Tolerance Mechanism Of Different Hot Pepper Cultivars To Cadmium Stress

Hot pepper?Capsicum annuum L.?is a popular vegetable and condiment,the cultivation of hot pepper has become a pillar industry in Jiangxi,Guizhou and Hunan provinces in China.However,in recent years,since Cd pollution in soil is becoming more and more serious,the quality and safety of pepper and its products have been seriously affected.The most economical and effective way to reduce Cd content of food is to reduce Cd content of edible parts through selecting and breeding cultivars with low Cd accumulation.Identification of key genes related to low Cd accumulation has guiding significance for the safe production of hot pepper.On the basis of previous research,this work selected high Cd accumulation cultivar Luojiao 318?Cow-horn pepper,purchased from Chengyan seed co.LTD,Luoyang?,medium Cd accumulation cultivar Daguo 99?Cow-horn pepper,purchased from Xiangyan seed co.LTD,Hunan?and low Cd accumulation cultivar X55?Line pepper,provided by vegetable and flower research institute of Academy of Agricultural Sciences,Chongqing?.Pot experiments were carried out to investigate the influence of different Cd levels(0,5,10 mg·kg^-1)on the growth,physiological effects,Cd absorption,transfer and bioconcentration,expression of genes related to Cd accumulation and tolerance in three cultivars of hot pepper?Luojiao 318,Daguo 99 and X55?,which could explore the molecular mechanism of low Cd accumulation in hot pepper,and obtained key genes related to Cd accumulation/tolerance process,so as to provide important genetic resources for the improvement of hot pepper cultivars.The main results are summarized as follows:1.Cd accumulation in hot pepper was expressed as vegetative organs>reproductive organ,the extent of damage showed as root>shoot,X55 was a precious genotype with high resistance to Cd and low accumulation of Cd in fruit.Under the same Cd level,Cd content in root,stem,leaf and fruit among three cultivars was in the order of X55>Daguo 99>Luojiao 318(Except stems under 10 mg·kg^-1 Cd and fruits under 5 mg·kg^-1 Cd).Under the same Cd level,the Cd accumulation of root was expressed as X55>Daguo 99>Luojiao 318,while the Cd accumulation of fruit was in the order of Daguo 99>Luojiao 318>X55.Under the same Cd level,Cd transfer factor?TF?of fruit was in the order of Luojiao 318>Daguo 99>X55 but Cd bioconcentration factor?BCF?of fruit showed no significant difference among three cultivars.High Cd level(10 mg·kg^-1)could significantly inhibit the growth of three cultivars of hot pepper,the inhibitory effect of growth of X55 on root and shoot was smaller than that of Luojiao 318 and Daguo 99.According to Cd content,Cd accumulation amount,Cd transfer and bioconcentration factor of fruit and biomass of hot pepper,X55 had the strongest ability to absorb Cd by roots,the lowest accumulation of Cd in fruit,the worst ability to transfer Cd to fruits,X55 was a precious genotype with high resistance to Cd and low accumulation of Cd in fruit.2.Membrane lipid peroxidation of pepper was induced by Cd stress,X55 had a strong ability to resist membrane lipid peroxidation.Malondialdehyde?MDA?content was in the order of Daguo 99>Luojiao 318>X55 under the same Cd level,the activity of peroxidase?POD?in root and leaf showed X55>Luojiao 318>Daguo 99,the activity of superoxide dismutase?SOD?showed no significant difference among three cultivars,the activity of catalase?CAT?showed X55>Luojiao318>Daguo 99.According to MDA content and the activity of antioxidant enzymes of hot pepper,X55 had a stronger tolerance to Cd.3.Photosynthetic rate and transpiration rate and stomatal conductance of hot pepper were generally decreased under the stress of Cd,but X55 was more resistant to Cd.Under different Cd levels,the content of chlorophyll a,chlorophyll b and carotenoid were all expressed as X55>Luojiao 318>Daguo 99,the net photosynthetic rate?Pn?,transpiration rate?Tr?and stomatal conductance?Gs?of hot pepper all showed X55>Luojiao 318>Daguo 99.According to the chlorophyll content and photosynthesis parameters of three cultivars,X55 was more resistant to Cd.4.The cell wall of hot pepper played an important role in the compartmentalization and resistance of Cd,the main chemical form of Cd in fruits was Cd_NaCl.Under different Cd levels,the Cd concentrations of subcellular fractions in root,stem,leaf and fruit of three cultivars all showed cell wall fraction?F1?>organelle fraction?F2?>soluble fraction?F3?,and the Cd concentrations of subcellular fractions in fruit was expressed as Daguo 99>Luojiao 318>X55,the main chemical form of Cd in fruits was Cd_NaCl.This indicated that hot pepper had its own unique heavy metal detoxification mechanism,and its cell wall played an important role in the compartmentalization and resistance of Cd.5.In weak acid conditions,the more acid soil became,the more active Cd would be,certainly the more toxic it was to plant.Under different Cd levels,the main chemical form of Cd in soil was EXC-Cd.Under the same Cd level,EXC-Cd of X55 was significantly lower than that of Luojiao 318 and Daguo 99.Under different Cd levels,Soil pH showed X55>Luojiao 318>Daguo99,the content of both effective Cd and whole Cd showed Daguo 99>Luojiao 318>X55.This indicated that in weak acid conditions,the extent of soil acidification was negatively correlated with the activity of Cd,the more acid soil became,and the more active Cd would be.6.FTP(Fe²⁺transport protein)/ZIP?Zinc and iron regulated transporter proteins?,HMA?Heavy metal ATPase?,NRAMP?Natural resistance-associated macrophage proteins?and PCS?Phytochelatin synthase?genes had different divisions of labor in Cd accumulation and tolerance mechanism,and NRAMP was a key site for Cd accumulation of hot pepper fruits.The expression of FTP1-2 and FTP1-3 genes in all three pepper cultivars were expressed as root>stem>fruit under different Cd levels,which indicated that both FTP1-2 and FTP1-3 genes were involved in the absorption of Cd by the root of three cultivars.Under the same Cd level,the expression of HMA1 and HMA2 genes of all three cultivars in stems were significantly more than roots and fruits,which suggested that HMA1 and HMA2 genes were involved in the transport of Cd from root to the shoot.Under the same Cd level,the expression of NRAMP1 gene?Except X55?and NRAMP3 gene in the stems of the three cultivars was significantly higher than that in the root and fruit,which showed that NRAMP1 gene and NRAMP3 gene mainly participated in the transportation of Cd of three cultivars from root to shoot.The expression of NRAMP2,NRAMP5 and NRAMP6genes in the root of three cultivars were the highest,indicated that NRAMP2,NRAMP5 and NRAMP6 genes were mainly involved in the Cd absorption process of three cultivars.The expression of Under the same Cd level,the expression of NRAMP2,NRAMP3 and NRAMP6 showed Daguo 99>Luojiao 318>X55 in the root,stem and fruit of different cultivars,meanwhile,X55 had the lowest accumulation of Cd in fruit,which indicated that NRAMP2,NRAMP3 and NRAMP6genes were the key genes mediating the low accumulation of Cd in fruits.Under the same Cd level,the expression of PCS gene in root,stem and fruit of all three cultivars was in the order of X55>Luojiao 318>Daguo 99,and showed root>stem>fruit in all three cultivars,which indicated that PCS genes may had induced the synthesis of PC in root,and root of X55 had a strong synthesis ability of PC.