We have studied the use of a glucocorticoid receptor-based inducible gene e
xpression system in the monocotyledonous model plant rice (Oryza sativa L.)
. This system, originally developed by T. Aoyama and N.-H. Chua [(1997) Pla
nt J 11: 605-612], is based on the chimaeric transcriptional activator GVG.
consisting of the yeast Gal4 DNA-binding domain, the VP16 activation domai
n and the glucocorticoid receptor domain. For application in rice, we desig
ned an optimized binary vector series (pINDEX) and tested this with the fl-
glucuronidase (gusA) reporter gene. GUS expression was tightly controlled a
nd relatively low concentrations (1-10 muM) of the glucocorticoid hormone d
examethasone (DEX) were able to induce GUS activities to levels comparable
to those conferred by the strong cauliflower mosaic virus (CaMV) 35S promot
er. DEX was taken up efficiently by the roots of tissue-cultured plantlets
or mature plants in hydroponic culture, and induced GUS activity throughout
the whole plant. DEX-induced GUS expression patterns were consistent in al
l lines and their TI progeny. The phenotype of tissue-cultured rice plantle
ts was not affected when inductions with 10-100 muM DEX were limited to 1-4
days or when 2-week inductions were performed with I muM DEX. which was al
ready sufficient to reach near-maximal GUS activity. However. 2-week induct
ions with 10 muM DEX caused growth retardation and developmental defects. A
s the severity of these effects varied between different lines, we could se
lect lines with a mild phenotype for future use as activator lines in cross
es with 'target', plants.