A Trypsin-Like Serine Protease ZmNAL1a Fine-Tunes Maize Floral Transition and Flowering Time

The floral transition is a crucial phase in flowering plants that initiates reproductive development. Florigen, a key regulator of this transition, is expressed in the leaves and transmits environmental signals by trafficking to the vegetative shoot apical meristem, thereby promoting the floral transition. However, whether additional signals, expressed outside the meristem, control the flowering transition remains to be explored. This study identified another floral transition signal, ZmNAL1a, which encodes a trypsin-like serine protease and can move from the leaf to the shoot apical meristem via plasmodesmata to regulate floral transition in maize. Mutation of ZmNAL1a suppresses the expression of key flowering genes in the shoot apical meristem, resulting in a delayed floral transition and flowering. ZmNAL1a interacts with and degrades RAMOSA1 ENHANCER LOCUS2 (REL2), a TOPLESS-like corepressor, which can regulate the expression of flowering genes by affecting histone acetylation and transcriptional regulation alongside ZmEREBP147, an AP2/EREBP transcription factor. These findings suggest that ZmNAL1a is a diffusible signal that regulates the floral transition and flowering via a conserved NAL1-TOPLESS epigenetic regulation module and through transcriptional regulation. This discovery broadens the understanding of flowering control, offering potential targets for improving adaptation and crop yield through precise manipulation of flowering time.