The promoter of the potato chitinase C gene directs expression to epidermal cells

Chitinases are ubiquitous proteins that occur in all plants in multiple isoforms. We have isolated the ChtC2 gene encoding an unusual, basic (class I) chitinase from potato (Solanum tuberosum L.). In contrast to other chitinase genes, ChtC2 is not activated by infection, but rather constitutively expressed in leaves and stems where it is restricted to epidermal cells. Sequence analysis revealed a number of potential regulatory elements in the promoter, but most striking was the presence of a 319-bp direct repeat located between -333 and -968 upstream of the transcription start site. For a functional analysis, a 1,322-bp promoter fragment and two 5' deletions of 782 bp and 162 bp in length were translationally fused to the beta-glucuronidase (GUS) reporter gene and used for transient expression studies by particle bombardment. All promoter constructs conferred expression of GUS activity in different epidermal cell types of potato leaves. Expression in parenchyma cells of the leaf mesophyll was not detectable with any of the ChtC2 gene promoter constructs, in contrast to the pattern observed with the 35S promoter from cauliflower mosaic virus. The epidermis-specific expression of the reporter gene was confirmed using transgenic potato plants containing the fusion of the entire ChtC2 promoter with the GUS reporter. Histochemical analysis indicated that the promoter was only active in epidermal cells of leaves.