Repairing TALEN-mediated double-strand break by microhomology-mediated recombination in tobacco plastids generates abundant subgenomic DNA

Genome Editing of Plant Mitochondrial and Chloroplast Genomes

Plastids (including chloroplasts) and mitochondria are remnants of endosymbiotic bacteria, yet they maintain their own genomes, which encode vital components for photosynthesis and respiration, respectively. Organellar genomes have distinctive features, such as being present as multicopies, being mostly inherited maternally, having characteristic genomic structures and undergoing frequent homologous recombination. To date, it has proven to be challenging to modify these genomes. For example, while CRISPR/Cas9 is a widely used system for editing nuclear genes, it has not yet been successfully applied to organellar genomes. Recently, however, precise gene-editing technologies have been successfully applied to organellar genomes. Protein-based enzymes, especially transcription activator-like effector nucleases (TALENs) and artificial enzymes utilizing DNA-binding domains of TALENs (TALEs), have been successfully used to modify these genomes by harnessing organellar-targeting signals. This short review introduces and discusses the use of targeted nucleases and base editors in organellar genomes, their effects and their potential applications in plant science and breeding.

TALE-based organellar genome editing and gene expression in plants

KEY MESSAGE

TALE-based editors provide an alternative way to engineer the organellar genomes in plants. We update and discuss the most recent developments of TALE-based organellar genome editing in plants. Gene editing tools have been widely used to modify the nuclear genomes of plants for various basic research and biotechnological applications. The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 editing platform is the most commonly used technique because of its ease of use, fast speed, and low cost; however, it encounters difficulty when being delivered to plant organelles for gene editing. In contrast, protein-based editing technologies, such as transcription activator-like effector (TALE)-based tools, could be easily delivered, expressed, and targeted to organelles in plants via Agrobacteria-mediated nuclear transformation. Therefore, TALE-based editors provide an alternative way to engineer the organellar genomes in plants since the conventional chloroplast transformation method encounters technical challenges and is limited to certain species, and the direct transformation of mitochondria in higher plants is not yet possible. In this review, we update and discuss the most recent developments of TALE-based organellar genome editing in plants.

Base editing of organellar DNA with programmable deaminases

Mitochondria and chloroplasts are organelles that include their own genomes, which encode key genes for ATP production and carbon dioxide fixation, respectively. Mutations in mitochondrial DNA can cause diverse genetic disorders and are also linked to ageing and age-related diseases, including cancer. Targeted editing of organellar DNA should be useful for studying organellar genes and developing novel therapeutics, but it has been hindered by lack of efficient tools in living cells. Recently, CRISPR-free, protein-only base editors, such as double-stranded DNA deaminase toxin A-derived cytosine base editors (DdCBEs) and adenine base editors (ABEs), have been developed, which enable targeted organellar DNA editing in human cell lines, animals and plants. In this Review, we present programmable deaminases developed for base editing of organellar DNA in vitro and discuss mitochondrial DNA editing in animals, and plastid genome (plastome) editing in plants. We also discuss precision and efficiency limitations of these tools and propose improvements for therapeutic, agricultural and environmental applications.

Characterization and development of a plastid genome base editor, ptpTALECD

The modification of photosynthesis-related genes in plastid genomes may improve crop yields. Recently, we reported that a plastid-targeting base editor named ptpTALECD, in which a cytidine deaminase DddA functions as the catalytic domain, can homoplasmically substitute a targeted C to T in plastid genomes of Arabidopsis thaliana. However, some target Cs were not substituted. In addition, although ptpTALECD could substitute Cs on the 3' side of T and A, it was unclear whether it could also substitute Cs on the 3' side of G and C. In this study, we identified the preferential positions of the substituted Cs in ptpTALECD-targeting sequences in the Arabidopsis plastid genome. We also found that ptpTALECD could substitute Cs on the 3' side of all four bases in plastid genomes of Arabidopsis. More recently, a base editor containing an improved version of DddA (DddA11) was reported to substitute Cs more efficiently, and to substitute Cs on the 3' side of more varieties of bases in human mitochondrial genomes than a base editor containing DddA. Here, we also show that ptpTALECD_v2, in which a modified version of DddA11 functions as the catalytic domain, more frequently substituted Cs than ptpTALECD in the Arabidopsis plastid genome. We also found that ptpTALECD_v2 tended to substitute Cs at more positions than ptpTALECD. Our results reveal that ptpTALECD can cause a greater variety of codon changes and amino acid substitutions than previously thought, and that ptpTALECD and ptpTALECD_v2 are useful tools for the targeted base editing of plastid genomes.

DNA base editing in nuclear and organellar genomes

Genome editing continues to revolutionize biological research. Due to its simplicity and flexibility, CRISPR/Cas-based editing has become the preferred technology in most systems. Cas nucleases tolerate fusion to large protein domains, thus allowing combination of their DNA recognition properties with new enzymatic activities. Fusion to nucleoside deaminase or reverse transcriptase domains has produced base editors and prime editors that, instead of generating double-strand breaks in the target sequence, induce site-specific alterations of single (or a few adjacent) nucleotides. The availability of protein-only genome editing reagents based on transcription activator-like effectors has enabled the extension of base editing to the genomes of chloroplasts and mitochondria. In this review, we summarize currently available base editing methods for nuclear and organellar genomes. We highlight recent advances with improving precision, specificity, and efficiency and discuss current limitations and future challenges. We also provide a brief overview of applications in agricultural biotechnology and gene therapy.

A Transcriptomic Analysis of Tobacco Leaf with the Functional Loss of the Plastid rpoB Operon Caused by TALEN-Mediated Double-Strand Breakage

At least two sets of RNA polymerase (RNAP), nucleus (NEP)- and plastid (PEP)-encoded polymerases, recognizing distinct promoters exist in the plastids of land plants. Most plastid genes are regulated by multiple promoters with different strengths in their response to developmental stages and environmental cues. Recently, we applied chloroplast-targeted transcription activator-like effector nuclease (cpTALEN) technology to site-specifically cause double-strand DNA breaks in the rpoB gene of tobacco, which encodes the β-subunit of PEP. The repair of damaged chloroplast DNA (cpDNA) through microhomology-mediated recombination caused the functional loss of the rpoB operon and resulted in the heterotrophic growth of an albino plant. We conducted a genome-wide analysis of the steady state of gene expression in the leaf tissue of PEP-deficient tobacco by RNA-Seq and compared it with that of wild-type plants. The expression of NEP genes was up-regulated in PEP-deficient tobacco; in particular, the level of RpoT3 transcripts encoding the specifically plastid-targeted NEP was significantly increased. Alongside most housekeeping genes, NEP also plays an important role in the regulation of gene expression involved in photosynthesis. In contrast, alongside the photosynthesis-related genes, PEP also plays an important role in the regulation of gene expression involved in some housekeeping functions. Furthermore, the mitochondrial DNA copy number and the level of most mitochondrial protein-coding transcripts were slightly increased in PEP-deficient tobacco. The disruption of PEP function not only affected plastid gene expression, but also nuclear and mitochondrial gene expression. This study demonstrated the intercompartmental retrograde signaling in the regulation of gene expression.

Biotechnological Advances to Improve Abiotic Stress Tolerance in Crops

The major challenges that agriculture is facing in the twenty-first century are increasing droughts, water scarcity, flooding, poorer soils, and extreme temperatures due to climate change. However, most crops are not tolerant to extreme climatic environments. The aim in the near future, in a world with hunger and an increasing population, is to breed and/or engineer crops to tolerate abiotic stress with a higher yield. Some crop varieties display a certain degree of tolerance, which has been exploited by plant breeders to develop varieties that thrive under stress conditions. Moreover, a long list of genes involved in abiotic stress tolerance have been identified and characterized by molecular techniques and overexpressed individually in plant transformation experiments. Nevertheless, stress tolerance phenotypes are polygenetic traits, which current genomic tools are dissecting to exploit their use by accelerating genetic introgression using molecular markers or site-directed mutagenesis such as CRISPR-Cas9. In this review, we describe plant mechanisms to sense and tolerate adverse climate conditions and examine and discuss classic and new molecular tools to select and improve abiotic stress tolerance in major crops.