Transgenic Advances toward Induction of Precocious Flowering in Citrus

摘要

Prior to transitioning to mature flowering and fruiting trees, citrus development proceeds through a juvenile period lasting up to 10 years. The small globular protein encoded by the Arabidopsis Flowering Locus T (FT) gene has been shown to have a vital role in initiating floral transition. This graft transmissible protein delivers the signal to produce flowers when it is transported from phloem companion cells in leaves to shoot apical meristems. Ectopic expression of a citrus FT (ciFT) transgene in combination with grafting methods could therefore lead to a simple, efficient transfer of induced in vitro precocious flowering to juvenile scion breeding material and might also be utilized to manipulate flowering of commercial trees. We are exploring the roles of each of three ciFT orthologs for inducing precocious flowering and other developmental effects. Stable transformants with genomic clones of the ciFT orthologs under the control of a constitutive 34 FMV promoter have been produced in both citrus and tobacco. Direct regeneration of flowers on Agrobacterium-mediated transformed 'Carrizo' citrange (Citrus sinensis [L.] Osb. x Poncirus trifoliata [L.] Raf.) epicotyl explants has been observed. We have also recovered TO tobacco plants with a dwarf, multi-branching phenotype reflecting one previously reported for ciFT transformed Poncirus trifoliata. To gain tighter control of flowering and to avoid the possibility of detrimental pleiotropic effects that might accompany constitutive ciFT gene expression, we are testing citrus for the feasibility of employing an estrogen receptor-based chemical-inducible system.