Characterization of the Populus PLCP Gene Family and the Function of PtrXCP2A in Wood Formation

Papain-like cysteine proteases (PLCPs) are a class of proteolytic enzymes that are involved in plant stress, immunity, growth and development processes. Compared to the extensive research on herbaceous plants, little is known about the PLCPs in trees, and the involvement of PLCPs in wood formation still needs to be confirmed. Here, 40 PLCPs were recognized in the genome of Populus trichocarpa, which can be divided into seven subfamilies with identified members: RD21, CEP, XCP, XBCP3, SAG12, RD19 and ALP. The genes in each subfamily exhibited similar gene structures, conserved motifs and domain compositions. Chromosome mapping analysis revealed that tandem and segmental duplications played crucial roles in expanding the PLCP family in Populus. Expression profiling, RT-qPCR verification and GUS activity analysis revealed that several PtrPLCPs were preferentially or highly expressed in the secondary xylem of the stem. Furthermore, we characterised PtrXCP2A overexpression in poplars and found that overexpression of PtrXCP2A accelerates the loss of xylem fiber viability, reduces fiber cell wall thickness and lignin content, but increases the saccharification yield of wood cell walls in transgenic poplar. These findings offer valuable information for future functional studies on the PLCP gene family, and an alternative gene for engineering lignocellulosic biomass for wood utilisation.