The QKY-SYP121 complex controls long-distance florigen movement ($) (Plant Cell)

In Arabidopsis thaliana, changes in day-length (photoperiod) activate the expression and transport of phloem-mobile florigen (FT, FLOWERING LOCUS T) to the shoot apical meristem to trigger the transition to flowering. While the role of FT as a long-distance signal is well-established, the underlying mechanisms controlling its movement from phloem companion cells (CCs) to sieve elements (SEs) for access to the phloem translocation stream is not well understood. Liu et al. investigated the role of membrane-resident SNARE proteins controlling endosomal vesicular trafficking for potential roles in regulating the CC-to-SE movement of FT, starting with the observation of a late-flowering phenotype in syp121-4 (syntaxin of plants 121) mutants. They subsequently showed that SYP121 interacts with the MCTP (Multiple C2 domain-containing Transmembrane Protein) family protein QUIRKY (QKY), much like the related MCTP family member, FT-INTERACTING PROTEIN1 (FTIP1). They observed FT movement defects in syp121, qky, and ftip1 mutants relative to wild-type controls. Collectively, this work reveals that a QKY-SYP121 complex mediates FT movement from CCs-to-SEs through endosomal vesicular trafficking at the plasma membrane, which occurs alongside FTIP1-mediated transport. (Summary by Phil Carella) Plant Cell 10.1105/tpc.18.00960