Alterations in chlorophyll a/b binding proteins in Solanaceae cybrids

Genetic and Molecular Genetic Basis of Nuclear-Plastid Incompatibilities

Genetic analysis of nuclear-cytoplasm incompatibilities is not straightforward and requires an elaborated experimental design. A number of species have been genetically studied, but notable advances in genetic mapping of nuclear loci involved in nuclear-plastid incompatibility have been achieved only in wheat and pea. This review focuses on the study of the genetic background underlying nuclear-plastid incompatibilities, including cases where the molecular genetic basis of such incompatibility has been unveiled, such as in tobacco, Oenothera, pea, and wheat.

Absence of photosynthetic state transitions in alien chloroplasts

MAIN CONCLUSION

The absence of state transitions in a Nt(Hn) cybrid is due to a cleavage of the threonine residue from the misprocessed N-terminus of the LHCII polypeptides. The cooperation between the nucleus and chloroplast genomes is essential for plant photosynthetic fitness. The rapid and specific interactions between nucleus-encoded and chloroplast-encoded proteins are under intense investigation with potential for applications in agriculture and renewable energy technology. Here, we present a novel model for photosynthesis research in which alien henbane (Hyoscyamus niger) chloroplasts function on the nuclear background of a tobacco (Nicotiana tabacum). The result of this coupling is a cytoplasmic hybrid (cybrid) with inhibited state transitions-a mechanism responsible for balancing energy absorption between photosystems. Protein analysis showed differences in the LHCII composition of the cybrid plants. SDS-PAGE analysis revealed a novel banding pattern in the cybrids with at least one additional 'LHCII' band compared to the wild-type parental species. Proteomic work suggested that the N-terminus of at least some of the cybrid Lhcb proteins was missing. These findings provide a mechanistic explanation for the lack of state transitions-the N-terminal truncation of the Lhcb proteins in the cybrid included the threonine residue that is phosphorylated/dephosphorylated in order to trigger state transitions and therefore crucial energy balancing mechanism in plants.

Multiple recent horizontal transfers of the cox1 intron in Solanaceae and extended co-conversion of flanking exons

Background

The most frequent case of horizontal transfer in plants involves a group I intron in the mitochondrial gene cox1, which has been acquired via some 80 separate plant-to-plant transfer events among 833 diverse angiosperms examined. This homing intron encodes an endonuclease thought to promote the intron's promiscuous behavior. A promising experimental approach to study endonuclease activity and intron transmission involves somatic cell hybridization, which in plants leads to mitochondrial fusion and genome recombination. However, the cox1 intron has not yet been found in the ideal group for plant somatic genetics - the Solanaceae. We therefore undertook an extensive survey of this family to find members with the intron and to learn more about the evolutionary history of this exceptionally mobile genetic element.

Results

Although 409 of the 426 species of Solanaceae examined lack the cox1 intron, it is uniformly present in three phylogenetically disjunct clades. Despite strong overall incongruence of cox1 intron phylogeny with angiosperm phylogeny, two of these clades possess nearly identical intron sequences and are monophyletic in intron phylogeny. These two clades, and possibly the third also, contain a co-conversion tract (CCT) downstream of the intron that is extended relative to all previously recognized CCTs in angiosperm cox1. Re-examination of all published cox1 genes uncovered additional cases of extended co-conversion and identified a rare case of putative intron loss, accompanied by full retention of the CCT.

Conclusions

We infer that the cox1 intron was separately and recently acquired by at least three different lineages of Solanaceae. The striking identity of the intron and CCT from two of these lineages suggests that one of these three intron captures may have occurred by a within-family transfer event. This is consistent with previous evidence that horizontal transfer in plants is biased towards phylogenetically local events. The discovery of extended co-conversion suggests that other cox1 conversions may be longer than realized but obscured by the exceptional conservation of plant mitochondrial sequences. Our findings provide further support for the rampant-transfer model of cox1 intron evolution and recommend the Solanaceae as a model system for the experimental analysis of cox1 intron transfer in plants.

Influence of mitochondria on gene expression in a citrus cybrid

The production of cybrids, combining nucleus of a species with alien cytoplasmic organelles, is a valuable method used for improvement of various crops. Several citrus cybrids have been created by somatic hybridization. These genotypes are interesting models to analyze the impact of cytoplasmic genome change on nuclear genome expression. Herein, we report genome-wide gene expression analysis in leaves of a citrus cybrid between C. reticulata cv 'Willowleaf mandarin' and C. limon cv 'Eureka lemon' compared with its lemon parent, using a Citrus 20K cDNA microarray. Molecular analysis showed that this cybrid possesses nuclear and chloroplast genomes of Eureka lemon plus mitochondria from Willowleaf mandarin and, therefore, can be considered as a lemon bearing foreign mitochondria. Mandarin mitochondria influenced the expression of a large set of lemon nuclear genes causing an over-expression of 480 of them and repression of 39 genes. Quantitative real-time RT-PCR further confirmed the credibility of microarray data. Genes over-expressed in cybrid leaves are predominantly attributed to the functional category "cellular protein metabolism" whereas in the down-regulated none functional category was enriched. Overall, mitochondria replacement affected different nuclear genes including particularly genes predicted to be involved in mitochondrial retrograde signaling. Mitochondria regulate all cell structures even chloroplast status. These results suggest that nuclear gene expression is modulated with respect to new information received from the foreign organelle, with the final objective to suit specific needs to ensure better cell physiological balance.

Genetic transformation of plastids of different Solanaceae species using tobacco cells as organelle hosts

The plastid genome of angiosperms represents an attractive target for genetic manipulations. However plastid transformation of higher plants, especially of agriculturally valuable crops is an extremely difficult problem. Transformation protocols developed for tobacco 15 years ago failed to produce similar results with more than a handful of other species so far. We have analyzed plastid transformability of remote cytoplasmic hybrids (cybrids) that combine nuclei of tobacco, an easily transformable species, and plastids of some other, recalcitrant Solanaceae species. Here, we demonstrate that the plastids of five species of Solanaceae family, representing two subfamilies and three tribes, can be easily transformed if the plastids of these species are transferred into a cell of a transformable species (tobacco). The results can be considered to be an alternative approach to the development of plastid transformation technologies for recalcitrant species using a transformable intermediary ("clipboard") host.

Maintenance of plastid RNA editing activities independently of their target sites

RNA editing in plant organelles is mediated by site-specific, nuclear-encoded factors. Previous data suggested that the maintenance of these factors depends on the presence of their rapidly evolving cognate sites. The surprising ability of allotetraploid Nicotiana tabacum (tobacco) to edit a foreign site in the chloroplast ndhA messenger RNA was thought to be inherited from its diploid male ancestor, Nicotiana tomentosiformis. Here, we show that the same ndhA editing activity is also present in Nicotiana sylvestris, which is the female diploid progenitor of tobacco and which lacks the ndhA site. Hence, heterologous editing is not simply a result of tobacco's allopolyploid genome organization. Analyses of other editing sites after sexual or somatic transfer between land plants showed that heterologous editing occurs at a surprisingly high frequency. This suggests that the corresponding editing activities are conserved despite the absence of their target sites, potentially because they serve other functions in the plant cell.

Pigment deficiency in nightshade/tobacco cybrids is caused by the failure to edit the plastid ATPase alpha-subunit mRNA

The subgenomes of the plant cell, the nuclear genome, the plastome, and the chondriome are known to interact through various types of coevolving macromolecules. The combination of the organellar genome from one species with the nuclear genome of another species often leads to plants with deleterious phenotypes, demonstrating that plant subgenomes coevolve. The molecular mechanisms behind this nuclear-organellar incompatibility have been elusive, even though the phenomenon is widespread and has been known for >70 years. Here, we show by direct and reverse genetic approaches that the albino phenotype of a flowering plant with the nuclear genome of Atropa belladonna (deadly nightshade) and the plastome of Nicotiana tabacum (tobacco) develops as a result of a defect in RNA editing of a tobacco-specific editing site in the plastid ATPase alpha-subunit transcript. A plastome-wide analysis of RNA editing in these cytoplasmic hybrids and in plants with a tobacco nucleus and nightshade chloroplasts revealed additional defects in the editing of species-specific editing sites, suggesting that differences in RNA editing patterns in general contribute to the pigment deficiencies observed in interspecific nuclear-plastidial incompatibilities.

The inheritance of chilling tolerance in tomato (Lycopersicon spp.)

During the past 25 years, chilling tolerance of the cultivated (chilling-sensitive) tomato Lycopersicon esculentum and its wild, chilling-tolerant relatives L. peruvianum and L. hirsutum (and, less intensively studied, L. chilense) has been the object of several investigations. The final aim of these studies can be seen in the increase in chilling tolerance of the cultivated genotypes. In this review, we will focus on low-temperature effects on photosynthesis and the inheritance of these traits to the offspring of various breeding attempts. While crossing L. peruvianum (male symbol) to L. esculentum (female symbol) so far has brought the most detailed insight with respect to physiological questions, for practical purposes, e.g., the readily cross ability, crossing programmes with L. hirsutum as pollen donor at present seem to be a promising way to achieve higher chilling-tolerant genotypes of the cultivated tomato. This perspective is due to the progress that has been made with respect to the genetic basis of chilling tolerance of Lycopersicon spp. over the past five years.

Extensive developmental and metabolic alterations in cybrids Nicotiana tabacum (+ Hyoscyamus niger) are caused by complex nucleo-cytoplasmic incompatibility

The genetic basis of multiple phenotypic alterations was studied in cell-engineered cybrids Nicotiana tabacum (+ Hyoscyamus niger) combining the nuclear genome of N. tabacum, plastome of H. niger and recombinant mitochondria. The plants possess a complex, maternally inheritable syndrome of nucleo-cytoplasmic incompatibility, severely affecting growth, metabolism and development. In vivo, the syndrome was manifested as: late germination of seeds; dramatic decrease of chlorophyll and carotenoids in cotyledons and leaves; altered morphology of cotyledons, leaves and flowers; and dwarfism. The leaf phenotype depended on light intensity. In 'green flowers' (an extreme phenotype), homeotic function B was downregulated. In vitro, the incompatibility syndrome was restricted to the pigment deficiency of cotyledons. Electron microscopy revealed perturbations in the differentiation of chloroplasts and palisade parenchyma cells in bleached leaves. The pigment deficiency accompanied by retarded growth is discussed as a result of plastome-genome incompatibility, whereas other features are likely to be due to nucleo-mitochondrial incompatibilities.