Functional Study Of The Key PPR591 Gene In Seed Development In Maize

Seed development is closely related to crop yield.Analysis of key genes and mechanism of plant seed development is the basis of molecular breeding for high-yield crops.Maize(Zea mays L.),as the main source of food,feed and industrial raw materials,is the world’s largest crop in China and the world.In scientific research,maize has a long history as a model plant in various scientific discoveries.Mitochondria provide energy and metabolites for the cell.The maize mitochondrial genome encodes a total of 58 genes,whose correct expression is crucial to mitochondrial function and cell activity.The expression of mitochondrial genes usually requires extensive RNA processing that includes intron splicing,RNA editing,RNA terminal processing,etc.These processes require a large number of nuclear-coded proteins.Defects in RNA post-transcriptional processing can easily lead to dysfunction of mitochondria,which ultimately affects embryogenesis and endosperm development.Therefore,investigation of the molecular mechanism of RNA post-transcriptional processing in mitochondria is essential to elucidate the mechanism of mitochondrial gene expression and seed development.The defect of RNA post-processing usually leads to embryo lethality and the lack of homozygous materials.In addition,due to the technical limitations of the transformation of mitochondrial genome by exogenous genes,it is difficult to directly study the molecular mechanism of mitochondrial RNA post-processing.With the advanced genome sequencing of maize and the construction of various maize mutant libraries,it has become feasible for elucidation of the molecular mechanism of maize seed development.Maize is an ideal material for studying the processing of mitochondrial RNA because of its large mutants and synchronous development,which makes it easier to extract homozygous materials from heterozygotes.Recent studies have found that a large number of pentatricopeptide repeat(PPR)proteins encoded by nuclear genes are involved in the regulation of mitochondrial post-transcriptional processing.We identified ppr591 mutant from maize UniformMu mutant library.PPR591 encodes a P-type PPR protein which is located in mitochondria.PPR591 mutation resulted in the arrested embryogenesis and endosperm development.The PPR591 mutation is closely linked to the phenotype of the ppr591 mutant.By analyzing the expression of mitochondrial coding genes,intron splicing and editing sites,it was found that the mutation of PPR591 caused the expression of nad2,nad6 and apt6 in mitochondria,but did not participate in intron splicing and editing.The 5’-and 3’-end of nad2 and nad6 were studied.It was found that the transcripts of nad2 and nad6 at the 3’end of the mutant and the 5’-end of the wild-type were significantly reduced.We further analyzed the 5’-end in detail and determined its specific processing sites.We infer that the increased expression of nad2 and nad6 is due to the feedback effect caused by the abnormal processing of the 5’-terminus,and the increase of atp6 is due to the co-transcription of nad6 and atp6.The mutation of PPR591 results in the loss of activity of mitochondrial respiratory chain complex I and the failure to assemble properly.Alternate oxidation pathways are thus activated and AOX accumulates in large quantities.These results suggest that PPR591 is essential for the maturation of the 5’-end of mitochondrial nad2 and nad6 transcription,and is essential for the function of mitochondria and the development of maize seeds.Up to now,little is known about the molecular mechanism and proteins involved in the 5’ terminal processing of mitochondrial RNA in higher plants.The elucidation of the function of PPR591 provides a favorable direction for the further study of the processing of the 5’ terminal of mitochondrial RNA.PPR591 is a P-type PPR protein,there is no additional domain at its end,and the protein itself has no enzymatic activity,we speculate that PPR591 binds to the leading sequence of nad2 and nad6,possibly by recruiting other nucleases with endonuclease activity to mediate the breakage of the 5’-end of RNA.