Expression of a membrane-anchored endo-1,4-beta-glucanase from Brassica napus, orthologous to KOR from Arabidopsis thaliana, is inversely correlated to elongation in light-grown plants

A PCR fragment derived from a membrane-anchored endo-1,4-beta -glucanase cD NA was amplified using degenerated oligonucleotides and mRNA from oilseed r ape (Brassica napus L.) siliques. Sequence analysis of the corresponding ge ne, Cel16, showed that the predicted Cel16 protein has high identity with t he Arabidopsis KOR protein (94%). High-stringency genomic Southern analysis further revealed that Cel16 and KOR are most likely orthologous genes perf orming a similar function in both species. Northern blot and GUS analysis o f transgenic Arabidopsis containing a fusion between a 2.0 kb Cel16 promote r fragment and the GUS reporter gene showed that Cel16 was expressed at a l ow level in the primary raceme, the young lateral stems, the elongation zon e of the primary root and the older root base. By contrast, a high level of Cel16 mRNA accumulation was found in the young root and in the main stem c arrying flowers and young siliques. Cel16 transcripts were localized to the apical meristem, cambium, primary xylem and cortex of oilseed rape stem ti ssue by in situ RT-PCR. A similar pattern of activity was found in the GUS analysis of transgenic Arabidopsis. Cel16 mRNA accumulation in the main stem was lower in the zone of most rapi d cell elongation than in the subjacent, fully elongated internodes. Simila rly, Cel16 transcripts accumulated to a higher level in leaves as they reac hed full size than during early leaf expansion. Analysis of the expression pattern in elongating, light-grown seedlings showed that Cel16 mRNA accumul ated at a lower level in the elongating upper third than elsewhere in the h ypocotyl. This is contrary to etiolated hypocotyls, where we found a higher expression level in the rapidly elongating upper part. This difference in expression most probably reflects a difference in cell wall assembly betwee n light- and dark-grown seedlings.