Sequences homologous to a
potato cathepsin D inhibitor cDNA, p749, were identified in the genomic
DNA of tomato (Lycopersicon esculentum) and of two non-tuber-bearing potato species (Solanum etuberosum and S. brevidens) by means of Southern blot analysis. The expression of these p749 genes in leaves was induced at the
RNA level in response to wounding. High levels of p749 transcripts were detected in
polyadenylated RNA extracted from locally wounded leaves 12 h after wounding. Systemic induction of the
cathepsin D inhibitor gene also occurred in nonwounded leaves of wounded plants. Both potato and tomato leaves treated with the
oligosaccharide chitosan showed an induced accumulation of p749 transcripts. Even though the
cathepsin D inhibitor genes from tomato and from non-tuber-bearing potato species are
wound inducible, they could not be induced in leaf explants cultured on medium containing very high concentrations of
sucrose. Only leaf explants from the tuber-bearing potato (S. tuberosum) accumulated p749 transcripts when cultured on high
sucrose medium. A sequence related to the 22-kD potato
proteinase inhibitor
cDNA, p34021, was identified in tomato by means of genomic Southern blot analysis. Northern blot hybridization showed that p34021 transcripts accumulated in potato (S. tuberosum) leaf explants, but not in tomato explants, when cultured on high
sucrose medium. This study demonstrates that the expression of a
potato cathepsin D inhibitor gene in tomato and in non-tuber-bearing potato species is
wound inducible, but not
sucrose inducible.