Small Kernel1Encodes A PPR Protein Required For Mitochondrial Nad7Transcript Editing And Seed Development In Maize

Grain yield will have to be improved to feed the increasing globalpopulation. The seeds of maize and rice are main grains. Grain yield dependsmainly on seed size. Seed development play a important role in the life cycle offlowering plant. In plant, RNA editing modifies cytidines (C) to uridines (U) atspecific sites in the transcripts of mitochondria and plastids, altering the amino acidspecified by the DNA sequence. RNA editing is essential for posttranscriptionalregulation in mitochondria and plastids. The PLS subfamily of PRR proteins arerequired for RNA editing and plant growth and development.Here, we selected small kernel1(smk1) mutant from22smk mutants fromUniformMu population based on co-segregation analysis, using mutatortransposon tagging by DNA blot. And then, we identified a critical editing factor ofmitochondrial nad7transcript via molecular characterization of smk1mutant in maize.Mutations in Smk1arrest both the embryo and endosperm development.Cloning ofSmk1by mutator transposon tagging indicates that it encodes an E-subclasspentatricopeptide repeat (PPR) protein that is targeted to mitochondria.Comparativeanalysis of the mitochondrial transcripts revealed that loss of SMK1function abolishesthe C-to-U editing at the nad7-836site, leading to retention of a proline codon that isedited to encode leucine in the wild-type.The smk1mutant showed dramatically reducedcomplex I assembly and NADH dehydrogenase activity and abnormal biogenesis of themitochondria.Analysis of the ortholog in rice (Oryza sativa) reveals that rice SMK1hasa conserved function in C-to-U editing of the mitochondrial nad7-836site.T-DNAknockout mutants showed abnormal embryo and endosperm development, resulting inembryo or seedling lethality.The leucine at NAD7-279is highly conserved frombacteria to flowering plants, and analysis of genome sequences from many plantsrevealed a molecular coevolution between the requirement for C-to-U editing at this siteand the existence of a SMK1homolog.These results demonstrate that Smk1encodes a PPR-E protein that is required fornad7-836editing, and this editing is critical to NAD7function in complex I assembly inmitochondria and hence to embryo and endosperm development in maize and rice.