Phytochrome A overexpression in transgenic tobacco. Correlation of dwarf phenotype with high concentrations of phytochrome in vascular tissue and attenuated gibberellin levels.

Phytochromes are a family of related chromoproteins that regulate photomorphogenesis in plants. Ectopic overexpression of the phytochrome A in several plant species has pleiotropic effects, including substantial dwarfing, increased pigmentation, and delayed leaf senescence. We show here that the dwarf response is related to a reduction in active gibberellins (GAs) in tobacco (Nicotiana tabacum) overexpressing oat phytochrome A under the control of the cauliflower mosaic virus (CaMV) 35S promoter and can be suppressed by foliar applications of gibberellic acid. In transgenic seedlings, high concentrations of oat phytochrome A were detected in stem and petiole vascular tissue (consistent with the activity of the CaMV 35S promoter), implicating vascular tissue as a potential site of phytochrome A action. To examine the efficacy of this cellular site, oat phytochrome A was also expressed using Arabidopsis chlorophyll a/b-binding protein (CAB) and the Arabidopsis ubiquitin (UBQ1) promoters. Neither promoter was as effective as CaMV 35S in expressing phytochrome in vascular tissue or in inducing the dwarf phenotype. Collectively, these data indicate that the spatial distribution of ectopic phytochrome is important in eliciting the dwarf response and suggest that the phenotype is invoked by elevated levels of the far-red-absorbing form of phytochrome within vascular tissue repressing GA biosynthesis.