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The Positive Roles of Exogenous Putrescine on Chlorophyll Metabolism and Xanthophyll Cycle in Salt-Stressed Cucumber Seedlings

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The Positive Roles of Exogenous Putrescine on Chlorophyll Metabolism and Xanthophyll Cycle in Salt-Stressed Cucumber Seedlings DOI: 10.1007/s11099-017-0712-5 PHOTOSYNTHETICA 56 (2): 557-566, 2018 The positive roles of exogenous putrescine on chlorophyll metabolism and xanthophyll cycle in salt-stressed cucumber seedlings R.N. YUAN*, S. SHU*, S.R. GUO*,**,+, J. SUN*,**, and J.Q. WU* Key Laboratory of Southern Vegetable Crop Genetic Improvement, Ministry of Agriculture, College of Horticulture, Nanjing Agricultural University, Nanjing 210095, China* Nanjing Agricultural University (Suqian) Academy of Protected Horticulture, Suqian 223800, China** Abstract The effects of foliar spray of putrescine (Put; 8 mM) on chlorophyll (Chl) metabolism and xanthophyll cycle in cucumber seedlings were investigated under saline conditions of 75 mM NaCl. Exogenous Put promoted the conversion of uroporhyrinogen III to protoporphyrin IX and alleviated decreases in Chl contents and in a size of the xanthophyll cycle pool under salt stress. Moreover, the Put treatment reduced the activities of uroporphyrinogen III synthase, chlorophyllase, and Mg-dechelatase and downregulated the transcriptional levels of glutamyl-tRNA reductase, 5-aminolevulinate dehydratase, uroporphyrinogen III synthase, uroporphyrinogen III decarboxylase, and chlorophyllide a oxygenase, but significantly increased the expression levels of non-yellow coloring 1-like, pheide a oxygenase, red chlorophyll catabolite reductase, and violaxanthin de-epoxidase. Taken together, these results suggest that Put might improve Chl metabolism and xanthophyll cycle by regulating enzyme activities and mRNA transcription levels in a way that improved the salt tolerance of cucumber plants. Additional key words: chlorophyll biosynthesis; chlorophyll degradation; de-epoxidation; energy dissipation; light-harvesting complex; pigments. Introduction More than 800 million hectares of land (6% of the world abiotic stresses in recent years. Several studies have shown total land area) is affected by salt (Wang et al. 2015). As that PAs can alleviate the inhibition of plant physiological one of the most adverse environmental stresses on plants, processes induced by stresses, such as the inhibition of salt stress has a negative effect on important plant photosynthetic apparatus activities and polyamines physiological processes, including growth, photosyn- biosynthesis at physiological and molecular levels thesis, water absorption, nutrient imbalance, and yield (Ioannidis and Kotzabasis 2007, Hu et al. 2012). The (Parihar et al. 2015). photosystem comprises Chl and carotenoid (Car) The most common polyamines (PAs) are diamine Put, pigments, which harvest light energy and transform it into triamine spermidine (Spd), and tetraamine spermine biochemical energy in higher plants. Li et al. (2015) (Spm). These low molecular-mass aliphatic amines exist showed that exogenous Spd protects tomato seedlings in a wide range of plants (Bagni and Tassoni 2001). Many against the damage induced by saline-alkaline stress by studies have focused on the relationships between PAs and regulating Chl metabolism. In addition, Chl metabolism ——— Received 18 May 2016, accepted 14 November 2016, published as online-first 13 March 2017. +Corresponding author; phone: +(86)25-84395267, e-mail: [email protected] Abbreviations: A – antheraxanthin; ALA – δ-aminolevulinic acid; ALAD – 5-aminolevulinate dehydratase; Car – carotenoids; CAO – chlorophyllide a oxygenase; Chl – chlorophyll; CBR – chlorophyll b reductase; Chlase – chlorophyllase; CHLH – Mg chelatase H subunit; DEPS – de-epoxidation of the xanthophyll cycle; DOE – days of experiment; FM – fresh mass; HEMA1 – glutamyl-tRNA reductase; MDCase – Mg dechelatase; Mg-Proto IX – Mg proporphyrin IX; NOL – NYC1-like; NPQ – nonphotochemical quenching; NYC1 – non-yellow coloring 1; PAO – pheide a oxygenase; PAs – polyamines; PBG – porphobilinogen; PBGD – porphobilinogen deaminase; PChl – protochlorophyllide; PPH – pheophytinase; PPO – protoporphyrinogen IX oxidase; ProtoIX – protoporphyrin IX; Put – putrescine; RCCR – red chlorophyll catabolite reductase; Spd – spermidine; Spm – spermine; UroIII – uroporhyrinogen III; UROD – uroporphyrinogen III decarboxylase; UROS – uroporphyrinogen III synthase; V – violaxanthin; VAZ – xanthophyll cycle pool; VDE – violaxanthin de-epoxidase; Z – zeaxanthin. Acknowledgements: This work was funded by the National Natural Science Foundation of China (No. 31471869, No. 31672199 and No. 31401919) and the Jiangsu Province Scientific and Technological Achievements into Special Fund (BA2014147) and was sponsored by the China Agriculture Research System (CARS-25-C-03). 557 R.N. YUAN et al. not only influences the formation of Chl-protein com- epoxidation of the xanthophyll cycle (DEPS) rapidly plexes but also regulates other physiological processes, increased under high light stress. However, it remains including cell death and chloroplast–nucleus communi- unclear whether a close relationship exists between Put cation (Tanaka and Tanaka 2006). and salt tolerance through changes in the Chl metabolism Car comprise a class of pigments that are widespread pathway and the xanthophyll cycle. in living organisms. Moreover, Car transfer light energy to In our previous study, Put was found to regulate Chls and play a crucial role in photoprotection against cucumber growth, glycolysis, the Krebs cycle metabolism, oxidative damage (Horton and Ruban 2005). The carbohydrate metabolism, and photosynthetic efficiency xanthophyll cycle exists in higher plants and involves three under salt stress (Shu et al. 2012, Yuan et al. 2015, Zhong pigments: violaxanthin (V), antheraxanthin (A), and et al. 2015). In this study, we examined the effects of zeaxanthin (Z) (Hieber et al. 2000). Z has been exogenous Put on pigment contents, enzyme activities, the convincingly shown to establish tolerance of photo- expressions of genes involved in Chl-metabolism oxidative stress through an independent process of pathways and the xanthophyll cycle in leaves of cucumber nonphotochemical quenching (NPQ), which protects the seedlings in order to further evaluate the roles of Put in thylakoid membrane from oxidative damage (Johnson et regulating Chl metabolism and the xanthophyll cycle. The al. 2007). Li et al. (2013) cloned the violaxanthin de- purpose of this study was to elucidate the protective epoxidase (VDE) gene of cucumber, and their results mechanism of Put by altering the contents of photo- showed that the gene expression of CsVDE and the de- synthetic pigments in salt-stressed cucumber seedlings. Materials and methods Plant material and growth conditions: The cucumber (pH 6.5 ± 0.1, EC of 2.0–2.2 dS m–1) when two true leaves seeds (Cucumis sativus L., cv. Jingyou No. 4) were had expanded. The nutrient solution was aerated using an germinated in a thermostat at 28°C and sown in quartz air pump at intervals of 30 min to maintain dissolved sand. The experiments were performed in a plant growth oxygen concentration at 8.0 ± 0.2 mg L–1 throughout the chamber, which was maintained at 28 ± 1°C (day) and experiment. 18 ± 1°C (night) with a maximum PPFD of approximately 1,200 μmol m–2 s–1 and a relative humidity of 60–70%. The Experimental approach: After precultivation for 3 d in seedlings were transplanted to plastic containers (40 × the nutrient solution, the seedlings were treated as follows: 30 × 15 cm) containing half-strength Hoagland solution Treatment Seedling growth Leaf spraying Control (C) half-strength Hoagland solution distilled water Put half-strength Hoagland solution 8 mM Put NaCl half-strength Hoagland solution + 75 mM NaCl distilled water NaCl + Put half-strength Hoagland solution + 75 mM NaCl 8 mM Put The Hoagland solution was renewed every two days. The collected and immediately ground to a fine powder, which containers were arranged in a completely randomized was extracted in 1 mL of 100% (v/v) acetone. The block design with three replicates per treatment (36 plants homogenate was centrifuged at 15,000 × g for 10 min at per treatment). The cucumber leaves were sprayed with 4°C, and the supernatant was collected. Fifteen microliters 100 mL of 8 mM Put every day until the end of the of the filtered (0.2 μm nylon filter) supernatant was experiment. An equal volume of distilled water was also subjected to HPLC (Agilent Technologies 1200 series, sprayed on the leaves in the control and salt stress Agilent Technologies, California, USA) and separated on treatment. After 1, 3, 5, and 7 d of the experiment (DOE), a Spherisorb C18 column (4.6 by 250 mm, 5 µm Kromasil) the pigment content of the third fully expanded leaf from at 30°C. Pigments were eluted using a linear gradient from the top of each plant was analyzed. The enzyme activities 100% (v/v) solvent A [acetonitrile:methanol:Tris-HCl and expression of genes involved in Chl metabolism were buffer (0.1 M, pH 8.0), 84:2:14] to 100% (v/v) solvent B analyzed after treating the cucumber seedlings for 7 DOE. (methanol:ethyl acetate, 68:32) for 12 min, followed by 6 min of solvent B, followed by a gradient from 100% B Pigment determination: We analyzed the effects of Put to 100% A for the next 7 min. The solvent flow rate was on the pigment content of cucumber seedlings under salt 1.2 mL min–1, and pigment absorption was detected at stress using HPLC, using a modified version of the method 445 nm. The injection volume was 20 µL. De-epoxidation of García-Plazaola and Becerril (1999). Leaf discs were of the xanthophyll cycle (DEPS) was calculated as 558 PHOTOSYNTHETIC CHARACTERISTICS IMPROVEMENT VIA PGRs (A + Z)/(V + A + Z) (%). Chl a and Chl b were extracted
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