Low PH-induced Alterations Of Proteome And Related Physiological Parameters In Citrus Leaves

Soil acidity is a major factor limiting crop growth and productivity,with approximately 30%of the world's ice-free land and 12%of crops affected by soil acidity.Citrus,which belong to evergreen tropical and subtropical fruit tree,are mainly cultivated in the acidic and strong acidic soils of southern China,in which they are often affected by soil acidity.Seedlings of 'Sour pummelo'(Citrus grandis)and 'Xuegan'(Citrus sinensis)were irrigated daily with nutrient solution at a pH of 2.5,3,or 6(control).C.sinensis seedlings were slightly more tolerant to low pH than C.grandis seedlings.Nine months after pH treatments,two-dimensional electrophoresis(2-DE)based proteome and related physiological parameters in leaves were investigated in order to reveal the adaptive mechanisms of Citrus tolerated low pH(H+)toxicity at the proteomic and physiological levels,and provide a scientific basis for the high yield and quality cultivation of citrus.The main results are as follows:1.C.sinensis and C.grandis seedling growth was greatly inhibited only at pH 2.5,many fibrous roots became black and rotten.Also,a few leaves became bleached in pH 2.5-treated C.grandis seedlings,and that early shedding of basal leaves occurred in a portion of pH 2.5-treated C.sinensis seedlings.No seedlings were killed for the two Citrus species.CO2 assimilation,stomatal conductance and transpiration rate were greatly decreased in pH 2.5-treated C.grandis and C.sinensis leaves,but they were unaffected in pH 3-treated ones.2.The concentrations of glucose,fructose,sucrose,total soluble sugars,starch,and total nonstructural carbohydrates(TNC)in C.grandis and C.sinensis leaves were elevated at pH 2.5.The only exception was that sucrose concentration in C.sinensis leaves did not change significantly in response to pH.The concentrations of ascorbate(ASC)+ hydroascorbate(DHA)and ASC,and the ratio of ASC/(ASC +DHA)in C.grandis and C.sinensis leaves were decreased at pH 2.5,but they were unaltered significantly at pH 3.Malonaldehyde(MDA)concentration in C.grandis and C.sinensis leaves increased as pH decreased from 6 to 2.5.The pH 2.5-induced increases in the concentrations of carbohydrates and MDA,and decreases,in the concentrations of ASC + DHA and ASC and in the ratio of ASC/(ASC+ DHA)ratio were more pronounced in C.grandis leaves than those in C.sinensis leaves.This might be related to the slightly higher acid-tolerance of C.sinensis seedlings relative to C.grandis seedlings.3.Using 2-DE based mass spectrometry(MS)approach,this study successfully identified 49 and 44 differentially abundant proteins(DAP)from pH 2.5 and pH 3-treated C.sinensis and C.grandis leaves,respectively.These DAP were mainly associated with carbohydrate and energy metabolism,antioxidation and detoxification,stress response,protein and amino acid metabolism,lipid metabolism,cellular transport,signal transduction and nucleic acid metabolism.4.pH 2.5 treatment decreased the abundances of proteins involved in ribulose bisphosphate carboxylase/oxygenase(Rubisco)activation,Calvin cycle,carbon fixation,chlorophyll biosynthesis and electron transport,hence lowering chlorophyll level,photosynthetic electron transport rate(ETR)and photosynthesis.The abundances of proteins related to jasmonic acid(JA)biosynthesis and signal transduction were increased and decreased in pH 2.5-treated C.sinensis and C.grandis leaves,respectively.In conclusion,we found that the higher oxidative damage(MDA level)in pH 2.5-treated C.sinensis and C.grandis leaves might be due to a combination of factors including higher production of reactive oxygen species(ROS),lower ASC level,and more proteins decreased in abundance involved in antioxidation and detoxification.This study revealed the preliminary causes for the tolerance of Citrus seedlings to low pH(H+),and provided a scientific basis for further stduy on the responses of high plants to low pH.