Patterned secondary cell-wall assembly in tracheary elements occurs in a self-perpetuating cascade

The behaviours are presented of newly-synthesized xylan and putative glycine-rich protein during patterned secondary cell-wall biogenesis in drug-treated tracheary elements (TEs) differentiating in culture from isolated mesophyll cells of Zinnia elegans. The normal secondary wall thickenings contain cellulose, xylan, and lignin, and the results reported here suggest that they also contain glycine-rich protein (GRP). However, qualifications to this definitive interpretation are discussed. The specific cellulose synthesis inhibitors, 2,6- dichlorobenzonitrile (DCB) and isoxaben, were applied near the onset of differentiation. When they were fully effective in inhibiting deposition of detectable cellulose in the thickenings, no labelling of the thickenings was observed with probes for xylan (xylanase and an antibody to xylose) or GRP (an antibody). When the drugs were partially effective, a small amount of detectable cellulose was still deposited in the thickenings. In such TEs, patches of xylan and GRP were observed between thickenings, suggesting that these components were exocytosed but not able to localize at the altered thickenings. A model for cell-wall assembly is presented in which some molecules themselves are able to mediate the patterning of others, so that patterned secondary cell-wall assembly partly occurs by a self-perpetuating cascade.