Functional analysis of SLO2 provides new insight into the role of plant PPR proteins

The pentatricopeptide repeat protein GEND1 is required for root development and high temperature tolerance in Arabidopsis thaliana

Pentatricopeptide repeat (PPR) proteins are a large family in land plants that play a role in organellular RNA processing, editing, and splicing. Here, we identify an Arabidopsis thaliana mutant, gend1-1, which exhibits a short root phenotype with reduced meristem size and cell numbers. Positional cloning of GEND1 revealed that it encodes a PPR protein, and functional analysis showed that GEND1 can bind and edit mitochondrial ccmFn-1 mRNA, causing gend1 mutants to have decreased levels of cytochrome C. GEND1 was up-regulated by high temperature conditions, to which gend1 mutants were hypersensitive. Analysis of a set of PPR mutants under high temperature showed that mutants with defects in cytochrome C had comparable temperature sensitivity to gend1. Collectively, these results suggest that cytochrome C plays an important role in root development and high temperature response in Arabidopsis.

The PPR protein SLOW GROWTH 4 is involved in editing of nad4 and affects the splicing of nad2 intron 1

SLO4 is a mitochondrial PPR protein that is involved in editing nad4, possibly required for the efficient splicing of nad2 intron1. Pentatricopeptide repeat (PPR) proteins constitute a large protein family in flowering plants and are thought to be mostly involved in organellar RNA metabolism. The subgroup of PLS-type PPR proteins were found to be the main specificity factors of cytidine to uridine RNA editing. Identifying the targets of PLS-type PPR proteins can help in elucidating the molecular function of proteins encoded in the organellar genomes. In this study, plants lacking the SLOW GROWTH 4 PPR protein were characterized. Slo4 mutants were characterized as having restricted root growth, being late flowering and displaying an overall delayed growth phenotype. Protein levels and activity of mitochondrial complex I were decreased and putative complex I assembly intermediates accumulated in the mutant plants. An editing defect, leading to an amino acid change, in the mitochondrial nad4 transcript, encoding for a complex I subunit, was identified. Furthermore, the splicing efficiency of the first intron of nad2, encoding for another complex I subunit, was also decreased. The change in splicing efficiency could however not be linked to any editing defects in the nad2 transcript.

Expansion and Function of Repeat Domain Proteins During Stress and Development in Plants

The recurrent repeats having conserved stretches of amino acids exists across all domains of life. Subsequent repetition of single sequence motif and the number and length of the minimal repeating motifs are essential characteristics innate to these proteins. The proteins with tandem peptide repeats are essential for providing surface to mediate protein-protein interactions for fundamental biological functions. Plants are enriched in tandem repeat containing proteins typically distributed into various families. This has been assumed that the occurrence of multigene repeats families in plants enable them to cope up with adverse environmental conditions and allow them to rapidly acclimatize to these conditions. The evolution, structure, and function of repeat proteins have been studied in all kingdoms of life. The presence of repeat proteins is particularly profuse in multicellular organisms in comparison to prokaryotes. The precipitous expansion of repeat proteins in plants is presumed to be through internal tandem duplications. Several repeat protein gene families have been identified in plants. Such as Armadillo (ARM), Ankyrin (ANK), HEAT, Kelch-like repeats, Tetratricopeptide (TPR), Leucine rich repeats (LRR), WD40, and Pentatricopeptide repeats (PPR). The structure and functions of these repeat proteins have been extensively studied in plants suggesting a critical role of these repeating peptides in plant cell physiology, stress and development. In this review, we illustrate the structural, functional, and evolutionary prospects of prolific repeat proteins in plants.

The Pentatricopeptide Repeat Proteins TANG2 and ORGANELLE TRANSCRIPT PROCESSING439 Are Involved in the Splicing of the Multipartite nad5 Transcript Encoding a Subunit of Mitochondrial Complex I

Pentatricopeptide repeat proteins constitute a large family of RNA-binding proteins in higher plants (around 450 genes in Arabidopsis [Arabidopsis thaliana]), mostly targeted to chloroplasts and mitochondria. Many of them are involved in organelle posttranscriptional processes, in a very specific manner. Splicing is necessary to remove the group II introns, which interrupt the coding sequences of several genes encoding components of the mitochondrial respiratory chain. The nad5 gene is fragmented in five exons, belonging to three distinct transcription units. Its maturation requires two cis- and two trans-splicing events. These steps need to be performed in a very precise order to generate a functional transcript. Here, we characterize two pentatricopeptide repeat proteins, ORGANELLE TRANSCRIPT PROCESSING439 and TANG2, and show that they are involved in the removal of nad5 introns 2 and 3, respectively. To our knowledge, they are the first two specific nad5 splicing factors found in plants so far.

The Arabidopsis thaliana RNA editing factor SLO2, which affects the mitochondrial electron transport chain, participates in multiple stress and hormone responses

Recently, we reported that the novel mitochondrial RNA editing factor SLO2 is essential for mitochondrial electron transport, and vital for plant growth through regulation of carbon and energy metabolism. Here, we show that mutation in SLO2 causes hypersensitivity to ABA and insensitivity to ethylene, suggesting a link with stress responses. Indeed, slo2 mutants are hypersensitive to salt and osmotic stress during the germination stage, while adult plants show increased drought and salt tolerance. Moreover, slo2 mutants are more susceptible to Botrytis cinerea infection. An increased expression of nuclear-encoded stress-responsive genes, as well as mitochondrial-encoded NAD genes of complex I and genes of the alternative respiratory pathway, was observed in slo2 mutants, further enhanced by ABA treatment. In addition, H2O2 accumulation and altered amino acid levels were recorded in slo2 mutants. We conclude that SLO2 is required for plant sensitivity to ABA, ethylene, biotic, and abiotic stress. Although two stress-related RNA editing factors were reported very recently, this study demonstrates a unique role of SLO2, and further supports a link between mitochondrial RNA editing events and stress response.

Surrogate mutants for studying mitochondrially encoded functions

Although chloroplast transformation is possible in some plant species, it is extremely difficult to create or select mutations in plant mitochondrial genomes, explaining why few genetic studies of mitochondrially encoded functions exist. In recent years it has become clear that many nuclear genes encode factors with key functions in organelle gene expression, and that often their action is restricted to a single organelle gene or transcript. Mutations in one of these nuclear genes thus leads to a specific primary defect in expression of a single organelle gene, and the nuclear mutation can be used as a surrogate for a phenotypically equivalent mutation in the organelle genome. These surrogate mutations often result in defective assembly of respiratory complexes, and lead to severe phenotypes including reduced growth and fertility, or even embryo-lethality. A wide collection of such mutants is now available, and this review summarises the progress in basic knowledge of mitochondrial biogenesis they have contributed to and points out areas where this resource has not been exploited yet.