Abstract: We conducted our study on tobacco plants for altered rates of biosynthesis and catabolism of GA constitutively or in specific tissues by transgenic approach and RNAi gene silencing technology. In the present study, PtGA20ox and PtGA2ox1 were over-expressed under the control of 35S promoter or root meristem-specific promoter (TobRB7) respectively and NtGA20ox was silenced. Root tips of transgenic plants and NtGA20ox RNAi lines were microscopic observation (n 100). It showed that transgenic plants compared with control had a very significant difference. This difference can be reversed by the exogenous application of GA3 and gibberellin synthesis inhibitor PAC. Moreover, similar phenotypes were obtained for root tip meristem-specific up-and down-regulation and constitutive regulation of endogenous gibberellin levels. The results provide evidence that GA negative regulates the root tip diameter through effects on the meristem