GOLVEN peptide signalling through RGI receptors and MPK6 restricts asymmetric cell division during lateral root initiation

During lateral root initiation, lateral root founder cells undergo asymmetric cell divisions that generate daughter cells with different sizes and fates, a prerequisite for correct primordium organogenesis. An excess of the GLV6/RGF8 peptide disrupts these initial asymmetric cell divisions, resulting in more symmetric divisions and the failure to achieve lateral root organogenesis. Here, we show that loss-of-function GLV6 and its homologue GLV10 increase asymmetric cell divisions during lateral root initiation, and we identified three members of the RGF1 INSENSITIVE/RGF1 receptor subfamily as likely GLV receptors in this process. Through a suppressor screen, we found that MITOGEN-ACTIVATED PROTEIN KINASE6 is a downstream regulator of the GLV pathway. Our data indicate that GLV6 and GLV10 act as inhibitors of asymmetric cell divisions and signal through RGF1 INSENSITIVE receptors and MITOGEN-ACTIVATED PROTEIN KINASE6 to restrict the number of initial asymmetric cell divisions that take place during lateral root initiation. The authors demonstrate the negative role of GOLVEN peptides during lateral root initiation in Arabidopsis, at the very early stage of the first asymmetric cell division of lateral root founder cells, and identify the receptors for these peptides

Expression of aspartyl protease and C3HC4-type RING zinc finger genes are responsive to ascorbic acid in Arabidopsis thaliana

Ascorbate (AsA) is a redox buffer and enzyme cofactor with various proposed functions in stress responses and growth. The aim was to identify genes whose transcript levels respond to changes in leaf AsA. The AsA-deficient Arabidopsis mutant vtc2-1 was incubated with the AsA precursor L-galactono-1,4-lactone (L-GalL) to increase leaf AsA concentration. Differentially expressed genes screened by DNA microarray were further characterized for AsA responsiveness in wild-type plants. The analysis of 14 candidates by real-time PCR identified an aspartyl protease gene (ASP, At1g66180) and a C3HC4-type RING zinc finger gene (AtATL15, At1g22500) whose transcripts were rapidly responsive to increases in AsA pool size caused by L-GalL and AsA supplementation and light. Transgenic Arabidopsis plants expressing an AtATL15 promoter::luciferase reporter confirmed that the promoter is L-GalL, AsA, and light responsive. The expression patterns of ASP and AtATL15 suggest they have roles in growth regulation. The promoter of AtATL15 is responsive to AsA status and will provide a tool to investigate the functions of AsA in plants further

Arabidopsis Phosphomannose Isomerase 1, but Not Phosphomannose Isomerase 2, Is Essential for Ascorbic Acid Biosynthesis*S⃞

We studied molecular and functional properties of Arabidopsis phosphomannose isomerase isoenzymes (PMI1 and PMI2) that catalyze reversible isomerization between d-fructose 6-phosphate and d-mannose 6-phosphate (Man-6P). The apparent Km and Vmax values for Man-6P of purified recombinant PMI1 were 41.3 ± 4.2 μm and 1.89 μmol/min/mg protein, respectively, whereas those of purified recombinant PMI2 were 372 ± 13 μm and 22.5 μmol/min/mg protein, respectively. Both PMI1 and PMI2 were inhibited by incubation with EDTA, Zn2+, Cd2+, and l-ascorbic acid (AsA). Arabidopsis PMI1 protein was constitutively expressed in both vegetative and reproductive organs under normal growth conditions, whereas the PMI2 protein was not expressed in any organs under light. The induction of PMI1 expression and an increase in the AsA level were observed in leaves under continuous light, whereas the induction of PMI2 expression and a decrease in the AsA level were observed under long term darkness. PMI1 showed a diurnal expression pattern in parallel with the total PMI activity and the total AsA content in leaves. Moreover, a reduction of PMI1 expression through RNA interference resulted in a substantial decrease in the total AsA content of leaves of knockdown PMI1 plants, whereas the complete inhibition of PMI2 expression did not affect the total AsA levels in leaves of knock-out PMI2 plants. Consequently, this study improves our understanding of the molecular and functional properties of Arabidopsis PMI isoenzymes and provides genetic evidence of the involvement of PMI1, but not PMI2, in the biosynthesis of AsA in Arabidopsis plants

Comparative proteomic analysis of mitochondria isolated from Euglena gracilis under aerobic and hypoxic conditions.

The unicellular microalga Euglena gracilis produces wax esters for ATP acquisition under low-oxygen conditions. The regulatory mechanism of wax ester production is not yet understood. Indeed, our previous transcriptomic analysis showed that transcript levels of genes involved in the wax ester synthesis hardly changed under hypoxic conditions, suggesting contribution of post-transcriptional regulation. In this study, we conducted a proteome analysis of E. gracilis mitochondria, as this organelle employs the fatty-acid synthesis pathway under hypoxic conditions. Mitochondria were isolated from E. gracilis SM-ZK strain treated with both aerobic and hypoxic conditions and used for shotgun proteomic analysis. Three independent proteomic analyses succeeded in identifying a total of 714 non-redundant proteins. Of these, 229 were detected in common to all experiments, and 116 were significantly recognized as differentially expressed proteins. GO enrichment analysis suggested dynamic changes in mitochondrial metabolic pathways and redox reactions under aerobic and hypoxic conditions. Protein levels of bifunctional enzymes isocitrate lyase and malate synthase in glyoxylate cycle were 1.35-fold higher under hypoxic conditions. Abundances of the propionyl-CoA synthetic enzymes, succinyl-CoA synthetase and propionyl-CoA carboxylase, were also 1.35- and 1.47-fold higher, respectively, under hypoxic conditions. Protein levels of pyruvate:NADP+ oxidoreductase, a key enzyme for anaerobic synthesis of acetyl-CoA, which serves as a C2 donor for fatty acids, showed a 1.68-fold increase under hypoxic conditions, whereas those of pyruvate dehydrogenase subunits showed a 0.77-0.81-fold decrease. Protein levels of the fatty-acid synthesis enzymes, 3-ketoacyl-CoA thiolase isoforms (KAT1 and KAT2), 3-hydroxyacyl-CoA dehydrogenases, and acyl-CoA dehydrogenase were up-regulated by 1.20- to 1.42-fold in response to hypoxic treatment. Overall, our proteomic analysis revealed that wax ester synthesis-related enzymes are up-regulated at the protein level post-transcriptionally to promote wax ester production in E. gracilis under low-oxygen conditions