Stigma- and Vascular-Specific Expression of the PR-10a Gene of Potato: A Novel Pattern of Expression of a Pathogenesis-Related Gene. C. Peter Constabel. Department of Biochemistry, Universite de Montreal, Montreal, Canada H3C 3J7. Normand Brisson. Department of Biochemistry, Universite de Montreal, Montreal, Canada H3C 3J7. MPMI 8:104-113. Accepted 26 September 1994. Copyright 1995 The American Phytopathological Society.

The tissue- and organ-specific expression of PR-10a (formerly STH-2), a pathogenesis-related gene of potato, was investigated using an anti-PR- 10a antibody and transgenic potato plants transformed with PR-I0a promoter-(-glu-curonidase (GUS) chimeric gene. Strong PR-10a expression was observed in tubers, stolons, stems, and petioles following infection with the potato pathogen Phytophlhora infestans or elicitation with P. infestans homogenate. These tissues also responded to different degrees to wounding. Induction in leaves was weak and detectable only with the elicitor treatment. Hislochcmical GUS staining indicated that PR-10a induction was associated with vascular bundles. Tuber storage parenchyma responded locally to wounding and infection but to a much smaller extent than vascular bundles. In infected petioles and stems, GUS activity was seen in most cell types beneath the infection site, with strongest expression in vascular cells. PR-10a expression was very vascular-specific in wounded stems and petioles, as well as in elicitor-treated leaves. In healthy, unstressed plants, PR-10a was expressed exclusively in the stigma. Neither GUS activity nor PR-10a protein was detected in vegetative shoot tissues, or in style, ovary, sepal, petal, or anther extracts. The stigma-specific expression was shown to be develop-mentally regulated, with more PR-10a protein accumulating in the stigmas of fully open than unopened flowers. Histochemical GUS staining indicated strongest PR-10a expression in the papillae and uppermost cell layers of the stigma, with lower levels of expression in cell layers below. Although the general pattern of expression of PR-10a resembles the pattern of expression of genes encoding cell wall protein genes, its organ- and tissue-specific expression differentiates it from most other defense-related proteins and genes. This suggests that PR-IOa plays a unique role in plant defense.