We have isolated
cDNA clones encoding
class I chitinase (
ChtC) from potato leaves which share a high degree of
nucleotide and amino acid sequence similarity to other, previously described basic (class I)
chitinases (ChtB) from potato. Despite this similarity, characteristic features distinguish
ChtC from ChtB, including an extended
proline-rich linker region between the
hevein and catalytic domains and presence of a potential glycosylation site (NDT) in the deduced
protein. These differences are in accordance with the properties of purified
chitinase C which is glycosylated and hence has a higher molecular mass in comparison to
chitinase B. In contrast to the coding sequences, the 3'-untranslated regions of
ChtC and ChtB exhibited a low degree of similarity, which allowed us to generate gene-specific probes to study the genomic organization and expression of both types of gene. Genomic
DNA blots suggest that
ChtC and ChtB are each encoded by one or two genes per haploid genome.
RNA blot analysis showed that in healthy potato plants
ChtC mRNA is most abundant in young leaves, the organs which also contain high levels of
chitinase C. By contrast, ChtB
mRNA abundance is highest in old leaves, which accumulate
chitinase B. By in situ
RNA hybridization with gene-specific probes we could demonstrate that
ChtC mRNA in leaves is restricted to epidermal cells, whereas ChtB
mRNA showed no distinct pattern of cell-type-specific localization.
Infection of potato leaves with Phytophthora infestans, or treatment with fungal elicitor,
ethylene, or wounding resulted in accumulation of both
ChtC and ChtB mRNAs; however, for
ChtC, in contrast to ChtB, no corresponding accumulation of the encoded
protein could be detected, suggesting a post-transcriptional mechanism of regulation.
Salicylic acid treatment did not induce accumulation of either
mRNA. The possible functional implications of these findings for pathogen defence and developmental processes are discussed.