| Mining and metallurgy of uranium,decommissioning of nuclear facilities,mining and metallurgy of metals and nonmetals afford large amounts of radionuclides(e.g.,uranium,U)and related heavy metals(e.g.,cadmium,Cd),which have become the main source of environmental pollution,especially soil,water radioactivity,and heavy-metal pollution.Meanwhile,the migration behavior of U and Cd at the soil–plant interface and the dual toxic effects of biological radioactivity and heavy metals have always been the focus of crop edible safety evaluation and environmental risk control.Storage roots of tuberous root(stem)plants such as medicinal plants and energy crops,which are important food crops,are in direct contact with the soil and suffer from a high risk of radioactive and heavy-metal contamination.However,the uptake mechanism and transport differences,as well as the physiological response of U and Cd in the tuberous root(stem)plants,are still unclear.In this study,different accumulation types of sweet potatoes(?Violet?purple sweet potatoes,?Sushu 8?sweet potatoes)were used as the starting materials,and U,Cd,and U+Cd treatments were set up.In this study,various environmental omics technologies such as transcriptomics,metabolomics,and ionomics,as well as microbial diversity,etc.,were employed.In this study,the following key scientific problems need to be clarified:(1)The uptake,transport,and micro-area distribution of U and Cd in the soil–tuberous root plant system need to be investigated;(2)The physiological response of tuberous root plants to U and Cd stress needs to be revealed;and(3)Key factors affecting and regulating the bioavailability of U and Cd need to be identified.The main results were as follows:1.At different growth periods,the tuberous roots of the two sweet potatoes were the main accumulating organs(90%of the whole plant)for U/Cd(U,100 mg·kg-1g;Cd,15 mg·kg-1).After growing the sweet potato for 120 days,the accumulation characteristics of Cd in various organs and tissues decreased in the order of lateral root>tuber bark>stem>tuber storage tissue>leaf.The U accumulation characteristics decreased in the order of lateral root>tuber bark>stem>leaf>tuber storage tissue.The accumulation amount of Cd in the tuber storage tissue of sweet potato was 0.80–1.17 mg·kg-1,and the pollution index(PI)was 1.01–1.93(light pollution,NY 861-2004,?0.2 mg·kg-1).However,the accumulation amount of U in the tuber storage tissue of sweet potato was 1.10–1.97 mg·kg-1,which was less than the food limit standard(?1.9 mg·kg-1,GB 14882-1994).The results revealed that the storage tuber of sweet potato is in direct contact with the contaminated soil,bearing the highest risk of being contaminated by soil radionuclides and heavy metals,but the accumulated risk of radionuclides and heavy metals in edible parts is low under growing in U/Cd contaminated soil.2.Significant differences between the ionome of two sweet potatoes at different growth times and different organs under U/Cd treatment(P<0.05)were noted.After U/Cd treatment,differences in the mineral metabolism and heavy-metal accumulation of sweet potatoes were noted.At the lateral root–tuber bark–tuber storage tissue interface,the absorption,transport,and accumulation processes of U and Cd by sweet potato were non-selective in nature.The difference in the ionomes caused by the difference in species(relative content of mineral elements decreased in the order of purple sweet potato>sweet potato)was one of the key factors that affect the accumulation of U and Cd.By the addition of U and Cd,the content of available U and Cd in the rhizosphere soil of the two sweet potatoes increased by 3.65–4.58 and2.33–3.75 times,respectively(P<0.01).The available content of U/Cd and competitive absorption of mineral elements in the soil directly led to the difference in the accumulation of U and Cd in sweet potato.Rhizomic analysis revealed that U and Cd treatments cause differences in the content of primary metabolites(such as carbohydrates,amino acids,and nucleotides,etc.)in the rhizosphere soil of sweet potato.3.U and Cd led to the abnormal growth of roots,photosynthetic metabolism,and mineral metabolism disorders of sweet potato under hydroponic conditions.Pathological section analysis reveals that U mainly destroys the edge cell layer of sweet potato roots,and U and Cd induced root-cell ultrastructure damage.Based on the subcellular compartment model,the cell wall was the most important accumulation site for U,and 71%–80%of U was blocked in the cell wall of the roots.Cd was mainly distributed in the cell wall(48%–55%),vacuoles(27%–39%),and other detoxification areas of sweet potato,alleviating the damage of Cd to the organelles.The analysis of the element occurrence forms reveals that residues(85%–87%)and hydrochloric acid extracts(12%–14%)are the main chemical forms in sweet potato roots;however,Na Cl extracts(34%–53%)and acetic acid extracts(14%–17%)were the main chemical forms of Cd in sweet potato roots.As the main accumulation site of U and Cd in sweet potato roots,the cell wall prevented the entry of U and Cd into the cytoplasm.U was chelated to the cell wall and existed in an insoluble form,making its transfer into sweet potato roots difficult.4.Non-targeted metabolomics analysis revealed that 4,625 and 4,865 metabolites are identified in the two sweet potato roots.Based on the analysis of the orthogonal partial least squares discriminant analysis(OPLS-DA)model,under U treatment,643and 191 differential metabolites(DEMs)were identified in purple sweet potato and common sweet potato,respectively;under Cd treatment,526 and 426 DEMs were identified in the corresponding sweet potatoes;under U+Cd treatment,634 and 463DEMs were identified in the two sweet potatoes,respectively.c AMP and c GMP,etc.,were the key signal sensing and transduction factors in response to U and Cd in sweet potato.Plant growth regulators such as plant hormones and their derivatives(e.g.,indole acetic acid analogs,zeatin,salicylic acid,and abscisic acid)were involved in the signal transduction pathway of U and Cd.At the metabolic level,the phytotoxic mechanisms of U and Cd mainly occurred by the induction of metabolic abnormalities,including the photosynthesis and tricarboxylic acid cycle(TCA),thereby affecting the abnormal primary metabolic pathways such as purine and pyrimidine synthesis,nitrogen assimilation,and amino acid metabolism.5.RNA-seq analysis reveals that by U and Cd treatments,the two differential genes of sweet potato were significantly enriched in ribosome synthesis and photosynthetic carbon metabolism pathways(P<0.05).In the glycolysis pathway,phosphoenol-pyruvate carboxylase kinase(PPCK),fructose pyrophosphate-6-phosphotransferase,phosphoenolpyruvate carboxylase(PEPC),and 3-phospho-glycerate kinase.The PGK gene was significantly down-regulated(P<0.05),thereby reducing the conversion efficiency of the carbon source of the hexose phosphate pool into triose phosphate.In the TCA cycle pathway,U and Cd significantly inhibited the expression of the cyt MDH gene and inhibited the decomposition of malate into oxaloacetate.In the mitochondrial oxidative phosphorylation pathway,U and Cd significantly inhibited the expression of ATP synthase(F1-ATPase)gene,leading to the cell ATP synthesis disorder and abnormal cell energy metabolism.This corresponded to the molecular mechanism of root necrosis of sweet potato caused by U and Cd.In the auxin signal transduction pathway,U and Cd induced the disorder expression of key transcription factors(e.g.,auxin influx carrier(AUX1),auxin response factor(ARF),auxin-responsive protein IAA(AUX/IAA),auxin responsive GH3 gene family(GH3),and SAUR family protein(SAUR),which was one of the internal mechanisms leading to the abnormal growth of sweet potato.The number of up-regulated genes related to inorganic ion transporters(e.g.,the divalent cation transporter,ABC transporter family,and gated channel family)in purple potato roots(up-regulated 156)was significantly greater than that of sweet potato(up-regulated68).The results revealed that purple sweet potato roots exhibit high absorption and transport capacities for U,Cd,and mineral elements,which was one of the reasons for the high accumulation of U and Cd in purple sweet potato.6.The physiological response mechanism of sweet potato to risk avoidance under U/Cd stress is described as follows:(1)At the rhizosphere soil–sweet potato root interface,the microbial communities tolerant to U and Cd pollution remolded the metabolism of rhizosphere soil to change its bioavailability and reduce the absorption of U and Cd in the sweet potato.(2)The water and mineral element absorption ascending network formed by leaf transpiration pull was relatively independent of the photosynthetic storage descending network,which was the physiological mechanism of the difference in accumulation of U and Cd in each part of sweet potato.(3)At the lateral root–outer epidermis–tuber storage tissue interface,the physiological barrier that blocks the transport of U and Cd was the holding effect of the root cell wall,thereby making the transfer of U and Cd to the central pillar difficult and subsequently blocking their transportation to,and storage in,the tuber storage tissue.(4)The synthesis of stress signal molecules such as c AMP,c GMP,plant hormones,and other metabolites,and the expression of key genes such as AUX1,AUC/IAA,GH3 and other stress genes at the molecular level were the mechanisms of sweet potato response to U and Cd toxicity. |
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