Jin Sun

Effects of exogenous spermine on chlorophyll fluorescence, antioxidant system and ultrastructure of chloroplasts in Cucumis sativus L. under salt stress.

The effects of exogenous spermine (Spm) on plant growth, chlorophyll fluorescence, ultrastructure and anti-oxidative metabolism of chloroplasts were investigated in Cucumis sativus L. under NaCl stress. Salt stress significantly reduced plant growth, chlorophylls content and F(v)/F(m). These changes could be alleviated by foliar spraying with Spm. Salt stress caused an increase in malondialdehyde (MDA) content and superoxide anion [Formula: see text] generation rate in chloroplasts. Application of Spm significantly increased activities of superoxidase dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), and ascorbate peroxidase (APX, EC 1.11.1.11) which decreased the levels of [Formula: see text] and MDA in the salt-stressed chloroplasts. Salt stress decreased the activities of dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) in the chloroplasts and reduced the contents of dehydroascorbate (DAsA) and glutathione (GSH), but increased monodehydroascorbate reductase (MDAR, EC 1.6.5.4) activity. On the other hand, Spm significantly increased the activities of antioxidant enzymes and levels of antioxidants in the salt-stressed chloroplasts. Further analysis of the ultrastructure of chloroplasts indicated that salinity induced destruction of the chloroplast envelope and increased the number of plastoglobuli with aberrations in thylakoid membranes. However, Spm application to salt-stressed plant leaves counteracted the adverse effects of salinity on the structure of the photosynthetic apparatus. These results suggest that Spm alleviates salt-induced oxidative stress through regulating antioxidant systems in chloroplasts of cucumber seedlings, which is associated with an improvement of the photochemical efficiency of PSII.

Effects of salt stress on the structure and function of the photosynthetic apparatus in Cucumis sativus and its protection by exogenous putrescine

With the objective to clarify the physiological significance of polyamines (PAs) in the photosynthetic apparatus, the present study investigated the effects of salt stress with and without foliar application of putrescine (Put) on the structure and function of the photosynthetic apparatus in cucumber. Salt stress at 75 mM NaCl for 7 days resulted in a severe reduction of photosynthesis. The fast chlorophyll afluorescence transient analysis showed that salt stress inhibited the maximum quantum yield of PSII photochemistry (F(v)/F(m)), mainly due to damage at the receptor side of PSII. In addition, salt stress decreased the density of active reaction centers and the structure performance. The microscopic analysis revealed that salt stress-induced destruction of the chloroplast envelope and increased the number of plastoglobuli along with aberrations in thylakoid membranes. Besides, salt stress caused a decrease in the content of endogenous PAs, conjugated and bound forms of spermidine and spermine in particular, in thylakoid membranes. However, applications of 8 mM Put alleviated the salt stress-mediated decrease in net photosynthetic rates (Pn) and actual efficiency of PSII(Φ(PSII)). Put increased PAs in thylakoid membranes and overcame the damaging effects of salt stress on the structure and function of the photosynthetic apparatus in salt-stressed plant leaves. Put application to control plants neither increased PAs in thylakoid membranes nor affected photosynthesis. These results indicate that PAs in chloroplasts play crucial roles in protecting the thylakoid membranes against the deleterious influences of salt stress. In addition, the present results point to the probability that the salt-induced dysfunction of photosynthesis is largely attributable to the loss of PAs in the photosynthetic apparatus.

Proteomics reveal cucumber Spd-responses under normal condition and salt stress.

To investigate the effects of exogenous Spd on proteomic changes under normal condition and NaCl stress of 3 days in cucumber seedling leaves, a 2-DE gel electrophoresis and MALDI-TOF/TOF MS was performed. A total of 63 differentially expressed proteins responded to salt stress or exogenous Spd treatments, and they were all successfully identified by MALDI-TOF/TOF MS. Many changes were observed in the levels of proteins involved in energy and metabolic pathways, protein metabolic, stress defense, and other functional proteins. Increased salt tolerance by exogenous Spd would contribute to higher expressions of proteins involved in the SAMs metabolism, protein biosynthesis, and defense mechanisms on antioxidant and detoxification. Meanwhile, the regulation of Calvin cycle, protein folding assembly and the inhibition of protein proteolysis by Spd might play important roles in salt tolerance. This study provides insight that may facilitate a better understanding of the salt resistance by Spd in cucumber seedlings.

Proteomic study participating the enhancement of growth and salt tolerance of bottle gourd rootstock-grafted watermelon seedlings.

An insertion grafting technique to do research on salt tolerance was applied using watermelon (Citrullus lanatus [Thunb.] Mansf. cv. Xiuli) as a scion and bottle gourd (Lagenaria siceraria Standl. cv. Chaofeng Kangshengwang) as a rootstock. Rootstock-grafting significantly relieved the inhibition of growth and photosynthesis induced by salt stress in watermelon plants. Proteomic analysis revealed 40 different expressed proteins in response to rootstock-grafting and/or salt stress. These proteins were involved in Calvin cycle, amino acids biosynthesis, carbohydrate and energy metabolism, ROS defense, hormonal biosynthesis and signal transduction. Most of these proteins were up-regulated by rootstock-grafting and/or susceptible to salt stress. The enhancement of the metabolic activities of Calvin cycle, biosynthesis of amino acids, carotenoids and peroxisomes, glycolytic pathway and tricarboxylic acid cycle will probably contribute to intensify the biomass and photosynthetic capacity in rootstock-grafted seedlings under condition without salt. The accumulation of key enzymes included in these biological processes described above seems to play an important role in the enhancement of salt tolerance of rootstock-grafted seedlings. Furthermore, leucine-rich repeat transmembrane protein kinase and phospholipase may be involved in transmitting the internal and external stimuli induced by grafting and/or salt stress.

Effects of proline on photosynthesis, root reactive oxygen species (ROS) metabolism in two melon cultivars ( Cucumis melo L.) under NaCl stress

Effects of 0.2 mM proline applied to saline nutrient solution on biomass, chlorophyll content, photosynthetic parameters, reactive oxygen species and antioxidant enzymes activities of two melon cultivars (cv. Yuhuang and cv. Xuemei) were examined. Results indicate that exogenous proline increased the fresh and dry weights of both melon cultivars under NaCl stress, raised their chlorophyll content, net photosynthetic rate (Pn), actual efficiency of photosystem II (ΦPSII), enhanced the activity of SOD, POD, CAT, APX, DHAR and GR in their roots, lowered the superoxide anion radical level and reduced the hydrogen peroxide (H 2 O 2 ) content and malondialdehyde (MDA) content. Exogenous proline also alleviate salinity-induced damage of membrane in both melon cultivars. In conclusion, proline treatment enhanced the salinity tolerance of both melon plants and alleviated their salinity-induced damage. However, all the above effects of proline were markedly more significant in cv. Xuemei than in cv. Yuhuang, suggesting that proline had different effects on different cultivars of melon plants. Key words : Proline, photosynthesis, reactive oxygen species, antioxidant enzymes, salt tolerance, NaCl stress.

Identification of hypoxic-responsive proteins in cucumber roots using a proteomic approach.

To elaborate the mechanisms of response of cucumber (Cucumis sativus L. cv.) seedlings to hypoxic stress, plants were grown under either normoxic conditions or hypoxic stress. As expected, plant biomass was significantly reduced under hypoxic stress. Proteomic profiles of cucumber roots were studied at 72 h after treatment; 316 and 425 protein spots were detected on polyacrylamide gels from normoxic and hypoxic-treated plants, respectively. Compared with normoxic-treated plants, protein abundance of 22 proteins was significantly upregulated while protein abundance of 12 proteins decreased in the hypoxic-treated plants. Twenty one of the proteins whose abundance was altered were identified by MALDI-TOF/TOF MS analysis, and categorized into classes corresponding to energy and metabolism proteins, transcription factor proteins, defense stress proteins, structural proteins and regulatory proteins. Under hypoxic stress, glycolysis was induced; energy was channeled to primary metabolism, while secondary pathways and nitrogen metabolism pathways were inhibited. Cucumber plants scavenged reactive oxygen species by antioxidase, and increased Acyl-[acyl-carrier-protein] desaturase which defend against reactive oxygen species damage to plant cell structure. This study provides insight that may facilitate a better understanding of the response mechanisms of cucumber plant to hypoxic stress.

Effects of 24-epibrassinolide on nitrogen metabolism in cucumber seedlings under Ca(NO3)2 stress

Ca(NO(3))(2) accumulation is a major factor that limits greenhouse production in China. The present investigation was carried out to study the effect of 24-epibrassinolide (EBL) on nitrogen metabolism (including contents of NO(3)(-), NH(4)(+) and amino acids and related enzymes activities) in cucumber seedlings (Cucumis sativus L. cv. Jinyou No. 4) under 80 mM Ca(NO(3))(2) stress. This study found that exogenous EBL significantly reduced the accumulation of NO(3)(-) and NH(4)(+) by Ca(NO(3))(2), and enhanced the inactivated enzymes activities involved in the nitrogen metabolism. In addition, EBL alleviated the inhibition of photosynthesis nitrogen-use efficiency by Ca(NO(3))(2). Increased total amino acids by EBL under stress increased the precursor of proteins biosynthesis, thus promoting the biosynthesis nitrogen containing compounds. The presence of Ca(NO(3))(2) increased polyamines level, which might result from the increased content of free putrescine that is harmful to plant growth. However, exogenous EBL induced a further increase in total polyamines. The increase is likely caused by the elevated contents of conjugated and bound forms of polyamines. In summary, exogenously EBL compensated for the damage/losses by Ca(NO(3))(2) stress to some extent through the regulation of nitrogen metabolism and metabolites.

The role of putrescine in the regulation of proteins and fatty acids of thylakoid membranes under salt stress.

Polyamines can alleviate the inhibitory effects of salinity on plant growth by regulating photosynthetic efficiency. However, little information is available to explain the specific mechanisms underlying the contribution of polyamines to salt tolerance of the photosynthetic apparatus. Here, we investigated the role of putrescine (Put) on the photosynthetic apparatus of cucumber seedlings under salt stress. We found that NaCl stress resulted in severe ion toxicity and oxidative stress in cucumber chloroplasts. In addition, salinity caused a significant increase in the saturated fatty acid contents of thylakoid membranes. Put altered unsaturated fatty acid content, thereby alleviating the disintegration of thylakoid grana lamellae and reducing the number of plastoglobuli in thylakoid membranes. BN-PAGE revealed Put up-regulated the expression of ATP synthase, CP47, D1, Qb, and psbA proteins and down-regulated CP24, D2, and LHCII type III in NaCl-stressed thylakoid membranes. qRT-PCR analysis of gene expression was used to compare transcript and protein accumulation among 10 candidate proteins. For five of these proteins, induced transcript accumulation was consistent with the pattern of induced protein accumulation. Our results suggest that Put regulates protein expression at transcriptional and translational levels by increasing endogenous polyamines levels in thylakoid membranes, which may stabilise photosynthetic apparatus under salt stress.

Effects of exogenous spermidine on photosynthetic capacity and expression of Calvin cycle genes in salt-stressed cucumber seedlings

We investigated the effects of exogenous spermidine (Spd) on growth, photosynthesis and expression of the Calvin cycle-related genes in cucumber seedlings (Cucumis sativus L.) exposed to NaCl stress. Salt stress reduced net photosynthetic rates (PN), actual photochemical efficiency of PSII (ΦPSII) and inhibited plant growth. Application of exogenous Spd to salinized nutrient solution alleviated salinity-induced the inhibition of plant growth, together with an increase in PN and ΦPSII. Salinity markedly reduced the maximum carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Vcmax), the maximal velocity of RuBP regeneration (Jmax), triose-phosphate utilization capacity (TPU) and carboxylation efficiency (CE). Spd alleviated the negative effects on CO2 assimilation induced by salt stress. Moreover, Spd significantly increased the activities and contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose-1,6-biphosphate aldolase (ALD; aldolase) in the salt-stressed cucumber leaves. On the other hand, salinity up-regulated the transcriptional levels of ribulose-1,5-bisphosphate (RCA), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribrokinase (PRK) and down-regulated the transcriptional levels of ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (RbcL), ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit (RbcS), ALD, triose-3-phosphate isomerase (TPI), fructose-1,6-bisphosphate phosphatase (FBPase) and 3-phosphoglyceric acid kinase (PGK). However, Spd application to salt-stressed plant roots counteracted salinity-induced mRNA expression changes in most of the above-mentioned genes. These results suggest that Spd could improve photosynthetic capacity through regulating gene expression and activity of key enzymes for CO2 fixation, thus confers tolerance to salinity on cucumber plants.

Bottle gourd rootstock-grafting affects nitrogen metabolism in NaCl-stressed watermelon leaves and enhances short-term salt tolerance

The plant growth, nitrogen absorption, and assimilation in watermelon (Citrullus lanatus [Thunb.] Mansf.) were investigated in self-grafted and grafted seedlings using the salt-tolerant bottle gourd rootstock Chaofeng Kangshengwang (Lagenaria siceraria Standl.) exposed to 100mM NaCl for 3d. The biomass and NO3(-) uptake rate were significantly increased by rootstock while these values were remarkably decreased by salt stress. However, compared with self-grafted plants, rootstock-grafted plants showed higher salt tolerance with higher biomass and NO3(-) uptake rate under salt stress. Salinity induced strong accumulation of nitrate, ammonium and protein contents and a significant decrease of nitrogen content and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT) in leaves of self-grafted seedlings. In contrast, salt stress caused a remarkable decrease in nitrate content and the activities of GS and GOGAT, and a significant increase of ammonium, protein, and nitrogen contents and NR activity, in leaves of rootstock-grafted seedlings. Compared with that of self-grafted seedlings, the ammonium content in leaves of rootstock-grafted seedlings was much lower under salt stress. Glutamate dehydrogenase (GDH) activity was notably enhanced in leaves of rootstock-grafted seedlings, whereas it was significantly inhibited in leaves of self-grafted seedlings, under salinity stress. Three GDH isozymes were isolated by native gel electrophoresis and their expressions were greatly enhanced in leaves of rootstock-grafted seedlings than those of self-grafted seedlings under both normal and salt-stress conditions. These results indicated that the salt tolerance of rootstock-grafted seedlings might (be enhanced) owing to the higher nitrogen absorption and the higher activities of enzymes for nitrogen assimilation induced by the rootstock. Furthermore, the detoxification of ammonium by GDH when the GS/GOGAT pathway was inhibited under salt stress might play an important role in the release of salt stress in rootstock-grafted seedlings.