Two Arabidopsis thaliana genes, KOR2 and KOR3, which encode membrane-anchored endo-1,4-beta-D-glucanases, are differentially expressed in developing leaf trichomes and their support cells

The Arabidopsis KOR gene encodes a membrane-anchored endo-1,4-beta -D-gluca nase involved in cell wall assembly. To obtain a more detailed knowledge of the small gene family encoding membrane-anchored endo-1,4-beta -D-glucanas es in Arabidopsis thaliana, we have characterized two additional membrane-a nchored endo-1,4-beta -D-glucanase genes. Sequence comparison indicates tha t KOR2 is distantly related to KOR and other plant membrane-anchored endo-1 ,4-beta -D-glucanases. The expression of KOR2 and KOR3 was followed by the beta -glucuronidase (gusA) reporter-gene method. While the KOR gene is most often expressed throughout the plant, KOR2::gusA and KOR3::gusA are active only in restricted cell types. We demonstrate that KOR2::gusA is expressed very early in the development of root hairs within the root differentiatio n zone (specialization zone) but not in the root-hair-bearing epidermal cel ls at the root/shoot junction (transition zone). Furthermore, KOR2::gusA is expressed in the proximal parts of leaves and floral organs (rosette and c auline leaves, sepals, petals and stamens), and in trichomes, as they devel op at the tip of young leaves and later in more basal regions of the leaf b lade. The KOR3::gusA construct is expressed in the trichome support cells t hat form a ring at the base of each trichome and in the bundle sheath cells which surround the vascular bundle within the leaf mesophyll tissue. Rever se transcription-polymerase chain reaction of Arabidopsis RNA confirmed the expression of KOR2::gusA and KOR3::gusA. In conclusion, although KOR2 and KOR3 have more restricted expression patterns than the previously character ized KOR gene, they are expressed in cell types at time points where cell w all assembly is likely to occur and, interestingly, differentially expresse d in leaf trichomes and their support cells.