Arabidopsis fibrillin 1-2 subfamily members exert their functions via specific protein-protein interactions.

Fibrillins (FBNs) are plastidial proteins found in photosynthetic organisms from cyanobacteria to higher plants. The function of most FBNs remains unknown. Here, we focused on members of the FBN subgroup comprising FBN1a, FBN1b, and FBN2. We show that these three polypeptides interact between each other, potentially forming a network around the plastoglobule surface. Both FBN2 and FBN1s interact with allene oxide synthase, and the elimination of any of these FBNs results in a delay in jasmonate-mediated anthocyanin accumulation in response to a combination of moderate high light and low temperature. Mutations in the genes encoding FBN1s or FBN2 also affect the protection of PSII under the combination of these stresses. Fully developed leaves of these mutants have lower maximum quantum efficiency of PSII (Fv/Fm) and higher oxidative stress than wild-type plants. These effects are additive, and the fbn1a-1b-2 triple mutant shows a stronger decrease in Fv/Fm and a greater increase in oxidative stress than fbn1a-1b or fbn2 mutants. Co-immunoprecipitation analysis indicated that FBN2 also interacts with other proteins involved in different metabolic processes. We propose that these fibrillins facilitate accurate positioning of different proteins involved in distinct metabolic processes, and that their elimination leads to dysfunction of those proteins.

Regulation by S-nitrosylation of the Calvin-Benson cycle fructose-1,6-bisphosphatase in Pisum sativum.

Redox regulation is of great importance in chloroplasts. Many chloroplast enzymes, such as those belonging to the Calvin-Benson cycle (CBC), have conserved regulatory cysteines which form inhibitory disulphide bridges when physiological conditions become unfavourable. Amongst these enzymes, cFBP1, the CBC fructose-1,6-bisphosphatase (FBPase) isoform, is well known to be redox activated by thioredoxin f through the reduction of a disulphide bridge involving Cys153 and Cys173. Moreover, data obtained during recent years point to S-nitrosylation as another redox post-translational modification putatively regulating an increasing number of plant enzymes, including cFBP1. In this study we have shown that the Pisum sativum cFBP1 can be efficiently S-nitrosylated by GSNO and SNAP, triggering the formation of the regulatory disulphide. Using in vivo experiments with P. sativum we have established that cFBP1 S-nitrosylation only occurs during the light period and we have elucidated by activity assays with Cys-to-Ser mutants that this enzyme may be inactivated through the S-nitrosylation of Cys153. Finally, in the light of the new data, we have proposed an extended redox-regulation model by integrating the S-nitrosylation and the TRX f-mediated regulation of cFBP1.

A novel NADPH thioredoxin reductase, localized in the chloroplast, which deficiency causes hypersensitivity to abiotic stress in Arabidopsis thaliana.

Plants contain three thioredoxin systems. Chloroplast thioredoxins are reduced by ferredoxin-thioredoxin reductase, whereas the cytosolic and mitochondrial thioredoxins are reduced by NADPH thioredoxin reductase (NTR). There is high similarity among NTRs from plants, lower eukaryotes, and bacteria, which are different from mammal NTR. Here we describe the OsNTRC gene from rice encoding a novel NTR with a thioredoxin-like domain at the C terminus, hence, a putative NTR/thioredoxin system in a single polypeptide. Orthologous genes were found in other plants and cyanobacteria, but not in bacteria, yeast, or mammals. Full-length OsNTRC and constructs with truncated NTR and thioredoxin domains were expressed in Escherichia coli as His-tagged polypeptides, and a polyclonal antibody specifically cross-reacting with the OsNTRC enzyme was raised. An in vitro activity assay showed that OsNTRC is a bifunctional enzyme with both NTR and thioredoxin activity but is not an NTR/thioredoxin system. Although the OsNTRC gene was expressed in roots and shoots of rice seedlings, the protein was exclusively found in shoots and mature leaves. Moreover, fractionation experiments showed that OsNTRC is localized to the chloroplast. An Arabidopsis NTRC knock-out mutant showed growth inhibition and hypersensitivity to methyl viologen, drought, and salt stress. These results suggest that the NTRC gene is involved in plant protection against oxidative stress.