Glucocorticoid-inducible gene expression in rice

We have studied the use of a glucocorticoid receptor-based inducible gene expression system in the monocotyledonous model plant rice (Oryza sativa L.). This system, originally developed by T. Aoyama and N.-H. Chua [(1997) Plant J 11: 605-612], is based on the chimaeric transcriptional activator GVG. consisting of the yeast Gal4 DNA-binding domain, the VP16 activation domain and the glucocorticoid receptor domain. For application in rice, we designed an optimized binary vector series (pINDEX) and tested this with the fl-glucuronidase (gusA) reporter gene. GUS expression was tightly controlled and relatively low concentrations (1-10 muM) of the glucocorticoid hormone dexamethasone (DEX) were able to induce GUS activities to levels comparable to those conferred by the strong cauliflower mosaic virus (CaMV) 35S promoter. 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 all lines and their TI progeny. The phenotype of tissue-cultured rice plantlets 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 already sufficient to reach near-maximal GUS activity. However. 2-week inductions with 10 muM DEX caused growth retardation and developmental defects. As the severity of these effects varied between different lines, we could select lines with a mild phenotype for future use as activator lines in crosses with 'target', plants.