Pentatricopepetide repeat (PPR) proteins are a large family of RNA-binding proteins involved in RNA meta-bolism in plant organelles.Although many PPR proteins have been functionally studied,few of them are identified with a function in mitochondrial RNA stability.By using a reverse genetic approach,we characterized the role of the mitochondrion-targeted PPR78 protein in nad5 mature mRNA stability and maize (Zea mays) seed development.Loss of PPR78 function leads to a dramatic reduction in the steady-state level of mitochondrial nad5 mature mRNA,blocks the assembly of complex I in the electron transport chain,and causes an arrest in embryogenesis and endosperm development.Characterization of a second strong allele confirms the function of PPR78 in nad5 mRNA accumulation and maize seed development.The generation of mature nad5 requires the assembly of three distinct precursor RNAs via trans-splicing reactions,and the accumulation of nad5T1 precursor is reduced in the ppr78 mutants.However,it is the instability of mature nad5 rather than nad5T1 causing loss of the full-length nad5 transcript,and degradation of nad5 losing both translation start and stop codons is enriched in the mutant.Our data imply the assembly of mature nad5 mRNA precedes the protection of PPR78.