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Mitochondria execute key pathways of central metabolism and serve as cellular sensing and signaling entities - functions that depend upon interactions between mitochondrial and nuclear genetic systems. This is exemplified in cytoplasmic male sterility type S (CMS-S) of Zea mays, where novel mitochondrial open reading frames are associated with a po...
Citations
... Nearly all RF-related PPRs are members of the P subclass except for PPR13 in sorghum, however, in most cases, lack or mutation in these PPR genes have no influences on growth in normal cytoplasm. Therefore, it is considered as a gain-of-function mutations for Rf-related genes (Gabay-Laughnan et al., 2018). P-family RF proteins are usually devoid of RNA binding sites, thus they often recruit additional components to form a restoration complex (Gillman et al., 2007;Hu et al., 2012;Huang et al., 2015). ...
A fast evolution within mitochondria genome(s) often generates discords between nuclear and mitochondria, which is manifested as cytoplasmic male sterility (CMS) and fertility restoration ( Rf ) system. The maize CMS‐C trait is regulated by the chimeric mitochondrial gene, atp6c , and can be recovered by the restorer gene ZmRf5 . Through positional cloning in this study, we identified the nuclear restorer gene, ZmRf5 , which encodes a P‐type pentatricopeptide repeat (PPR) family protein. The over‐expression of ZmRf5 brought back the fertility to CMS‐C plants, whereas its genomic editing by CRISPR/Cas9 induced abortive pollens in the restorer line. ZmRF5 is sorted to mitochondria, and recruited RS31A, a splicing factor, through MORF8 to form a cleaving/restoring complex, which promoted the cleaving of the CMS‐associated transcripts atp6c by shifting the major cleavage site from 480th nt to 344 th nt for fast degradation, and preserved just right amount of atp6c RNA for protein translation, providing adequate ATP6C to assembly complex V, thus restoring male fertility. Interestingly, ATP6C in the sterile line CMo17A, with similar cytology and physiology changes to YU87‐1A, was accumulated much less than it in NMo17B, exhibiting a contrary trend in the YU87‐1 nuclear genome previously reported, and was restored to normal level in the presence of ZmRF5. Collectively these findings unveil a new molecular mechanism underlying fertility restoration by which ZmRF5 cooperates with MORF8 and RS31A to restore CMS‐C fertility in maize, complemented and perfected the sterility mechanism, and enrich the perspectives on communications between nucleus and mitochondria.
... It is not yet known whether the translational or post-translational controls that limit accumulation of these respiratory proteins also regulate accumulation of mitochondria-encoded ORF355 such that endogenous ORF355 accumulates at only at the BCP stage, or whether ORF355 accumulates in CMS-S MSP but cannot effect pollen collapse without concomitant mitochondrial events. CMS-S maize is unique in that it can be reversed by numerous nuclear restorer-of-fertility mutations, many of which compromise pollen mitochondrial gene expression [86]. The effects of these mutations on both the accumulation of mitochondria-encoded ORF355 and the phenotype of transgenic plants expressing mitochondria-targeted ORF355 could be useful in dissecting the relative independence of this protein as an executor of PCD in developing maize pollen. ...
Background
Cytoplasmic male sterility (CMS) is a maternally inherited failure to produce functional pollen that most commonly results from expression of novel, chimeric mitochondrial genes. In Zea mays, cytoplasmic male sterility type S (CMS-S) is characterized by the collapse of immature, bi-cellular pollen. Molecular and cellular features of developing CMS-S and normal (N) cytoplasm pollen were compared to determine the role of mitochondria in these differing developmental fates.
Results
Terminal deoxynucleotidyl transferase dUTP nick end labeling revealed both chromatin and nuclear fragmentation in the collapsed CMS-S pollen, demonstrating a programmed cell death (PCD) event sharing morphological features with mitochondria-signaled apoptosis in animals. Maize plants expressing mitochondria-targeted green fluorescent protein (GFP) demonstrated dynamic changes in mitochondrial morphology and association with actin filaments through the course of N-cytoplasm pollen development, whereas mitochondrial targeting of GFP was lost and actin filaments were disorganized in developing CMS-S pollen. Immunoblotting revealed significant developmental regulation of mitochondrial biogenesis in both CMS-S and N mito-types. Nuclear and mitochondrial genome encoded components of the cytochrome respiratory pathway and ATP synthase were of low abundance at the microspore stage, but microspores accumulated abundant nuclear-encoded alternative oxidase (AOX). Cytochrome pathway and ATP synthase components accumulated whereas AOX levels declined during the maturation of N bi-cellular pollen. Increased abundance of cytochrome pathway components and declining AOX also characterized collapsed CMS-S pollen. The accumulation and robust RNA editing of mitochondrial transcripts implicated translational or post-translational control for the developmentally regulated accumulation of mitochondria-encoded proteins in both mito-types.
Conclusions
CMS-S pollen collapse is a PCD event coincident with developmentally programmed mitochondrial events including the accumulation of mitochondrial respiratory proteins and declining protection against mitochondrial generation of reactive oxygen species.
Background
Cytoplasmic male sterility (CMS) is a trait of economic importance in the production of hybrid seeds. In CMS-S maize, exerted anthers appear frequently in florets of field-grown female populations where only complete male-sterile plants were expected. It has been reported that these reversions are associated with the loss of sterility-conferring regions or other rearrangements in the mitochondrial genome. However, the relationship between mitochondrial function and sterility stability is largely unknown.
Results
In this study, we determined the ratio of plants carrying exerted anthers in the population of two CMS-S subtypes. The subtype with a high ratio of exerted anthers was designated as CMS-Sa, and the other with low ratio was designated as CMS-Sb. Through next-generation sequencing, we assembled and compared mitochondrial genomes of two CMS-S subtypes. Phylogenetic analyses revealed strong similarities between the two mitochondrial genomes. The sterility-associated regions, S plasmids, and terminal inverted repeats (TIRs) were intact in both genomes. The two subtypes maintained high transcript levels of the sterility gene orf355 in anther tissue. Most of the functional genes/proteins were identical at the nucleotide sequence and amino acid sequence levels in the two subtypes, except for NADH dehydrogenase subunit 1 (nad1). In the mitochondrial genome of CMS-Sb, a 3.3-kilobase sequence containing nad1-exon1 was absent from the second copy of the 17-kb repeat region. Consequently, we detected two copies of nad1-exon1 in CMS-Sa, but only one copy in CMS-Sb. During pollen development, nad1 transcription and mitochondrial biogenesis were induced in anthers of CMS-Sa, but not in those of CMS-Sb. We suggest that the impaired mitochondrial function in the anthers of CMS-Sb is associated with its more stable sterility.
Conclusions
Comprehensive analyses revealed diversity in terms of the copy number of the mitochondrial gene nad1-exon1 between two subtypes of CMS-S maize. This difference in copy number affected the transcript levels of nad1 and mitochondrial biogenesis in anther tissue, and affected the reversion rate of CMS-S maize. The results of this study suggest the involvement of mitochondrial robustness in modulation of sterility stability in CMS-S maize.
Pentatricopeptide repeat (PPR) proteins form a large family of proteins targeted to organelles, where they post‐transcriptionally modulate gene expression through binding to specific RNA sequences. Among them, the mitochondria‐targeted restorer‐of‐fertility (Rf) PPRs inhibit peculiar mitochondrial genes that are detrimental to male gametes and cause cytoplasmic male sterility (CMS). Here, we revealed three nuclear loci involved in CMS in a cross between two distant Arabidopsis thaliana strains, Sha and Cvi‐0. We identified the causal gene at one of these loci as RFL24, a conserved gene encoding a PPR protein related to known Rf PPRs. By analysing fertile revertants obtained in a male sterile background, we demonstrate that RFL24 promotes pollen abortion, in contrast with the previously described Rf PPRs, which allow pollen to survive in the presence of a sterilizing cytoplasm. We show that the sterility caused by the RFL24 Cvi‐0 allele results from higher expression of the gene during early pollen development. Finally, we predict a binding site for RFL24 upstream of two mitochondrial genes, the CMS gene and the important gene cob. These results suggest that the conservation of RFL24 is linked to a primary role of ensuring a proper functioning of mitochondria, and that it was subsequently diverted by the CMS gene to its benefit.
Supporting Information
Coordination between mitochondria and nucleus is crucial for fertility determination in cytoplasmic male sterility (CMS) plant. Using yeast one-hybrid screening, we identified a transcription factor ZmDREB1.7 that is highly expressed in sterile microspores at the large vacuole stage and activates CMS gene orf355 expression in mitochondria. Δpro, a weak allele of ZmDREB1.7 promoter due to the loss of key UPR motif, partially restores male fertility of CMS-S maize. ZmDREB1.7 expression increases rapidly in response to antimycin A treatment, but this response is attenuated in the present of Δpro allele. We further show that expression of orf355 in mitochondria activates mitochondrial retrograde signal, which in turn induces ZmDREB1.7 expression. These findings demonstrate a positive feedback transcriptional regulation between nuclear regulator and mitochondrial CMS gene in male sterility determination and provide insight into nucleus-mitochondria communication in plants.
