PRX2 and PRX25, peroxidases regulated by COG1, are involved in seed longevity in Arabidopsis.

Permeability is a crucial trait that affects seed longevity and is regulated by different polymers including proanthocyanidins, suberin, cutin and lignin located in the seed coat. By testing mutants in suberin transport and biosynthesis, we demonstrate the importance of this biopolymer to cope with seed deterioration. Transcriptomic analysis of cog1-2D, a gain-of-function mutant with increased seed longevity, revealed the upregulation of several peroxidase genes. Reverse genetics analysing seed longevity uncovered redundancy within the seed coat peroxidase gene family; however, after controlled deterioration treatment, seeds from the prx2 prx25 double and prx2 prx25 prx71 triple mutant plants presented lower germination than wild-type plants. Transmission electron microscopy analysis of the seed coat of these mutants showed a thinner palisade layer, but no changes were observed in proanthocyanidin accumulation or in the cuticle layer. Spectrophotometric quantification of acetyl bromide-soluble lignin components indicated changes in the amount of total polyphenolics derived from suberin and/or lignin in the mutant seeds. Finally, the increased seed coat permeability to tetrazolium salts observed in the prx2 prx25 and prx2 prx25 prx71 mutant lines suggested that the lower permeability of the seed coats caused by altered polyphenolics is likely to be the main reason explaining their reduced seed longevity.

Involvement of the eIF2α Kinase GCN2 in UV-B Responses.

GCN2 (general control nonrepressed 2) is a serine/threonine-protein kinase that regulates translation in response to stressors such as amino acid and purin deprivation, cold shock, wounding, cadmium, and UV-C exposure. Activated GCN2 phosphorylates the α-subunit of the eukaryotic initiation factor 2 (eIF2) leading to a drastic inhibition of protein synthesis and shifting translation to specific mRNAs. To investigate the role of GCN2 in responses to UV-B radiation its activity was analyzed through eIF2α phosphorylation assays in mutants of the specific UV-B and stress signaling pathways of Arabidopsis thaliana. EIF2α phosphorylation was detectable 30 min after UV-B exposure, independent of the UV-B photoreceptor UV RESISTANCE LOCUS8 and its downstream signaling components. GCN2 dependent phosphorylation of eIF2α was also detectable in mutants of the stress related MAP kinases, MPK3 and MPK6 and their negative regulator map kinase phosphatase1 (MKP1). Transcription of downstream components of the UV-B signaling pathway, the Chalcone synthase (CHS) was constitutively higher in gcn2-1 compared to wildtype and further increased upon UV-B while GLUTATHIONE PEROXIDASE7 (GPX7) behaved similarly to wildtype. The UVR8 independent FAD-LINKED OXIDOREDUCTASE (FADox) had a lower basal expression in gcn2-1 which was increased upon UV-B. Since high fluence rates of UV-B induce DNA damage the expression of the RAS ASSOCIATED WITH DIABETES PROTEIN51 (RAD51) was quantified before and after UV-B. While the basal expression was similar to wildtype it was significantly less induced upon UV-B in the gcn2-1 mutant. This expression pattern correlates with the finding that gcn2 mutants develop less cyclobutane pyrimidine dimers after UV-B exposure. Quantification of translation with the puromycination assay revealed that gcn2 mutants have an increased rate of translation which was also higher upon UV-B. Growth of gcn2 mutants to chronic UV-B exposure supports GCN2's role as a negative regulator of UV-B responses. The elevated resistance of gcn2 mutants towards repeated UV-B exposure points to a critical role of GCN2 in the regulation of translation upon UV-B.

Protein kinase GCN2 mediates responses to glyphosate in Arabidopsis.

BACKGROUND: The increased selection pressure of the herbicide glyphosate has played a role in the evolution of glyphosate-resistance in weedy species, an issue that is becoming a threat to global agriculture. The molecular components involved in the cellular toxicity response to this herbicide at the expression level are still unidentified. RESULTS: In this study, we identify the protein kinase GCN2 as a cellular component that fosters the action of glyphosate in the model plant Arabidopsis thaliana. Comparative studies using wild-type and gcn2 knock-out mutant seedlings show that the molecular programme that the plant deploys after the treatment with the herbicide, is compromised in gcn2. Moreover, gcn2 adult plants show a lower inhibition of photosynthesis, and both seedlings and adult gcn2 plants accumulate less shikimic acid than wild-type after treatment with glyphosate. CONCLUSIONS: These results points to an unknown GCN2-dependent factor involved in the cascade of events triggered by glyphosate in plants. Data suggest either that the herbicide does not equally reach the target-enzyme in a gcn2 background, or that a decreased flux in the shikimate pathway in a gcn2 plants minimize the impact of enzyme inhibition.