Two divergent xyloglucan endotransglycosylases exhibit mutually exclusive patterns of expression in nasturtium

A cDNA encoding a xyloglucan endotransglycosylase (XET) homolog was isolated from nasturtium (Tropaeolum majus) epicotyl RNA. The deduced protein encoded by the corresponding gene, termed XET1, was substantially divergent from a previously isolated nasturtium XET (NXG1) expressed in germinating seed cotyledons but was highly homologous to XET genes isolated from vegetative tissues of several distantly related species. XET1 was expressed at the level of mRNA accumulation in all vegetative tissues examined (root, epicotyl, stem, and leaf) except in germinating cotyledons. Conversely, NXG1 exhibited the opposite pattern of expression and its mRNA was detected exclusively in cotyledons. Both XET1 and NXG1 were apparently encoded by single genes. Protein extracts from epicotyls or germinating cotyledons, in which XET1 or NXG1 are specifically expressed respectively, exhibited XET activity when assayed using two different xyloglucan substrates. XET activity from epicotyl extracts used nonfucosylated seed amyloid xyloglucan or fucosylated stem xyloglucan as a substrate with equal facility, whereas XET activity from cotyledon extracts had a significantly higher activity against nonfucosylated xyloglucan. The existence in a single species of two XETs possessing divergent amino acid sequences, mutually exclusive patterns of expression, and potentially different activities against xyloglucan substrates demonstrates the existence of different classes of XET and suggests differing roles in vivo.