Mechanisms Of A Arabidopsis Photorespiratory Mutant Sgat-1 With Increased Branches

Photorespiration is a metablic process in which green plants obsorb O₂and release CO₂under light conditions.This process runs through mitochondria,chloroplasts,peroxisomes and cytoplasm.It interwaves with various metabolisms in plants to form a cross-linking metabolic network.It has been reported that photorespiration could regulate photosynthesis,maintain cell redox state,affect nitrogen metabolism,etc.However there are few reports on whether photorespiration affects and how to affect auxin metabolism.Recently a mutant with typical photorespiratory phenotype was identified,which also showed auxin-deficient phenotype.The mechanisms of the mutant showing dwarf and more branches were analyzed in this study.Main results were described as follow:(1)Photorespiratory mutants sgat-1,sgat-2,mtkas and fsd2 were identifiedIn this study,variants 30-6,30-7,30-8 and 34-10 with photorespiratory phenotype were identified from the Arabidopsis thaliana EMS mutagenesis mutant library using high to low CO₂comparison methods.The results of map cloning,genome resequencing and genetic complementation experiment showed that the photorespiration phenotype of30-6 and 30-7 mutants was caused by the mutation in SGAT(Serine:glycerate aminotransferase).A single-nucleotide C-to-T transition at the 49th position in SGAT of30-6 mutants,which caused a Pro to Ser change at 17th position,SGAT activity in the leaves of 30-6 mutants was reduced to about 30%of that of wild type.C-to-T transition at 71th position of SGAT of 30-7 resulted in a Gly to Asp at 71th position,and SGAT activity was almost lost in the leaves of 30-6 mutants,indicating that Gly at the 71th position is required for maintaining the activity of SGAT.The photorespiration phenotype of 30-8 is due to the mutation of mt KAS(?-ketoacyl-acyl carrier protein synthase),which affected the acylation of the H subunit of GDC(Glycine decarboxylase complex)protein.G-to-A transition at 50th position of FSD2(Fe superoxide dismutase 2)of 34-10 resulted in a Trp at 260th to stop code.Therefore,the above mutants were named sgat-1,sgat-2,mtkas and fsd2,respectively.(2)Defective mutation in SGAT resulted in dwarf,etiolation and increasing branches in Arabidopsis thaliana.sgat-1 mutants can survive in the atmospheric condition,however,the appearance of the true leaves is postponed,mutant plants were dwarf and etiolated,had a significantly higher number of branches than wild type plants,and unable to properly fructify.The phenotype of sgat-1mutants showed no significant difference compared with wild type plants grown in a 3000 ppm CO₂enviroment.The SGAT activity was restored,even higher than of wild type,in the leaves of the complementary lines p SGAT:SGAT/sgat-1.In addition,the leaf color,plant height and branches of the complemented plants also exhibited no significant difference with that of wild type.Taken together,a Pro changed to Ser at the 17th of SGAT is responsible for the phenotypes displayed in sgat-1 mutants.(3)The increase in branches of sgat-1 dwarf mutants may be due to the changes in IAA level by interfering redox state.Exogenous application of IBA or NAA,the relative root length of sgat-1 mutants grown in the atmosphere was lower than that of Col-0 and its complementary lines.On the other hand,sgat-1,its complementary lines and wild type plants grown in 3000 ppm CO₂environment had a similar relative root length.These results indicated that sgat-1 mutants were more sensitive to IBA and NAA in the atmosphere,which may be related to the accumulation of certain products in the photorespiratory pathway.The content of IAA in root tips,leaf margins and flowers of sgat-1 mutants grown in the atmosphere was significantly lower than that of wild type and its genetic complementary lines assayed by DR5::GUS staining.However,the IAA content in the root tip of sgat-1 increased after the addition of Ser to the meditium,addtionally,the exogenous application of Ser did not increase the number of branches of Col-0.These data implied that the accumulation of Ser may not be the reason for the increase in branches of sgat-1 mutants.DAB and NBT staining showed that the accumulation of H₂O₂and O₂.^-increased significantly in the leaves of sgat-1 mutants,but the catalase(CAT)activity in the leaves of sgat-1 mutants was higher than that of Col-0 and its genetic complementary lines,and the glycolate oxidase(GLO)activity of sgat-1 mutants was lower than that of Col-0.Since the accumulation of H₂O₂can affect the synthesis of auxin by inhibiting the expression of genes involved in auxin synthesis.Thus the increase in branch of sgat-1 mutants may be due to the accumulation in H₂O₂and O₂.^-affecting the IAA level.