Do plants have cellulases?
At least some plants have cellulases which can be used to cleave primers from mature cellulose chains.
Cellulose synthesis requires chain initiation and elongation (the two processes are separate). CesA glucosyltransferase initiates cellulose polymerization using a steroid primer and UDP-glucose. Cellulose synthase utilizes UDP-D-glucose precursors to elongate the growing cellulose chain. In some plants cellulase is then used to cleave the primer from the mature chain.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4119035/