Functional identification of MdPIF1 as a Phytochrome Interacting Factor in Apple

Light plays a central role in regulating both apple plant yield and fruit quality formation; however, the Phytochrome interacting factors (PIFs), which are the main components of Phytochrome-mediated light signal transduction in apple, have rarely been characterized. Here, we isolated and identified a PIF-like protein(MdPIF1) in apple, which is similar to AtPIF1. MdPIF1 was constitutively expressed at different levels in various apple tissues, and the transcription level of MdPIF1 was significantly induced during seed germination. A functional complementation assay in the Arabidopsis PIF1-deletion mutant pil5 suggested that MdPIF1 was a negative regulator in the Phy-mediated inhibition of hypocotyl elongation under far-red light conditions. MdPIF1-overexpression promoted hypocotyl elongation, while inhibiting seed germination and PIF1 deletion-induced the bleaching phenotype in the pil5 mutant. In addition, expression analysis indicated that MdPIF1 was involved in the germination of apple seeds and dormancy breaking of apple buds. Moreover, MdPIF1 inhibited the growth of apple calli via Phy-mediated pathways. These findings build a solid foundation for studies on Phytochrome-mediated light signal transduction and molecular breeding in apple.