Decreased GA(1) Content Caused by the Overexpression of OSH1 Is Accompanied by Suppression of GA 20-Oxidase Gene Expression

We previously reported that overexpression of the rice homeobox gene OSH1 led to altered morphology and hormone levels in transgenic tobacco (Nicotiana tabacum L.) plants. Among the hormones whose levels were changed, GA(1) was dramatically reduced. Here we report the results of our analysis on the regulatory mechanism(s) of OSH1 on GA metabolism. GA(53) and GA(20), precursors of GA(1), were applied separately to transgenic tobacco plants exhibiting severely changed morphology due to overexpression of OSH1. Only treatment with the end product of GA 20-oxidase, GA(20), resulted in a striking promotion of stem elongation in transgenic tobacco plants. The internal GA(1) and GA(20) contents in OSH1-transformed tobacco were dramatically reduced compared with those of wild-type plants, whereas the level of GA(19), a mid-product of GA 20-oxidase, was 25% of the wild-type level. We have isolated a cDNA encoding a putative tobacco GA 20-oxidase, which is mainly expressed in vegetative stem tissue. RNA-blot analysis revealed that GA 20-oxidase gene expression was suppressed in stem tissue of OSH1-transformed tobacco plants. Based on these results, we conclude that overexpression of OSH1 causes a reduction of the level of GA(1) by suppressing GA 20-oxidase expression.