to promote thermomorphogenesis in Arabidopsis.