Mediation of clathrin-dependent trafficking during cytokinesis and cell expansion by Arabidopsis stomatal cytokinesis defective proteins.

Stomatal cytokinesis defective1 (SCD1) encodes a putative Rab guanine nucleotide exchange factor that functions in membrane trafficking and is required for cytokinesis and cell expansion in Arabidopsis thaliana. Here, we show that the loss of SCD2 function disrupts cytokinesis and cell expansion and impairs fertility, phenotypes similar to those observed for scd1 mutants. Genetic and biochemical analyses showed that SCD1 function is dependent upon SCD2 and that together these proteins are required for plasma membrane internalization. Further specifying the role of these proteins in membrane trafficking, SCD1 and SCD2 proteins were found to be associated with isolated clathrin-coated vesicles and to colocalize with clathrin light chain at putative sites of endocytosis at the plasma membrane. Together, these data suggest that SCD1 and SCD2 function in clathrin-mediated membrane transport, including plasma membrane endocytosis, required for cytokinesis and cell expansion.

Cis-Golgi cisternal assembly and biosynthetic activation occur sequentially in plants and algae.

The cisternal progression/maturation model of Golgi trafficking predicts that cis-Golgi cisternae are formed de novo on the cis-side of the Golgi. Here we describe structural and functional intermediates of the cis cisterna assembly process in high-pressure frozen algae (Scherffelia dubia, Chlamydomonas reinhardtii) and plants (Arabidopsis thaliana, Dionaea muscipula; Venus flytrap) as determined by electron microscopy, electron tomography and immuno-electron microscopy techniques. Our findings are as follows: (i) The cis-most (C1) Golgi cisternae are generated de novo from cisterna initiators produced by the fusion of 3-5 COPII vesicles in contact with a C2 cis cisterna. (ii) COPII vesicles fuel the growth of the initiators, which then merge into a coherent C1 cisterna. (iii) When a C1 cisterna nucleates its first cisterna initiator it becomes a C2 cisterna. (iv) C2-Cn cis cisternae grow through COPII vesicle fusion. (v) ER-resident proteins are recycled from cis cisternae to the ER via COPIa-type vesicles. (vi) In S. dubia the C2 cisternae are capable of mediating the self-assembly of scale protein complexes. (vii) In plants, ∼90% of native α-mannosidase I localizes to medial Golgi cisternae. (viii) Biochemical activation of cis cisternae appears to coincide with their conversion to medial cisternae via recycling of medial cisterna enzymes. We propose how the different cis cisterna assembly intermediates of plants and algae may actually be related to those present in the ERGIC and in the pre-cis Golgi cisterna layer in mammalian cells.

Cytoskeletal and membrane dynamics during higher plant cytokinesis.

Following mitosis, cytoplasm, organelles and genetic material are partitioned into daughter cells through the process of cytokinesis. In somatic cells of higher plants, two cytoskeletal arrays, the preprophase band and the phragmoplast, facilitate the positioning and de novo assembly of the plant-specific cytokinetic organelle, the cell plate, which develops across the division plane and fuses with the parental plasma membrane to yield distinct new cells. The coordination of cytoskeletal and membrane dynamics required to initiate, assemble and shape the cell plate as it grows toward the mother cell cortex is dependent upon a large array of proteins, including molecular motors, membrane tethering, fusion and restructuring factors and biosynthetic, structural and regulatory elements. This review focuses on the temporal and molecular requirements of cytokinesis in somatic cells of higher plants gleaned from recent studies using cell biology, genetics, pharmacology and biochemistry.

Bridging the divide between cytokinesis and cell expansion.

Two of the most fundamental processes in plant development are cytokinesis, by which new cells are formed, and cell expansion, by which existing cells grow and establish their functional morphology. In this review we summarize recent progress in understanding the pathways necessary for cytokinesis and cell expansion, including the role of the cytoskeleton, cell wall biogenesis, and membrane trafficking. Here, we focus on genes and lipids that are involved in both cytokinesis and cell expansion and bridge the divide between these two processes. In addition, we discuss our understanding of and controversies surrounding the role of endocytosis in both of these processes.