The DEAD-box RNA helicase ZmRH48 is required for the splicing of multiple mitochondrial introns,mitochondrial complex biosynthesis,and seed development in maize

RNA helicases participate in nearly all aspects of RNA metabolism by rearranging RNAs or RNA–protein complexes in an adenosine triphosphatedependent manner.Due to the large RNA helicase families in plants,the precise roles of many RNA helicases in plant physiology and development remain to be clarified.Here,we show that mutations in maize(Zea mays)DEAD-box RNA helicase48(Zm RH48)impair the splicing of mitochondrial introns,mitochondrial complex biosynthesis,and seed development.Loss of Zm RH48 function severely arrested embryogenesis and endosperm development,leading to defective kernel formation.Zm RH48 is targeted to mitochondria,where its deficiency dramatically reduced the splicing efficiency of five cis-introns(nad5 intron 1;nad7 introns 1,2,and 3;and ccm Fc intron 1)and one trans-intron(nad2 intron 2),leading to lower levels of mitochondrial complexes I andⅢ.Zm RH48 interacts with two unique pentatricopeptide repeat(PPR)proteins,PPR-SMR1 and SPR2,which are required for the splicing of over half of all mitochondrial introns.PPR-SMR1 interacts with SPR2,and both proteins interact with P-type PPR proteins and Zm-m CSF1 to facilitate intron splicing.These results suggest that Zm RH48 is likely a component of a splicing complex and is critical for mitochondrial complex biosynthesis and seed development.

SMALL KERNEL4 is required for mitochondrial cox1 transcript editing and seed development in maize

In land plants,cytidine-to-uridine(C-to-U)editing of organellar transcripts is an important posttranscriptional process,which is considered to remediate DNA genetic mutations to restore the coding of functional proteins.Pentatricopeptide repeat(PPR)proteins have key roles in C-to-U editing.Owing to its large number,however,the biological functions of many PPR proteins remain to be identified.Through characterizing a small kernel4(smk4)mutant,here we report the function of Smk4 and its role in maize growth and development.Null mutation of Smk4 slows plant growth and development,causing small plants,delayed flowering time,and small kernels.Cloning revealed that Smk4 encodes a new E-subclass PPR protein,and localization indicated that SMK4 is exclusively localized in mitochondria.Loss of Smk4 function abolishes C-to-U editing at position 1489 of the cytochrome c oxidase1(cox1)transcript,causing an amino acid change from serine to proline at 497 in Cox1.Cox1 is a core component of mitochondrial complex IV.Indeed,complex IV activity is reduced in the smk4,along with drastically elevated expression of alternative oxidases(AOX).These results indicate that SMK4 functions in the C-to-U editing of cox1-1489,and this editing is crucial for mitochondrial complex IV activity,plant growth,and kernel development in maize.