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

Cell expansion is a critical determinant of the final shape and size of wood cells. In this study, the role of N6-methyladenosine (m⁶A), an abundant mRNA modification, in regulating xylem cell expansion was investigated. Treatment with the m⁶A methylation inhibitor STM2457 triggered global m⁶A hypomethylation in larch and significantly enlarged the xylem cell cross-sectional area. Analysis of m⁶A-seq data indicated that the hypomethylated m⁶A 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 m⁶A 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 m⁶A RNA modification and wood formation.