The mRNA of a Knotted1-like transcription factor of potato is phloem mobile

Potato Homeobox1 (POTH1) is a Knotted1-like transcription factor from the Three Amino Acid Loop Extension (TALE) superfamily that is involved in numerous aspects of development in potato (Solanum tuberosum L). POTH1 interacts with its protein partner, StBEL5, to facilitate binding to specific target genes to modulate hormone levels, mediate leaf architecture, and enhance tuber formation. In this study, promoter analyses show that the upstream sequence of POTH1 drives beta-glucuronidase activity in response to light and in association with phloem cells in both petioles and stems. Because POTH1 transcripts have previously been detected in phloem cells, long-distance movement of its mRNA was tested. Using RT-PCR and transgenic potato lines over-expressing POTH1, in vitro micrografts demonstrated unilateral movement of POTH1 RNA in a rootward direction. Movement across a graft union into leaves from newly arising axillary shoots and roots of wild type stocks was verified using soil-grown tobacco heterografts. Leaves from the wild type stock containing the mobile POTH1 RNA exhibited a reduction in leaf size relative to leaves from wild type grafts. Both untranslated regions of POTH1 when fused to an expression marker beta-glucuronidase, repressed its translation in tobacco protoplasts. RNA/protein binding assays demonstrated that the UTRs of POTH1 bind to two RNA-binding proteins, a polypyrimidine tract-binding protein and an alba-domain type. Conserved glycerol-responsive elements (GRE), specific to alba-domain interaction, are duplicated in both the 5' and 3' untranslated regions of POTH1. These results suggest that POTH1 functions as a mobile signal in regulating development.