The ‘ins’ and ‘outs’ of flavonoid transport

The sites of plant flavonoid biosynthesis, storage and final function often differ at the subcellular, cell, and even tissue and organ levels. Efficient transport systems for flavonoids across endomembranes and the plasma membrane are therefore required. However, a clear picture of the dynamic trafficking of flavonoids is only now beginning to emerge and appears to have many players. Here, we review current hypotheses for flavonoid transport, discuss whether these are mutually exclusive, highlight the importance of flavonoid efflux from vacuoles to the cytosol and consider future efforts to catch flavonoids ‘in the act’ of moving within and between cells. An improved understanding of transport mechanisms will facilitate the successful metabolic engineering of flavonoids for plant protection and human health. The sites of plant flavonoid biosynthesis, storage and final function often differ at the subcellular, cell, and even tissue and organ levels. Efficient transport systems for flavonoids across endomembranes and the plasma membrane are therefore required. However, a clear picture of the dynamic trafficking of flavonoids is only now beginning to emerge and appears to have many players. Here, we review current hypotheses for flavonoid transport, discuss whether these are mutually exclusive, highlight the importance of flavonoid efflux from vacuoles to the cytosol and consider future efforts to catch flavonoids ‘in the act’ of moving within and between cells. An improved understanding of transport mechanisms will facilitate the successful metabolic engineering of flavonoids for plant protection and human health. a large superfamily of ATP-binding cassette (ABC) proteins. They usually contain a nucleotide-binding domain and a transmembrane domain for mediating MgATP-energized transmembrane transport and/or regulation of other transporters [41]. Different subfamilies of ABC transporters have essential and diverse roles in the transport of metal ions and primary and secondary metabolites across membranes. intravacuolar bodies of varying sizes containing concentrated anthocyanins. AVIs are observable in petal cells of carnation (Dianthus caryophyllus) and lisianthus (Eustoma grandiflorum), and in other anthocyanin-accumulating cells in grapevine, sweet potato (Ipomoea batatas), and maize. AVIs do not have membrane boundaries, but contain membrane lipids and a protein matrix bound to anthocyanins, particularly acylated anthocyanins [19]. initially used to describe cytoplasmic membrane-bound vesicles containing high levels of anthocyanins and regarded as anthocyanin biosynthetic sites. However, later studies confused them with anthocyanic vacuolar inclusion (AVIs), which are more widely observed inside the vacuoles of many plant species. Anthocyanoplasts are found exclusively in the cytoplasm in grapevine cells, and in protoplasts prepared from red radish (Raphanus sativus) seedlings. multidrug and toxic compound extrusion (MATE) family transporters. They use H+/Na+ gradients across membranes as a force to drive waste or toxic compounds out of the cytoplasm. MATE transporters perform conserved and basic transport functions in most prokaryotes and eukaryotes. an endocytic multivesicle compartment involved in ER-Golgi-vacuole vesicle trafficking. It carries proteins and other metabolites to fuse to the large central vacuole. The PVC has SNAREs or vacuolar-sorting receptors to accept precursor vesicles and then fuse to the vacuole. A specific type of PVC in lisianthus epidermal cells contains anthocyanin and was proposed to transport anthocyanins into the central vacuole (19). a type of vacuole and compound organelle formed during plant seed development and maturation and containing large amounts of storage proteins. PSVs contain vacuolar-sorting receptors to recognize cargo molecules. A PSV marker co-localizes with anthocyanins, leading to the suggestion that anthocyanins are transported directly via ER-derived vesicle trafficking in a Golgi-independent manner (22). soluble N-ethylmaleimide-sensitive factor attachment protein receptors. These are small but abundant integral membrane proteins that mediate vesicle fusion and reside on the surface of the transport vesicle (v-SNAREs) and target membrane (t-SNAREs). They have a cytosolic domain called a SNARE motif, which assembles with another SNARE motif into parallel four-helix bundles within SNARE complexes and brings the transmembrane anchors and the two membrane vesicles into close proximity. abundant organelles in the tapetum cells of anthers during the active stage of pollen maturation in Brassicaceae species. Tapetosomes originate from massive ER cisternae, which release lipid droplets that are fused into large vesicles. These lipid vesicles further fuse with ER-derived vesicles containing flavonoids. Upon tapetum cell death, tapetosomes release alkanes and oleosins in lipid droplets, along with flavonoids, to the pollen surface. a dynamic series of membrane compartments at the trans- face of the Golgi stacks. The TGN mainly processes and sorts various proteins and glycolipids at the interface of the biosynthetic and endosomal pathways. The generation and maintenance of apical and basolateral membranes relies on sorting events that occur in the TGN. integral membrane proteins responsible for the proper targeting of cargo proteins to their destination compartments. VSRs are localized to ER, PVC, TGN and Golgi apparatus.