Molecular And Physiological Mechanisms Of Aluminum-Tolerance And Sulfur-Mediated-Alleviation Of Aluminum-Toxicity In Citrus

Aluminum?Al?is the most abundant metal element on the earth.It exists as soluble and available Al³⁺ in acid soil?pH<5.0?and is the main growth limiting factor.Al toxicity inhibits root and shoot growth,damages plant photosynthesis and antioxidant systems,and affects mineral nutrient uptake.Plants mainly resist Al toxicity through external and internal detoxification of Al.Supply of exogenous sulfur?S?and and the upregulation of S metabolism confer plant Al-tolerance.In China,Citrus are mainly grown in low pH soils with high active Al affected easily by Al-toxicity.Thus,study on the molecular and physiological mechanisms of Al-tolerance and the regulation of Al-toxicity in Citrus has important theoretical and practical significance for the cultivation of Citrus.To my best knowledge,data available on Citrus Al-tolerance and-toxicity are rare.Most of research in model plants has focused on the Al-induced secretion of organic acid?OA?anion and antioxidant system,little information is are available on the effects of Al-toxicity on plant S metabolism and the S-mediated alleviation of Al-toxicity.Seedlings of Al-tolerant 'Xuegan'?Citrus sinensis?and Al-intolerant 'Shatian pummelo'?Citrus grandis?were grown in pots containing clean river sand and were treated for 18 weeks.Thereafter,the author investigated?1?the physiological and biochemical responses of C.grandis and C.sinensis seedlings to Al-toxicity;?2?the responses of transcriptome to Al-toxicity in roots and leaves of C.sinensis and C.grandis,and?3?the S-mediated-alleviation of Al-toxicity in C.grandis.The objectives of this study were to understand the Al-tolerance mechanisms of Citrus plants at physiological,biochemical and and molecular levels,and the regulation of Al-toxicity by S.1 The physiological and biochemical responses of C.sinensis and C.grandis seedlings to Al-toxicityC.sinensis and C.grandis seedlings were fertilized for 18 weeks with nutrient solution containing 0 mM?control?or 1 mM?Al toxicity?AlCl6·6H₂O.The Al-induced decreases of biomass,leaf photosynthesis,relative water content?RWC?and total soluble protein levels in roots and leaves,and increases of methylglyoxal?MG?levels in roots and leaves only occurred in C.grandis.Besides,the Al-induced decreases of pigments and alterations of chlorophyll?Ch1?a fluorescence?OJIP?transients and fluorescence parameters were greater in C.grandis leaves than those in C.sinensis leaves.Al-treated C.grandis had higher stem and leaf Al levels and similar root Al levels relative to Al-treated C.sinensis,but lower Al distribution in roots and Al-uptake per plant.Al toxicity decreased nitrogen?N?,phosphorus?P?,potassium?K?,calcium?Ca?,magnesium?Mg?and sulfur?S?uptake per plant in C.grandis and C.sinensis seedlings with the exception of Al-treated C.sinensis seedlings exhibiting increased S uptake per plant and unaltered Mg uptake per plant.Under Al-stress,macroelement uptake per plant was higher in C.sinensis than that in C.grandis.Al toxicity decreased the ratios of reduced glutathione?GSH?/[GSH + oxidized glutathione?GSSG?and of ASC/[ASC + dehydroascorbate?DHA?]only in C.grandis roots and leaves.The activities of most antioxidant enzymes,S metabolism-related enzymes and glyoxalases and the concentrations of S-containing compounds were higher in Al-treated C.sinensis roots and leaves than those in Al-treated C.grandis ones.Under Al-treatment,C.sinensis roots secreted more citrate and malate than C.grandis ones.To conclude,C.sinensis displayed higher Al tolerance than C.grandis.The higher Al tolerance of C.sinensis might involve:?a?more Al accumulation in roots and less transport of Al from roots to shoots;?b?efficient maintenance of nutrient?Ca,P and Mg?homeostasis;Al-treated C.sinensis seedlings displayed a higher level of S in roots and leaves possibly due to increased uptake and decreased export of S;?c?efficient maintenance of redox homeostasis via detoxification systems of reactive oxygen species?ROS?and MG;and?d?Al-treated C.sinensis seedlings had a higher external Al detoxification capacity via enhanced Al-induced secretion of organic acid?OA?anions in roots.2 The responses of transcriptome to Al-toxicity in C.sinensis and C.grandis roots C.sinensis and C.grandis seedlings were fertilized for 18 weeks with nutrient solution containing 0 mM?control?or 1 mM?Al toxicity?AlCl₃·6H₂O.Using RNA-seq,we isolated 1905 and 2670 differentially expressed genes?DEGs?from Al-treated C.sinensis than C.grandis roots,respectively.Among these DEGs,only 649 DEGs were shared by the two species.Further analysis suggested that the following several aspects conferred C.sinensis higher Al-tolerance:?a?cell wall and cytoskeleton metabolism,energy and carbohydrate metabolism and signal transduction displayed higher adaptative responses to Al in C.sinensis than in C.grandis roots;and?b?more upregulated than downregulated genes related to fatty acid and amino acid metabolisms were isolated from Al-treated C.sinensis roots,but the reverse was the case for Al-treated C.grandis roots.3 The responses of transcriptome to Al-toxicity in C.sinensis and C.grandis leavesC.sinensis and C.grandis seedlings were fertilized for 18 weeks with nutrient solution containing 0 mM?control?or 1 mM?Al toxicity?AlCl₃·6H₂O.By sequencing the transcriptome from leaves with or without Al,we obtained much more Al-responsive genes from Al-treated C.grandis leaves than those from Al-treated C.sinensis leaves,indicating that gene expression profiles were less affected in the latter,which agrees with the results that C.sinensis had higher Al-tolerance than did C.grandis.Functional analysis showed that the following several aspects might account for the higher Al-tolerance of C.sinensis,including:?a?Al-treated C.sinensis leaves had higher capacity to maintain the homeostasis of energy,the stability of lipid composition and the integrity of cell wall than did Al-treated C.grandis leaves;?b?Al-triggered production of ROS and the other cytotoxic compounds was less in Al-treated C.sinensis leaves than that in Al-treated C.grandis leaves;?c?Al-treated C.sinensis leaves could keep a better balance between protein phosphorylation and dephosphorylation than did Al-treated C.grandis leaves;and?d?both the equilibrium of hormones and hormone-mediated signal transduction were greatly disrupted in Al-treated C.grandis leaves,but less altered in Al-treated C.sinensis leaves.Finally,we discussed the differences in Al-responsive genes between Citrus roots and leaves.4 S-mediated-alleviation of Al-toxicity in C.grandisC.grandis seedlings were irrigated for 18 weeks with 0.5 mM MgSO₄ or 0.5 mM MgSO₄ + 0.5 mM Na₂SO₄,and 0?-Al?or 1 mM AICl3·6H₂O?+Al,Al-toxicity?.Under Al-toxicity,S decreased the level of Al in leaves;increased root and leaf RWC,P,Ca and Mg uptake per plant,root and shoot dry weight?DW?,root DW/shoot DW ratio,and the Al-induced secretion of citrate from root;and alleviated the Al-induced inhibition of photosynthesis via mitigating the Al-induced decrease of electron transport capacity resulting from the impaired photosynthetic electron transport chain.In addition to decreasing the Al-stimulated H₂O2 production,the S-induced upregulation of both S metabolism-related enzymes and antioxidant enzymes also contributed to the S-mediated-alleviation of oxidative damage in Al-treated roots and leaves.Decreased transport of Al from roots to shoots and relatively little accumulation of Al in leaves,and increased leaf and root RWC and P,Ca,and Mg uptake per plant might also play a role in the S-mediated-alleviation of Al-toxicity.