Exogenous spermidine is enhancing tomato tolerance to salinity-alkalinity stress by regulating chloroplast antioxidant system and chlorophyll metabolism

Abstract

Background: Salinity-alkalinity stress is known to adversely affect a variety of processes in plants, thus inhibiting growth and decreasing crop yield. Polyamines protect plants against a variety of environmental stresses. However, whether exogenous spermidine increases the tolerance of tomato seedlings via effects on chloroplast antioxidant enzymes and chlorophyll metabolism is unknown. In this study, we examined the effect of exogenous spermidine on chlorophyll synthesis and degradation pathway intermediates and related enzyme activities, as well as chloroplast ultrastructure, gene expression, and antioxidants in salinity-alkalinity-stressed tomato seedlings.

Results: Salinity-alkalinity stress disrupted chlorophyll metabolism and hindered uroorphyrinogen III conversion to protoporphyrin IX. These effects were more pronounced in seedlings of cultivar Zhongza No. 9 than cultivar Jinpengchaoguan. Under salinity-alkalinity stress, exogenous spermidine alleviated decreases in the contents of total chlorophyll and chlorophyll a and b in seedlings of both cultivars following 4 days of stress. With extended stress, exogenous spermidine reduced the accumulation of δ-aminolevulinic acid, porphobilinogen, and uroorphyrinogen III and increased the levels of protoporphyrin IX, Mg-protoporphyrin IX, and protochlorophyllide, suggesting that spermidine promotes the conversion of uroorphyrinogen III to protoporphyrin IX. The effect occurred earlier in cultivar Jinpengchaoguan than in cultivar Zhongza No. 9. Exogenous spermidine also alleviated the stress-induced increases in malondialdehyde content, superoxide radical generation rate, chlorophyllase activity, and expression of the chlorophyllase gene and the stress-induced decreases in the activities of antioxidant enzymes, antioxidants, and expression of the porphobilinogen deaminase gene. In addition, exogenous spermidine stabilized the chloroplast ultrastructure in stressed tomato seedlings.

Conclusions: The tomato cultivars examined exhibited different capacities for responding to salinity-alkalinity stress. Exogenous spermidine triggers effective protection against damage induced by salinity-alkalinity stress in tomato seedlings, probably by maintaining chloroplast structural integrity and alleviating salinity-alkalinity-induced oxidative damage, most likely through regulation of chlorophyll metabolism and the enzymatic and non-enzymatic antioxidant systems in chloroplast. Exogenous spermidine also exerts positive effects at the transcription level, such as down-regulation of the expression of the chlorophyllase gene and up-regulation of the expression of the porphobilinogen deaminase gene.

Fig. 1

Effect of exogenous Spd on chlorophyll content in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a, c and e represent cv. Zhongza No.9; (b, d and f) represent cv. Jinpengchaoguan

Fig. 2

Effect of Spd on ALA content in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a represents cv. Zhongza No.9; b represents cv. Jinpengchaoguan

Fig. 3

Effect of Spd on URO III and PBG content in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a and c represent cv. Zhongza No.9; b and d represent cv. Jinpengchaoguan

Fig. 4

Effect of Spd on Proto IX, Mg–proto IX and Pchl content in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a, c and e represent cv. Zhongza No.9; b, d and f) represent Jinpengchaoguan

Fig. 5

Effect of Spd on Chlase activity in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a represents cv. Zhongza No.9; b represents cv. Jinpengchaoguan

Fig. 6

Effect of Spd on MDA content and O₂ ^–⋅ generation rate in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a and c represent cv. Zhongza No.9; b and d represent cv. Jinpengchaoguan

Fig. 7

Effect of exogenous Spd on SOD activity in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a represents cv. Zhongza No. 9; b represents cv. Jinpengchaoguan

Fig. 8

Effect of exogenous Spd on APX, MDHAR, DHAR and GR activity in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a, c, e and g represent cv. Zhongza No.9; (b, d, f and h) represent cv. Jinpengchaoguan

Fig. 9

Effect of exogenous Spd on AsA and GSH content in tomato seedlings. CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a and c represent cv. Zhongza No.9; (b and d) represent cv. Jinpengchaoguan

Fig. 10

Effect of exogenous Spd on chloroplast ultrastructure in tomato seedlings grown under salinity–alkalinity stress. cv. ZZ, cv. Zhongza No. 9; cv. JP, cv. Jinpengchaoguan; CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl: Na₂SO₄: NaHCO₃: Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. Data were obtained from the second expanded leaves (numbered basipetally) after salinity–alkalinity treatment for 6 days. SL, stroma lamellae; GL, grana lamellae; SG, starch grains; P, plastoglobuli. Scale bars for chloroplasts and thylakoids are 0.5 and 0.1 μm, respectively. a represents chloroplast of CK treated cv. Zhongza No.9; b represents thylakoid of CK treated cv. Zhongza No.9; c represents chloroplast of CK treated cv. Jinpengchaoguan; d represents thylakoid of CK treated cv. Jinpengchaoguan; e represents chloroplast of S treated cv. Zhongza No.9; f represents thylakoid of S treated cv. Zhongza No.9; g represents chloroplast of S treated cv. Jinpengchaoguan; h represents thylakoid of S treated cv. Jinpengchaoguan; i represents chloroplast of SS treated cv. Zhongza No.9; j represents thylakoid of SS treated cv. Zhongza No.9; k represents chloroplast of SS treated cv. Jinpengchaoguan; l represents thylakoid of SS treated cv. Jinpengchaoguan

Fig. 11

Effect of exogenous Spd on the expression of chlorophyll metabolism enzyme genes. cv. ZZ, cv. Zhongza No. 9; cv. JP, cv. Jinpengchaoguan; CK, 1/2 Hoagland’s solution; S, 75 mM saline–alkaline solution (NaCl:Na₂SO₄:NaHCO₃:Na₂CO₃ = 1:9:9:1); SS, sprayed with 0.25 mM Spd and treated with 75 mM saline–alkaline solution. a, c and e represent cv. Zhongza No.9; b, d and f represent cv. Jinpengchaoguan