Overexpression of chloroplastic monodehydroascorbate reductase enhanced tolerance to temperature and methyl viologen-mediated oxidative stresses.

A tomato (Lycopersicon esculentum Mill.) monodehydroascorbate reductase gene (LeMDAR) was isolated. The LeMDAR-green fluorescence protein (GFP) fusion protein was targeted to chloroplast in Arabidopsis mesophyll protoplast. RNA and protein gel blot analyses confirmed that the sense- and antisense- LeMDAR were integrated into the tomato genome. The MDAR activities and the levels of reduced ascorbate (AsA) were markedly increased in sense transgenic lines and decreased in antisense transgenic lines compared with wild-type (WT) plants. Under low and high temperature stresses, the sense transgenic plants showed lower level of hydrogen peroxide (H(2)O(2)), lower thiobarbituric acid reactive substance (TBARS) content, higher net photosynthetic rate (P(n)), higher maximal photochemical efficiency of PSII (F(v)/F(m)) and fresh weight compared with WT plants. The oxidizable P700 decreased more obviously in WT and antisense plants than that in sense plants at chilling temperature under low irradiance. Furthermore, the sense transgenic plants exhibited significantly lower H(2)O(2) level, higher ascorbate peroxidase (APX) activity, greater P(n) and F(v)/F(m) under methyl viologen (MV)-mediated oxidative stresses. These results indicated that overexpression of chloroplastic MDAR played an important role in alleviating photoinhibition of PSI and PSII and enhancing the tolerance to various abiotic stresses by elevating AsA level.

Tomato SlGGP-LIKE gene participates in plant responses to chilling stress and pathogenic infection.

Plants are always exposed to abiotic and biotic stresses which can adversely affect their growth and development. As an important antioxidant, AsA plays a vital role in plant defence against damage caused by stresses. In this study, we cloned a tomato GDP-L-galactose phosphorylase-like (SlGGP-LIKE) gene and investigated its role in resistance to abiotic and biotic stresses by using antisense transgenic (AS) tomato lines. The AsA content in AS plants was lower than that in WT plants. Under chilling stress, the growth of AS plants was inhibited significantly, and they yielded higher levels of ROS, REC and MDA but demonstrated weaker APX activity than that shown by WT plants. Additionally, the declined values of Pn, Fv/Fm, oxidisable P700, and D1 protein content of PSII in AS lines were significant. Furthermore, the effect on xanthophyll cycle of AS plants was more severe than that on WT plants, and the ratio of zeaxanthin (Z)/(V + A + Z) and (Z + 0.5 A)/(V + A + Z) in AS lines was lower than that in WT plants. In spite of chilling stress, under Pseudomonas syringae pv.tomato (Pst) DC3000 strain infection, AS plants showed lesser bacterial cell growth and dead cells than those shown by WT plants. This finding indicated that AS plants demonstrated stronger resistance against pathogenic infection. Results suggest that SlGGP-LIKE gene played an important role in plant defence against chilling stress and pathogenic infection.

Overexpression of tomato SlGGP-LIKE gene improves tobacco tolerance to methyl viologen-mediated oxidative stress.

Ascorbate (AsA) is very important in scavenging reactive oxygen species in plants. AsA can reduce photoinhibition by xanthophyll cycle to dissipate excess excitation energy. GGP is an important enzyme in AsA biosynthesis pathway in higher plants. In this study, we cloned a gene, SlGGP-LIKE, that has the same function but different sequence compared with SlGGP. The function of SlGGP-LIKE gene in response to oxidative stress was investigated using transgenic tobacco plants overexpressed SlGGP-LIKE under methyl viologen treatment. After oxidative stress treatment, transgenic tobacco lines exhibited higher levels of reduced AsA content and APX activity than WT plants. Under oxidative stress, transgenic tobacco plants accumulated less ROS and exhibited lower degrees of REC and MDA. Consequently, relatively higher levels of Pn, Fv/Fm, de-epoxidation status of xanthophyll cycle and D1 protein were maintained in transgenic tobacco plants. Hence, overexpression of SlGGP-LIKE gene enhances AsA biosynthesis and can alleviate the photoinhibition of PSII under oxidative stress.

[Effects of root-applied glycinebetaine on the composition and function of wheat thylakoid membrane under drought stress].

Two wheat cultivars (Triticum aestivum L.), HF9703 tolerant to drought and SN215953 sensitive to drought, were used to study the effects of glycinebetaine on the composition and function of thylakoid membrane. The wheat seedlings with two leaves were pretreated with Hoagland solution containing 1.5 mmol/L glycinebetaine (GB) for 72 h, then cultured with Hoagland solution containing 15% PEG-6000 for 48 h. The seedling leaves were used for mensuration. The results indicated that the chlorophyll, monogalactosyl diaylglycerol (MGDG), digalactosyl diacylglycerol (DGDG) and phosphatidylglycerol (PG) contents of the two wheat cultivars decreased significantly (P<0.05) under drought stress. GB alleviated their decrease. The sulfoquinovosyl diacylglycerol (SQDG) content, trans-hexadecenoic [16:1(3t)] and saturated fatty acid content of MGDG in HF9703 increased significantly (P<0.05), while in SN215953, the sulfoquinovosyl diacylglycerol (SQDG) and trans-hexadecenoic [16:1(3t)] contents decreased significantly under drought stress, but the saturated fatty acid content of MGDG increased slightly. These differences between the two wheat cultivars might account for much of the difference in drought tolerance between them. Ca2+-ATPase activity of thylakoid membrane, Hill-reaction activity and photosynthesis were decreased markedly (P<0.05) by drought stress. GB ameliorated these effects on thylakoid membrane, and the effect of GB on SN215953 was stronger than on HF9703. Discussion was made on the possible mechanism of the alleviating effect of root-applied GB on the composition and function of thylakoid membrane.

Plastid-expressed choline monooxygenase gene improves salt and drought tolerance through accumulation of glycine betaine in tobacco.

Glycine betaine (GlyBet), a quaternary ammonium compound, functions as an osmoprotectant in many organisms including plants. Previous research has shown that over-expression of enzymes for GlyBet biosynthesis in transgenic plants improved abiotic stress tolerance, but so far no study on the effects of plastid-expression of choline monooxygenase, the enzyme that catalyzes the conversion of choline into betaine aldehyde, has been reported. In the present study, tobacco (Nicotiana tabacum L. cv Wisconsin 38) plants were transformed with a gene for choline monooxygenase (BvCMO) from beet (Beta vulgaris) via plastid genetic engineering. Transplastomic plants constitutively expressing BvCMO under the control of the ribosomal RNA operon promoter and a synthetic T7 gene G10 leader were able to accumulate GlyBet in leaves, roots and seeds, and exhibited improved tolerance to toxic level of choline and to salt/drought stress when compared to wild type plants. Transplastomic plants also demonstrated higher net photosynthetic rate and apparent quantum yield of photosynthesis in the presence of 150 mM NaCl. Salt stress caused no significant change on the maximal efficiency of PSII photochemistry (Fv/Fm) in both wild type and transplastomic plants, but a decrease in the actual efficiency of PSII (PhiPSII) was observed, and such a decrease was much greater in wild type plants. Our results demonstrate the feasibility of improving salt and drought tolerance in plants through plastid transformation with BvCMO gene.