SBT4.1 promotes xylem cell expansion and coordinates secondary wall deposition and PCD by modulating PME61 activity downstream of SND1/VND6/MYB46 in Arabidopsis thaliana.

Abstract

Xylem development involves a series of coordinated processes, including cell differentiation, expansion, secondary cell wall (SCW) deposition, and programmed cell death (PCD). Here, we characterize a subtilisin-like serine protease gene, SBT4.1, which exhibits specific expression in stem xylem. Loss of SBT4.1 function results in reduced xylem cell number, smaller cell size, thinner secondary walls, and delayed organelle degradation during PCD. In contrast, SBT4.1 overexpression accelerates cellular clearance and increases xylem cell number, size, and wall thickness, indicating its critical roles in differentiation, SCW synthesis, and PCD. Consistent with a positive regulatory function, pectinase activity and the expression of pectin-related genes are decreased in the mutant but increased in overexpressors. Accordingly, SBT4.1 promotes cell expansion, at least partly, by modulating pectinase activity, as evidenced by enhanced pectin methylesterase (PME) activity upon co-expression with specific PME genes in Nicotiana benthamiana. Molecular analyses revealed that SBT4.1 is a direct transcriptional target of the key SCW regulators SND1, VND6, and MYB46. Furthermore, the expression of SCW synthesis-related genes and PCD-associated protease genes is downregulated in the mutant and upregulated in overexpressing plants. Taken together, our findings demonstrate that SBT4.1, transcriptionally activated by SND1/VND6/MYB46, coordinates xylem cell differentiation, SCW deposition, and PCD, while also promoting cell expansion through modulation of pectin metabolism.