Two Pentatricopeptide Repeat Proteins Are Required for the Splicing of nad5 Introns in Maize.

Huanhuan Yang,Feng Sun,Le Wang,Bao-Cai Tan,Zhihui Xiu,Xiulan Li,Shi-Kai Cao

doi:10.3389/fpls.2020.00732

Abstract

Mitochondrial genes in flowering plants contain predominantly group II introns that require precise splicing before translation into functional proteins. Splicing of these introns is facilitated by various nucleus-encoded splicing factors. Due to lethality of mutants, functions of many splicing factors have not been revealed. Here, we report the function of two P-type PPR proteins PPR101 and PPR231, and their role in maize seed development. PPR101 and PPR231 are targeted to mitochondria. Null mutation of PPR101 and PPR231 arrests embryo and endosperm development, generating empty pericarp and small kernel phenotype, respectively, in maize. Loss-of-function in PPR101 abolishes the splicing of nad5 intron 2, and reduces the splicing of nad5 intron 1. Loss-of-function in PPR231 reduces the splicing of nad5 introns 1, 2, 3 and nad2 intron 3. The absence of Nad5 protein eliminates assembly of complex I, and activates the expression of alternative oxidase AOX2. These results indicate that both PPR101 and PPR231 are required for mitochondrial nad5 introns 1 and 2 splicing, while PPR231 is also required for nad5 intron 3 and nad2 intron 3. Both genes are essential to complex I assembly, mitochondrial function, and maize seed development. This work reveals that the splicing of a single intron involves multiple PPRs.

Highlights

Mitochondria are highly metabolically active organelles in eukaryotic cells that perform multiple cellular functions, including basic energy supply and redox regulation, ion transmembrane transport, and metabolic pathways integration (Sweetlove et al, 2007)
We further prove that PPR101 and PPR231 are essential to mitochondrial complex I biogenesis and seed development in maize
PPR101 and PPR231 Are P-Type pentatricopeptide repeat (PPR) Proteins Targeted to Mitochondria

Summary

Introduction

Mitochondria are highly metabolically active organelles in eukaryotic cells that perform multiple cellular functions, including basic energy supply and redox regulation, ion transmembrane transport, and metabolic pathways integration (Sweetlove et al, 2007). Mitochondria are derived from ancient α-proteobacteria via endosymbiosis and the majority of bacterial genes are either transferred to the host nuclear genome or lost during evolution (Martin and Herrmann, 1998). The present mitochondria in higher plants host only 5% of the genes encoding proteins essential for mitochondrial biogenesis and functions (Gray et al, 1999; Martin et al, 2012). The maize mitochondrial genome contains 58 genes encoding 22 proteins for respiratory chain, 9 ribosomal proteins (RP), a transporter (MttB), a maturase (Mat-r), 3 rRNAs (rrn, rrn, rrn26), and 21 tRNAs (for 14 amino acids) (Clifton et al, 2004). Expression of the mitochondrial genes is strictly regulated by the nuclear genome, at post-transcriptional level which includes intron splicing, RNA editing, RNA cleavage, RNA stabilization and translation (Fujii and Small, 2011; Barkan and Small, 2014).

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