Progeny Plants Research Articles

Germination ability and seedling vigour in the progeny of heat-stressed wheat plants

Heat stress during the grain-filling period has a substantial effect on embryo development, and on the size and chemical composition of the grain. The lines of a doubled haploid population arising from a cross between a heat-sensitive (Plainsman V) and a heat-tolerant (Mv Magma) wheat variety were analysed to determine how these changes influenced the germination of kernels formed during heat stress and the initial development of the seedlings. Heat stress during the early grain development of the main spike had a significant influence on the yield components, which differed however for the main and side spikes. Considerable differences were observed in the extent to which the yield components declined in the individual lines. Averaged over the population, the germination percentage, and the shoot and root length and root number of the seedlings did not differ significantly for seed originating from heat-stressed and control plants.

Substitution of Hordeum marinum ssp. gussoneanum chromosome 7HL into wheat homoeologous group-7

Ditelosomic (Dt) 7HLmar(7D) and monotelosomic (Mt) 7HLmar(7A) and 7HLmar(7B) wheat–barley substitution lines were developed by crossing monosomic 7A, 7B and 7D lines of common wheat cv. Saratovskaya 29 with disomic wheat–barley addition lines (2n = 44) that carry telocentric chromosomes 7HLmar from Hordeummarinum ssp. gussoneanum 4×. Genomic in situ hybridisation confirmed the presence of barley chromosomes in the wheat genome. The compensating ability of the telosome in each combination was assessed by its transmission rate to progenies of plants with 2n = 41 + t chromosomes. Seed set and transmission rates of the telosome depended on the identity of the competing wheat homoeologue. Of the three chromosomes wheat, the telosome 7HLmar compensated better for chromosome 7D and poorly for 7B. These and other data are discussed with respect to the phylogenetic relationships between the wheat chromosomes of group 7 and the chromosome of H.marinum, and the practical utility of these lines for wheat improvement is evaluated.

Orchis × colemanii hybridization: Molecular and morphological evidence, seed set success, and evolutionary importance

Despite highly specialized pollination strategies, hybridization is a common phenomenon among Mediterranean deceptive orchids. Food-deceptive species sire a progeny of F1 unfertile plants, which work as a late post-zygotic barrier. Conversely, when pre-zygotic barriers of sexually deceptive (Ophrys) species are absent, the hybrids are fertile and an extensive introgression may occur. Here, we have performed molecular analysis and hand pollination treatments to characterize a hybrid zone of two food-deceptive species, O. mascula and O pauciflora. Hybrids (called O.×colemanii) have shown different amounts of parental nrDNA, strongly supporting that they are F2 and/or successive hybrid generations. Comparable high levels of reproductive success have been detected in natural conditions and in experimental crosses suggesting the absence of effective reproductive barriers either between hybrids, either between hybrids and parental species. In light of ecological and distributional features of O.×colemanii across its distribution range, we hypothesize that these populations have originated by secondary contact in the periglacial belt of Apennines. Moreover, the rare and localized O. pauciflora could benefit a genetic enrichment by hybridizing with a widespread related species. O.×colemanii is not a dead end population, but may have a role as potential reserve of adaptive variability and is an unusual stage along the speciation process.

Transgenerational inheritance of modified DNA methylation patterns and enhanced tolerance induced by heavy metal stress in rice (Oryza sativa L.).

BackgroundDNA methylation is sensitive and responsive to stressful environmental conditions. Nonetheless, the extent to which condition-induced somatic methylation modifications can impose transgenerational effects remains to be fully understood. Even less is known about the biological relevance of the induced epigenetic changes for potentially altered well-being of the organismal progenies regarding adaptation to the specific condition their progenitors experienced.Methodology/Principal FindingsWe analyzed DNA methylation pattern by gel-blotting at genomic loci representing transposable elements and protein-coding genes in leaf-tissue of heavy metal-treated rice (Oryza sativa) plants (S0), and its three successive organismal generations. We assessed expression of putative genes involved in establishing and/or maintaining DNA methylation patterns by reverse transcription (RT)-PCR. We measured growth of the stressed plants and their unstressed progenies vs. the control plants. We found (1) relative to control, DNA methylation patterns were modified in leaf-tissue of the immediately treated plants, and the modifications were exclusively confined to CHG hypomethylation; (2) the CHG-demethylated states were heritable via both maternal and paternal germline, albeit often accompanying further hypomethylation; (3) altered expression of genes encoding for DNA methyltransferases, DNA glycosylase and SWI/SNF chromatin remodeling factor (DDM1) were induced by the stress; (4) progenies of the stressed plants exhibited enhanced tolerance to the same stress their progenitor experienced, and this transgenerational inheritance of the effect of condition accompanying heritability of modified methylation patterns.Conclusions/SignificanceOur findings suggest that stressful environmental condition can produce transgenerational epigenetic modifications. Progenies of stressed plants may develop enhanced adaptability to the condition, and this acquired trait is inheritable and accord with transmission of the epigenetic modifications. We suggest that environmental induction of heritable modifications in DNA methylation provides a plausible molecular underpinning for the still contentious paradigm of inheritance of acquired traits originally put forward by Jean-Baptiste Lamarck more than 200 years ago.

Transgenerational maintenance of transgene body CG but not CHG and CHH methylation

In plants, RNA-directed DNA methylation (RdDM) can target both transgene promoters and coding regions/gene bodies. RdDM leads to methylation of cytosines in all sequence contexts: CG, CHG and CHH. Upon segregation of the RdDM trigger, at least CG methylation can be maintained at promoter regions in the progeny. So far, it is not clear whether coding region methylation can be also maintained. We showed that the body of Potato spindle tuber viroid (PSTVd) transgene constructs became densely de novo methylated at CG, CHG and CHH sites upon PSTVd infection. In this study, we demonstrate that in viroid-free progeny plants, asymmetric CHH and CHG methylation was completely lost. However, symmetric CG methylation was stably maintained for at least two generations. Importantly, the presence of transgene body methylation did not lead to an increase of dimethylation of histone H3 lysine 9 or a decrease of acetylation of H3. Our data supports the view that CG methylation can be maintained not only in promoters but also in the body of transgenes. They further suggest that maintenance of methylation may occur independently of tested chromatin modifications.

Genetics of Leaf Rust Resistance in the Winter Wheat Line CI13227

ABSTRACTThe winter wheat (Triticum aestivum L.) line CI13227 with slow rusting resistance to leaf rust, caused by Puccinia triticina Eriks., was crossed and backcrossed to ‘Thatcher’ (Tc) wheat. Backcross F2 families were tested for seedling resistance and segregated in a 1:1 ratio for families that segregated for resistance and families that were homozygous susceptible. The BC1F2 resistant seedlings were progeny tested for infection type (IT) as BC1F3 lines and compared for IT with near‐isogenic lines of Tc with alleles at the Lr3 locus. The BC1F3 lines and the Tc line with Lr3ka had nearly identical IT. The F2 progeny of a resistant BC1F3 plant and a Tc line with Lr3ka did not segregate for IT confirming that CI13227 had an allele at the Lr3 locus. The BC1F2 plants from families that were homozygous susceptible as seedlings were tested as adults for leaf rust IT. Resistant BC1F2 adult plants were selected and advanced to BC1F6 by single seed descent. A single BC1F6 plant was crossed with Tc, and the Tc*3/CI13227 population advanced to F6. The F6 recombinant inbred lines (RILs) were tested for leaf rust resistance at two locations. Molecular mapping with Diversity Arrays Technology (DArT) and simple sequence repeat (SSR) markers detected a major locus for resistance on chromosome 1BL. A polymerase chain reaction (PCR) marker specific for Lr46 also mapped in the 1BL region associated with resistance. Quantitative trait loci on 2DS, 2B, and 7BL for slow rusting resistance in CI13227 detected in previous experiments were not detected in this study. CI13227 has seedling resistance gene Lr3ka and likely has the adult plant resistance gene Lr46.

Cytogenetic abnormalities in Norway spruce (Picea abies (L.) Karst.) seedlings from natural populations and an introduction plantation

Comparative studies of the frequency and spectrum of pathological mitoses (PMs), as well as of the nucleolus number in the interphase nucleus, in cells of Norway spruce (Picea abies (L.) Karst.) seedlings were carried out in two natural populations of Ukrainian Polesie and plants from an introduction plantation in Donbass. Low levels of PMs in the seed progeny of the populations (0.32–0.38%) and a slightly higher level in the plantation progeny (0.40%) are established. The number of nucleoli was somewhat higher in the progeny of the populations (5.35–5.85) than that of the plantation plants (4.95). The PMs frequency was higher (0.43%) in the progeny of low-heterozygous plants than in highly heterozygous ones (0.28%).

Nitrogen Use Efficiency in Plants

This disclosure provides transgenic plants, including crop plants and methods for their production. The transgenic plants comprise recombinant DNA constructs for the expression of polypeptides encoded by the DNA constructs. The polypeptides are capable of conferring improved traits to the transgenic plants when expressed under the control of a developmentally-regulated promoter. This disclosure also pertains to transgenic plants and their progeny plants, wherein the transgenic or progeny plants comprise the recombinant DNA constructs and are selected for having enhanced nitrogen use efficiency and/or nitrogen remobilization. Seed of the transgenic plants that can be grown into a plant that comprise the disclosed recombinant DNA constructs and exhibits having enhanced nitrogen use efficiency, and which may be selected for this trait, are also envisioned.

Insect resistance of a full sib family of tetraploid switchgrass Panicum virgatum L. with varying lignin levels

Little information is available on insect resistance mechanisms and inheritance in biomass grasses. Although reduction of lignin in biomass grasses in order to increase the efficiency of fermentation may result in increased susceptibility to insect feeding, other resistance mechanisms may be more important. Field grown leaves of two tetraploid parent (Kanlow N1, Summer) and 14 progeny switchgrass (Panicum virgatum L.) plant clones selected for a diversity of plant form and ranges in lignin levels were tested for leaf resistance to feeding by the fall armyworm (Spodoptera frugiperda J. E. Smith), a grass feeding insect pest. Although lignin generally appeared important as a resistance mechanism only at early season stages, replicate clones of two low lignin progeny plants generally remained resistant to fall armyworm feeding. Mechanical damaging increased resistance to fall armyworm feeding in several of these plants. Degrees of resistance were sometimes associated with leaf form of progeny. These results indicate there are likely multiple insect resistance mechanisms operating at different stages in switchgrass, and that segregation of some mechanisms appears related to growth form of the plants.

In vitro flowering and seed formation in lentil (Lens culinaris Medik.)

IntroductionGrain legumes, commonly known as pulses, are one of themost important agricultural crops worldwide. Pulses areamongst the ancient food crops that have been traditionallycultivated, particularly in Asia, Latin America, and Africa.In many developing nations, grain legumes are gainingincreased importance in view of the acute shortage in pro-duction of animal proteins and widespread protein malnu-trition (Bressani 1973). Apart from this, grain legumes arealso considered as the main source of protein for livestockfeed and inland fish production. Moreover, grain legumeshave the unique ability to fix nitrogen symbiotically therebyimprove overall soil fertility.Among the pulses, lentil is the most popular and ranksfirst in terms of human consumption in Indian subcontinent.The protein and nutritional qualities of lentil varieties havebeen reported to be superior to other pulse crops (Singh etal. 1968; Bhatty 1988). Thus lentil plays an integral role inagricultural systems of South Asian countries, with increas-ing annual demands due to its nutritional quality and popu-larity. Even so, this crop is typically characterized by lowyield potential.Several factors arebelieved toberesponsiblefor the reduced yields, including susceptibility to pests anddiseases, massive flower drop, and post-harvest losses(Erskine 1984). Diseases of lentil, at various stages ofgrowth, are caused by fungi, bacteria, viruses, and nematodeinfestations (Gowda and Kaul 1982).A number of attempts have been made to improve lentilvarieties using conventional breeding techniques, but theseefforts have failed to achieve desired results due to a narrowgenetic base and lack of available and suitable germplasmfor lentil improvement. Under these circumstances, there isconsiderable scope to embrace biotechnological approachesfor lentil improvement. Indeed, a number of studies havebeen previously conducted to develop suitable in vitro re-generation systems, and to explore the possibility of apply-ing biotechnology to lentil improvement programmes(Saxena and King 1987; Polanco et al. 1988; Warkentinand Mchughen 1993; Sarker et al. 2003).However, improvement of lentil through biotechnologi-cal methods has proven to be difficult due to its recalcitrantnature, particularly pertaining to the development of aneffective in vitro root induction system. Previous studieshave indicated that rooting in microsperma type of lentil isespecially difficult and appears to be a major constraint inobtaining fully regenerated plants through in vitro techni-ques (Khanam et al. 1995;Sarkeret al. 2003). Micro-grafting has been attempted to obtain in vitro-derived plant-lets (Gulati et al. 2001), but this approach was fraught withdifficulties especially in case of microsperma lentil varieties.Under these circumstances, in vitro flowering and fullydeveloped seed formation have been considered as a viablealternative to obtain plant progenies. It is believed that invitro flowering and pod production may significantly con-tribute towards the genetic improvement of lentil through abiotechnological approach.The main objective of the present investigation was todevelop an alternative regeneration system for Lens culina-ris Medik. from regenerated shoots of lentil, avoiding the invitro root formation stage. The necessary first step was todetermine suitable media for in vitro flowering and seedformation. Several reportsare available onin vitroflowering

Genetic parameters and correlation in early measurement cycles in rubber trees

Knowledge of the traits present in plant populations of a species is essential to carry out selection and breeding. The objective of the present study was to estimate the genetic parameters and correlate genetically and phenotypically the variables rubber yield, annual trunk girth increase, number of latex vessel rings and bark thickness in early measurement cycles. A total of 22 rubber tree progenies (Hevea brasiliensis) were assessed in three locations during three early measurement cycles. The bark thickness variable correlated significantly for the three increase measurements made in the locations of Jau, Pindorama and Votuporanga. The progeny mean based joint hereditability showed mean values greater than individual plants and within progeny plants of 0.848, 0.286 and 0.242, respectively. The variables annual trunk girth increase and bark thickness are genetically correlated variables. Rubber yield is correlated genetically with bark thickness that permits simultaneous selection for the two characteristics. The early measurement cycles were efficacious for finding genetic variability among the genotypes.

Fine mapping of Co, a gene controlling columnar growth habit located on apple (Malus×domestica Borkh.) linkage group 10

With 2 figures and 3 tablesAbstractTree architecture and shoot morphology are important factors in apple production systems. Because a columnar growth habit facilitates high‐density planting and labour‐saving fruit production methods, columnar apple breeding programmes are spreading worldwide. Columnar growth habit is controlled by a single dominant gene, Co, on linkage group 10. Fine mapping of Co was conducted using 1000 F1 progeny plants from 31 populations. Novel simple sequence‐repeat (SSR) markers were developed based on the genome sequence of apple ‘Golden Delicious’ and analysed for linkage to Co. Graphical genotyping of 22 progeny with recombination event in the vicinity of Co identified three SSR markers (Mdo.chr10.12, Mdo.chr10.13 and Mdo.chr10.14) that co‐segregated with Co. Mdo.chr10.12 and Mdo.chr10.14 are well suited for marker‐assisted selection because the sizes of the alleles linked to Co are different from those found in common ancestors of current apple breeding germplasm. The Co region was delimited to a 196‐kb region of the apple genome, between markers Mdo.chr10.11 and Mdo.chr10.15.

Rapid generation of rice mutants via the dominant negative suppression of the mismatch repair protein OsPMS1

Mismatch repair (MMR) is a conservative pathway for maintaining the genome integrity of different organisms. Although suppression of MMR has resulted in various mutation phenotypes in Arabidopsis, the use of this strategy for mutation breeding in major crops has not been reported. Here, we overexpressed a truncated version of the OsPMS1 protein in rice; this approach is expected to suppress the rice MMR system through a dominant negative mechanism. We observed a wide spectrum of mutation phenotypes in the progeny of the transgenic plants during seed germination and the plant growth stages. Genomic variations were detected with inter-simple sequence repeat (ISSR), and sequencing of the differential ISSR bands revealed that the mutation occurred as a point mutation or as microsatellite instability at high frequencies. Plant lines with agronomically important traits, such as salt and drought tolerance, various tiller number, and early flowering, were obtained. Furthermore, we obtained mutants with important traits that are free of the transgene. Together, these results demonstrate that MMR suppression can be used as an efficient strategy for mutation breeding in rice.

Inducing male sterility of tomato using two component system

Production of hybrid seeds and pursuing heterosis breeding of many crops have been accomplished using male sterile lines. However, not all crops have valuable male sterile lines due to instability of male sterility and absence of a restorer system. In this study, male sterile lines have been induced using a two-component system. The extracellular ribonuclease Barnase was cleaves into two inactive yet complementary fragments, designated as ‘Bn-5’ and ‘Bn-3’. Both components were controlled by a TA29 promoter. They were transferred into the tomato inbred line ‘Yellow tomato’ by Agrobacterium method. Southern blotting identified that 11 transgenic Bn-5 plants (T0) and 10 transgenic Bn-3 plants (T0) were obtained. The vegetative phenotypes of all T0 plants were similar to wild-type, and they were capable of producing viable pollen grains and normal fruit with seeds, indicating that Barnase had lost its function after it being split two partial fragments. After self-pollination, homozygous progenies (T1) of transgenic Bn-5 and Bn-3 plants were chosen to cross each other, Barnase could be reconstituted and co-expressed in the same cell, which caused the hybrid plants to produce collapsed pollen grains with no viability and thus100 % male sterile plants were obtained. Stamens of male sterile plants were shorter than those of the wild type plants. PCR detection demonstrated that all male sterile plants contained Barnase, but male fertile plants did not. The male sterile plants were crossed with the male fertile inbred lines, and the result showed that hybrid (F1) plants were capable of producing normal fruit with seeds, and their pollen grain fertility was restored. The co-segregation ratio of Bn-5 and Bn-3 fragments showed 1:1 among hybrid plants. In conclusion, the results verified that the male sterility could be generated by two component system and be used in hybrid seed production. The F1 between the male sterile plant and the inbred line showed heterotic comparing to both parents. This system needs not breed restoration line.

Coat protein-mediated transgenic resistance in tomato against a IB subgroup Cucumber mosaic virus strain

Transgenic tomato plants containing the coat protein (CP) gene of Cucumber mosaic virus (CMV) of subgroup IB were developed through Agrobacterium-mediated transformations. The progenies of transgenic plants showed the presence of transgene, its expression and translation of 26 KDa CP. The T1 and T2 generation plants were evaluated for resistance against challenge inoculations by a homologous strain of CMV. Visual observations of challenged transgenic plants categorized them into resistant, tolerant and susceptible as compared with untransformed control plants. Out of 33 plants of the T1 generation, 36.3% showed resistance and remained symptomless throughout their life, 48.4% showed tolerance which developed delayed symptoms of mild mosaic, and 15.1% showed susceptibility to CMV which developed severe systemic mosaic and leaf distortion symptoms after 30 days of virus challenge. Out of 120 plants of the T2 generation, 60% showed resistance, 26.6% were tolerant and only 13.3% were found susceptible to challenge inoculations of CMV. Resistant transgenic plants also showed less CP accumulation in systemic upper leaves as compared with challenged untransformed plants. In this study, CP of a CMV subgroup IB strain has demonstrated a significant level of resistance in transgenic tomato plants against the CMV strain. The strategy may be applied for better quality and productivity of tomato crops.

Loss of XRN4 Function Can Trigger Cosuppression in a Sequence-Dependent Manner

OLE1 encodes an oleosin isoprotein, a major membrane protein of the lipid-reserve organelle in seeds known as the oil body. Transgenic Arabidopsis were generated to contain an artificial chimeric transgene composed of OLE1 and green fluorescent protein (GFP). Overexpression of the fusion protein allowed visualization of the oil body size and structure in living cells using fluorescence microscopy. Two mutants, xrn4-8(OleG) and xrn4-9(OleG), accumulating enlarged oil bodies with reduced GFP fluorescence were isolated from the mutagenized progeny of a transgenic plant. Both mutants contained a defect in EXORIBONUCLEASE4 (XRN4), a gene known to encode a ribonuclease that specifically degrades uncapped mRNAs. Transgene expression was silenced in these mutants, as demonstrated by the reduced levels of the transgene mRNA and its product, OLE1-GFP. XRN4 loss of function also triggered cosuppression, i.e. simultaneous reduction in expression of the transgene and an endogenous OLE1 gene that shared a region of identical sequence. The enlarged oil bodies exhibiting reduced GFP fluorescence were formed in the xrn4-8(OleG) and xrn4-9(OleG) mutants due to the reduction of the endogenous OLE1 and the transgene product, OLE1-GFP, respectively. Cosuppression triggered by the xrn4 mutation also occurs for other genes such as PYK10, which encodes an endoplasmic reticulum (ER) body-resident β-glucosidase. The overall results indicate that a loss of XRN4 function can potentially trigger the cosuppression in a sequence-dependent manner.

ASPECTS OF FEMALE HEREDITY IN THE BREEDING PROGENY OF DIFFERENT OLIVE CULTIVARS

Although the olive is one of the oldest cultivated tree-crop their large number of cultivars is mostly the result of selection from the endemic vegetation and thus, very close to the wild type. Specific cross breeding to improve or modify olive cultivars for specific new growing conditions started about 60 years ago. As most olive cultivars are only partly self compatible and even less so at elevated temperatures during flowering a methodology for cross breeding without flower castration was developed on that basis. In the present study we were able to demonstrate the impact of the female parent on its breeding progeny disregarding the effect of the male parent. Female heredity was clearly dominant over the male one for a considerable number of different parameters in the progeny of most female parentage tested. The most predominant characters of the female parent expressed in its progeny were tree form, size, leaf shape, fruit form, size, oil content and various others. Some mother cultivars induced the expression of specific characters in their progeny considerably more than others. Specifically pronounced characters of the female parent cultivar were in most cases even further amplified in its progeny. This was true for most specific extreme characters except for some clearly receive ones such as lack of anthocyanine biosynthesis in the fruit and in various cases resistance to diseases. For some characters and cultivars 80-90% of the progeny showed a very high similarity with that in the mother plant. For example the oil content in the progeny of ‘Kalamata’, fruit size in the progeny of ‘Kadesh’ and tree size in the progeny of ‘Manzanillo’. It was of particular interest that fruit characters in most of the progeny plants from any female parent tested were above the level of that expressed in the mother plants.

Transcriptional activation of Brassica napus β‐ketoacyl‐ACP synthase II with an engineered zinc finger protein transcription factor

Targeted gene regulation via designed transcription factors has great potential for precise phenotypic modification and acceleration of novel crop trait development. Canola seed oil composition is dictated largely by the expression of genes encoding enzymes in the fatty acid biosynthetic pathway. In the present study, zinc finger proteins (ZFPs) were designed to bind DNA sequences common to two canola β-ketoacyl-ACP Synthase II (KASII) genes downstream of their transcription start site. Transcriptional activators (ZFP-TFs) were constructed by fusing these ZFP DNA-binding domains to the VP16 transcriptional activation domain. Following transformation using Agrobacterium, transgenic events expressing ZFP-TFs were generated and shown to have elevated KASII transcript levels in the leaves of transgenic T(0) plants when compared to 'selectable marker only' controls as well as of T(1) progeny plants when compared to null segregants. In addition, leaves of ZFP-TF-expressing T(1) plants contained statistically significant decreases in palmitic acid (consistent with increased KASII activity) and increased total C18. Similarly, T(2) seed displayed statistically significant decreases in palmitic acid, increased total C18 and reduced total saturated fatty acid contents. These results demonstrate that designed ZFP-TFs can be used to regulate the expression of endogenous genes to elicit specific phenotypic modifications of agronomically relevant traits in a crop species.

Development of AFLP-derived SCAR markers associated with resistance to two races of southern root-knot nematode in sweetpotato

The southern root-knot nematode (SRKN) Meloidogyne incognita severely damages yield and quality in sweetpotato production, and host plant resistance is one of the primary options for SRKN control. Segregation of F1 progeny resistant and susceptible to the SP1 and SP2 races of SRKN suggested that the race-specific resistance of the sweetpotato cultivar “Hi-Starch” is mostly controlled by single genes and that the genes for resistance against each race are closely located. Bulked segregant analysis and subsequent analysis of 86 F1 progeny plants identified nine amplified fragment-length polymorphism markers associated with SRKN resistance and a single linkage map consisting of seven of these markers. Quantitative trait locus (QTL) analysis using the segregating resistance data of the F1 progeny allowed mapping of both a locus with a large effect on resistance to the SRKN race SP1 and another affecting resistance to SP2 to the region around E33M53_090 that was designated as qRmi(t). Two AFLP markers in the vicinity of qRmi(t), E33M53_090 and E41M32_206, were converted to locus-specific sequence-characterized amplified region markers based on their internal and adjacent DNA sequences. These markers might be useful for marker-assisted selection of SRKN resistance in sweetpotato breeding and as a first step to map-based cloning of the responsible QTL(s).

RecA-Dependent DNA Repair Results in Increased Heteroplasmy of the Arabidopsis Mitochondrial Genome

Plant mitochondria have very active DNA recombination activities that are responsible for its plastic structures and that should be involved in the repair of double-strand breaks in the mitochondrial genome. Little is still known on plant mitochondrial DNA repair, but repair by recombination is believed to be a major determinant in the rapid evolution of plant mitochondrial genomes. In flowering plants, mitochondria possess at least two eubacteria-type RecA proteins that should be core components of the mitochondrial repair mechanisms. We have performed functional analyses of the two Arabidopsis (Arabidopsis thaliana) mitochondrial RecAs (RECA2 and RECA3) to assess their potential roles in recombination-dependent repair. Heterologous expression in Escherichia coli revealed that RECA2 and RECA3 have overlapping as well as specific activities that allow them to partially complement bacterial repair pathways. RECA2 and RECA3 have similar patterns of expression, and mutants of either display the same molecular phenotypes of increased recombination between intermediate-size repeats, thus suggesting that they act in the same recombination pathways. However, RECA2 is essential past the seedling stage and should have additional important functions. Treatment of plants with several DNA-damaging drugs further showed that RECA3 is required for different recombination-dependent repair pathways that significantly contribute to plant fitness under stress. Replication repair of double-strand breaks results in the accumulation of crossovers that increase the heteroplasmic state of the mitochondrial DNA. It was shown that these are transmitted to the plant progeny, enhancing the potential for mitochondrial genome evolution.