Chloroplast proteostasis:A story of birth,life,and death

Protein homeostasis(proteostasis)is a dynamic balance of protein synthesis and degradation.Because of the endosymbiotic origin of chloroplasts and the massive transfer of their genetic information to the nucleus of the host cell,many protein complexes in the chloroplasts are constituted from subunits encoded by both genomes.Hence,the proper function of chloroplasts relies on the coordinated expression of chloroplast-and nucleus-encoded genes.The biogenesis and maintenance of chloroplast proteostasis are dependent on synthesis of chloroplast-encoded proteins,import of nucleus-encoded chloroplast proteins from the cytosol,and clearance of damaged or otherwise undesired“old”proteins.This review focuses on the regulation of chloroplast proteostasis,its interactionwith proteostasis of the cytosol,and its retrograde control over nuclear gene expression.We also discuss significant issues and perspectives for future studies and potential applications for improving the photosynthetic performance and stress tolerance of crops.

Global Analysis of Gene Expression Profiles in Brassica napus Developing Seeds Reveals a Conserved Lipid Metabolism Regulation with Arabidopsis thaliana

In order to study Brassica napus fatty acid （FA） metabolism and relevant regulatory networks, a systematic identification of fatty acid （FA） biosynthesis-related genes was conducted. Following gene identification, gene expression profiles during B. napus seed development and FA metabolism were performed by cDNA chip hybridization （〉8000 EST clones from seed）. The results showed that FA biosynthesis and regulation, and carbon flux, were conserved between B. napus and Arabidopsis. However, a more critical role of starch metabolism was detected for B. napus seed FA metabolism and storage-component accumulation when compared with Arabidopsis. In addition, a crucial stage for the transition of seed-to-sink tissue was 17-21 d after flowering （DAF）, whereas FA biosynthesis-related genes were highly expressed pri- marily at 21 DAF. Hormone （auxin and jasmonate） signaling is found to be important for FA metabolism. This study helps to reveal the global regulatory network of FA metabolism in developing B. napus seeds.

Tuning the Johari-Goldstein β-Relaxation and Its Separation from α-Relaxation of Poly(n-alkyl methacrylate)s by Small Molecule-bridged Hydrogen Bonds

Introducing small molecule-bridged hydrogen bonds(HBs)between polymer chains has been reported to effectively reduce the interchain cooperativity despite of strengthening the intermolecular interaction.Here,a systematic investigation on tuning the Johari-Goldsteinβ(βJG)relaxation by adding various low-molecular-weight phenols in poly(n-alkyl methacrylate)s is carried out to further clarify the anomalous dynamics.Given these small molecules capable of coupling the motion with pendent groups of host polymers due to forming at least two HBs per molecule,poly(n-alkyl methacrylate)mixtures exhibit rich dynamic changes in theβJG-properties andα,βJG separations.An increased loading of phenols with a small size and strong inter-HB strength(Δυi)clearly benefits for significant retardation and suppression of theβJG-relaxation,narrows theα,βJG separation and converges theβJG-peak with theα-peak,which demonstrates the alleviation of inter-chain topological constraints.However,small molecules with a relatively big size and weakΔυi are found to amplify the magnitude of theα,βJG separation of poly(butyl methacrylate),even though experimental results of changes inα-dispersion and dynamic fragility confirm a reduction of the coupling factor n in all of these hybrids.The counterintuitive phenomenon suggests that the crossover time tc in the Coupling Model is no longer a universal quantity if the inter-chain interaction of polymers is strengthened by HBs.These compelling findings shed vital insights into the HBinduced anomalous dynamics,and provide essential guidance for tailoring theβJG behavior and designing glassy polymeric materials.

Nanoparticle Dispersion and Glass Transition Behavior of Polyimide-grafted Silica Nanocomposites

How to control the spatial distribution of nanoparticles to meet different performance requirements is a constant challenge in the field of polymer nanocomposites.Current studies have been focused on the flexible polymer chain systems.In this study,the rigid polyimide(PI) chain grafted silica particles with different grafting chain lengths and grafting densities were prepared by "grafting to" method,and the influence of polymerization degree of grafted chains(N),matrix chains(P),and grafting density(a) on the spatial distribution of nanoparticles in the PI matrix was explored.The glass transition temperature(Tg) of PI composites was systematically investigated as well.The results show that silica particles are well dispersed in polyamic acid composite systems,while aggregation and small clusters appear in PI nanocomposites after thermal imidization.Besides,the particle size has no impact on the spatial distribution of nanoparticles.When σ·N0.5<<(N/P)2,the grafted and matrix chains interpenetrate,and the frictional resistance of the segment increases,resulting in restricted relaxation kinetics and Tg increase of the PI composite system.In addition,smaller particle size and longer grafted chains are beneficial to improving Tg of composites These results are all propitious to complete the microstructure control theory of nanocomposites and make a theoretical foundation for the high performance and multi-function of PI nanocomposites.