Xylan Plays a Critical Role in Patterned Secondary Cell Wall Formation
Numerous cellular processes act in a coordinated manner in order to deposit patterned secondary cell walls (SCWs) to reinforce xylem vessel cells. Arabidopsis protoplasts, induced to regenerate SCWs, were used to investigate the role of xylan in pattern formation. Cellulose and xylan patterns were examined with fluorescence labeling and confocal microscopy. Protoplasts with shortened xylan backbones (irx9-2, irx14, & endo-xylanase-treated) produced distorted SCWs bands with coincident cellulose and xylan patterning. Protoplasts isolated from esk1, where cellulose-xylan interactions are limited, also produced distorted SCWs but, uniquely, the cellulose and xylan patterns did not coincide. Our data shows that SCW patterning is disrupted by altered xylan structure. We hypothesize that a cellulose-xylan network is established during early wall synthesis that acts as a scaffold to direct subsequent microtubule-independent cellulose deposition. Short-chain xylans do not properly tether cellulose microfibrils, which likely results in polymer drift, yielding an indistinct scaffold that is reinforced throughout synthesis. When cellulose and xylan do not interact, their patterns are perpetually misaligned. Our findings point to the existence of a secondary cell wall-plasma membrane-cytoskeleton continuum, previously only demonstrated for primary cell walls. Finally, we present SCW-regenerating protoplasts as a novel platform to research the deposition of xylem vessel-type SCWs.