Developmental phenotypes of plants expressing phosphovariants of .

The transcription factor Myeloblastosis protein 75 (MYB75, AT1G56650) is a well-established transcriptional activator of genes required for anthocyanin and flavonoid production, and a repressor of lignin and other secondary cell wall biosynthesis genes. MYB75 is itself tightly regulated at the transcriptional, translational and post-translational levels, including protein phosphorylation by MAP kinases Examination of the behavior of different phosphovariant versions of MYB75 and revealed that overexpression of the MYB75 phosphovariant had a particularly marked effect on global changes in gene expression suggesting that phosphorylated MYB75 could be involved in a broader range of functions than previously recognized. Here, we describe a range of distinct developmental phenotypes observed among lines expressing various phosphovariant forms of MYB75.

Biological impacts of phosphomimic AtMYB75.

The phosphorylation status of MYB75 at T-131 affects protein stability, flavonoid profiles, and patterns of gene expression. The Arabidopsis transcription factor Myeloblastosis protein 75 (MYB75, AT1G56650) is known to act as a positive transcriptional regulator of genes required for flavonoid and anthocyanin biosynthesis. MYB75 was also shown to negatively regulate lignin and other secondary cell wall biosynthetic genes (Bhargava et al.

Analysis of MAPK and MAPKK gene families in wheat and related Triticeae species.

Background: The mitogen-activated protein kinase (MAPK) family is involved in signal transduction networks that underpin many different biological processes in plants, ranging from development to biotic and abiotic stress responses. To date this class of enzymes has received little attention in Triticeae species, which include important cereal crops (wheat, barley, rye and triticale) that represent over 20% of the total protein food-source worldwide.

Results: The work presented here focuses on two subfamilies of Triticeae MAPKs, the MAP kinases (MPKs), and the MAPK kinases (MKKs) whose members phosphorylate the MPKs.

Identification and Characterization of Arabidopsis Seed Coat Mucilage Proteins.

Plant cell wall proteins are important regulators of cell wall architecture and function. However, because cell wall proteins are difficult to extract and analyze, they are generally poorly understood. Here, we describe the identification and characterization of proteins integral to the Arabidopsis (Arabidopsis thaliana) seed coat mucilage, a specialized layer of the extracellular matrix composed of plant cell wall carbohydrates that is used as a model for cell wall research.

Nucleocytoplasmic partitioning of tobacco N receptor is modulated by SGT1.

SGT1 (Suppressor of G2 allele of SKP1) is required to maintain plant disease Resistance (R) proteins with Nucleotide-Binding (NB) and Leucine-Rich Repeat (LRR) domains in an inactive but signaling-competent state. SGT1 is an integral component of a multi-protein network that includes RACK1, Rac1, RAR1, Rboh, HSP90 and HSP70, and in rice the Mitogen-Activated Protein Kinase (MAPK), OsMAPK6. Tobacco (Nicotiana tabacum) N protein, which belongs to the Toll-Interleukin Receptor (TIR)-NB-LRR class of R proteins, confers resistance to Tobacco Mosaic Virus (TMV).

The interacting MYB75 and KNAT7 transcription factors modulate secondary cell wall deposition both in stems and seed coat in Arabidopsis.

The Arabidopsis thaliana KNAT7 (KNOX family) and MYB75 (MYB family) transcription factors were each shown earlier to interact in yeast two-hybrid assays, and to modulate secondary cell wall formation in inflorescence stems. We demonstrate here that their interaction also occurs in vivo, and that specific domains of each protein mediate this process. The participation of these interacting transcription factors in secondary cell wall formation was then extended to the developing seed coat through the use of targeted transcript analysis and SEM in single loss-of-function mutants.

Immunoprofiling reveals unique cell-specific patterns of wall epitopes in the expanding Arabidopsis stem.

The Arabidopsis inflorescence stem undergoes rapid directional growth, requiring massive axial cell-wall extension in all its tissues, but, at maturity, these tissues are composed of cell types that exhibit markedly different cell-wall structures. It is not clear whether the cell-wall compositions of these cell types diverge rapidly following axial growth cessation, or whether compositional divergence occurs at earlier stages in differentiation, despite the common requirement for cell-wall extensibility. To examine this question, seven cell types were assayed for the abundance and distribution of 18 major cell-wall glycan classes at three developmental stages along the developing inflorescence stem, using a high-throughput immunolabelling strategy.

Postal code for a plant MAPK.

Plants contain hundreds of protein kinases that are believed to provide cellular signal transduction services, but the identities of the proteins they are targeting are largely unknown. Using an Arabidopsis MAPK (mitogen-activated protein kinase) (MPK6) as a model, Sörensson et al. describe in this issue of the Biochemical Journal how arrayed combinatorial peptide scanning offers an efficient route to discovery of new potential kinase substrates.

Mitogen-activated protein kinase signaling in plant-interacting fungi: distinct messages from conserved messengers.

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved proteins that function as key signal transduction components in fungi, plants, and mammals. During interaction between phytopathogenic fungi and plants, fungal MAPKs help to promote mechanical and/or enzymatic penetration of host tissues, while plant MAPKs are required for activation of plant immunity. However, new insights suggest that MAPK cascades in both organisms do not operate independently but that they mutually contribute to a highly interconnected molecular dialogue between the plant and the fungus.

The Class II KNOX gene KNAT7 negatively regulates secondary wall formation in Arabidopsis and is functionally conserved in Populus.

• The formation of secondary cell walls in cell types such as tracheary elements and fibers is a defining characteristic of vascular plants. The Arabidopsis transcription factor KNAT7 is a component of a transcription network that regulates secondary cell wall biosynthesis, but its function has remained unclear. • We conducted anatomical, biochemical and molecular phenotypic analyses of Arabidopsis knat7 loss-of-function alleles, KNAT7 over-expression lines and knat7 lines expressing poplar KNAT7.

AtMKK6 and AtMPK13 are required for lateral root formation in Arabidopsis.

The mitogen-activated protein (MAP) kinase cascades are important signaling components that mediate various biological pathwaysin all eukaryotic cells. In our recent publication,1 we identified AtMPK4 as one of the downstream targets of AtMKK6 that is required for executing male-specific meiotic cytokinesis. Here we provide evidence that another target, AtMPK13, is developmentally co-expressed with AtMKK6 in Arabidopsis, and both AtMPK13 and AtMKK6 display high Promoter::GUS activity in the primary root tips and at the lateral root primordia.

Arabidopsis ovate family proteins, a novel transcriptional repressor family, control multiple aspects of plant growth and development.

Background: The Arabidopsis genome contains 18 genes that are predicted to encode Ovate Family Proteins (AtOFPs), a protein family characterized by a conserved OVATE domain, an approximately 70-amino acid domain that was originally found in tomato OVATE protein. Among AtOFP family members, AtOFP1 has been shown to suppress cell elongation, in part, by suppressing the expression of AtGA20ox1, AtOFP4 has been shown to regulate secondary cell wall formation by interact with KNOTTED1-LIKE HOMEODOMAIN PROTEIN 7 (KNAT7), and AtOFP5 has been shown to regulate the activity of a BEL1-LIKEHOMEODOMAIN 1(BLH1)-KNAT3 complex during early embryo sac development, but little is known about the function of other AtOFPs.

Methodology/principal Findings: We demonstrated here that AtOFP proteins could function as effective transcriptional repressors in the Arabidopsis protoplast transient expression system.

AtMPK4 is required for male-specific meiotic cytokinesis in Arabidopsis.

Mitogen-activated protein kinase (MAPK) cascades have been implicated in regulating various aspects of plant development, including somatic cytokinesis. The evolution of expanded plant MAPK gene families has enabled the diversification of potential MAPK cascades, but functionally overlapping components are also well documented. Here we report that Arabidopsis MPK4, an MAPK that was previously described as a regulator of disease resistance, can interact with and be phosphorylated by the cytokinesis-related MAP kinase kinase, AtMKK6.

Involvement of Arabidopsis RACK1 in protein translation and its regulation by abscisic acid.

Earlier studies have shown that RACK1 functions as a negative regulator of abscisic acid (ABA) responses in Arabidopsis (Arabidopsis thaliana), but the molecular mechanism of the action of RACK1 in these processes remains elusive. Global gene expression profiling revealed that approximately 40% of the genes affected by ABA treatment were affected in a similar manner by the rack1 mutation, supporting the view that RACK1 is an important regulator of ABA responses. On the other hand, coexpression analysis revealed that more than 80% of the genes coexpressed with RACK1 encode ribosome proteins, implying a close relationship between RACK1's function and the ribosome complex.

MYB75 functions in regulation of secondary cell wall formation in the Arabidopsis inflorescence stem.

Deposition of lignified secondary cell walls in plants involves a major commitment of carbon skeletons in both the form of polysaccharides and phenylpropanoid constituents. This process is spatially and temporally regulated by transcription factors, including a number of MYB family transcription factors. MYB75, also called PRODUCTION OF ANTHOCYANIN PIGMENT1, is a known regulator of the anthocyanin branch of the phenylpropanoid pathway in Arabidopsis (Arabidopsis thaliana), but how this regulation might impact other aspects of carbon metabolism is unclear.

MAP kinases MPK9 and MPK12 are preferentially expressed in guard cells and positively regulate ROS-mediated ABA signaling.

Reactive oxygen species (ROS) mediate abscisic acid (ABA) signaling in guard cells. To dissect guard cell ABA-ROS signaling genetically, a cell type-specific functional genomics approach was used to identify 2 MAPK genes, MPK9 and MPK12, which are preferentially and highly expressed in guard cells. To provide genetic evidence for their function, Arabidopsis single and double TILLING mutants that carry deleterious point mutations in these genes were isolated.

Novel and expanded roles for MAPK signaling in Arabidopsis stomatal cell fate revealed by cell type-specific manipulations.

Mitogen-activated protein kinase (MAPK) signaling networks regulate numerous eukaryotic biological processes. In Arabidopsis thaliana, signaling networks that contain MAPK kinases MKK4/5 and MAPKs MPK3/6 function in abiotic and biotic stress responses and regulate embryonic and stomatal development. However, how single MAPK modules direct specific output signals without cross-activating additional downstream processes is largely unknown.

Suppression of MKK5 reduces ozone-induced signal transmission to both MPK3 and MPK6 and confers increased ozone sensitivity in Arabidopsis thaliana.

In Arabidopsis thaliana, ozone-induced signaling has been shown to involve the mitogen-activated protein kinases (MAPKs) MPK3 and MPK6. To identify a possible ozone-induced mitogen-activated protein kinase kinase (MAPKK) involved in the activation of these specific MAPKs, we employed RNA interference-(RNAi)-based suppression of MKK5, a known cognate MAPKK to both MPK3 and MPK6. When exposed to ozone, activation of both MPK3 and MPK6 was markedly reduced in the MKK5-suppressed plants compared to WT.

Quantitative proteomics identifies oxidant-induced, AtMPK6-dependent changes in Arabidopsis thaliana protein profiles.

In Arabidopsis thaliana, oxidant-induced signalling has been shown to utilize the mitogen-activated protein kinase (MAPK), AtMPK6. To identify proteins whose accumulation is altered by ozone in an AtMPK6-dependent manner we employed isotope-coded affinity tagging (ICAT) technology to investigate the impact of AtMPK6-suppression on the protein profiles in Arabidopsis both before (air control) and during continuous ozone (O(3)) fumigation (500 nL L(-1) for 8 h). Among the 150 proteins positively identified and quantified in the O(3)-treated plants, we identified thirteen proteins whose abundance was greater in the AtMPK6-suppressed genotype than in wild-type (WT).

Comprehensive analysis of protein-protein interactions between Arabidopsis MAPKs and MAPK kinases helps define potential MAPK signalling modules.

The Arabidopsis genome encodes a 20-member gene family of mitogen-activated protein kinases (MPKs) but biological roles have only been identified for a small subset of these crucial signalling components. In particular, it is unclear how the MPKs may be organized into functional modules within the cell. To gain insight into their potential relationships, we used the yeast two-hybrid system to conduct a directed protein-protein interaction screen between all the Arabidopsis MPKs and their upstream activators (MAPK kinases; MKK).

An update on the nomenclature for the cellulose synthase genes in Populus.

Cellulose synthase (CesA) is a central catalyst in the generation of the plant cell wall biomass and is, therefore, the focus of intense research. Characterization of individual CesA genes from Populus species has led to the publication of several different naming conventions for CesA gene family members in this model tree. To help reduce the resulting confusion, we propose here a new phylogeny-based CesA nomenclature that aligns the Populus CesA gene family with the established Arabidopsis thaliana CesA family structure.

Arabidopsis mitogen-activated protein kinase MPK12 interacts with the MAPK phosphatase IBR5 and regulates auxin signaling.

Mitogen-activated protein kinase (MAPK) phosphatases are important negative regulators in the MAPK signaling pathways responsible for many essential processes in plants, including development, stress management and hormonal responses. A mutation in INDOLE-3-BUTYRIC ACID-RESPONSE5 (IBR5), which is predicted to encode a dual-specificity MAPK phosphatase, was previously reported to confer reduced sensitivity to auxin and ABA in Arabidopsis roots. To further characterize IBR5, and to understand how it might help integrate MAPK cascades with hormone signaling, we searched for IBR5-interacting MAPKs.

The GCR2 gene family is not required for ABA control of seed germination and early seedling development in Arabidopsis.

Background: The plant hormone abscisic acid (ABA) regulates diverse processes of plant growth and development. It has recently been proposed that GCR2 functions as a G-protein-coupled receptor (GPCR) for ABA. However, the structural relationships and functionality of GCR2 have been challenged by several independent studies.

GCR2 is a new member of the eukaryotic lanthionine synthetase component C-like protein family.

GCR2 was recently proposed to represent a G-protein-coupled receptor (GPCR) for the plant hormone, abscisic acid (ABA). We and others provided evidence that GCR2 is unlikely to be a bona fide GPCR because it is not clearly predicted to contain seven transmembrane domains, a structural hallmark for classical GPCRs. Instead, GCR2 shows significant sequence similarity to homologs of bacterial lanthionine synthetase component C (LanC).