An alternate route for cellulose microfibril biosynthesis in plants

Like cellulose synthases (CESAs), cellulose synthase-like D (CSLD)
proteins synthesize β-1,4 glucan in plants. CSLDs are important for tip
growth and cytokinesis, but it was unknown whether they form membrane
complexes in vivo or produce microfibrillar cellulose. We produced viable
CESA-deficient mutants of the moss Physcomitrium patens and used them to
investigate CSLD function in the absence of interfering CESA activity.
Microscopy and spectroscopy showed that the CESA-deficient mutants
synthesize cellulose microfibrils that are indistinguishable from those in
vascular plants. Correspondingly, freeze-fracture electron microscopy
revealed rosette-shaped particle assemblies in the plasma membrane that
are indistinguishable from CESA-containing rosette cellulose synthesis
complexes (CSCs). Our data show that proteins other than CESAs, most
likely CSLDs, produce cellulose microfibrils in P. patens protonemal
filaments. They also suggest that the specialized roles of CSLDs in
cytokinesis and tip growth are based on differential expression and
different interactions with microtubules and possibly Ca2+, rather than
structural differences in the microfibrils they produce.