Lingyun Yuan

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.

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.

A Review: Polyamines and Photosynthesis

Polyamines (PAs) are low molecular weight ubiquitous nitrogenous compounds found in all living organisms (Kaur-Sawhney et al., 2003). In higher plants, the most common polyamines are spermidine (Spd), spermine (Spm) and their diamine obligate precursor putrescine (Put). They are formed by aliphatic hydrocarbons substituted with two or more amino groups (Figure.1). Because of the polycationic nature at physiological pH, PAs are present in the free form or as conjugates bound to phenolic acids and other low molecular weight compounds or to proteins and nucleic acids (Childs et al., 2003). Like hormones, PAs displaying high biological activity are involved in a wide array of fundamental processes in plants, such as replication and gene expression, growth and development, senescence, membrane stabilization, enzyme activity modulation and adaptation to abiotic and biotic stresses (Galston et al., 1997; Bais and Ravishankar, 2002; Zapata et al., 2008). Although, according to these reports, PAs seem to be important growth regulators, their precise physiological function and mechanism of action still remain unclear. It has been shown that chloroplasts and photosynthetic subcomplexes including thylakoids, LHCII complex and PSII membranes are enriched with three major polyamines, while PSII core and the reaction center of PSII are exclusively rich in Spm (Kotzabasis et al., 1993; Navakoudis et al., 2003). The potential role of polyamines in maintaining the photochemical efficiency of plants has become a research focus. These studies mainly focused on the effect PAs exert a positive role in the photosynthesis of plants in response to various environmental stresses. In green alga, it was shown that the bound Put content of the thylakoid membrane was increased in environments with high CO2 concentrations, which caused an increase in reaction center density and led to an increased photosynthetic rate (Logothetis et al., 2004). An increase in conjugated Put content can stabilize the thylakoid membrane, thus enhancing resistance of tobacco plants to ozone pollution (Navakoudis et al., 2003) and UV-B radiation (Lutz et al., 2005). Low temperature stress reduced the content of Put as well as the Put/Spm ratio in thylakoids and the light-harvesting complexes LHCII in Phaseolus vulgaris L., leading to a decrease in photosynthetic electron transport rate and inactivation of the PSII reaction center (Sfakianaki et al., 2006). Put is also involved in the induction of a photosynthetic apparatus owning high concentration of reaction center with a small functional antenna that leads to enhance photochemical quenching of the absorbed light energy (Kotzabasis et al., 1999). PAs biosynthesis is controlled by light and the Spm/Put ratio is correlated to the structure and function of the photosynthetic apparatus

Effects of 24-epibrassinolide on ascorbate–glutathione cycle and polyamine levels in cucumber roots under Ca(NO3)2 stress

The effects of 24-epibrassinolide (EBL) applied by spraying the leaves of Cucumis sativus L (cv. Jinyou No.4) on the roots of ascorbate–glutathione (AsA–GSH) cycle and levels and forms of polyamines were investigated under Ca(NO3)2 stress. Ca(NO3)2 stress caused significant decreases in ascorbate acid (AsA) and glutathione (GSH) levels and the activities of antioxidant enzymes involved in the AsA–GSH cycle, including ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2), monodehydroascorbate reductase (MDAR, EC 1.6.5.4) and dehydroascorbate reductase (DHAR, EC 1.6.5.4). In contrast, exogenous EBL in NB combination increased AsA and GSH levels through enhancing activities of APX, GR, MDAR and DHAR. This alleviated the declines of AsA/DHA and GSH/GSSG ratios, which could maintain cellular homeostasis. In addition, EBL application reduced the accumulation of putrescine (Put) induced by excess Ca(NO3)2, and promoted the conversion of Put into spermidine (Spd) and/or spermine (Spm) able to bind to cell membrane structures. These results showed that EBL could alleviate the damage from oxidative stress by up-regulating the capacity of the AsA–GSH cycle and altering polyamine levels and forms.

The effect of exogenous calcium on mitochondria, respiratory metabolism enzymes and ion transport in cucumber roots under hypoxia

Hypoxia induces plant stress, particularly in cucumber plants under hydroponic culture. In plants, calcium is involved in stress signal transmission and growth. The ultimate goal of this study was to shed light on the mechanisms underlying the effects of exogenous calcium on the mitochondrial antioxidant system, the activity of respiratory metabolism enzymes, and ion transport in cucumber (Cucumis sativus L. cv. Jinchun No. 2) roots under hypoxic conditions. Our experiments revealed that exogenous calcium reduces the level of reactive oxygen species (ROS) and increases the activity of antioxidant enzymes in mitochondria under hypoxia. Exogenous calcium also enhances the accumulation of enzymes involved in glycolysis and the tricarboxylic acid (TCA) cycle. We utilized fluorescence and ultrastructural cytochemistry methods to observe that exogenous calcium increases the concentrations of Ca2+ and K+ in root cells by increasing the activity of plasma membrane (PM) H+-ATPase and tonoplast H+-ATPase and H+-PPase. Overall, our results suggest that hypoxic stress has an immediate and substantial effect on roots. Exogenous calcium improves metabolism and ion transport in cucumber roots, thereby increasing hypoxia tolerance in cucumber.

Regulation of 2,4-epibrassinolide on mineral nutrient uptake and ion distribution in Ca(NO3)2 stressed cucumber plants.

2,4-Epibrassinolide (EBL) is a plant hormone that plays a pivotal role in regulation of plants growth and development processes under abiotic stress. The investigation was carried out to study the effect of EBL on mineral nutrients uptake and distribution with ion element analysis and X-ray microanalysis in cucumber seedlings (Cucumis sativus L. cv. Jinyou No.4) under 80 mM Ca(NO3)2 stress. Our study found EBL significantly alleviated the inhibitory of P, K, Na, Mg, Fe, Mn, or Cl uptake in shoot or root by Ca(NO3)2 stress. Under Ca(NO3)2 stress, X-ray microanalysis showed that high levels of Ca by EBL treatment accumulated in the epidermal cells of root and gradually decreased from epidermal cells to stellar parenchyma. K(+) levels were restored in different cross section of roots and high K(+) level mostly accumulated in stellar parenchyma. The results of Ca(2+) ultra-structural localization showed Ca(2+) particles of antimonite precipitate by EBL were partly decreased in mesophyll and root cells, and Ca(2+) precipitate distributed in intercellular spaces again. Increased Ca(2+)-ATPase activity and ATP content by EBL were also contributed to extrude excess Ca(2+) from the cytoplasm. These results suggested that EBL could alleviate the ion damage from excess Ca(2+) through regulating mineral nutrients uptake and distribution.