Evidence that endo‐1,4‐β‐glucanases act on cellulose in suspension‐cultured poplar cells

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Abstract
Suspension‐cultured poplar (Populus alba) cells produce two distinct endo‐1,4‐β‐glucanases, one of which is released in the extracellular culture medium and the other localized in their walls. Two cDNA clones, PopCel1 and PopCel2, isolated from a poplar cDNA library, encode the extracellular and the wall‐bound endo‐1,4‐β‐glucanases, respectively, based upon deduced amino acid sequences. The products of these two genes contained domains conserved in endo‐1,4‐β‐glucanase (family 9) and showed 91.5% amino acid identity. The levels of both PopCel1 and PopCel2 mRNAs increased during the lag phase of growth and decreased rapidly during the linear phase. After the levels had decreased, they were again increased by addition of sucrose to the culture medium and further enhanced by the addition of 2,4‐dichlorophenoxyacetic acid (2,4‐D) in the presence of sucrose. The accumulation of the mRNAs was correlated with the solubilization of cello‐oligosaccharides. Cello‐oligosaccharides and xyloglucan were also solubilized from the wall preparations of poplar cells incubated with enzyme preparations from the extracellular culture medium and walls. An antibody against both PopCel proteins reduced the production of cello‐oligosaccharides by the extracellular enzyme by 90% and that by the wall‐bound enzyme by 55%, and also prevented xyloglucan solubilization. The results show that the accumulation of poplar endo‐1,4‐β‐glucanases is regulated indirectly by auxin in the presence of sucrose and can act on cellulose in suspension‐cultured poplar cells.