KNOTTED1 mRNA undergoes long-distance transport and interacts with movement protein binding protein 2C in pear (Pyrus betulaefolia)

KNOTTED1 (KN1)-like homologous box is a homeodomain transcription factor that is transported intercellularly through plasmodesmata and over long distances via phloem transport in plants. In this study, we investigated KN1 long-distance transport and its regulation by the viral movement protein binding protein of ‘Du Li’ (Pyrus betulaefolia, PbMPB2C) in two species of fruit trees, ‘Du Li’ (rootstock) and ‘Ya Li’ (P. bretschneideri, scion), using grafting experiments. The results showed that PbKN1 mRNA was present in the phloem in both transport directions across the graft junctions of tissue-cultured grafted plants and orchard saplings. To investigate the relationship between PbKN1 and PbMPB2C, we conducted interaction assays to demonstrate their physical interaction. An RNA–protein pull-down assay revealed that PbKN1 mRNA interacted with PbMPB2C protein in vitro but not with PbKN1 and that PbKN1 repressed the binding of PbMPB2C to PbKN1 mRNA. Additionally, using p35S:GFP-PbKN1 transgenic tobacco grafted with wild-type plants, we found that the movement of PbKN1 was increased by the silencing of NtMPB2C. Based on the above findings, we first demonstrate evidence of the bidirectional transport of PbKN1 mRNA and propose that PbMPB2C could act as a negative regulator of the phloem-mediated long-distance transport of PbKN1 mRNA.