| Thirty percent of the arable land of the world belongs to calcareous alkaline soils prone to iron(Fe)deficiency,and approximately 40% of the soil grows crops with serious yield losses due to Fe deficiency.On the other hand,the increasingly serious cadmium(Cd)pollution of soil seriously restricts the sustainable development of agriculture and food production security.Improving micronutrients and reducing the accumulation of toxic heavy metals by using modern biotechnology has always been a hot topic in botanical research.Nanobiotics to enhance crop stress resistance are a safe and sustainable strategy to increase crop yield.Zinc oxide nanoparticles(Zn O NPs)function in improving crop stress resistance has attracted wide attention,with the application of Zn O NPs as a slow release factor of fertilizer in agricultural production.Our previous studies showed that Zn O NPs treatment significantly induced Fe-responsive gene expression in Arabidopsis thaliana and promoted Fe accumulation in plants.In addition,the expression patterns of many metal transporter genes also changed.These results indicate that Zn O NPs affect the absorption,transport and accumulation of mineral elements by plants.Tomato is sensitive to Fe deficiency and easily accumulates Cd,so it is an ideal material to study the mechanism of Fe deficiency and Cd tolerance in vegetable crops.In this study,we used Micro-Tom(Solanum lycopersicum L.cv.Micro-Tom)as material to explore the plant growth mediated by Zn O NPs and the physiological and molecular mechanism by which Zn O NPs alleviate the plant response to Fe deficiency stress and Cd toxicity.The main results of this research are as follows:1.Morphological analysis showed that foliar spraying with Zn O NPs significantly increased tomato seedling height(8.9%),root length(11.9%),chlorophyll content(9.2%),photosynthetic efficiency parameter Y(II)(18.4%)and plastoquinone redox state parameter1-q L(26.3%).Thus,the growth of tomato seedlings is promoted.2.High-throughput sequencing technology was used to identify differentially expressed genes(DEGs)in the roots and leaves of tomato seedlings sprayed with Zn O NPs.Compared with the control,808 genes were upregulated and 103 genes were downregulated in the leaves after foliar spraying with Zn O NPs.In the roots,395 genes were upregulated,and 1127 genes were downregulated.Further analysis showed that foliar spraying with Zn O NPs significantly induced the expression of genes related to mineral element uptake,transport and accumulation,such as Nramp5-like,IRT1 and IRT2,and thus increased the accumulation level of mineral elements in the plants.3.Gene Ontology(GO)biological process analysis revealed that genes related to reactive oxygen species(ROS)metabolism and antioxidant activity were significantly enriched after foliar spraying with Zn O NPs,which affected ROS metabolism and stress resistance in tomato seedlings.4.Kyoto Encyclopedia of Genes and Genomes(KEGG)pathway enrichment analysis revealed that genes related to keratin,lignan and wax synthesis,flavonoid synthesis and carbohydrate metabolism pathways were significantly enriched after foliar spraying with Zn O NPs.Physiological analysis showed that the levels of soluble sugars and free amino acids in tomato seedlings were generally increased after Zn O NPs were sprayed on the leaves.These results indicate that Zn O NPs regulate primary and secondary metabolism.5.Fe deficiency inhibited the growth of tomato seedlings.Foliar spraying with Zn O NPs significantly increased the plant height(25.4%)and root length(26.8%)of Fe-deficient seedlings,effectively alleviated the reduction in chlorophyll content caused by Fe deficiency,and improved the photosynthetic efficiency under Fe deficiency stress.Fe deficiency also induced oxidative damage in tomato seedlings.Foliar spraying with Zn O NPs significantly decreased the levels of H2O2,malondialdehyde(MDA)and relative electrical conductivity(RC)induced by Fe deficiency stress and increased the activities of superoxide dismutase(SOD),catalase(CAT),peroxidase(POD)and ascorbate peroxidase(APX).Therefore,the oxidative damage caused by Fe deficiency stress can be effectively alleviated.Foliar spraying with Zn O NPs increased the accumulation of Fe,Zn,Ca and other mineral elements in the leaves and roots of tomato seedlings under Fe deficiency stress and thus improved the mineral nutrition status of tomato seedlings under Fe deficiency stress.Leaf spraying Zn O NPs also significantly increased the contents of sucrose,starch,glucose and amino acids in the leaves and roots of Fe-deficient tomato seedlings.6.Transcriptome analysis showed that 796 genes were upregulated and 296 genes were downregulated in Fe-deficient tomato leaves compared with control leaves.In the roots,826 genes were upregulated,and 629 genes were downregulated.Compared with Fe-deficient seedlings,there were 775 upregulated genes(83.9%)and 149 downregulated genes(16.1%)in leaves and 602 upregulated genes(58.1%)and 435 downregulated genes(41.9%)in roots after foliar spraying with Zn O NPs.Further analysis showed that foliar spraying with Zn O NPs induced Zn/Fe transporter gene expression in tomato leaves,such as VIT4-like,Nramp2-like,ZIP-like,and YSL7,compared with Fe-deficient seedlings.The expression of the Zn/Fe transporter genes IRT1,IRT2,Nramp3-like,and Nramp5-like and the Fe response transcription factor b HLH100 was induced in roots.In addition,foliar spraying with Zn O NPs induced the expression of genes involved in glucose metabolism,amino acid metabolism and secondary metabolism in Fe-deficient tomato leaves,as well as the genes of aquaporin TIP3-2-like,TIP2-1-like,PIP-type p TOM75 and TIP1-1,and C/N metabolism-related gene expression in roots.These results indicate that foliar spraying with Zn O NPs can improve the growth and development of tomato seedlings under Fe deficiency stress by regulating the expression of metal transporter genes,core metabolic pathways and secondary metabolic pathways and improving the absorption and accumulation of mineral elements and the synthesis of sugars and amino acids.7.Cd toxicity significantly inhibited the growth and development of tomato seedlings.Under Cd toxicity,Zn O NPs sprayed on the foliage significantly increased the height(20.9%),fresh weight(52.6%)and dry weight(56.9%)of tomato seedlings.The chlorophyll content was increased(14.4%),and the photosynthetic efficiency was improved.Foliar spraying with Zn O NPs significantly reduced the oxidative damage caused by Cd toxicity and decreased the content of MDA(30%),thus alleviating the inhibitory effect of Cd toxicity on the growth of tomato seedlings.ICP?MS analysis showed that foliar spraying with Zn O NPs significantly decreased the content of Cd in the roots(40.2%)and leaves(34.5%)of tomato seedlings.The contents of Mn(267.8%)and Zn(33.9%)in the roots and Cu(13%),Mn(60.8%)and Zn(78.6%)in the leaves increased,which improved the mineral nutrition status of tomato seedlings under Cd toxicity.8.We analyzed the effects of foliar spraying with Zn O NPs on metabolites in the leaves and roots of tomato seedlings under Cd toxicity using extensively targeted metabolomics techniques.The results showed that compared with the control,36 differentially accumulated metabolites(DAMs)in tomato leaves were upregulated under Cd toxicity,while 46 DAMs were downregulated.In addition,29 DAMs were upregulated,and 108 DAMs were downregulated in roots.Compared with the control,25 DAMs were upregulated and 37 DAMs were downregulated in tomato leaves after foliar spraying with Zn O NPs.Nineteen DAMs were upregulated,and 45 DAMs were downregulated in the roots.Compared with Cd toxicity,17 DAMs were upregulated and 17 DAMs were downregulated in tomato leaves after foliar spraying with Zn O NPs.Thirty DAMs were upregulated,and 28 DAMs were downregulated in roots.Further analysis showed that compared with the control,the contents of seven flavonoids,four alkaloids,eight phenylpropyl compounds,three amino acid derivatives and eight nucleotide derivatives were increased in Cd toxicity tomato leaves.The contents of 12 flavonoids,seven alkaloids,16 phenylpropanoids,three amino acid derivatives and four nucleotide derivatives were increased in roots.Compared with Cd toxicity,foliar spraying with Zn O NPs significantly reduced the contents of nine flavonoids,five alkaloids,three phenylpropyl,three amino acid derivatives and five nucleotide derivatives in tomato leaves.It reduced the contents of nine flavonoids,five alkaloids,three phenylpropyl,one amino acid derivative and six nucleotide derivatives in the roots.Correlation analysis between the physiological phenotype and metabolome showed that the growth parameters,including plant height,leaf fresh weight,leaf dry weight,and chlorophyll content,were negatively correlated with nucleotide derivatives.Primary root length,root fresh weight and dry weight were negatively correlated with flavonoids and phenylpropyl.KEGG pathway enrichment analysis revealed that compared with Cd toxicity,foliar spraying with Zn O NPs reduced adenosine 5’-monophosphate levels in leaves of Cd toxicity tomato seedlings and reduced the accumulation levels of flavonoids and alkaloids in roots.We also found that flavonoids and alkaloid metabolites may play an important role in alleviating Cd toxicity by Zn O NPs.In conclusion,this study revealed that(1)Zn O NPs improved the growth of tomato seedlings by regulating the absorption of mineral elements,core metabolism and secondary metabolism.(2)Zn O NPs can reduce the oxidative damage of plants caused by Fe deficiency,increase the content of mineral elements,induce the expression of antioxidant enzyme genes,transporter genes,and genes involved in primary and secondary metabolism,and thus improve the antioxidant capacity,mineral element content,sugar and amino acid content,and the level of secondary metabolites of Fe-deficient seedlings.Thus,the tolerance of seedlings to Fe deficiency stress was improved.(3)Foliar spraying with Zn O NPs effectively reduced Cd accumulation in tomato seedlings,alleviated oxidative damage,improved absorption of trace elements,and reduced metabolic fluctuations caused by Cd toxicity,thus alleviating growth inhibition of tomato seedlings caused by Cd toxicity.The results of this study reveal the physiological and molecular mechanisms by which Zn O NPs regulate the plant abiotic stress response and provide a theoretical basis for improving crop growth and reducing Cd accumulation by Zn O NPs.This study has important research significance for expanding the application of Zn O NPs in agricultural production,improving land use efficiency and ensuring food production security. |
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