A cDNA clone corresponding to a transcript that was induced by auxin i
n tomato seedling roots encodes a 223 amino acid protein belonging to
the proteinase inhibitor II family of tomato and potato. The TR8 open
reading frame has an N-terminal region with characteristics of a signa
l peptide and three copies of a 64 amino acid segment that is also pre
sent in multiple copies in other PI-II family members. Comparison to o
ther serine proteinase inhibitors indicates that all three domains are
trypsin-specific. Transcripts homologous to TR8 increased in abundanc
e within 24 h of auxin treatment and continued to increase through 72
h, at which point the induction was approximately ten-fold. Auxin indu
ction was observed in roots and hypocotyls, but not in cotyledons or e
picotyls. Induction may therefore be correlated with lateral and adven
titious root initiation, which occurs over a similar time frame in the
same tissues. RFLPs detected with the TR8 probe were mapped to the lo
ng arm of chromosome 11.