2 DIVERGENT XYLOGLUCAN ENDOTRANSGLYCOSYLASES EXHIBIT MUTUALLY EXCLUSIVE PATTERNS OF EXPRESSION IN NASTURTIUM

A cDNA encoding a xyloglucan endotransglycosylase (XET) homolog was is olated from nasturtium (Tropaeolum majus) epicotyl RNA. The deduced pr otein encoded by the corresponding gene, termed XET1, was substantiall y 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 vegetati ve tissues examined (root, epicotyl, stem, and leaf) except in germina ting cotyledons. Conversely, NXG1 exhibited the opposite pattern of ex pression and its mRNA was detected exclusively in cotyledons. Both XET 1 and NXG1 were apparently encoded by single genes. Protein extracts f rom epicotyls or germinating cotyledons, in which XET1 or NXG1 are spe cifically expressed respectively, exhibited XET activity when assayed using two different xyloglucan substrates. XET activity from epicotyl extracts used nonfucosylated seed amyloid xyloglucan or fucosylated st em xyloglucan as a substrate with equal facility, whereas XET activity from cotyledon extracts had a significantly higher activity against n onfucosylated xyloglucan. The existence in a single species of two XET s possessing divergent amino acid sequences, mutually exclusive patter ns of expression, and potentially different activities against xyloglu can substrates demonstrates the existence of different classes of XET and suggests differing roles in vivo.