HY5, a positive regulator of light signaling, negatively controls the unfolded protein response in Arabidopsis.

Light influences essentially all aspects of plant growth and development. Integration of light signaling with different stress response results in improvement of plant survival rates in ever changing environmental conditions. Diverse environmental stresses affect the protein-folding capacity of the endoplasmic reticulum (ER), thus evoking ER stress in plants. Consequently, the unfolded protein response (UPR), in which a set of molecular chaperones is expressed, is initiated in the ER to alleviate this stress. Although its underlying molecular mechanism remains unknown, light is believed to be required for the ER stress response. In this study, we demonstrate that increasing light intensity elevates the ER stress sensitivity of plants. Moreover, mutation of the ELONGATED HYPOCOTYL 5 (HY5), a key component of light signaling, leads to tolerance to ER stress. This enhanced tolerance of hy5 plants can be attributed to higher expression of UPR genes. HY5 negatively regulates the UPR by competing with basic leucine zipper 28 (bZIP28) to bind to the G-box-like element present in the ER stress response element (ERSE). Furthermore, we found that HY5 undergoes 26S proteasome-mediated degradation under ER stress conditions. Conclusively, we propose a molecular mechanism of crosstalk between the UPR and light signaling, mediated by HY5, which positively mediates light signaling, but negatively regulates UPR gene expression.

UV-Induced cell death in plants.

Plants are photosynthetic organisms that depend on sunlight for energy. Plants respond to light through different photoreceptors and show photomorphogenic development. Apart from Photosynthetically Active Radiation (PAR; 400-700 nm), plants are exposed to UV light, which is comprised of UV-C (below 280 nm), UV-B (280-320 nm) and UV-A (320-390 nm). The atmospheric ozone layer protects UV-C radiation from reaching earth while the UVR8 protein acts as a receptor for UV-B radiation. Low levels of UV-B exposure initiate signaling through UVR8 and induce secondary metabolite genes involved in protection against UV while higher dosages are very detrimental to plants. It has also been reported that genes involved in MAPK cascade help the plant in providing tolerance against UV radiation. The important targets of UV radiation in plant cells are DNA, lipids and proteins and also vital processes such as photosynthesis. Recent studies showed that, in response to UV radiation, mitochondria and chloroplasts produce a reactive oxygen species (ROS). Arabidopsis metacaspase-8 (AtMC8) is induced in response to oxidative stress caused by ROS, which acts downstream of the radical induced cell death (AtRCD1) gene making plants vulnerable to cell death. The studies on salicylic and jasmonic acid signaling mutants revealed that SA and JA regulate the ROS level and antagonize ROS mediated cell death. Recently, molecular studies have revealed genes involved in response to UV exposure, with respect to programmed cell death (PCD).

The influence of light quality, circadian rhythm, and photoperiod on the CBF-mediated freezing tolerance.

Low temperature adversely affects crop yields by restraining plant growth and productivity. Most temperate plants have the potential to increase their freezing tolerance upon exposure to low but nonfreezing temperatures, a process known as cold acclimation. Various physiological, molecular, and metabolic changes occur during cold acclimation, which suggests that the plant cold stress response is a complex, vital phenomenon that involves more than one pathway. The C-Repeat Binding Factor (CBF) pathway is the most important and well-studied cold regulatory pathway that imparts freezing tolerance to plants. The regulation of freezing tolerance involves the action of phytochromes, which play an important role in light-mediated signalling to activate cold-induced gene expression through the CBF pathway. Under normal temperature conditions, CBF expression is regulated by the circadian clock through the action of a central oscillator and also day length (photoperiod). The phytochrome and phytochrome interacting factor are involved in the repression of the CBF expression under long day (LD) conditions. Apart from the CBF regulon, a novel pathway involving the Z-box element also mediates the cold acclimation response in a light-dependent manner. This review provides insights into the progress of cold acclimation in relation to light quality, circadian regulation, and photoperiodic regulation and also explains the underlying molecular mechanisms of cold acclimation for introducing the engineering of economically important, cold-tolerant plants.

In silico study on Arabidopsis BAG gene expression in response to environmental stresses.

BAG (Bcl-2 athanogene) family proteins are conserved in a wide range of eukaryotes, and they have been proposed to play a crucial role in plant programmed cell death (PCD). During the past decade, with the help of advanced bioinformatics tools, seven homologs of BAG genes have been identified in the Arabidopsis genome; these genes are involved in pathogen attack and abiotic stress conditions. In this study, gene expression of Arabidopsis BAG family members under environmental stresses was analyzed using the Botany Array Resource (BAR) expression browser tool and the in silico data were partially confirmed by qRT-PCR analysis for the selected stress- and hormone-treated conditions related to environmental stresses. Particularly, the induction of AtBAG6 gene in response to heat shock was confirmed by using GUS reporter lines. The loss of the AtBAG6 gene resulted into impairment in basal thermotolerance of plant and showed enhanced cell death in response to heat stress. To elucidate the regulatory mechanisms of BAG genes, we analyzed ∼1-kbp promoter regions for the presence of stress-responsive elements. Our transcription profiling finally revealed that the Arabidopsis BAG genes differentially respond to environmental stresses under the control of specifically organized upstream regulatory elements.

Dual functions of Arabidopsis sulfiredoxin: acting as a redox-dependent sulfinic acid reductase and as a redox-independent nuclease enzyme.

Based on the fact that the amino acid sequence of sulfiredoxin (Srx), already known as a redox-dependent sulfinic acid reductase, showed a high sequence homology with that of ParB, a nuclease enzyme, we examined the nucleic acid binding and hydrolyzing activity of the recombinant Srx in Arabidopsis (AtSrx). We found that AtSrx functions as a nuclease enzyme that can use single-stranded and double-stranded DNAs as substrates. The nuclease activity was enhanced by divalent cations. Particularly, by point-mutating the active site of sulfinate reductase, Cys (72) to Ser (AtSrx-C72S), we demonstrate that the active site of the reductase function of AtSrx is not involved in its nuclease function.

Molecular and functional properties of three different peroxiredoxin isotypes in Chinese cabbage.

Peroxiredoxins (Prxs), which are classified into three isotypes in plants, play important roles in protection systems as peroxidases or molecular chaperones. The three Prx isotypes of Chinese cabbage, namely C1C-Prx, C2C-Prx, and C-PrxII, have recently been identified and characterized. The present study compares their molecular properties and biochemical functions to gain insights into their concerted roles in plants. The three Prx isotype genes were differentially expressed in tissue- and developmental stage-specific manners. The transcript level of the C1C-Prx gene was abundant at the seed stage, but rapidly decreased after imbibitions. In contrast, the C2C-Prx transcript was not detected in the seeds, but its expression level increased at germination and was maintained thereafter. The C-PrxII transcript level was mild at the seed stage, rapidly increased for 10 days after imbibitions, and gradually disappeared thereafter. In the localization analysis using GFP-fusion proteins, the three isotypes showed different cellular distributions. C1C-Prx was localized in the cytosol and nucleus, whereas C2C-Prx and C-Prx were found mainly in the chloroplast and cytosol, respectively. In vitro thiol-dependent antioxidant assays revealed that the relative peroxidase activities of the isotypes were CPrxII > C2C-Prx > C1C-Prx. C1C-Prx and C2C-Prx, but not C-PrxII, prevented aggregation of malate dehydrogenase as a molecular chaperone. Taken together, these results suggest that the three isotypes of Prx play specific roles in the cells in timely and spatially different manners, but they also cooperate with each other to protect the plant.

The 1-Cys peroxiredoxin, a regulator of seed dormancy, functions as a molecular chaperone under oxidative stress conditions.

Peroxiredoxins are antioxidative enzymes that catalyze the reduction of alkyl hydroperoxides to alcohols and hydrogen peroxide to water. 1-Cys peroxiredoxins (1-Cys Prxs) perform important roles during late seed development in plants. To characterize their biochemical functions in plants, a 1Cys-Prx gene was cloned from a Chinese cabbage cDNA library and designated as "C1C-Prx". Glutamine synthetase (GS) protection and hydrogen peroxide reduction assays indicated that C1C-Prx was functionally active as a peroxidase. Also C1C-Prx prevented the thermal- or chemical-induced aggregation of malate dehydrogenase and insulin. Hydrogen peroxide treatment changed the mobility of C1C-Prx on a two-dimensional gel, which implies overoxidation of the conserved Cys residue. Furthermore, after overoxidation, the chaperone activity of C1C-Prx increased approximately two-fold, but its peroxidase activity decreased to the basal level of the reaction mixture without enzyme. However, according to the structural analysis using far-UV circular dichroism spectra, intrinsic tryptophan fluorescence spectra, and native-PAGE, overoxidation did not lead to a conformational change in C1C-Prx. Therefore, our results suggest that 1-Cys Prxs function not only to relieve mild oxidative stresses but also as molecular chaperones under severe conditions during seed germination and plant development, and that overoxidation controls the switch in function of 1-Cys-Prxs from peroxidases to molecular chaperones.