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Previous studies showed that ORANGE (OR) plays a key role in carotenoid biosynthesis and response to various stress. In this study, we present our discovery of the role of Arabidopsis ORANGE (AtOR) in regulating the ABA-dependent adaptive response to drought stress. For drought tolerance treatment, two weeks old seedlings were transplanted to pots filled with compost soil and subjected to progressive drought treatment for 20 days and re-watered for 2 days. The growth conditions of Arabidopsis were recorded after drought treatment and survival rates were analyzed after re-watering. Quantitative RT-PCR was used to investigate the transcripts levels of AtOR gene under different treatment conditions. Subcellular localization results were obtained by laser scanning confocal microscopy. Overexpression of AtOR significantly enhanced the tolerance to drought stress in Arabidopsis and AtOR was induced by drought and mannitol stresses. Compared with the Col-0 plants, overexpression lines showed slower water loss and smaller stomatal pore size after ABA treatment. Subcellular localization imaging showed that AtOR protein was localized in the chloroplasts. AtOR regulates the expression of ABA- and stress-responsive genes (RAB18, RD29B, RD26, ADH1, ABI2, ABF2) to enhance the tolerance to drought stress.

Hydrogen gas (H2) has recently been recognized as an important plant bio-regulator, inducing tolerance to several abiotic stresses. However, any role for H2 in amelioration of high-temperature-induced stress remains largely unknown. We investigated the mechanism of hydrogen-rich water (HRW)-mediated enhancement of heat-tolerance in cucumber seedlings exposed to high-temperature stress. The 3 weeks seedlings were pretreated with 50 or 100% HRW for 7 days, the effects on photosynthesis, chlorophyll content, chlorophyll fluorescence parameters, electrolyte leakage, lipid peroxidation, and antioxidant activity were examined when cucumber seedlings subjected for 3 days to heat stress treatment. With respect to samples treat with high temperature stress alone, HRW pretreatment remarkably alleviated stress-induced effects on the above parameters. Furthermore, after 3 days of treatment, HRW pretreatment also significantly increased the activities of antioxidative enzymes, promoted high-level accumulation of osmoprotectants, and upregulated HSP70 expression in cucumber leaves. All of these data suggest that pretreatment with exogenous HRW partially alleviated the detrimental effects of high-temperature stress on the growth of cucumber seedlings by improving the photosynthetic capacity, increasing the antioxidant response, and promoting the accumulation of HSP70 and osmolytes. Therefore, HRW pretreatment may improve cucumber seedlings heat-tolerance.

In this study, we investigated the possible role of hemin in alleviating zinc (Zn), lead (Pb) and chromium (Cr) toxicity in rice seedlings grown hydroponically by analyzing the morphological and physiological parameters. Our results showed that exposure of rice seedlings to excess Zn, Pb or Cr could cause severe leaf chlorosis, inhibit photosynthetic activity and consequently suppress plant growth. The concentration of O2 •− and H2O2 significantly increased and the activities of antioxidative enzymes decreased in roots of rice seedlings under metal exposure. The combined treatments (hemin + ZnSO4, hemin + Pb(NO3)2 and hemin + K2Cr2O7), on the other hand, significantly enhanced the photosynthesis- and plant growth-related parameters compared with their corresponding heavy-metal-stress alone. Combined treatments dramatically stimulated the activities of superoxide dismutase (SOD), ascorbic peroxidase (APX) and glutathione reductase (GR) as well as the concentrations of ascorbic acid (AsA) and glutathione (GSH) as compared with the metal- stress alone. The concentrations of reactive oxygen species (ROS, e.g. O2 •− and H2O2) were significantly reduced in the metal plus hemin treatments. Hemin addition also reduced metal accumulation in rice seedlings especially in root tissues. These findings suggest that hemin-elevated levels of antioxidants, activities of antioxidative enzymes and hemin-reduced accumulation of heavy-metal could confer resistance against Zn, Pb, and Cr stress in rice seedlings, resulting in improved pigments accumulation, photosynthetic attributes and plant growth.

To understand how light quality influences plant photosynthesis, we investigated chloroplastic ultrastructure, chlorophyll fluorescence and photosynthetic parameters, Rubisco and chlorophyll content and photosynthesis-related genes expression in cucumber seedlings exposed to different light qualities: white, red, blue, yellow and green lights with the same photosynthetic photon flux density of 100 μmol m−2 s−1. The results revealed that plant growth, CO2 assimilation rate and chlorophyll content were significantly reduced in the seedlings grown under red, blue, yellow and green lights as compared with those grown under white light, but each monochromatic light played its special role in regulating plant morphogenesis and photosynthesis. Seedling leaves were thickened and slightly curled; Rubisco biosynthesis, expression of the rca, rbcS and rbcL, the maximal photochemical efficiency of PSII (Fv/Fm) and quantum yield of PSII electron transport (ФPSII) were all increased in seedlings grown under blue light as compared with those grown under white light. Furthermore, the photosynthetic rate of seedlings grown under blue light was significantly increased, and leaf number and chlorophyll content of seedlings grown under red light were increased as compared with those exposed to other monochromatic lights. On the contrary, the seedlings grown under yellow and green lights were dwarf with the new leaves etiolated. Moreover, photosynthesis, Rubisco biosynthesis and relative gene expression were greatly decreased in seedlings grown under yellow and green light, but chloroplast structural features were less influenced. Interestingly, the Fv/Fm, ФPSII value and chlorophyll content of the seedlings grown under green light were much higher than those grown under yellow light.

The effect of Al stress on H2O2 production of rice (Oryza sativa L.) seedlings and difference in responses of antioxidant enzymes between Al-tolerant variety (Azucena) and Al-sensitive rice one (IR 64) were investigated. Aluminum-induced H2O2 production and malondialdehyde (MDA) content were more pronounced for IR 64 than for Azucena. In the presence of 2 mM Al, addition of 10 mM imidazole (inhibitor of NADPH oxidase) and 1 mM azide (inhibitor of peroxidase) significantly decreased H2O2 production by 16% and 43% for Azucena, and 21% and 68% for IR 64, respectively. Under Al treatment, the Al-tolerant variety Azucena had significantly higher activities of catalase, ascorbate peroxidase, dehydroascorbate reducase, glutathione peroxidase and glutathione reductase, and higher concentrations of reduced glutathione than the Al-sensitive one IR 64. Treatment with buthionine sulfoximine, a specific inhibitor of GSH synthesis, significantly increased H2O2 production in both varieties in the presence and absence of Al. In contrast, the treatment with GSH significantly decreased the production of H2O2 induced by Al stress. Results suggest that GSH may play an important role in scavenging H2O2 caused by Al stress.

Copper (Cu) accumulation and tolerance mechanisms in Elsholtzia haichowensis, an indicator plant of Cu mines, were investigated under hydroponics supplied with different concentrations (0.32, 50.0, 100.0 and 200.0 μM) of Cu for 8 days. Cu at 100 and 200 μM significantly decreased the root dry weight, but had no significant effect on shoot dry weight. The plants grown in the presence of 200 μM Cu accumulated 288 and 7626 μg g−1 DW total Cu in the shoots and roots, respectively. A greater proportion of accumulated Cu was water-soluble accounting for 42–93% of the total Cu content in the shoots. The concentrations of reduced glutathione (GSH) and protein thiols were significantly enhanced under excess Cu supply. However, the concentrations of these compounds, particularly protein thiols, were much higher in the leaves than that in the roots. Three UV-absorbing peaks could be eluted out through gel filtration chromatography on Sephadex G-50. A large amount of Cu was detected in the UV-absorbing peaks in 40–50 and 70–90 ml elution fractions of the root extract, and in 40–50 and 120–140 ml elution fractions of the leaf extract. The results suggested that the adaptive Cu tolerance mechanism in E. haichowensis might involve the active participation of protein thiols which had a more important role in the leaves than in the roots.