Wound signaling pathways in plants are mediated by mitogen-activated protein kinases (MAPKs) and stress hormones, such as ethylene and jasmonates. In Arabidopsis thaliana, the transmission of wound signals by MAPKs has been the subject of detailed investigations; however, the involvement of specific phosphatases in wound signaling is not known. Here, we show that AP2C1, an Arabidopsis Ser/Thr phosphatase of type 2C, is a novel stress signal regulator that inactivates the stress-responsive MAPKs MPK4 and MPK6. Mutant ap2c1 plants produce significantly higher amounts of jasmonate upon wounding and are more resistant to phytophagous mites (Tetranychus urticae). Plants with increased AP2C1 levels display lower wound activation of MAPKs, reduced ethylene production, and compromised innate immunity against the necrotrophic pathogen Botrytis cinerea. Our results demonstrate a key role for the AP2C1 phosphatase in regulating stress hormone levels, defense responses, and MAPK activities in Arabidopsis and provide evidence that the activity of AP2C1 might control the plant's response to B. cinerea.

In plant post-embryonic epidermis mitogen-activated protein kinase (MAPK) signaling promotes differentiation of pavement cells and inhibits initiation of stomata. Stomata are cells specialized to modulate gas exchange and water loss. Arabidopsis MAPKs MPK3 and MPK6 are at the core of the signaling cascade; however, it is not well understood how the activity of these pleiotropic MAPKs is constrained spatially so that pavement cell differentiation is promoted only outside the stomata lineage. Here we identified a PP2C-type phosphatase termed AP2C3 (Arabidopsis protein phosphatase 2C) that is expressed distinctively during stomata development as well as interacts and inactivates MPK3, MPK4 and MPK6. AP2C3 co-localizes with MAPKs within the nucleus and this localization depends on its N-terminal extension. We show that other closely related phosphatases AP2C2 and AP2C4 are also MAPK phosphatases acting on MPK6, but have a distinct expression pattern from AP2C3. In accordance with this, only AP2C3 ectopic expression is able to stimulate cell proliferation leading to excess stomata development. This function of AP2C3 relies on the domains required for MAPK docking and intracellular localization. Concomitantly, the constitutive and inducible AP2C3 expression deregulates E2F-RB pathway, promotes the abundance and activity of CDKA, as well as changes of CDKB1;1 forms. We suggest that AP2C3 downregulates the MAPK signaling activity to help maintain the balance between differentiation of stomata and pavement cells.

Anthocyanins are essential contributors to fruit coloration, an important quality feature and a breed determining trait of a sweet cherry fruit. It is well established that the biosynthesis of anthocyanins is regulated by an interplay of specific transcription factors belonging to MYB and bHLH families accompanied by a WD40 protein. In this study, we isolated and analyzed PaWD40, PabHLH3, PabHLH33, and several closely related MYB10 gene variants from different cultivars of sweet cherry, analyzed their expression in fruits with different anthocyanin levels at several developmental stages, and determined their capabilities to modulate anthocyanin synthesis in leaves of two Nicotiana species. Our results indicate that transcription level of variant PaMYB10.1-1 correlates with fruit coloration, but anthocyanin synthesis in Nicotiana was induced by another variant, PaMYB10.1-3, which is moderately expressed in fruits. The analysis of two fruit-expressed bHLH genes revealed that PabHLH3 enhances MYB-induced anthocyanin synthesis, whereas PabHLH33 has strong inhibitory properties.

The anaerobic spore-forming bacterium Clostridium perfringens is a source of one of the most common food-borne illnesses in the United States and Europe. The costs associated with disease management are high and interventions are limited; therefore, effective and safe antimicrobials are needed to control food contamination by C. perfringens. A viable solution to this problem could be bacteriophage lysins used as food additives or food processing aids. Such antimicrobials could be produced cost-effectively and in ample supply in green plants. By using edible plant species as production hosts the need for expensive product purification can be reduced or obviated. We describe the first successful expression in plants of C. perfringens-specific bacteriophage lysins. We demonstrate that six lysins belonging to two different families (N-acetylmuramoyl-L-alanine amidase and glycosyl hydrolase 25) are active against a panel of enteropathogenic C. perfringens strains under salinity and acidity conditions relevant to food preparation environments. We also demonstrate that plant-expressed lysins prevent multiplication of C. perfringens on cooked meat matrices far better than nisin, the only currently approved bacteriocin food preservative to control this pathogen.

Mitogen-activated protein kinase (MAPK) cascades transmit environmental signals and induce stress and defence responses in plants. These signalling cascades are negatively controlled by specific phosphatases of the type 2C Ser/Thr protein phosphatase (PP2C) and dual-specificity phosphatase (DSP) families that inactivate stress-induced MAPKs; however, the interplay between phosphatases of these different types has remained unknown. Our work reveals that different Arabidopsis MAPK phosphatases, the PP2C-type AP2C1 and the DSP-type MKP1, exhibit both specific and overlapping functions in plant stress responses. Each single mutant and ap2c1 mkp1 double mutant displayed enhanced stress-induced activation of MAPKs MPK3, MPK4, and MPK6, as well as induction of a set of transcription factors. Moreover, ap2c1 mkp1 double mutants show an autoimmune-like response, associated with elevated levels of the stress hormones salicylic acid and ethylene, and of the phytoalexin camalexin. Interestingly, this phenotype is reduced in ap2c1 mkp1 mpk3 and ap2c1 mkp1 mpk6 triple mutants, suggesting that the autoimmune-like response is due to MAPK misregulation. We conclude that the evolutionarily distant MAPK phosphatases AP2C1 and MKP1 contribute crucially to the tight control of MAPK activities, ensuring appropriately balanced stress signalling and suppression of autoimmune-like responses during plant growth and development.

Horticultural crops of the Ribes genus are valued for their anthocyanin-rich fruits, but until now, there were no data about the genes and regulation of their flavonoid pathway. In this study, the coding sequences of flavonoid pathway enzymes and their putative regulators MYB10, bHLH3 and WD40 were isolated, and their expression analyzed in fruits with varying anthocyanin levels from different cultivars of four species belonging to the Ribes genus. Transcription levels of anthocyanin synthesis enzymes and the regulatory gene RrMYB10 correlated with fruit coloration and anthocyanin quantities of different Ribes cultivars. Regulatory genes were tested for the ability to modulate anthocyanin biosynthesis during transient expression in the leaves of two Nicotiana species and to activate Prunus avium promoters of late anthocyanin biosynthesis genes in N. tabacum. Functional tests showed a strong capability of RrMyb10 to induce anthocyanin synthesis in a heterologous system, even without the concurrent expression of any heterologous bHLH, whereas RrbHLH3 enhanced MYB-induced anthocyanin synthesis. Data obtained in this work facilitate further analysis of the anthocyanin synthesis pathway in key Ribes species, and potent anthocyanin inducer RrMyb10 can be used to manipulate anthocyanin expression in heterologous systems.

No abstract

The adaptation of plants to the environment is a key property for survival. Adaptation responses to environmental cues are generated in cells by signaling initiated from cell receptors. Signal transduction is based on protein phosphorylation that is employed in mitogen-activated protein kinase (MAPK) cascades to integrate signals from receptors to cellular responses. MAPK activity is determined by phosphorylation of amino acid residues within the kinase activation loop and their dephosphorylation by phosphatases is essential to control signal duration and intensity.Monitoring protein-protein interactions (PPIs) of MAPKs with MAPK phosphatases in vivo provides valuable information about specificity and intracellular localization of the protein complex. Here, we report studying PPIs between Arabidopsis MAPKs and PP2C-type MAPK phosphatases using bimolecular fluorescent complementation (BiFC) in suspension cell protoplasts. The interactions of the MAPKs MPK3, MKP4 and MPK6 with the phosphatases AP2C1 and AP2C3 have been tested.