Expression of a xyloglucan endo-transglycosylase gene is closely related to grape berry softening

Abstract

Six partial cDNAs for cell wall degradation-related enzymes, xyloglucan endo-transglycosylase (XET), polygalacturonase (PG), pectin methylesterase (PME), pectate lyase (PL), cellulase (Cel), and β- d-galactosidase (GAL), were isolated from a véraison-specific subtractive library made from Kyoho grape ( Vitis labruscana) berries. During the development of Kyoho grape berries, the expression of these genes was analyzed. Among the genes analyzed, XET gene expression was closely related to berry softening; slight XET gene expression was detected before véraison and was markedly increased at véraison (the stage of the onset of berry softening). In addition, the expression of the gene was berry specific. In the other genes, except for PL, however, no expression was detected during the berry development; PL was detected only after the coloring stage began. These observations suggest that XET plays an important role in grape berry softening. To obtain further information about this gene, a full-length cDNA clone ( VXET 1) encoding XET was isolated from a cDNA library of Kyoho grape berries and characterized. The VXET 1 was 1326 bp in length and contained a 5′-untranslated region of 67 bp, an open reading frame of 873 bp, and a 3′-untranslated region of 386 bp. The deduced amino acid sequence of the VXET 1 showed 73.5% identity with the corresponding XET (NXG 1) from nasturtium ( Tropaeolum majus) that has been shown to have endo-glucanase activity. These findings suggest that the VXET 1 product cleaves a cellulose–xyloglucan network of cell wall and induces the softening of Kyoho grape berries at véraison.