Pectin methylesterases

Because methylation of pectins affects their structural properties in the cell wall as well as their susceptibility to pectinases, expression of pectin methylesterases (PMEs) has been altered to modify pectin metabolism of tomato fruit (Tieman et al., 1992; Tieman and Handa, 1994; Tieman et al., 1995). Suppression of the expression of a single PME in tomato by the transgenic introduction of a truncated sense tomato PME gene resulted in significantly higher molecular weight pectins isolated from the fruit cell walls (Thakur et al., 1996a). Processed tomato products made from the PME suppressed transgenic lines also exhibited increased serum viscosity and reduced serum separation (Thakur et al., 1996b; Errington et al., 1998). Analysis of the PME mRNA in the transgenic plants suggested that the reduction in the endogenous PME mRNA resulted from interference by the transgenic mRNA with post-transcriptional processing of the endogenous PME mRNA (Mishra and Handa, 1998). Transgenic expression of the sequence encoding 71 amino acids of tomato fruit PG linked to the tomato PME sequence under control of the 35S CaMV promoter resulted in suppression of both PG and PE simultaneously (Seymour et al., 1993).