Eukaryotic cell is subdivided into functionally distinct, membrane-enclosed compartments including endoplasmic reticulum (ER), Golgi apparatus, endosomes, vacuole, and so on. Vesicle traffic is a major transport system in eukaryotic cells. It connects the interiors of most endomembrane compartments to each other and to the outside of the cell; mediates the sorting and targeting of ER-synthesized proteins and lipids to specific compartments for distinct functions; and contributes to the changes in cell volume. Transport vesicles bud off from one compartment and fuse with another. Vesicle positioning and motility is governed by the cytoskeleton elements and motor proteins. Membrane fusion between the transport vesicle and the particular target membrane requires specialized fusion proteins named SNAREs (Soluble NSF attachment protein receptor).
Plant cell polar growth requires polarized vesicle-targeting and active exocytosis at specialized growth sites. However, how exocytic trafficking is spatially and temporally controlled remains elusive. Our recent study reveals that a Qa-SNARE, SYP121, acts as an effector of Rho of Plants 2 (ROP2), mediating regulation of root hair tip growth. Active ROP2 promotes SYP121 targeting to the apical plasma membrane (PM). Moreover, ROP2 directly interacts with SYP121 and promotes the interaction between SYP121 and the R-SNARE VAMP722 to form a SNARE complex, likely by counteracting with the Sec1/Munc18 protein SEC11 which suppresses SYP121’s function. Thus, the ROP2-SYP121 pathway facilitates exocytic trafficking during root hair tip growth. Our new finding uncovers a direct link between a ROP GTPase and vesicular trafficking and a new mechanism controlling apical exocytosis, whereby ROP GTPase signaling spatially regulates SNARE complex assembly and the polar distribution of a Q-SNARE.
Stomata are small pores surrounded by a pair of guard cells and are present on the epidermis of all aerial parts of almost all higher plants. By modulating stomatal opening and closure, plants control gas exchange, water loss, and photosynthesis in response to various environmental signals. During light-induced stomatal opening, the transport of ions and solutes across the PM of the surrounding guard cells results in an increase in turgor pressure, leading to cell swelling. Simultaneously, vesicles for exocytosis are delivered via membrane trafficking to compensate for the enlarged cell surface area and maintain an appropriate ion-channel density in the PM. It has been shown that delivery and positional anchoring of a K+ channel KAT1 to the PM is dependent on SYP121. Our recent work demonstrates that SYP121 directly binds to AtEB1, a microtubule plus-end binding protein, trafficking along with the growth of microtubules for targeting to the plasma membrane. Moreover, a light-induced phosphorylation of SYP121 is revealed, and we characterize the kinase which mediates the SYP121 phosphorylation in response to light and participates the regulation of exocytic trafficking during light-induced stomatal opening.
