ArabidopsisXTH4andXTH9contribute to wood cell expansion and secondary wall formation

In dicotyledons, xyloglucan is the major hemicellulose of primary walls affecting the load-bearing framework with participation of XTH enzymes. We used loss- and gain-of function approaches to study functions of abundant cambial region expressedXTH4andXTH9in secondary growth. In secondarily thickened hypocotyls, these enzymes had positive effects on vessel element expansion and fiber intrusive growth. In addition, they stimulated secondary wall thickening, but reduced secondary xylem production. Cell wall analyses of inflorescence stems revealed changes in lignin, cellulose, and matrix sugar composition, indicating overall increase in secondary versus primary walls in the mutants, indicative of higher xylem production compared to wild type (since secondary walls were thinner). Intriguingly, the number of secondary cell wall layers was increased inxth9and reduced inxth4, whereas the double mutantxth4x9displayed intermediate number of layers. These changes correlated with certain Raman signals from the walls, indicating changes in lignin and cellulose. Secondary walls were affected also in the interfascicular fibers where neitherXTH4norXTH9were expressed, indicating that these effects were indirect. Transcripts involved in secondary wall biosynthesis and in cell wall integrity sensing, includingTHE1andWAK2, were highly induced in the mutants, indicating that deficiency inXTH4andXTH9triggers cell wall integrity signaling, which, we propose, stimulates the xylem cell production and modulates secondary wall thickening. Prominent effects ofXTH4andXTH9on secondary xylem support the hypothesis that altered xyloglucan can affect wood properties both directly andviacell wall integrity sensing.SIGNIFICANCE STATEMENTXyloglucan is a ubiquitous component of primary cell walls in all land plants but has not been so far reported in secondary walls. It is metabolizedin muroby cell wall-residing enzymes - xyloglucan endotransglycosylases/hydrolases (XTHs), which are reportedly abundant in vascular tissues, but their role in these tissues is unclear. Here we report that two vascular expressed enzymes in Arabidopsis, XTH4 and XTH9 contribute to the secondary xylem cell radial expansion and intrusive elongation in secondary vascular tissues.Unexpectedly, deficiency in their activities highly affect chemistry and ultrastructure of secondary cell walls by non-cell autonomous mechanisms, including transcriptional induction of secondary wall-related biosynthetic genes and cell wall integrity sensors. These results link xyloglucan metabolism with cell wall integrity pathways, shedding new light on previous reports about prominent effects of xyloglucan metabolism on secondary walls.One sentence summaryXTH4 and XTH9 positively regulate xylem cell expansion and fiber intrusive tip growth, and their deficiency alters secondary wall formation via cell wall integrity sensing mechanisms.