The merging of distinct genomes, allopolyploidization, is a widespread phenomenon in plants. It generates adaptive potential through increased genetic diversity, but examples demonstrating its exploitation remain scarce. White clover (Trifolium repens) is a ubiquitous temperate allotetraploid forage crop derived from two European diploid progenitors confined to extreme coastal or alpine habitats. We sequenced and assembled the genomes and transcriptomes of this species complex to gain insight into the genesis of white clover and the consequences of allopolyploidization. Based on these data, we estimate that white clover originated ;15,000 to 28,000 years ago during the last glaciation when alpine and coastal progenitors were likely colocated in glacial refugia. We found evidence of progenitor diversity carryover through multiple hybridization events and show that the progenitor subgenomes have retained integrity and gene expression activity as they traveled within white clover from their original confined habitats to a global presence. At the transcriptional level, we observed remarkably stable subgenome expression ratios across tissues. Among the few genes that show tissue-specific switching between homeologous gene copies, we found flavonoid biosynthesis genes strongly overrepresented, suggesting an adaptive role of some allopolyploidy-associated transcriptional changes. Our results highlight white clover as an example of allopolyploidy-facilitated niche expansion, where two progenitor genomes, adapted and confined to disparate and highly specialized habitats, expanded to a ubiquitous global presence after glaciation-associated allopolyploidization.

Proanthocyanidins (PAs) are oligomeric flavonoids and one group of end products of the phenylpropanoid pathway. PAs have been reported to be beneficial for human and animal health and are particularly important in pastoral agricultural systems for improved animal production and reduced greenhouse gas emissions. However, the main forage legumes grown in these systems, such as Trifolium repens and Medicago sativa, do not contain any substantial amounts of PAs in leaves. We have identified from the foliar PA-accumulating legume Trifolium arvense an R2R3-MYB transcription factor, TaMYB14, and provide evidence that this transcription factor is involved in the regulation of PA biosynthesis in legumes. TaMYB14 expression is necessary and sufficient to up-regulate late steps of the phenylpropanoid pathway and to induce PA biosynthesis. RNA interference silencing of TaMYB14 resulted in almost complete cessation of PA biosynthesis in T. arvense, whereas Nicotiana tabacum, M. sativa, and T. repens plants constitutively expressing TaMYB14 synthesized and accumulated PAs in leaves up to 1.8% dry matter. Targeted liquid chromatography-multistage tandem mass spectrometry analysis identified foliar PAs up to degree of polymerization 6 in leaf extracts. Hence, genetically modified M. sativa and T. repens plants expressing TaMYB14 provide a viable option for improving animal health and mitigating the negative environmental impacts of pastoral animal production systems.

BackgroundWhite clover (Trifolium repens L.) is a temperate forage legume with an allotetraploid genome (2n=4×=32) estimated at 1093 Mb. Several linkage maps of various sizes, marker sources and completeness are available, however, no integrated map and marker set has explored consistency of linkage analysis among unrelated mapping populations. Such integrative analysis requires tools for homoeologue matching among populations. Development of these tools provides for a consistent framework map of the white clover genome, and facilitates in silico alignment with the model forage legume, Medicago truncatula.ResultsThis is the first report of integration of independent linkage maps in white clover, and adds to the literature on methyl filtered GeneThresher®-derived microsatellite (simple sequence repeat; SSR) markers for linkage mapping. Gene-targeted SSR markers were discovered in a GeneThresher® (TrGT) methyl-filtered database of 364,539 sequences, which yielded 15,647 SSR arrays. Primers were designed for 4,038 arrays and of these, 465 TrGT-SSR markers were used for parental consensus genetic linkage analysis in an F1 mapping population (MP2). This was merged with an EST-SSR consensus genetic map of an independent population (MP1), using markers to match homoeologues and develop a multi-population integrated map of the white clover genome. This integrated map (IM) includes 1109 loci based on 804 SSRs over 1274 cM, covering 97% of the genome at a moderate density of one locus per 1.2 cM. Eighteen candidate genes and one morphological marker were also placed on the IM. Despite being derived from disparate populations and marker sources, the component maps and the derived IM had consistent representations of the white clover genome for marker order and genetic length. In silico analysis at an E-value threshold of 1e-20 revealed substantial co-linearity with the Medicago truncatula genome, and indicates a translocation between T. repens groups 2 and 6 relative to M. truncatula.ConclusionsThis integrated genetic linkage analysis provides a consistent and comprehensive linkage analysis of the white clover genome, with alignment to a model forage legume. Associated marker locus information, particularly the homoeologue-specific markers, offers a new resource for forage legume research to enable genetic analysis and improvement of this forage and grassland species.

Proanthocyanidins (PAs) are polymeric flavonoids derived from the phenylpropanoid pathway, and they bind reversibly to forage proteins within the rumen, providing protection from bloat while enhancing protein utilisation and animal production. The occurrence of PAs varies greatly within forage legume species. Foliar PAs are present in Lotus corniculatus (birdsfoot trefoil) and Onobrychis viciifolia (sainfoin), but such species often show poor persistence under grazing. By contrast, Trifolium repens (white clover) and Medicago sativa (lucerne, or alfalfa) have good persistence but negligible amounts of foliar PAs. We altered the accumulation patterns present in lucerne and white clover by the overexpression of an R2R3-MYB transcription factor (TaMYB14) isolated from T. arvense (rabbit’s foot clover), a species with significant levels of leaf PA. Such plants effectively produced PAs of high degree of polymerisation (DP) in leaf tissue by upregulating genes of the PA pathway. By comparing transcriptome pools, we identified additional MYB transcription factors with putative involvement in PA synthesis in white clover and T. arvense, indicating that these species share a complex PA regulation system. Progress towards producing commercial cultivars of lucerne and white clover containing effective levels of PAs has begun. Such plants will provide a viable option for mitigating bloat in pastoral agriculture-based farming systems.

Perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) are forage species of the grass family (Poaceae) that are key components of temperate pasture-based agricultural systems. Proanthocyanidins (PAs) are oligomeric flavonoids that, when provided as part of a farm animal's diet, have been reported to improve animal production and health. Up to now, forage grasses have been deemed not to produce PAs. This paper reports for the first time the detection of polymerized PAs in aqueous methanolic extracts of seed tissue of both perennial ryegrass and tall fescue, using LC-MS/MS. We have determined the structure of the PAs to be trans-flavan-3-ol-based, consisting predominately of afzelechin and catechin and linked primarily by B-type bonds. Investigations into the leaf tissue of both species failed to detect any PAs. This discovery opens the possibility of using genetic engineering tools to achieve tannin accumulation in leaf tissue of perennial ryegrass and tall fescue.