Mitogen-activated protein kinase kinase kinases (MAPKKKs) play key roles in intra- and extracellular signaling in eukaryotes. Here we report that the MAPKKK MEKK1 regulates redox homeostasis in Arabidopsis. We show that MEKK1-deficient plants are misregulated in the expression of a number of genes involved in cellular redox control and accumulate reactive oxygen species (ROS). Most strikingly, homozygous mekk1 mutant plants exhibit a lethal phenotype when developing true leaves. MEKK1 kinase activity and protein stability was regulated by H(2)O(2) in a proteasome-dependent manner and mekk1 plants were compromised in ROS-induced MAPK MPK4 activation. Whereas mpk3 and mpk6 knock out plants showed no defects in development or changes in redox control genes, mpk4 null mutant shared several phenotypic and transcript profile features with mekk1 plants. In agreement with the concept that ROS negatively regulates auxin responses in plants, mekk1 and mpk4 mutants show reduced expression of several auxin-inducible marker genes. Overall, our data defines MPK4 as downstream target of MEKK1 and show that MEKK1 functions in integrating ROS homeostasis with plant development and hormone signaling.

Summary: The exocyst is a hetero‐oligomeric protein complex involved in exocytosis and has been extensively studied in yeast and animal cells. Evidence is now accumulating that the exocyst is also present in plants. Bioinformatic analysis of genes encoding plant homologs of the exocyst subunit, Exo70, revealed that three Exo70 subgroups are evolutionarily conserved among angiosperms, lycophytes and mosses. Arabidopsis and rice contain 22 and approximately 39 EXO70 genes, respectively, which can be classified into nine clusters considered to be ancient in angiosperms (one has been lost in Arabidopsis). We characterized two independent T‐DNA insertional mutants of the AtEXO70A1 gene (exo70A1‐1 and exo70A1‐2). Heterozygous EXO70A1/exo70A1 plants appear to be normal and segregate in a 1:2:1 ratio, suggesting that neither male nor female gametophytes are affected by the EXO70A1 disruption. However, both exo70A1‐1 and exo70A1‐2 homozygotes exhibit an array of phenotypic defects. The polar growth of root hairs and stigmatic papillae is disturbed. Organs are generally smaller, plants show a loss of apical dominance and indeterminate growth where instead of floral meristems new lateral inflorescences are initiated in a reiterative manner. Both exo70A1 mutants have dramatically reduced fertility. These results suggest that the putative exocyst subunit EXO70A1 is involved in cell and organ morphogenesis.

Plant pathogens are perceived by pattern recognition receptors, which are activated upon binding to pathogen-associated molecular patterns (PAMPs). Ubiquitination and vesicle trafficking have been linked to the regulation of immune signaling. However, little information exists about components of vesicle trafficking involved in immune signaling and the mechanisms that regulate them. In this study, we identified Arabidopsis thaliana Exo70B2, a subunit of the exocyst complex that mediates vesicle tethering during exocytosis, as a target of the plant U-box–type ubiquitin ligase 22 (PUB22), which acts in concert with PUB23 and PUB24 as a negative regulator of PAMP-triggered responses. We show that Exo70B2 is required for both immediate and later responses triggered by all tested PAMPs, suggestive of a role in signaling. Exo70B2 is also necessary for the immune response against different pathogens. Our data demonstrate that PUB22 mediates the ubiquitination and degradation of Exo70B2 via the 26S Proteasome. Furthermore, degradation is regulated by the autocatalytic turnover of PUB22, which is stabilized upon PAMP perception. We therefore propose a mechanism by which PUB22-mediated degradation of Exo70B2 contributes to the attenuation of PAMP-induced signaling.

Cell reproduction is a complex process involving whole cell structures and machineries in space and time, resulting in regulated distribution of endomembranes, organelles, and genomes between daughter cells. Secretory pathways supported by the activity of the Golgi apparatus play a crucial role in cytokinesis in plants. From the onset of phragmoplast initiation to the maturation of the cell plate, delivery of secretory vesicles is necessary to sustain successful daughter cell separation. Tethering of secretory vesicles at the plasma membrane is mediated by the evolutionarily conserved octameric exocyst complex. Using proteomic and cytologic approaches, we show that EXO84b is a subunit of the plant exocyst. Arabidopsis thaliana mutants for EXO84b are severely dwarfed and have compromised leaf epidermal cell and guard cell division. During cytokinesis, green fluorescent protein-tagged exocyst subunits SEC6, SEC8, SEC15b, EXO70A1, and EXO84b exhibit distinctive localization maxima at cell plate initiation and cell plate maturation, stages with a high demand for vesicle fusion. Finally, we present data indicating a defect in cell plate assembly in the exo70A1 mutant. We conclude that the exocyst complex is involved in secretory processes during cytokinesis in Arabidopsis cells, notably in cell plate initiation, cell plate maturation, and formation of new primary cell wall.