Regulatory Mechanism of Melatonin on Tomato Seed Germination Under Cd2+ Stress

doi:10.11983/CBB22202

Abstract

Abstract: Soil cadmium (Cd) pollution seriously restricts the yield and quality of facility vegetables. Melatonin (MT) can enhance the resistance to various stresses in plants. However, the downstream signals, which regulated by MT during tomato seed germination under Cd²⁺ stress remains unclear. The effects of Cd²⁺ stress and exogenous MT on seed germination were investigated using tomato (Solanum lycopersicum) wild-type Alisa Craig seeds. The results showed that the germination of tomato seeds and seedling growth were significantly inhibited by Cd²⁺ treatment with more than 0.5 mmol·L^-1. Exogenous MT (0.15 mmol·L^-1) reduced the content of Cd²⁺ in the underground and above-ground tissues of seedlings, effectively alleviating the inhibitory effect of Cd²⁺ stress on tomato seed germination and seedling length. The expression of phytochelatin and transporters-related genes (PCS, NRAMP1, ABCC3, HMA3, and ABCG5) in tomato radicles were significantly increased by MT under Cd²⁺ stress, showing the positive regulation of MT in transmembrane transport and vacuolar sequestration of Cd²⁺. In addition, MT alleviated the oxidative damage induced by Cd²⁺stress, which was related to the enhanced activities of CAT, APX, and ALDH enzymes apart from its own scavenging ability. Furthermore, MT significantly down-regulated the expressions of abscisic acid (ABA) synthesis genes (NCED1 and NCED2) under Cd²⁺ stress, and up-regulated the expression of ABA decomposition gene ABA8ox1, leading to the decrease of ABA content, which effectively regulated the GA/ABA ratio and promoted the germination of tomato seeds under Cd²⁺ stress.

Key words: melatonin, cadmium stress, tomato, seed germination

Cai Shuyu, Liu Jianxin, Wang Guofu, Wu Liyuan, Song Jiangping. Regulatory Mechanism of Melatonin on Tomato Seed Germination Under Cd²⁺ Stress[J]. Chinese Bulletin of Botany, 2023, 58(5): 720-732.

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URL: https://www.chinbullbotany.com/EN/10.11983/CBB22202

https://www.chinbullbotany.com/EN/Y2023/V58/I5/720

Figures/Tables 11

Figure1 Effect of cadmium concentrations on seed germination and seedling growth of tomato (A) Germination phenotypes of tomato seeds 3 days after germination(bars=1 cm); (B) Germination rate of tomato seeds 3 days after germination; (C) Phenotypes of tomato seedlings 5 days after germination (bar=1 cm); (D) Seedling length of tomato 5 days after germination. Different lowercase letters indicate significant differences among different treatments (P<0.05).

Figure 2 Effects of melatonin (MT) on tomato seed germination under Cd2+ stress (A) Germination phenotypes of seeds 3 days after germination(bars=1 cm); (B) Germination rate; (C) Phenotypes of tomato seedlings 5 days after germination (bar=1 cm); (D) Seedling length. Different lowercase letters indicate significant differences among different treatments (P<0.05).

Figure 3 Effects of melatonin (MT) treatment on endogenous melatonin metabolism during tomato seed germination under Cd2+ stress (A) MT content; (B) Tryptophan decarboxylase (TDC) activity; (C) Tryptamine-5-hydroxylase (T5H) activity; (D) 5-serotonin-N-acetyltransferae (SNAT) activity; (E) N-acetyl-5-serotonin-methyltransferase (ASMT) activity; (F) Caffeicacid-O-me- thyltransferase (COMT) activity. Different lowercase letters and * indicate significant differences among different treatments (P<0.05). Control, Cd2+, and Cd2+/MT are the same as shown in Figure 2.

Table 1 Cd2+ accumulation of tomato seeds during germination under Cd2+ stress

Treatment	Cd²⁺content in underground parts (mg·kg^-1 DW)		Cd²⁺ content in above-ground parts (mg·kg^-1 DW)
Treatment	3 d	5 d	3 d	5d
Control	0.074±0.004 c	0.098±0.012 c	0.034±0.005 b	0.047±0.003 c
Cd²⁺	0.138±0.011 a	0.176±0.009 a	0.041±0.006 a	0.068±0.004 a
Cd²⁺/MT	0.097±0.007 b	0.126±0.007 b	0.042±0.005 a	0.054±0.007 b

Figure 4 Effects of melatonin (MT) on the gene expression of Cd2+ uptake, transport and chelating metabolic enzymes in tomato seed germination under Cd2+ stress Control, Cd2+, and Cd2+/MT are the same as shown in Figure 2.

Table 2 Effects of melatonin (MT) on the accumulation of H2O2, O2-. and malonaldehyde (MDA) in tomato seed germination under Cd2+ stress

Treatment	H₂O₂(μmol·g^-1 FW)	O₂^-.(μmol·g^-1 FW)	MDA (μmol·g^-1 FW)
Control	13.24±0.714 c	2.43±0.154 b	6.26±0.514 c
Cd²⁺	25.37±1.325 a	4.79±0.328 a	10.13±0.758 a
Cd²⁺/MT	20.13±1.628 b	4.13±0.376 a	8.24±0.811 b

Table 3 Effects of melatonin (MT) on antioxidant enzyme activities during tomato seed germination under Cd2+ stress

Treatment	SOD (U·g^-1 FW)	POD (U·g^-1 FW)	CAT (U·g^-1 FW)	APX (U·g^-1 FW)	ALDH (U·g^-1 FW)
Control	2.13±0.175 b	0.105±0.078 b	3.57±0.295 c	0.274±0.037 c	1.37±0.116 c
Cd²⁺	2.74±0.213 a	0.157±0.024 a	4.19±0.182 b	0.357±0.042 b	2.17±0.133 b
Cd²⁺/MT	2.63±0.146 a	0.142±0.021 a	5.12±0.411 a	0.408±0.026 a	2.58±0.179 a

Figure 5 Effects of melatonin (MT) on the expression of antioxidant enzyme genes in tomato seed germination under Cd2+ stress Control, Cd2+, and Cd2+/MT are the same as shown in Figure 2.

Figure 6 Effects of melatonin (MT) on gibberellin acid (GA) metabolism during tomato seed germination under Cd2+stress (A) GA content; (B) GA-3-oxidase (GA3ox) activity; (C) GA-20-oxidase (GA20ox) activity; (D) GA-2-oxidase (GA2ox) activity. Different lowercase letters and * indicate significant differences at 0.05 level. Control, Cd2+, and Cd2+/MT are the same as shown in Figure 2.

Figure 7 Effects of melatonin (MT) on abscisic acid (ABA) metabolism during tomato seed germination under Cd2+stress (A) Abscisic acid content; (B) 9-cis-epoxycarotenoid dioxygenase (NCED) activity; (C) Zeaxanthin epoxidase (ZEP) activity; (D) ABA-aldehyde oxidase (AAO) activity; (E) ABA-8-hydroxylase (ABA8ox) activity. Different lowercase letters and * indicate significant differences at 0.05 level. Control, Cd2+, and Cd2+/MT are the same as shown in Figure 2.

Figure 8 Effects of melatonin (MT) on gene expression of gibberellin (GA) and abscisic acid (ABA) metabolism Control, Cd2+, and Cd2+/MT are the same as shown in Figure 2.

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Regulatory Mechanism of Melatonin on Tomato Seed Germination Under Cd²⁺ Stress

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