Pyrroline-5-carboxylate metabolism protein complex detected in Arabidopsis thaliana leaf mitochondria.

Proline dehydrogenase (ProDH) and pyrroline-5-carboxylate (P5C) dehydrogenase (P5CDH) catalyse the oxidation of proline into glutamate via the intermediates P5C and glutamate-semialdehyde (GSA), which spontaneously interconvert. P5C and GSA are also intermediates in the production of glutamate from ornithine and α-ketoglutarate catalysed by ornithine δ-aminotransferase (OAT). ProDH and P5CDH form a fused bifunctional PutA enzyme in Gram-negative bacteria and are associated in a bifunctional substrate-channelling complex in Thermus thermophilus; however, the physical proximity of ProDH and P5CDH in eukaryotes has not been described. Here, we report evidence of physical proximity and interactions between Arabidopsis ProDH, P5CDH, and OAT in the mitochondria of plants during dark-induced leaf senescence when all three enzymes are expressed. Pairwise interactions and localization of the three enzymes were investigated using bimolecular fluorescence complementation with confocal microscopy in tobacco and sub-mitochondrial fractionation in Arabidopsis. Evidence for a complex composed of ProDH, P5CDH, and OAT was revealed by co-migration of the proteins in native conditions upon gel electrophoresis. Co-immunoprecipitation coupled with mass spectrometry analysis confirmed the presence of the P5C metabolism complex in Arabidopsis. Pull-down assays further demonstrated a direct interaction between ProDH1 and P5CDH. P5C metabolism complexes might channel P5C among the constituent enzymes and directly provide electrons to the respiratory electron chain via ProDH.

Pyrroline-5-carboxylate dehydrogenase is an essential enzyme for proline dehydrogenase function during dark-induced senescence in Arabidopsis thaliana.

During leaf senescence, nitrogen is remobilized and carbon backbones are replenished by amino acid catabolism, with many of the key reactions occurring in mitochondria. The intermediate Δ1 -pyrroline-5-carboxylate (P5C) is common to some catabolic pathways, thus linking the metabolism of several amino acids, including proline and arginine. Specifically, mitochondrial proline catabolism involves sequential action of proline dehydrogenase (ProDH) and P5C dehydrogenase (P5CDH) to produce P5C and then glutamate. Arginine catabolism produces urea and ornithine, the latter in the presence of α-ketoglutarate being converted by ornithine δ-aminotransferase (OAT) into P5C and glutamate. Metabolic changes during dark-induced leaf senescence (DIS) were studied in Arabidopsis thaliana leaves of Col-0 and in prodh1prodh2, p5cdh and oat mutants. Progression of DIS was followed by measuring chlorophyll and proline contents for 5 days. Metabolomic profiling of 116 compounds revealed similar profiles of Col-0 and oat metabolism, distinct from prodh1prodh2 and p5cdh metabolism. Metabolic dynamics were accelerated in p5cdh by 1 day. Notably, more P5C and proline accumulated in p5cdh than in prodh1prodh2. ProDH1 enzymatic activity and protein amount were significantly down-regulated in p5cdh mutant at Day 4 of DIS. Mitochondrial P5C levels appeared critical in determining the flow through interconnected amino acid remobilization pathways to sustain senescence.

Proline oxidation fuels mitochondrial respiration during dark-induced leaf senescence in Arabidopsis thaliana.

Leaf senescence is a form of developmentally programmed cell death that allows the remobilization of nutrients and cellular materials from leaves to sink tissues and organs. Among the catabolic reactions that occur upon senescence, little is known about the role of proline catabolism. In this study, the involvement in dark-induced senescence of proline dehydrogenases (ProDHs), which catalyse the first and rate-limiting step of proline oxidation in mitochondria, was investigated using prodh single- and double-mutants with the help of biochemical, proteomic, and metabolomic approaches. The presence of ProDH2 in mitochondria was confirmed by mass spectrometry and immunogold labelling in dark-induced leaves of Arabidopsis. The prodh1 prodh2 mutant exhibited enhanced levels of most tricarboxylic acid cycle intermediates and free amino acids, demonstrating a role of ProDH in mitochondrial metabolism. We also found evidence of the involvement and the importance of ProDH in respiration, with proline as an alternative substrate, and in remobilization of proline during senescence to generate glutamate and energy that can then be exported to sink tissues and organs.

Proteomic and functional analysis of proline dehydrogenase 1 link proline catabolism to mitochondrial electron transport in Arabidopsis thaliana.

Proline accumulates in many plant species in response to environmental stresses. Upon relief from stress, proline is rapidly oxidized in mitochondria by proline dehydrogenase (ProDH) and then by pyrroline-5-carboxylate dehydrogenase (P5CDH). Two ProDH genes have been identified in the genome of the model plant Arabidopsis thaliana To gain a better understanding of ProDH1 functions in mitochondria, proteomic analysis was performed. ProDH1 polypeptides were identified in Arabidopsis mitochondria by immunoblotting gels after 2D blue native (BN)-SDS/PAGE, probing them with an anti-ProDH antibody and analysing protein spots by MS. The 2D gels showed that ProDH1 forms part of a low-molecular-mass (70-140 kDa) complex in the mitochondrial membrane. To evaluate the contribution of each isoform to proline oxidation, mitochondria were isolated from wild-type (WT) and prodh1, prodh2, prodh1prodh2 and p5cdh mutants. ProDH activity was high for genotypes in which ProDH, most likely ProDH1, was strongly induced by proline. Respiratory measurements indicate that ProDH1 has a role in oxidizing excess proline and transferring electrons to the respiratory chain.

The mitogen-activated protein kinase phosphatase PHS1 regulates flowering in Arabidopsis thaliana.

MAIN CONCLUSION: Arabidopsis PHS1, initially known as an actor of cytoskeleton organization, is a positive regulator of flowering in the photoperiodic and autonomous pathways by modulating both CO and FLC mRNA levels. Protein phosphorylation and dephosphorylation is a major type of post-translational modification, controlling many biological processes. In Arabidopsis thaliana, five genes encoding MAPK phosphatases (MKP)-like proteins have been identified. Among them, PROPYZAMIDE HYPERSENSITIVE 1 (PHS1) encoding a dual-specificity protein tyrosine phosphatase (DsPTP) has been shown to be involved in microtubule organization, germination and ABA-regulated stomatal opening. Here, we demonstrate that PHS1 also regulates flowering under long-day and short-day conditions. Using physiological, genetic and molecular approaches, we have shown that the late flowering phenotype of the knock-out phs1-5 mutant is linked to a higher expression of FLOWERING LOCUS C (FLC). In contrast, a decline of both CONSTANS (CO) and FLOWERING LOCUS T (FT) expression is observed in the knock-out phs1-5 mutant, especially at the end of the light period under long-day conditions when the induction of flowering occurs. We show that this partial loss of sensitivity to photoperiodic induction is independent of FLC. Our results thus indicate that PHS1 plays a dual role in flowering, in the photoperiodic and autonomous pathways, by modulating both CO and FLC mRNA levels. Our work reveals a novel actor in the complex network of the flowering regulation.

The central role of mosquito cytochrome P450 CYP6Zs in insecticide detoxification revealed by functional expression and structural modelling.

The resistance of mosquitoes to chemical insecticides is threatening vector control programmes worldwide. Cytochrome P450 monooxygenases (CYPs) are known to play a major role in insecticide resistance, allowing resistant insects to metabolize insecticides at a higher rate. Among them, members of the mosquito CYP6Z subfamily, like Aedes aegypti CYP6Z8 and its Anopheles gambiae orthologue CYP6Z2, have been frequently associated with pyrethroid resistance. However, their role in the pyrethroid degradation pathway remains unclear. In the present study, we created a genetically modified yeast strain overexpressing Ae. aegypti cytochrome P450 reductase and CYP6Z8, thereby producing the first mosquito P450-CPR (NADPH-cytochrome P450-reductase) complex in a yeast recombinant system. The results of the present study show that: (i) CYP6Z8 metabolizes PBAlc (3-phenoxybenzoic alcohol) and PBAld (3-phenoxybenzaldehyde), common pyrethroid metabolites produced by carboxylesterases, producing PBA (3-phenoxybenzoic acid); (ii) CYP6Z8 transcription is induced by PBAlc, PBAld and PBA; (iii) An. gambiae CYP6Z2 metabolizes PBAlc and PBAld in the same way; (iv) PBA is the major metabolite produced in vivo and is excreted without further modification; and (v) in silico modelling of substrate-enzyme interactions supports a similar role of other mosquito CYP6Zs in pyrethroid degradation. By playing a pivotal role in the degradation of pyrethroid insecticides, mosquito CYP6Zs thus represent good targets for mosquito-resistance management strategies.

Involvement of Phosphatidylinositol 3-kinase in the regulation of proline catabolism in Arabidopsis thaliana.

Plant adaptation to abiotic stresses such as drought and salinity involves complex regulatory processes. Deciphering the signaling components that are involved in stress signal transduction and cellular responses is of importance to understand how plants cope with salt stress. Accumulation of osmolytes such as proline is considered to participate in the osmotic adjustment of plant cells to salinity. Proline accumulation results from a tight regulation between its biosynthesis and catabolism. Lipid signal components such as phospholipases C and D have previously been shown to be involved in the regulation of proline metabolism in Arabidopsis thaliana. In this study, we demonstrate that proline metabolism is also regulated by class-III Phosphatidylinositol 3-kinase (PI3K), VPS34, which catalyses the formation of phosphatidylinositol 3-phosphate (PI3P) from phosphatidylinositol. Using pharmacological and biochemical approaches, we show that the PI3K inhibitor, LY294002, affects PI3P levels in vivo and that it triggers a decrease in proline accumulation in response to salt treatment of A. thaliana seedlings. The lower proline accumulation is correlated with a lower transcript level of Pyrroline-5-carboxylate synthetase 1 (P5CS1) biosynthetic enzyme and higher transcript and protein levels of Proline dehydrogenase 1 (ProDH1), a key-enzyme in proline catabolism. We also found that the ProDH1 expression is induced in a pi3k-hemizygous mutant, further demonstrating that PI3K is involved in the regulation of proline catabolism through transcriptional regulation of ProDH1. A broader metabolomic analysis indicates that LY294002 also reduced other metabolites, such as hydrophobic and aromatic amino acids and sugars like raffinose.

The intrinsically disordered C-terminal region of Arabidopsis thaliana TCP8 transcription factor acts both as a transactivation and self-assembly domain.

TCPs are plant specific transcription factors with non-canonical basic helix-loop-helix domains. While Arabidopsis thaliana has 24 TCPs involved in cell proliferation and differentiation, their mode of action has not been fully elucidated. Using bioinformatic tools, we demonstrate that TCP transcription factors belong to the intrinsically disordered proteins (IDP) family and that disorder is higher in class I TCPs than in class II TCPs. In particular, using bioinformatic and biochemical approaches, we have characterized TCP8, a class I TCP. TCP8 exhibits three intrinsically disordered regions (IDR) made of more than 50 consecutive residues, in which phosphorylable Ser residues are mainly clustered. Phosphorylation of Ser-211 that belongs to the central IDR was confirmed by mass spectrometry. Yeast two-hybrid assays also showed that the C-terminal IDR corresponds to a transactivation domain. Moreover, biochemical experiments demonstrated that TCP8 tends to oligomerize in dimers, trimers and higher-order multimers. Bimolecular fluorescence complementation (BiFC) experiments carried out on a truncated form of TCP8 lacking the C-terminal IDR indicated that it is effectively required for the pronounced self-assembly of TCP8. These data were reinforced by the prediction of a coiled coil domain in this IDR. The C-terminal IDR acts thus as an oligomerization domain and also a transactivation domain. Moreover, many Molecular Recognition Features (MoRFs) were predicted, indicating that TCP8 could interact with several partners to fulfill a fine regulation of transcription in response to various stimuli.