Xyloglucan Endotransglycosylation in Suspension-cultured Poplar Cells

Xyloglucan endotransglycosylase activity was identified and defined by transfer of a part of xyloglucan to reduced xyloglucan heptasaccharide ([3H]XXXGol) in an enzyme preparations from suspension-cultured poplar cells. Although the activity was distributed in buffer-soluble and buffer-insoluble fractions associated with cells and in the extracellular fraction, it was mostly recovered in the buffer-insoluble fraction, suggesting that the enzyme was bound to the cell wall. The affinity for acceptor XXXGol was increased at a higher concentration of donor xyloglucan with a constant Vmax. The Vmax for donor xyloglucan was increased at a higher concentration of the oligosaccharide without any change in affinity. These kinetic data suggest that the acceptor acts by combining with the enzyme independently of the donor. The velocity of the reaction decreased gradually as the heptasaccharide units was increased from two to four, suggesting that the xyloglucan endotransglycosylase reaction caused donor xyloglucan substantially to decrease in molecular size. The activity in buffer-soluble fraction was increased by ABA in auxin-starved cells, when cultured in MS medium containing various plant hormones. Nevertheless, the activity increased markedly at the exponential growth and decreased immediately at the stationary phase of cells in the presence of 2,4-D. The activity of xyloglucan endotransglycosylase is developmentally regulated during the growth but is not directly induced by plant hormones.