Yuefei Xu

Exogenous 5-aminolevulenic acid promotes seed germination in Elymus nutans against oxidative damage induced by cold stress.

The protective effects of 5-aminolevulenic acid (ALA) on germination of Elymus nutans Griseb. seeds under cold stress were investigated. Seeds of E. nutans (Damxung, DX and Zhengdao, ZD) were pre-soaked with various concentrations (0, 0.1, 0.5, 1, 5, 10 and 25 mg l−1) of ALA for 24 h before germination under cold stress (5°C). Seeds of ZD were more susceptible to cold stress than DX seeds. Both seeds treated with ALA at low concentrations (0.1–1 mg l−1) had higher final germination percentage (FGP) and dry weight at 5°C than non-ALA-treated seeds, whereas exposure to higher ALA concentrations (5–25 mg l−1) brought about a dose dependent decrease. The highest FGP and dry weight of germinating seeds were obtained from seeds pre-soaked with 1 mg l−1 ALA. After 5 d of cold stress, pretreatment with ALA provided significant protection against cold stress in the germinating seeds, significantly enhancing seed respiration rate and ATP synthesis. ALA pre-treatment also increased reduced glutathione (GSH), ascorbic acid (AsA), total glutathione, and total ascorbate concentrations, and the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), whereas decreased the contents of malondialdehyde (MDA) and hydrogen peroxide (H2O2), and superoxide radical (O2 •−) release in both germinating seeds under cold stress. In addition, application of ALA increased H+-ATPase activity and endogenous ALA concentration compared with cold stress alone. Results indicate that ALA considered as an endogenous plant growth regulator could effectively protect E. nutans seeds from cold-induced oxidative damage during germination without any adverse effect.

Nitric Oxide Mediates 5-Aminolevulinic Acid-Induced Antioxidant Defense in Leaves of Elymus nutans Griseb. Exposed to Chilling Stress

Nitric oxide (NO) and 5-aminolevulinic acid (ALA) are both extremely important signalling molecules employed by plants to control many aspects of physiology. In the present study, the role of NO in ALA-induced antioxidant defense in leaves of two sources of Elymus nutans Griseb. (Damxung, DX and Zhengdao, ZD) was investigated. Chilling stress enhanced electrolyte leakage, accumulation of malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide radical in two E. nutans, which were substantially alleviated by exogenous ALA and NO application. Pretreatment with NO scavenger PTIO or NOS inhibitor L-NNA alone and in combination with ALA induced enhancements in electrolyte leakage and the accumulation of MDA, H2O2 and superoxide radical in leaves of DX and ZD exposed to chilling stress, indicating that the inhibition of NO biosynthesis reduced the chilling resistance of E. nutans and the ALA-enhanced chilling resistance. Further analyses showed that ALA and NO enhanced antioxidant defense and activated plasma membrane (PM) H+-ATPase and decreased the accumulation of ROS induced by chilling stress. A pronounced increase in nitric oxide synthase (NOS) activity and NO release by exogenous ALA treatment was found in chilling-resistant DX plants exposed to chilling stress, while only a little increase was observed in chilling-sensitive ZD. Furthermore, inhibition of NO accumulation by PTIO or L-NNA blocked the protective effect of exogenous ALA, while both exogenous NO treatment and inhibition of endogenous NO accumulation did not induce ALA production. These results suggested that NO might be a downstream signal mediating ALA-induced chilling resistance in E. nutans.

Involvement of nitric oxide in 5-aminolevulinic acid-induced antioxidant defense in roots of Elymus nutans exposed to cold stress

Nitric oxide (NO) and 5-aminolevulinic acid (5ALA) play fundamental roles in plant responses to environmental stresses, but their cross-talk in antioxidant defense in cold-stressed Elymus nutans Griseb. have not been investigated. We herein report that 5ALA and NO donor, sodium nitroprusside (SNP), alleviated cold stress-induced plant growth inhibition and lipid peroxidation in roots of two E. nutans ecotypes (Damxung, DX and Zhengdao, ZD). However, application of an NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt (PTIO) differentially blocked these protective effects indicating that an inhibition of NO accumulation reduced 5ALA-enhanced cold resistance. Application of exogenous 5ALA or NO markedly up-regulated activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, enhanced reduced glutathione accumulation and reduced glutathione to oxidized glutathione ratio, activated plasma membrane (PM) H+-ATPase, and reduced Na+/K+ ratio in roots of the two E. nutans ecotypes. Moreover, in the presence of 5ALA, nitric oxide synthase (NOS) activity and NO release in cold-resistant DX were higher than those in cold-sensitive ZD. Conversely, both NO treatment and inhibition of endogenous NO accumulation by PTIO or NOS inhibitor Nω-nitro-L-arginine did not induce 5ALA production. These results suggest that NO might be acting as a downstream signal to mediate 5ALA-induced cold resistance by activating antioxidant defense and PM H+-ATPase and maintaining Na+ and K+ homeostasis.

Crosstalk of nitric oxide with calcium induced tolerance of tall fescue leaves to high irradiance

Calcium ion (Ca2+) is essential secondary messenger in plant signaling networks. In this study, the effect of Ca2+ on oxidative damage caused by a high irradiance (HI) was investigated in the leaves of two cultivars of tall fescue (Arid3 and Houndog5). Pretreatment of the tall fescue leaves with a CaCl2 solution significantly increased Ca2+ content and intrinsic HI tolerance due to a decreased ion leakage and content of malondialdehyde, hydrogen peroxide, and superoxide radicals. Moreover, the activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase increased in both the cultivars in the presence of Ca2+ under the HI stress. In contrast, treatments with a Ca2+ chelator ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA) or a plasma membrane Ca2+ channel blocker LaCl3 reversed these effects. On the other hand, a pronounced increase in nitric oxide synthase-like activity and NO release by exogenous Ca2+ treatment was observed in the tolerant Arid3 plants after exposure to the HI, whereas only a small increase was observed in more sensitive Houndog5. Moreover, the inhibition of NO production by 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide or Nω-nitro-L-arginine blocked the protective effect of exogenous Ca2+, whereas the inhibition of Ca2+ by EGTA or LaCl3 had no influence on the protective effect of NO. The results indicate that NO might be involved in the Ca2+-induced activities of antioxidant enzymes further protecting against HI-induced oxidative damage. This protective mechanism was found to be more efficient in Arid3 than in Houndog5.

Diffusion limitations and metabolic factors associated with inhibition and recovery of photosynthesis following cold stress in Elymus nutans Griseb.

We studied the effects of cold stress (5°C) and re-warming (25°C) on gas exchange, photosystem II, key photosynthetic enzyme activities, gene expression, and carbohydrate metabolite concentrations in two Elymus nutans genotypes differing in cold resistance (DX, cold-tolerant and ZD, cold-sensitive). Cold stress led to irreversible reductions in photosynthetic rate. This reduction was accompanied by declining stomatal and mesophyll conductance (gs and gm), transpiration rate (Tr) and photochemical efficiency in both genotypes, however there were smaller decreases in DX than in ZD. Cold-tolerant DX maintained higher photosynthetic enzyme activities and transcript levels, as well as higher reducing sugar concentrations and sucrose accumulation. The relationship between Pn and internal leaf CO2 concentration (Pn/Ci curve) during cold and re-warming was analyzed to estimate the relative influence of stomatal and non-stomatal components on photosynthesis. Stomatal limitation, non-stomatal limitation, and CO2 compensation point (CP) increased in both genotypes under cold stress, but to a lesser extent in DX. Maximum CO2 assimilation rate (Pmax), and carboxylation efficiency (CE) declined, but DX had significantly higher levels of Pmax and CE than ZD. Following cold-stress recovery, the maximum quantum yield of PSII (Fv/Fm), apparent electron transport rate (ETR), Rubisco activity, Rubisco activation state and CE in DX resumed to the control levels. In contrast, Pn, Pmax, gs, gm, and Tr recovered only partially for DX, suggesting that incomplete recovery of photosynthesis in DX may be mainly related to diffusion limitations. Higher Rubisco large subunit (RbcL) and Rubisco activase (RCA) transcript levels, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity, and carbohydrate accumulation contributed to higher photosynthetic recovery in DX. These results indicate that the maintenance of higher Pn and Pmax under cold stress and recovery in cold-tolerant DX could be attributed to reduced diffusion limitations and rapid recuperation of metabolic factors.

Exogenous nitric oxide alleviates shade-induced oxidative stress in tall fescue (Festuca arundinacea Schreb.)

Summary Nitric oxide (NO) is an important signalling molecule involved in a diverse range of physiological processes. In this study, the effects of sodium nitroprusside (SNP), a donor of NO, on the growth and anti-oxidant capacity of leaves of tall fescue [Festuca arundinacea (Schreb.) cv. Arid3] were investigated under low shade (LS), moderate shade (MS), and heavy shade (HS) applied for 7 d and 14 d in all cases. LS plants exhibited slight injury, while MS and HS caused a significant increase in cell membrane damage throughout the stress period, which was measured by enhanced electrolyte leakage (EL) and increased malondialdehyde (MDA) concentrations. Exogenous application of 0.1 mM SNP alleviated this oxidative stress, increased leaf widths and chlorophyll (Chl) concentrations, and decreased leaf lengths in plants grown under shade stress. Spray applications of of 0.1 mM SNP also increased the activities of the anti-oxidant enzymes, superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX), and the accumulation of proline in stressed leaves. Significant increases in endogenous NO concentration were found in tall fescue leaves after 7 d and 14 d of shade. SNP-treated plants grown under shade stress produced higher concentrations of NO than untreated, shaded plants. The application of SNP was more effective at reducing the detrimental effects of shade stress on tall fescue leaves under MS than under LS or HS stress. These results suggest that one or more of the breakdown product(s) of SNP might be signalling molecule(s) which enhance the activities of anti-oxidant enzymes and increase proline concentrations to alleviate oxidative damage in the leaves of tall fescue plants grown under shade stress.