Expression of an antisense GIGANTEA (GI) gene fragment in transgenic radish causes delayed bolting and flowering

A late-flowering transgenic radish has been produced by the expression of an antisense GIGANTEA (GI) gene fragment using a floral-dip method. Twenty-five plants were dipped into a suspension of Agrobacterium carrying a 2.5 kb antisense GI gene fragment from Arabidopsis, along with the gusA and bar reporter genes, all under the control of a CaMV 35S promoter. From a total of 1462 seeds harvested from these floral-dipped plants, 16 Basta-resistant T1 plants were found to have GUS activity (transformation efficiency of 1.1%). Southern analysis confirmed the integration of one or two copies of the gusA gene in these herbicide-resistant plants. Expression of the GI gene in T1 plants was much reduced compared to both wildtype plants and plants transformed with pCAMBIA3301 (positive control). In the progenies of eleven T1 plants analysed (T2 generation), all lines showed a significant delay in both bolting and flowering times compared to wildtype and positive control plants, and that, the level of GI transcript was inversely proportional to the time of bolting and flowering. At a maximum, bolting and flowering times were delayed by 17 and 18 days respectively, compared to wildtype plants (in positive control plants, the delay was 23 and 26 days, respectively). Ten of the 11 lines exhibited a significant reduction in plant height compared to wildtype and positive control plants. This study provides evidence that down-regulation of the GI gene by co-suppression could delay bolting in a cold-sensitive long-day (LD) plant. Production of late-flowering germplasms of radish may allow this important crop to be cultivated over an extended period and also provide further food to the famine countries of S/E Asia.