The LkMTA2 / LkMTB8 – LkERF1 – LkEXPA8 cascade regulates xylem cell expansion in larch

Summary Cell expansion is a critical determinant of the final shape and size of wood cells. In this study, the role of N6‐methyladenosine (m 6 A), an abundant mRNA modification, in regulating xylem cell expansion was investigated. Treatment with the m 6 A methylation inhibitor STM2457 triggered global m 6 A hypomethylation in larch and significantly enlarged the xylem cell cross‐sectional area. Analysis of m 6 A‐seq data indicated that the hypomethylated m 6 A peaks were primarily localized within coding sequences and correlated with decreased transcript abundance. Among these transcripts, we identified a gene associated with xylem cell expansion, ETHYLENE‐RESPONSIVE FACTOR 1 ( LkERF1 ), whose overexpression in poplar reduced the xylem cell cross‐sectional area. We characterized MRNA ADENOSINE METHYLASE 2 ( LkMTA2 ) and METHYLTRANSFERASE B8 ( LkMTB8 ), whose overexpression in poplar resulted in a significant reduction in vessel lumen cross‐sectional area. LkMTA2 and LkMTB8 form a heterodimeric complex, which enhances the mRNA stability and abundance of LkERF1 via m 6 A hypermethylation, while LkERF1 directly represses the expression of EXPANSIN A8 ( LkEXPA8 ), a key regulator of cell expansion. These findings provide novel insights into the LkMTA2/LkMTB8–LkERF1–LkEXPA8 cascade in regulating xylem cell expansion and reveal a molecular link between m 6 A RNA modification and wood formation.