Role of ?-galactosidase in cell wall metabolism of date palm (Phoenix dactylifera) endosperm

Abstract

The endosperm of developing date palm (Phoenix dactylifera) seeds was sampled at regular intervals from pollination to mature fruit. The galactose content of the cell wall mannans was assessed. Accumulation of α-galactosidase, a cell wall hydrolase, during endosperm development was analyzed by isoelectric focusing, sodium dodecyl sulfate polyacrylamide gel electrophoresis in combination with Western blotting and immunolocalization on tissue sections. N-terminal amino acid sequence of the first 15 amino acids showed homology with amino acids 71 to 85 of the sequence reported for the mature guar protein. Four forms of the enzyme with isoelectric points ranging from 4.4 to 5.2 appeared by 11 weeks after pollination, and all forms remained until maturity. A major band of 41 kDa and several lower Mr, lightly staining bands cross reacted with the anti-α-galactosidase antiserum. The major band remained until maturity while the lightly staining bands gradually disappeared. In the mobilizing endosperm of germinated seeds, two darkly staining bands were observed at 41 and 40 kDa. At 9 weeks after pollination, the endosperm was cellular and the silver enhanced gold label localizing α-galactosidase occurred predominantly in the cell periphery. By 11 weeks, the label was present in the cytoplasm, but lacking on the thickening cell wall. α-Galactosidase accumulated in the protein bodies along with the storage protein. At 13 to 17 weeks, the label accumulated and then was lost in a centrifugal pattern (from the middle lamella inward) from the cell walls as they matured and was lost in the cytoplasm. The mature endosperm cells had intense label present only over the protein bodies and over the inner cell wall. These observations suggest that α-galactosidase is synthesized during endosperm development and unique forms of the enzyme are associated with cell wall maturation and cell wall mobilization in this species.