A complex consisting of evolutionarily conserved FD, FLOWERING LOCUS T (FT) proteins is a regulator of floral transition. Intriguingly, FT orthologs are also implicated in developmental transitions distinct from flowering, such as photoperiodic control of bulbing in onions, potato tuberization, and growth cessation in trees. However, whether an FT-FD complex participates in these transitions and, if so, its mode of action, are unknown. We identified two closely related FD homologs, FD-like 1 (FDL1) and FD-like 2 (FDL2), in the model tree hybrid aspen. Using gain of function and RNAisuppressed FDL1 and FDL2 transgenic plants, we show that FDL1 and FDL2 have distinct functions and a complex consisting of FT and FDL1 mediates in photoperiodic control of seasonal growth. The downstream target of the FT-FD complex in photoperiodic control of growth is Like AP1 (LAP1), a tree ortholog of the floral meristem identity gene APETALA1. Intriguingly, FDL1 also participates in the transcriptional control of adaptive response and bud maturation pathways, independent of its interaction with FT, presumably via interaction with ABSCISIC ACID INSENSITIVE 3 (ABI3) transcription factor, a component of abscisic acid (ABA) signaling. Our data reveal that in contrast to its primary role in flowering, FD has dual roles in the photoperiodic control of seasonal growth and stress tolerance in trees. Thus, the functions of FT and FD have diversified during evolution, and FD homologs have acquired roles that are independent of their interaction with FT. 5), is critical for the formation of protein complexes to control flowering via transcriptional control of downstream targets [e.g., floral meristem identity genes, transcription factors APETALA1 (AP1) and OsMADS1]. Whereas the FT-FD complex promotes flowering at the shoot apical meristem (4), BRC1 appears to delay floral transition at the axillary meristem (5). These findings indicate that depending upon its interaction partner, FT-containing complexes can have distinct roles. The structure of FT-FD complex elucidated in rice has shown that a 14-3-3 protein mediates the interaction between rice FT homolog HEADING DATE 3a (Hd3a) and FD homolog OsFD1 via the C-terminally located SAP (serine alanine proline) motif in OsFD1 to generate the active nuclear localized florigen activation complex (3).Interestingly, FT homologs are also involved in the control of diverse developmental transitions distinct from flowering, such as tuberization in potatoes (6), bulb formation in onions (7), stomatal opening (8), and photoperiodic control of seasonal growth in trees (9-11). These observations suggest that complexes of FT are not only important to the control of flowering but have a broader functionality. However, the mechanisms underlying the functional diversity of the FT complexes and how they can participate in the control of developmental pathways distinct from flowering are not well understood because, in contrast to FT, FD or BRC1 (or other interactors of FT), which provide DNA binding ability to ...

The post-Golgi compartment trans-Golgi Network (TGN) is a central hub divided into multiple subdomains hosting distinct trafficking pathways, including polar delivery to apical membrane. Lipids such as sphingolipids and sterols have been implicated in polar trafficking from the TGN but the underlying mechanisms linking lipid composition to functional polar sorting at TGN subdomains remain unknown. Here we demonstrate that sphingolipids with α-hydroxylated acyl-chains of at least 24 carbon atoms are enriched in secretory vesicle subdomains of the TGN and are critical for de novo polar secretory sorting of the auxin carrier PIN2 to apical membrane of Arabidopsis root epithelial cells. We show that sphingolipid acyl-chain length influences the morphology and interconnections of TGN-associated secretory vesicles. Our results uncover that the sphingolipids acyl-chain length links lipid composition of TGN subdomains with polar secretory trafficking of PIN2 to apical membrane of polarized epithelial cells.

The plant hormone indole-acetic acid (auxin) is essential for many aspects of plant development. Auxin-mediated growth regulation typically involves the establishment of an auxin concentration gradient mediated by polarly localized auxin transporters. The localization of auxin carriers and their amount at the plasma membrane are controlled by membrane trafficking processes such as secretion, endocytosis, and recycling. In contrast to endocytosis or recycling, how the secretory pathway mediates the localization of auxin carriers is not well understood. In this study we have used the differential cell elongation process during apical hook development to elucidate the mechanisms underlying the post-Golgi trafficking of auxin carriers in Arabidopsis. We show that differential cell elongation during apical hook development is defective in Arabidopsis mutant echidna (ech). ECH protein is required for the trans-Golgi network (TGN)-mediated trafficking of the auxin influx carrier AUX1 to the plasma membrane. In contrast, ech mutation only marginally perturbs the trafficking of the highly related auxin influx carrier LIKE-AUX1-3 or the auxin efflux carrier PIN-FORMED-3, both also involved in hook development. Electron tomography reveals that the trafficking defects in ech mutant are associated with the perturbation of secretory vesicle genesis from the TGN. Our results identify differential mechanisms for the postGolgi trafficking of de novo-synthesized auxin carriers to plasma membrane from the TGN and reveal how trafficking of auxin influx carriers mediates the control of differential cell elongation in apical hook development.sorting | IAA | morphogenesis P olar auxin transport (PAT) plays a key role in plant development (1-5). PAT is mediated by plasma membrane localized auxin influx and efflux carriers of the auxin-resistant (AUX)/like-AUX (LAX), pin-formed (PIN), and ABCB families (6-12). Highly regulated tissue, cellular localization, and amount of auxin carriers at the plasma membrane (PM) provide directionality to the auxin transport and underlies the creation of auxin concentration gradient that is essential for controlling several aspects of plant development (13-18). One of the developmental programs in which auxin concentration gradient plays a central role is the formation of apical hook, a bending in the embryonic stem during early seedling germination (19). Hook formation involves differential elongation of cells on the two opposite sides of the hypocotyl. This process is mediated by the formation of an auxin maximum at the concave side of the hook, leading to the inhibition of cell elongation (20)(21)(22)(23)(24)(25). A model based on mutational analysis shows that auxin carriers including polarly localized auxin efflux and influx facilitators PIN3 and AUX1/LAX3, respectively, are important for hook development (23, 24). The amount of auxin carriers at the PM is important for the regulation of auxin concentration, and this depends on the balance between secretion, endocytosis, and recycling. The analysis...