The plant hormone auxin plays a central role in regulating many aspects of plant 
growth and development. This largely occurs as a consequence of changes in gene 
expression. The Aux/IAA genes are best characterized among the early 
auxin-responsive genes, which encode short-lived transcriptional repressors. In 
most plants examined, including Arabidopsis, soybean, and rice, the Aux/IAA 
genes constitute a large gene family. By screening the available databases, at 
least 15 expressed sequence tags (ESTs) have been identified from wheat 
(Triticum aestivum), which exhibit high sequence identity with Aux/IAA 
homologues in other species. One of these Aux/IAA genes, TaIAA1, harbouring all 
the four conserved domains characteristic of the Aux/IAA proteins, has been 
characterized in detail. The expression of TaIAA1 is light-sensitive, 
tissue-specific, and is induced within 15-30 min of exogenous auxin application. 
Also, the TaIAA1 transcript levels increase in the presence of a divalent 
cation, Ca(2+), and this effect is reversed by the calcium-chelating agent, 
EGTA. The TaIAA1 gene qualifies as the primary response gene because an increase 
in its transcript levels by auxin is unaffected by cycloheximide. In addition to 
auxin, the TaIAA1 gene is also induced by brassinosteroid, providing evidence 
that interplay between hormones is crucial for the regulation of plant growth 
and development.