At the end of the cell cycle, the nascent cross wall is laid down within a transient membrane compartment referred to as the cell plate. Tethering factors, which act by capturing vesicles and holding them in the vicinity of their target membranes, are likely to play an important role in the first stages of cell plate assembly. Factors required for cell plate biogenesis, however, remain to be identified. In this study, we used a reverse genetic screen to isolate tethering factors required for cytokinesis in Arabidopsis (Arabidopsis thaliana). We focused on the TRAPPI and TRAPPII (for transport protein particle) tethering complexes, which are thought to be required for the flow of traffic through the Golgi and for trans-Golgi network function, as well as on the GARP complex, thought to be required for the tethering of endocytotic vesicles to the trans-Golgi network. We found weak cytokinesis defects in some TRAPPI mutants and strong cytokinesis defects in all the TRAPPII lines we surveyed. Indeed, four insertion lines at the TRAPPII locus AtTRS120 had canonical cytokinesis-defective seedling-lethal phenotypes, including cell wall stubs and incomplete cross walls. Confocal and electron microscopy showed that in trs120 mutants, vesicles accumulated at the equator of dividing cells yet failed to assemble into a cell plate. This shows that AtTRS120 is required for cell plate biogenesis. In contrast to the TRAPP complexes, we found no conclusive evidence for cytokinesis defects in seven GARP insertion lines. We discuss the implications of these findings for the origin and identity of cell plate membranes.

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Highlights d Microtubule and F-actin needed for lateral root founder cell asymmetric expansion d Auxin-dependent, microtubule-mediated feedback constrains expansion d Cell polarization and asymmetric cell division require F-actin network d New genetic tools for targeted perturbation of microtubules or actin

This work describes the de-novo design of peptides that inhibit a broad range of plant pathogens. Four structurally different groups of peptides were developed that differ in size and position of their charged and hydrophobic clusters and were assayed for their ability to inhibit bacterial growth and fungal spore germination. Several peptides are highly active at concentrations between 0,1 and 1 µg/ml against plant pathogenic bacteria, such as Pseudomonas syringae, Pectobacterium carotovorum, and Xanthomonas vesicatoria. Importantly, no hemolytic activity could be detected for these peptides at concentrations up to 200 µg/ml. Moreover, the peptides are also active after spraying on the plant surface demonstrating a possible way of application. In sum, our designed peptides represent new antimicrobial agents and with the increasing demand for antimicrobial compounds for production of “healthy” food, these peptides might serve as templates for novel antibacterial and antifungal agents.

Cytokinesis, the partitioning of the cytoplasm following nuclear division, requires extensive coordination between membrane trafficking and cytoskeletal dynamics. In plants, the onset of cytokinesis is characterized by the assembly of a bipolar microtubule array, the phragmoplast, and of a transient membrane compartment, the cell plate. Little is known about the coordination between membrane deposition at the cell plate and the dynamics of phragmoplast microtubules. In this study, we monitor the localization dynamics of microtubule and membrane markers throughout cytokinesis. Our spatiotemporal resolution is consistent with the general view that microtubule dynamics drive membrane movements. Nonetheless, we provide evidence for active sorting at the cell plate and show that this is, at least in part, mediated by the TRAPPII tethering complex. We also characterize phragmoplast microtubule organization and cell plate formation in a suite of cytokinesis-defective mutants. Of four mutant lines with defects in phragmoplast microtubule organization, only mor1 microtubule-associated mutants exhibited aberrant cell plates. Conversely, the mutants with the strongest impairment in phragmoplast microtubule reorganization are keule alleles, which have a primary defect in membrane fusion. Our findings identify the SEC1/Munc18 protein KEULE as a central regulatory node in the coordination of membrane and microtubule dynamics during plant cytokinesis.