Krishna Nath

Stable platinum nanoclusters on genomic DNA–graphene oxide with a high oxygen reduction reaction activity

Nanosize platinum clusters with small diameters of 2–4 nm are known to be excellent catalysts for the oxygen reduction reaction. The inherent catalytic activity of smaller platinum clusters has not yet been reported due to a lack of preparation methods to control their size (<2 nm). Here we report the synthesis of platinum clusters (diameter ≤1.4 nm) deposited on genomic double-stranded DNA–graphene oxide composites, and their high-performance electrocatalysis of the oxygen reduction reaction. The electrochemical behaviour, characterized by oxygen reduction reaction onset potential, half-wave potential, specific activity, mass activity, accelerated durability test (10,000 cycles) and cyclic voltammetry stability (10,000 cycles) is attributed to the strong interaction between the nanosize platinum clusters and the DNA–graphene oxide composite, which induces modulation in the electronic structure of the platinum clusters. Furthermore, we show that the platinum cluster/DNA–graphene oxide composite possesses notable environmental durability and stability, vital for high-performance fuel cells and batteries.

Towards a critical understanding of the photosystem II repair mechanism and its regulation during stress conditions

Photosystem II (PSII) is vulnerable to high light (HL) illumination resulting in photoinhibition. In addition to photoprotection mechanisms, plants have developed an efficient PSII repair mechanism to save themselves from irreversible damage to PSII under abiotic stresses including HL illumination. The phosphorylation/dephosphorylation cycle along with subsequent degradation of photodamaged D1 protein to be replaced by the insertion of a newly synthesized copy of D1 into the PSII complex, is the core function of the PSII repair cycle. The exact mechanism of this process is still under discussion. We describe the recent progress in identifying the kinases, phosphatases and proteases, and in understanding their involvement in the maintenance of thylakoid structure and the quality control of proteins by PSII repair cycle during photoinhibition.

Interconnected Pt-nanodendrite/DNA/reduced-graphene-oxide hybrid showing remarkable oxygen reduction activity and stability.

Controlling the morphology and size of platinum nanodendrites (PtDs) is a key factor in improving their catalytic activity and stability. Here, we report the synthesis of PtDs on genomic-double-stranded-DNA/reduced-graphene-oxide (gdsDNA/rGO) by the NaBH4 reduction of H(2)PtCl(6) in the presence of plant gdsDNA. Compared to industrially adopted catalysts (i.e., state-of-the-art Pt/C catalyst, Pt/rGO, Pt(3)Co, etc.), the as-synthesized PtDs/gdsDNA/rGO hybrid displays very high oxygen reduction reaction (ORR) catalytic activities (much higher than the 2015 U.S. Department of Energy (DOE) target values), which are the rate-determining steps in electrochemical energy devices, in terms of onset-potential, half-wave potential, specific-activity, mass-activity, stability, and durability. Moreover, the hybrid exhibits a highly stable mass activity for the ORR over a wide pH range of 1-13. These exceptional properties would make the hybrid applicable in next-generation electrochemical energy devices.

ZEBRA-NECROSIS, a thylakoid-bound protein, is critical for the photoprotection of developing chloroplasts during early leaf development

The zebra-necrosis (zn) mutant of rice (Oryza sativa) produces transversely green/yellow-striped leaves. The mutant phenotype is formed by unequal impairment of chloroplast biogenesis before emergence from the leaf sheath under alternate light/dark or high/low temperatures (restrictive), but not under constant light and temperature (permissive) conditions. Map-based cloning revealed that ZN encodes a thylakoid-bound protein of unknown function. Virus-induced gene silencing of a ZN homolog in Nicotiana benthamiana causes leaf variegation with sporadic green/yellow sectors, indicating that ZN is essential for chloroplast biogenesis during early leaf development. Necrotic lesions often occur in the yellow sectors as a result of an excessive accumulation of reactive oxygen species (ROS). The phenotypic severity (leaf variegation and necrosis) and ROS levels are positively correlated with an increase in light intensity under restrictive conditions. In the mutant leaves, chlorophyll (Chl) metabolism, ROS scavenging activities, maximum quantum yield of photosystem II (PSII), and structures and functions of the photosynthetic complexes are normal in the Chl-containing cells, suggesting that ROS are mainly generated from the defective plastids of the Chl-free cells. The PSII activity of normal chloroplasts is hypersensitive to photoinhibition because the recovery rates of PSII are much slower. In the PSII repair, the degradation of damaged D1 is not impaired, suggesting a reduced activity of new D1 synthesis, possibly because of higher levels of ROS generated from the Chl-free cells by excess light. Together, we propose that ZN is required for protecting developing chloroplasts, especially during the assembly of thylakoid protein complexes, from incidental light after darkness.

Production of superoxide from Photosystem II in a rice ( Oryza sativa L.) mutant lacking PsbS

BackgroundPsbS is a 22-kDa Photosystem (PS) II protein involved in non-photochemical quenching (NPQ) of chlorophyll fluorescence. Rice (Oryza sativa L.) has two PsbS genes, PsbS1 and PsbS2. However, only inactivation of PsbS1, through a knockout (PsbS1-KO) or in RNAi transgenic plants, results in plants deficient in qE, the energy-dependent component of NPQ.ResultsIn studies presented here, under fluctuating high light, growth of young seedlings lacking PsbS is retarded, and PSII in detached leaves of the mutants is more sensitive to photoinhibitory illumination compared with the wild type. Using both histochemical and fluorescent probes, we determined the levels of reactive oxygen species, including singlet oxygen, superoxide, and hydrogen peroxide, in leaves and thylakoids. The PsbS-deficient plants generated more superoxide and hydrogen peroxide in their chloroplasts. PSII complexes isolated from them produced more superoxide compared with the wild type, and PSII-driven superoxide production was higher in the mutants. However, we could not observe such differences either in isolated PSI complexes or through PSI-driven electron transport. Time-course experiments using isolated thylakoids showed that superoxide production was the initial event, and that production of hydrogen peroxide proceeded from that.ConclusionThese results indicate that at least some of the photoprotection provided by PsbS and qE is mediated by preventing production of superoxide released from PSII under conditions of excess excitation energy.

Loss-of-function of OsSTN8 suppresses the photosystem II core protein phosphorylation and interferes with the photosystem II repair mechanism in rice (Oryza sativa).

STN8 kinase is involved in photosystem II (PSII) core protein phosphorylation (PCPP). To examine the role of PCPP in PSII repair during high light (HL) illumination, we characterized a T-DNA insertional knockout mutant of the rice (Oryza sativa) STN8 gene. In this osstn8 mutant, PCPP was significantly suppressed, and the grana were thin and elongated. Upon HL illumination, PSII was strongly inactivated in the mutants, but the D1 protein was degraded more slowly than in wild-type, and mobilization of the PSII supercomplexes from the grana to the stromal lamellae for repair was also suppressed. In addition, higher accumulation of reactive oxygen species and preferential oxidation of PSII reaction center core proteins in thylakoid membranes were observed in the mutants during HL illumination. Taken together, our current data show that the absence of STN8 is sufficient to abolish PCPP in osstn8 mutants and to produce all of the phenotypes observed in the double mutant of Arabidopsis, indicating the essential role of STN8-mediated PCPP in PSII repair.

Developmental stage-dependent differential gene expression of superoxide dismutase isoenzymes and their localization and physical interaction network in rice ( Oryza sativa L.)

Superoxide dismutase (SOD) isoenzymes are essential for scavenging excess reactive oxygen species in living organisms. So far, expression pattern of SOD isoenzymes genes along leaf development plus their sub-cellular localization and physical interaction network have not yet been clearly elucidated. Using multiple bioinformatics tools, we predicted the sub-cellular localizations of SOD isoforms and described their physical interactions in rice. Using in silico approaches, we obtained several evidences for existence of seven SOD genes and a SOD copper chaperone gene. Their transcripts were differentially expressed along with different developmental stage of rice leaf. Finally, we performed quantitative real time-polymerase chain reaction (qRT-PCR) to validate in silico differential expression pattern of SOD genes experimentally. Expression of two cytosolic cCuZn-SODs was high during the whole vegetative stage. Two plastidic Fe-SODs were found and their expression levels were very low and started to increase from the late vegetative stage. Their expression patterns were very similar to each other, indicating the formation of heterodimer. However, their expression patterns are different from those for ArabidopsisFe-SODs. The expression of pCuZn-SOD was very high in the early developmental stage, but qRT-PCR results were different, which remains for further study. From the results on the differential expression of SOD genes, we can understand the role of each SOD gene and even predict their role under certain circumstances based on in silico analysis.

Mobilization of Photosystem II-Light Harvesting Complex II Supercomplexes during High Light Illumination and State Transitions

The photosystem II (PSII) light harvesting complex (LHC) consists of a variety of pigment protein complexes which are involved in structural organization and regulation of photosynthetic unit. These LHC proteins encoded by a group of Lhcb genes are essential for the structural integrity of PSII supercomplex, the channeling the excitation energy to the reaction center of PSII and its redistribution to photosystem I by state transitions. Numerous studies with the help of recent technological advancements have enabled a significant progress in our understanding on the structure of PSII-LHCII supercomplexes and their mobilization under various light conditions. Here, we present a mini-review on the latest concepts and models depicting the structure of PSII-LHCII supercomplexes and the role of Lhcb proteins in their supra-molecular organization. Also we will review on the current understandings and remaining problems involved in the mobilization of the supercomplexes during state transitions and during high light illumination for controlling light energy distribution between the two photosystems.

Characterization of a T-DNA Inserted STN8 Kinase Mutant of Oryza sativa L.

In Arabidopsis Two Stt7-Like Proteins Exist, Stn7 And Stn8. Loss Of Stn8 Kinase Blocks Photosystem Ii (Psii) Core Protein Phosphorylation Whereas Stn7 Is Required For Lhc Ii Phosphorylation. In This Work, A T-Dna Inserted Stn8 Kinase Mutant Line Of Rice (Oryza Sativa L.) Was Selected And Characterized To Understand The Role Of Stn8 Kinase In Rice. The Mutant Plants Were Slightly Shorter Than Wildtype (Wt). The Transcript Level Was Observed Only In Photosynthetic Organs In Wt, But Not In The Mutant. Inactivation Of Photochemical Efficiency Under High Light Stress Was More Severe In The Stn8 Mutant Than In Wt. Recovery Of Psii Activity Was Also Slower In The Mutant. However, In The Presence Of Lincomycin Changes In Photochemical Efficiency And Its Recovery Process Of Mutants Was Similar To Wt. The Phosphorylation Of The Psii Core Proteins In The Stn8 Kinase Mutant Was Abolished Without Changing Lhc Ii Phosphorylation Indicating That Stn8 Kinase In Rice Is Required For Psii Core Phosphorylation But Not For Lhc Ii Phosphorylation And State Transition. The Discrepancy Observed In Stn8 Mutant Of A Model Monocot Plant, Rice, From Those Reported In A Model Dicot Plant, Arabidopsis, Is Discussed.