The HD-Zip IV gene ZmHB118 is required for basal endosperm transfer layer formation and maternal-to-filial nutrient...

The basal endosperm transfer layer (BETL) constitutes the indispensable gateway mediating maternal nutrient transport into developing maize kernels, a process that fundamentally determines grain-filling efficiency and final grain yield. To date, only a few transcription factors have been documented to directly regulate BETL formation and nutrient transport, and the transcriptional cascades governing BETL cell differentiation remain largely elusive. Here, we functionally characterized ZmHB118, an HD-Zip IV transcription factor with exclusive expression in the BETL. Loss of function of ZmHB118 produced smaller kernels, accompanied by impaired cell wall ingrowth (CWI) in the BETL and reduced grain-filling rate. Integrative RNA-seq and DAP-seq analyses identified two BETL-specific, ZmHB118 target genes, encoding non-canonical lipid transfer proteins. Indeed, ZmHB118 robustly transactivates these two downstream genes, named NCLTP1 and NCLTP2, suggesting its role in mediating lipid transport toward the cell periphery to maintain proper CWI formation. Overall, our findings establish ZmHB118 as a master transcriptional regulator of BETL formation and maternal-to-filial nutrient allocation during grain filling, thus providing a promising genetic target for and new insights for boosting maize grain yield.