Organelle Genetics in Plants

Eleven published articles (4 reviews, 7 research papers) are collected in the Special Issue entitled “Organelle Genetics in Plants.” This selection of papers covers a wide range of topics related to chloroplasts and plant mitochondria research: (i) organellar gene expression (OGE) and, more specifically, chloroplast RNA editing in soybean, mitochondria RNA editing, and intron splicing in soybean during nodulation, as well as the study of the roles of transcriptional and posttranscriptional regulation of OGE in plant adaptation to environmental stress; (ii) analysis of the nuclear integrants of mitochondrial DNA (NUMTs) or plastid DNA (NUPTs); (iii) sequencing and characterization of mitochondrial and chloroplast genomes; (iv) recent advances in plastid genome engineering. Here we summarize the main findings of these works, which represent the latest research on the genetics, genomics, and biotechnology of chloroplasts and mitochondria.

A photosynthesis operon in the chloroplast genome drives speciation in evening primroses

Incompatibility between the cytoplasm and the nucleus is considered as major factor in species formation, but mechanistic understanding is poor. In evening primroses, a model plant for organelle genetics and population biology, hybrid offspring regularly displays chloroplast-nuclear incompatibility. These incompatibilities affect photosynthesis, a trait under selection in changing environments. Here we show that light-dependent misregulation of the plastid psbB operon (encoding core subunits of photosystem II and the cytochrome b6f complex), can lead to hybrid incompatibility, thus ultimately driving speciation. This misregulation results in an impaired light acclimation response in incompatible plants. Moreover, as a result of their different chloroplast genotypes, the parental lines differ in their photosynthesis performance upon exposure to different light conditions. Significantly, the incompatible chloroplast genome is naturally found in xeric habitats with high light intensities, whereas the compatible one is limited to mesic habitats. Consequently, our data raise the possibility that the hybridization barrier evolved as a result of adaptation to specific climatic conditions.

Cybridization of Grapefruit with ‘Dancy’ Mandarin Leads to Improved Fruit Characteristics

In cybridization, new combinations of nuclear and cytoplasmic genes result in a unique genotype that may bring cellular, physical, physiological, and biochemical changes to the plant. This has been demonstrated in the unexpected cybrids generated from the fusion of citrus (Citrus sp.) protoplasts in two independent experiments. The first experiment was conducted to generate potentially seedless triploids by fusing diploid protoplasts of embryogenic ‘Dancy’ mandarin (Citrus reticulata) suspension culture cells with haploid ‘Ruby Red’ grapefruit (C. paradisi) protoplasts derived from tetrad-stage microspores. After multiple attempts, only one triploid was recovered, but several diploid plants with typical grapefruit morphology were also regenerated. In the second experiment, protoplasts derived from embryogenic ‘Dancy’ mandarin suspension culture were fused with nonembryogenic protoplasts from ‘Duncan’ grapefruit leaves in an effort to produce an allotetraploid somatic hybrid. The fruit from the resulting trees resembled grapefruit in morphology and type, and maintained excellent quality throughout the summer, when commercial grapefruit rapidly loses quality. Fruit on these trees remained firm with exceptional sweetness and good flavor into August, and without seed germination. The regenerants obtained in the protoplast fusion experiments were confirmed as cybrids by genetic marker analyses. The test grapefruit were identical to commercial ‘Ruby Red’ grapefruit at six nuclear simple sequence repeat (SSR) marker loci, but identical to ‘Dancy’ with respect to a mitochondrial intron marker. The plastid genomes of individual trees originated from either fusion partner. In the first experiment, haploid ‘Ruby Red’ protoplast preparations must have also contained contaminant diploid protoplasts. Apart from the value of altered fruit quality attributes in the marketplace, these plants provide an opportunity to understand the contributions of cytoplasmic organelle genetics to important citrus fruit-breeding objectives.

Use of protoplast fusion systems to study organelle genetics in a commercially important crop

Protoplast fusion can overcome the sexual incompatibility barriers that exist between different plant species and can give rise to novel plants that cannot be produced in nature. It therefore provides a new source of genetic variation, particularly within the cytoplasmic organelles. We review the mixtures, rearrangements, and recombinations of mitochondrial and chloroplast genomes in plants regenerated from protoplast fusion products. Particular emphasis is placed on canola fusion systems that are being utilized in the commercial production of hybrid seed for a worldwide market.