Chromosome pairing in triploid intergeneric hybrids ofFestuca pratensis withLolium multiflorum, revealed by GISH

Unambiguous chromosome identification reveals the factors impacting irregular chromosome behaviors in allotriploid AAC Brassica

KEY MESSAGE

The major irregular chromosome pairing and mis-segregation were detected during meiosis through unambiguous chromosome identification and found that allotriploid Brassica can undergo meiosis successfully and produce mostly viable aneuploid gametes. Triploids have played a crucial role in the evolution of species by forming polyploids and facilitating interploidy gene transfer. It is widely accepted that triploids cannot undergo meiosis normally and predominantly produce nonfunctional aneuploid gametes, which restricts their role in species evolution. In this study, we demonstrated that natural and synthetic allotriploid Brassica (AAC), produced by crossing natural and synthetic Brassica napus (AACC) with Brassica rapa (AA), exhibits basically normal chromosome pairing and segregation during meiosis. Homologous A chromosomes paired faithfully and generally segregated equally. Monosomic C chromosomes were largely retained as univalents and randomly entered daughter cells. The primary irregular meiotic behaviors included associations of homoeologs and 45S rDNA loci at diakinesis, as well as homoeologous chromosome replacement and premature sister chromatid separation at anaphase I. Preexisting homoeologous arrangements altered meiotic behaviors in both chromosome irregular pairing and mis-segregation by increasing the formation of A-genomic univalents and A-C bivalents, as well as premature sister chromatid separation and homologous chromosome nondisjunction. Meiotic behaviors depended significantly on the genetic background and heterozygous homoeologous rearrangement. AAC triploids mainly generated aneuploid gametes, most of which were viable. These results demonstrate that allotriploid Brassica containing an intact karyotype can proceed through meiosis successfully, broadening our current understanding of the inheritance and role in species evolution of allotriploid.

Remodeling of chloroplast proteome under salinity affects salt tolerance of Festuca arundinacea

Acclimation of photosynthetic apparatus to variable environmental conditions is an important component of tolerance to dehydration stresses, including salinity. The present study deals with the research on alterations in chloroplast proteome of the forage grasses. Based on chlorophyll fluorescence parameters, two genotypes of a model grass species-Festuca arundinacea with distinct levels of salinity tolerance: low salt tolerant (LST) and high salt tolerant (HST), were selected. Next, two-dimensional electrophoresis and mass spectrometry were applied under both control and salt stress conditions to identify proteins accumulated differentially between these two genotypes. The physiological analysis revealed that under NaCl treatment the studied plants differed in photosystem II activity, water content, and ion accumulation. The differentially accumulated proteins included ATPase B, ATP synthase, ribulose-1,5-bisphosphate carboxylase large and small subunits, cytochrome b6-f complex iron-sulfur subunit, oxygen-evolving enhancer proteins (OEE), OEE1 and OEE2, plastidic fructose-bisphosphate aldolase (pFBA), and lipocalin. A higher level of lipocalin, potentially involved in prevention of lipid peroxidation under stress, was also observed in the HST genotype. Our physiological and proteomic results performed for the first time on the species of forage grasses clearly showed that chloroplast metabolism adjustment could be a crucial factor in developing salinity tolerance.

Insight into cellular proteome of Lolium multiflorum/Festuca arundinacea introgression forms to decipher crucial mechanisms of cold acclimation in forage grasses

Frost tolerance is the main component of winter-hardiness. To express this trait, plants sense low temperature, and respond by activating the process of cold acclimation. The molecular mechanisms of this acclimation have not been fully understood in the agronomically important group of forage grasses, including Lolium-Festuca species. Herein, the introgression forms of L. multiflorum/F. arundinacea distinct with respect to their frost tolerance, were used as models for the comprehensive, proteomic and physiological, research to recognize the crucial components of cold acclimation in forage grasses. The obtained results stressed the importance of photosynthetic performance under acclimation to low temperature. The stable level of photochemical processes after three weeks of cold acclimation in the introgression form with a higher level of frost tolerance, combined simultaneously with only slightly (but not significantly) decreased level of CO₂ assimilation after that period, despite significantly lower stomatal conductance, indicated the capacity for that form to acclimate its photosynthesis to low temperature. This phenomenon was driven by the Calvin cycle efficiency, associated with revealed here accumulation profiles and activities of chloroplastic aldolase. The capacity to acclimate the photosynthetic machinery to cold could be one of the most crucial components of forage grass metabolism to improve frost tolerance.

Dissection of resistance to Microdochium nivale in Lolium multiflorum/Festuca arundinacea introgression forms

The potential of resistance to Microdochium nivale is still not recognized for numerous plant species. The forage grasses of Lolium-Festuca complex are important for grass-biomass production in the temperate regions. Lolium multiflorum is a grass with a high forage quality and productivity but also a relatively low resistance to M. nivale. On the contrary, F. arundinacea has a higher potential of resistance but simultaneously a significantly lower forage quality. These two species cross with each other and the intergeneric hybrids possess complementary characters of both genera. Herein, for the first time, we perform the research on L. multiflorum/F. arundinacea introgression forms to decipher mechanisms of resistance to M. nivale in that group of plants. Two forms with distinct levels of resistance were used as models in cytogenetic and biochemical studies. The resistant plant was shown to be a tetraploid with 28 L. multiflorum chromosomes, including one with three F. arundinacea introgressions. The susceptible introgression form revealed the unbalanced genomic structure and only 25 chromosomes. Twenty four chromosomes were shown to be L. multiflorum chromosomes, including one chromosome with F. arundinacea segment. One Festuca chromosome with additional two interstitial F. arundinacea segments, was also revealed in the susceptible form. The selected introgression forms differed in the accumulation profiles of total soluble carbohydrates, phytohormones, and phenolics in the leaf and crown tissue under the control and infection conditions. The higher amount of carbohydrates and salicylic acid in the leaves and crowns as well as a lower amount of abscisic acid in both studied organs and jasmonic acid in the crowns, were shown to be crucial for the expression of resistance to M. nivale in the analyzed hybrids.

Exploiting repetitive sequences and BAC clones in Festuca pratensis karyotyping

The Festuca genus is thought to be the most numerous genus of the Poaceae family. One of the most agronomically important forage grasses, Festuca pratensis Huds. is treated as a model plant to study the molecular mechanisms associated with tolerance to winter stresses, including frost. However, the precise mapping of the genes governing stress tolerance in this species is difficult as its karyotype remains unrecognized. Only two F. pratensis chromosomes with 35S and 5S rDNA sequences can be easily identified, but its remaining chromosomes have not been distinguished to date. Here, two libraries derived from F. pratensis nuclear DNA with various contents of repetitive DNA sequences were used as sources of molecular probes for fluorescent in situ hybridisation (FISH), a BAC library and a library representing sequences most frequently present in the F. pratensis genome. Using FISH, six groups of DNA sequences were revealed in chromosomes on the basis of their signal position, including dispersed-like sequences, chromosome painting-like sequences, centromeric-like sequences, knob-like sequences, a group without hybridization signals, and single locus-like sequences. The last group was exploited to develop cytogenetic maps of diploid and tetraploid F. pratensis, which are presented here for the first time and provide a remarkable progress in karyotype characterization.

Metabolite profiling during cold acclimation of Lolium perenne genotypes distinct in the level of frost tolerance

Abiotic stresses, including low temperature, can significantly reduce plant yielding. The knowledge on the molecular basis of stress tolerance could help to improve its level in species of relatively high importance to agriculture. Unfortunately, the complex research performed so far mainly on model species and also, to some extent, on cereals does not fully cover the demands of other agricultural plants of temperate climate, including forage grasses. Two Lolium perenne (perennial ryegrass) genotypes with contrasting levels of frost tolerance, the high frost tolerant (HFT) and the low frost tolerant (LFT) genotypes, were selected for comparative metabolomic research. The work focused on the analysis of leaf metabolite accumulation before and after seven separate time points of cold acclimation. Gas chromatography-mass spectrometry (GC/MS) was used to identify amino acids (alanine, proline, glycine, glutamic and aspartic acid, serine, lysine and asparagine), carbohydrates (fructose, glucose, sucrose, raffinose and trehalose) and their derivatives (mannitol, sorbitol and inositol) accumulated in leaves in low temperature. The observed differences in the level of frost tolerance between the analysed genotypes could be partially due to the time point of cold acclimation at which the accumulation level of crucial metabolite started to increase. In the HFT genotype, earlier accumulation was observed for proline and asparagine. The increased amounts of alanine, glutamic and aspartic acids, and asparagine during cold acclimation could be involved in the regulation of photosynthesis intensity in L. perenne. Among the analysed carbohydrates, only raffinose revealed a significant association with the acclimation process in this species.

Influence of short-term drought conditions and subsequent re-watering on the physiology and proteome of Lolium multiflorum/Festuca arundinacea introgression forms, with contrasting levels of tolerance to long-term drought

Festuca arundinacea is a drought tolerant species. Lolium multiflorum has better forage quality but lower tolerance to abiotic stresses. Their hybrids offer an opportunity to perform research on the molecular basis of tolerance to drought. The aim of this work was to recognise the mechanisms of response to short-term drought (11 days) in a glasshouse in two L. multiflorum/F. arundinacea introgression forms with distinct levels of tolerance to long-term drought (14 weeks) in the field. Measurements of physiological parameters, analyses of protein accumulation profiles using two-dimensional gel electrophoresis, and mass spectrometry identification of proteins, which were accumulated differentially between the selected genotypes during short-term drought, were performed. Genotype 7/6, with lower yield potential during 14 weeks of drought, and lower ability to re-grow after watering, had a higher capacity for photosynthesis during 11 days of drought. Genotype 4/10, more tolerant to long-term drought, was able to repair damaged cell membranes after watering and was also characterised by lower transpiration during short-term drought. A total of 455 proteins were analysed, and the 17 that were differentially accumulated between the two genotypes were identified. The results of physiological and proteomic research led to a hypothesis that the higher photosynthetic capacity of genotype 7/6 could be due to a more efficient Calvin cycle, supported by higher accumulation of crucial proteins involving chloroplast aldolase.

Changes in the chloroplast proteome following water deficit and subsequent watering in a high- and a low-drought-tolerant genotype of Festuca arundinacea

Festuca arundinacea is one of the most drought-tolerant species within the Lolium-Festuca complex and was used as a model for research aimed at identifying the chloroplast components involved in the proteomic response for drought stress in forage grasses. Individual F. arundinacea genotypes with contrasting levels of drought tolerance, the high-drought-tolerant (HDT) and the low-drought-tolerant (LDT) genotypes, were selected for comparative physiological and proteomic work. Measurements of water uptake, chlorophyll fluorescence, relative water content, electrolyte leakage, and gas exchange during drought and rewatering periods were followed by investigations on accumulation levels of chloroplast proteins before drought conditions, on d 3 and 11 of drought treatment, and after 10 d of subsequent watering, using two-dimensional gel electrophoresis. The proteins that were accumulated differentially between the selected plants were then identified by mass spectrometry. The LDT genotype revealed lower levels of water uptake and relative water content as drought progressed, and this was accompanied by lower levels of transpiration and net photosynthesis, and a higher level of electrolyte leakage observed in this genotype. Eighty-two protein accumulation profiles were compared between the HDT and LDT genotypes and ten proteins were shown to be differentially accumulated between them. The functions of the selected proteins in plant cells and their probable influence on the process of recovery after drought treatment in F. arundinacea are discussed.

Cytogenetics of Festulolium (Festuca x Lolium hybrids)

Grasses are the most important and widely cultivated crops. Among them, ryegrasses (Lolium spp.) and fescues (Festuca spp.) provide high quality fodder for livestock, are used for turf and amenity purposes, and play a fundamental role in environment protection. Species from the two genera display complementary agronomic characteristics and are often grown in mixtures. Breeding efforts to combine desired features in single entities culminated with the production of Festuca x Lolium hybrids. The so called Festuloliums enjoy a considerable commercial success with numerous cultivars registered all over the world. They are also very intriguing from a strictly cytogenetic point of view as the parental chromosomes recombine freely in hybrids. Until a decade ago this phenomenon was only known in general quantitative terms. The introduction of molecular cytogenetic tools such as FISH and GISH permitted detailed studies of intergeneric chromosome recombination and karyotyping of Festulolium cultivars. These tools were also invaluable in revealing the origin of polyploid fescues, and facilitated the development of chromosome substitution and introgression lines and physical mapping of traits of interest. Further progress in this area will require the development of a larger set of cytogenetic markers and high-resolution cytogenetic maps. It is expected that the Lolium-Festuca complex will continue providing opportunities for breeding superior grass cultivars and the complex will remain an attractive platform for fundamental research of the early steps of hybrid speciation and interaction of parental genomes, as well as the processes of chromosome pairing, elimination and recombination.

Chromosome pairing in allotetraploid hybrids of Festuca pratensis x Lolium perenne revealed by genomic in situ hybridization (GISH)

Genomic in situ hybridization (GISH) was used to make a detailed study of chromosome pairing at metaphase I (MI) of meiosis in six F(1) hybrid plants of the allotetraploid Festuca pratensis x Lolium perenne (2n = 4x = 28; genomic constitution FpFpLpLp). The mean chromosome configurations for all hybrids analysed were 1.13 univalents + 11.51 bivalents + 0.32 trivalents + 0.72 quadrivalents, and the mean chiasma frequency was 21.96 per cell. GISH showed that pairing was predominantly intragenomic, with mean numbers of L. perenne (Lp/Lp) and F. pratensis (Fp/Fp) bivalents being virtually equal at 5.41 and 5.48 per cell, respectively. Intergenomic pairing between Lolium and Festuca chromosomes was observed in 33.3% of Lp/Fp bivalents (0.62 per cell), in 79.7% of trivalents - Lp/Lp/Fp and Lp/Fp/Fp (0.25 per cell), and in 98.4% of quadrivalents - Lp/Lp/Fp/Fp and Lp/Lp/Lp/Fp (0.71 per cell). About 4.0% of the total chromosome complement analysed remained as univalents, an average 0.68 Lp and 0.45 Fp univalents per cell. It is evident that in these hybrids there is opportunity for recombination to take place between the two component genomes, albeit at a low level, and this is discussed in the context of compromising the stability of Festulolium hybrid cultivars and accounting for the drift in the balance of the genomes over generations. We speculate that genotypic differences between hybrids could permit selection for pairing control, and that preferences for homologous versus homoeologous centromeres in their spindle attachments and movement to the poles at anaphase I could form the basis of a mechanism underlying genome drift.