Perennial ryegrass is the most widely sown forage grass within the UK due to its ability to form productive swards which support grazing-based ruminant agriculture. When used in combination with forage legumes, such as red and white clover, these mixed grasslands can provide both a high energy and protein feed and allow for fewer inputs, due to the nitrogen-fixing properties of the legumes. However, the increasing variability of the climate means that the individual plants which make up these mixed grasslands must be able to persist in the face of a wider range of environmental challenges. Our research is aimed at identifying some of the genetic components of persistence and resilience in these grassland species.

 

Approach: Our approach involves using some of the considerable genetic resources that exist for ryegrasses and clovers to generate novel populations which show interesting variations in trait performances. For ryegrass we are using experiments on individual plants in controlled environments which look at response to water-logging and drought, combined with field based analyses of population performances, i.e., plants grown in dense swards. The results of these trials will indicate genes and genomic regions which may play important roles in conferring persistence and resilience in the face of abiotic stresses. For red clover, we have identified germplasm with contrasting growth habits ranging from erect to prostrate or even creeping, where roots form from nodes of shoots lying on the surface. The erect growth habit is chiefly a result of breeding, while the prostrate habit tends to be associated with natural populations or ecotypes. We believe that growth habit determines how well the plant is able to cope with the selective pressure of grazing or frequent cutting and have generated populations which are derived from crosses between erect versus prostrate growth habit which will allow us to identify key genes in controlling this trait.

 

Potential impact: Grasslands cover c. 70% of the utilised agricultural area within the UK and, therefore, have a major impact on many aspects of our environment. Because of the extent of these grasslands, incremental improvements in the performance of individual grass or clover plants could generate considerable benefits. For instance, longer-term persistence of mixed grass and legume swards and improved resilience in the face of environmental stresses can result in the need for fewer interventions to maintain or renew the grasslands, thus generating more stable natural environments with lower energy costs.

 

Key research insights and findings: We have generated populations of ryegrasses and clovers of varying genetic complexities which have been sown as experimental plots in the field. The unusual heat and drought during the summer of 2018 meant that these plots were also exposed to considerable abiotic stress during establishment. Work at the moment is characterising whether the conditions during this field establishment affected the balance of individuals and genes within these populations.

 

We have evaluated c. 200 individual ryegrass plants taken from genetic resources collections in controlled environments for their responses to drought and water-logging. This has allowed us to identify individuals for further study which will be used in controlled crosses to generate populations for further analysis.

 

We have conducted a genome-wide association study (GWAS) utilising 716 perennial ryegrass individuals from 90 different locations across Europe grown in the field in Aberystwyth over 2 years. By integrating the results of this study with the publication of a newly developed BAC-based physical map and minimum tiling path genome sequence, we have been able to identify a number of genomic locations associated with variations in flowering time, plant morphology and water soluble carbohydrate accumulation. The latter is a key trait for meeting the energy requirements of grazing animals (doi.org/10.1371/journal.pone.0207412).

 

We have set up field experiments at Gogerddan and at our upland research site Pwllpeirian using red  clover populations segregating for growth habit traits which are being managed for cutting for silage or simulated grazing. We will use these trials to assess changes in allele frequencies, using genotyping by sequencing on pooled plant material from each plot. We will use the genomic data to investigate how allele frequency changes may be used to identify regions of the genome under selection (Ergon, Skøt et al 2019; Frontiers in Plant Science 10:718. doi.org/10.3389/fpls.2019.00718). We are also collecting phenotypic data from the trials in the form of biomass yield, ground cover and quality, as well as other observational traits. Additionally, we are using a spaced plant trial on which we can make detailed phenotypic observations of the growth habit, and combining this with genotype data using GBS of each plant in the population. We will map traits associated with growth habit to the red clover genome using a GWAS approach. The data from the two experiments will provide us with further insight into this important complex domestication trait, and map genomic regions under selection and underlying the trait. The use of breeder relevant material means that the results will feed into the breeding programme.

 

We have undertaken an analysis of gene expression in perennial ryegrass shoots and roots in response to increasing water stress, using 3 different analysis programmes (DeSEQ2, edgeR, limma-voom) to identify differentially expressed genes (DEGs) and comparing associated gene ontology (GO) terms. The most striking findings from this study include that for the shoot: i) while approximately equal numbers of up and down-regulated DEGs were identified at each stage, up-regulated DEGs were more likely to be included in enriched GO terms at earlier stages but down-regulated DEGs more likely to be associated with enriched GO terms at later stages; ii) up-regulated DEGs were often associated with transcription factor and membrane transporter activities and down-regulated DEGs with more general metabolic processes. For the root: i) very few either up- or down-regulated DEGs were identified in earlier stages of drought; ii) at the late stage, heavy-metal associated membrane transporter and cytoskeleton-associated ATPase activities were differentially up- and down-regulated, respectively; v) for both shoot and root, KEGG pathway analysis indicated that ATPase and lignin biosynthesis-associated peroxidase activities were affected by increasing water-stress, doi.org/10.1371/journal.pone.0220518.

 

We have carried out an analysis of abiotic stress tolerance in a festulolium mapping family (a cross between perennial ryegrass and meadow fescue) which has been characterised both in terms of genetic markers and the relative contribution of the meadow fescue genome. We have discovered genomic associations with response to low phosphorus as well as plant growth and biomass under normal and stressed conditions. An interesting observation is that whole chromosome introgressions of chromosomes 3 and 4 from meadow fescue into the perennial ryegrass background (see image) seem to be particularly associated with plant vigour (doi.org/10.1093/aob/mcy230).

 

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