Phenotypic variation and quantitative trait loci for resistance to southern anthracnose and clover rot in red clover

Winter survival in red clover: experimental evidence for interactions among stresses

BACKGROUND

There is a lack of knowledge on the combined effects of different stresses on plants, in particular different stresses that occur during winter in temperate climates. Perennial herbaceous plants in temperate regions are exposed to many different stresses during winter, but except for the fact that cold temperatures induce resistance to a number of them, very little is known about their interaction effects. Knowledge about stress interactions is needed in order to predict effects of climate change on both agricultural production and natural ecosystems, and to develop adaptation strategies, e.g., through plant breeding. Here, we conducted a series of experiments under controlled conditions to study the interactions between cold (low positive temperature), clover rot infection (caused by Sclerotinia trifoliorum) and freezing, in red clover (Trifolium pratense) accessions. We also compared our results with winter survival in field experiments and studied associations between stress and shoot growth.

RESULTS

Exposure to low positive temperatures (cold acclimation) induced resistance to clover rot. There was a clear negative interaction effect between freezing stress and clover rot infection, resulting in up to 37% lower survival rate compared to what would have been expected from the additive effect of freezing and infection alone. Freezing tolerance could continue to improve during incubation under artificial snow cover at 3 °C in spite of darkness, and we observed compensatory shoot growth following freezing after prolonged incubation. At the accession level, resistance to clover rot was negatively correlated with growth in the field during the previous year at a Norwegian location. It was also negatively correlated with the shoot regrowth of control plants after incubation. Clover rot resistance tests under controlled conditions showed limited correlation with clover rot resistance observed in the field, suggesting that they may reveal variation in more specific resistance mechanisms.

CONCLUSIONS

We here demonstrate, for the first time, a strong negative interaction between freezing and infection with a winter pathogen. We also characterize the effects of cold acclimation and incubation in darkness at different temperatures on winter stress tolerance, and present data that support the notion that annual cycles of growth and stress resistance are associated at the genetic level.

A Cluster of Peronospora parasitica 13-like (NBS-LRR) Genes Is Associated with Powdery Mildew (Erysiphe polygoni) Resistance in Mungbean (Vigna radiata)

Powdery mildew (PM) caused by Erysiphe polygoni is an important foliar disease in mungbean (Vigna radiata). A previous study showed that QTL qPMRUM5-2 is a major locus for PM resistance in mungbean accession RUM5 (highly resistant). Bioinformatics analysis revealed that flanking markers of the qPMRUM5-2 covered a region of 1.93 Mb. In this study, we conducted fine mapping for the qPMRUM5-2 using the F2 population of 1156 plants of the cross between Chai Nat 60 (CN60; highly susceptible) and RUM5. PM resistance evaluation was performed under field conditions using F2:3 lines grown in three different environments. QTL analyses consistently located the qPMRUM5-2 to a 0.09 cm interval on linkage group 6 between InDel markers VrLG6-InDel05 and VrLG6-InDel10, which corresponded to a 135.0 kb region on chromosome 8 containing nine predicted genes of which five were NBS-LRR-type genes Recognition of Peronospora parasitica 13-like protein (RPP13L). Whole-genome re-sequencing of RUM5 and CN60 showed polymorphisms in four RPP13L genes predictively cause substantial amino acid changes, rendering them important candidate genes for PM resistance. The InDel markers VrLG6-InDel05 and VrLG6-InDel10 flanking to the qPMRUM5-2 would be useful for marker-assisted breeding of PM resistance in the mungbean.

Phenotypic characterization of drought responses in red clover (Trifolium pratense L.)

Introduction

Red clover (Trifolium pratense) is a protein-rich, short-lived perennial forage crop that can achieve high yields, but suffers increasingly from drought in different cultivation areas. Breeding for increased adaptation to drought is becoming essential, but at this stage it is unclear which traits breeders should target to phenotype responses to drought that allow them to identify the most promising red clover genotypes. In this study, we assessed how prolonged periods of drought affected plant growth in field conditions, and which traits could be used to distinguish better adapted plant material.

Methods

A diverse panel of 395 red clover accessions was evaluated during two growing seasons. We simulated 6-to-8-week drought periods during two consecutive summers, using mobile rain-out shelters, while an irrigated control field was established in an adjacent parcel. Plant growth was monitored throughout both growing seasons using multiple flights with a drone equipped with RGB and thermal sensors. At various observation moments throughout both growing seasons, we measured canopy cover (CC) and canopy height (CH). The crop water stress index (CWSI) was determined at two moments, during or shortly after the drought event.

Results

Manual and UAV-derived measurements for CH were well correlated, indicating that UAV-derived measurements can be reliably used in red clover. In both years, CC, CH and CWSI were affected by drought, with measurable growth reductions by the end of the drought periods, and during the recovery phase. We found that the end of the drought treatment and the recovery phase of approximately 20 days after drought were suitable periods to phenotype drought responses and to distinguish among genotypes.

Discussion

Multifactorial analysis of accession responses revealed interactions of the maturity type with drought responses, which suggests the presence of two independent strategies in red clover: 'drought tolerance' and 'drought recovery'. We further found that a large proportion of the accessions able to perform well under well-watered conditions were also the ones that were less affected by drought. The results of this investigation are interpreted in view of the development of breeding for adaptation to drought in red clover.

Multi-location trials and population-based genotyping reveal high diversity and adaptation to breeding environments in a large collection of red clover

Red clover (Trifolium pratense L.) is an outcrossing forage legume that has adapted to a wide range of climatic and growing conditions across Europe. Red clover is valued for its high yield potential and its forage quality. The high amount of genetic diversity present in red clover provides an invaluable, but often poorly characterized resource to improve key traits such as yield, quality, and resistance to biotic and abiotic stresses. In this study, we examined the genetic and phenotypic diversity within a diverse set of 395 diploid red clover accessions via genome wide allele frequency fingerprinting and multi-location field trials across Europe. We found that the genetic structure of accessions mostly reflected their geographic origin and only few cases were detected, where breeders integrated foreign genetic resources into their local breeding pools. The mean dry matter yield of the first main harvesting season ranged from 0.74 kg m^-2 in Serbia and Norway to 1.34 kg m^-2 in Switzerland. Phenotypic performance of accessions in the multi-location field trials revealed a very strong accession x location interaction. Local adaptation was especially prominent in Nordic red clover accessions that showed a distinct adaptation to the growing conditions and cutting regime of the North. The traits vigor, dry matter yield and plant density were negatively correlated between the trial location in Norway and the locations Great Britain, Switzerland, Czech Republic and Serbia. Notably, breeding material and cultivars generally performed well at the location where they were developed. Our results confirmed that red clover cultivars were bred from regional ecotypes and show a narrow adaptation to regional conditions. Our study can serve as a valuable basis for identifying interesting materials that express the desired characteristics and contribute to the adaptation of red clover to future climatic conditions.