COBRA-LIKE4 modulates cellulose synthase velocity and facilitates cellulose deposition in the secondary cell wall

Cellulose is a critical component of secondary cell walls (CWs) and woody tissues of plants. Cellulose synthase (CESA) complexes (CSCs) produce cellulose as they move within the plasma membrane, extruding glucan chains into the CW that coalesce and often crystallize into cellulose fibrils. Here we examine COBRA-LIKE4 (COBL4), a GPI-anchored protein on the outer leaflet of the plasma membrane that is required for normal cellulose deposition in secondary CWs. Characterization of the Arabidopsis (Arabidopsis thaliana) cobl4 mutant alleles called irregular xylem6, irx6-2 and irx6-3, showed reduced α-cellulose content and lower crystallinity, supporting a role for COBL4 in maintaining cellulose quantity and quality. In live-cell imaging, mNeon Green-tagged CESA7 moved in the plasma membrane at higher speeds in the irx6-2 background compared to wild-type. To test conservation of COBL4 function between herbaceous and woody plants, poplar (Populus trichocarpa) COBL4 homologs PtCOBL4a and PtCOBL4b were transformed into, and rescued, the Arabidopsis irx6 mutants. Using the Arabidopsis secondary CW-inducible VND7-GR system to study poplar COBL4 dynamics, YFP-tagged PtCOBL4a localized to the plasma membrane in regions of high cellulose deposition in secondary CW bands. As predicted for a lipid-linked protein, COBL4 was more mobile in the plane of the plasma membrane than CESA7 or a control plasma membrane marker. Following programmed cell death, COBL4 anchored to the secondary CW bands. These data support a role for COBL4 as a modulator of cellulose organization in the secondary CW, influencing cellulose production, and CSC velocity at the plasma membrane.