Five quantitative trait loci and one epistatic interaction were associated with heat tolerance in a doubled haploid population of broccoli evaluated in three summer field trials. Predicted rising global temperatures due to climate change have generated a demand for crops that are resistant to yield and quality losses from heat stress. Broccoli (Brassica oleracea var. italica) is a cool weather crop with high temperatures during production decreasing both head quality and yield. Breeding for heat tolerance in broccoli has potential to both expand viable production areas and extend the growing season but breeding efficiency is constrained by limited genetic information. A doubled haploid (DH) broccoli population segregating for heat tolerance was evaluated for head quality in three summer fields in Charleston, SC, USA. Multiple quantitative trait loci (QTL) mapping of 1,423 single nucleotide polymorphisms developed through genotyping-by-sequencing identified five QTL and one positive epistatic interaction that explained 62.1% of variation in heat tolerance. The QTL identified here can be used to develop markers for marker-assisted selection and to increase our understanding of the molecular mechanisms underlying plant response to heat stress.
Landraces have the potential to provide a reservoir of genetic diversity for crop improvement to combat the genetic erosion of the food supply. A landrace collection of the vitamin-rich specialty crop collard (Brassica oleracea L. var. viridis) was genetically characterized to assess its potential for improving the diverse crop varieties of B. oleracea. We used the Illumina 60K Brassica SNP BeadChip array with 52,157 single nucleotide polymorphisms (SNPs) to (i) clarify the relationship of collard to the most economically important B. oleracea crop types, (ii) evaluate genetic diversity and population structure of 75 collard landraces, and (iii) assess the potential of the collection for genome-wide association studies (GWAS) through characterization of genomic patterns of linkage disequilibrium. Confirming the collection as a valuable genetic resource, the collard landraces had twice the polymorphic markers (11,322 SNPs) and 10 times the varietyspecific alleles (521 alleles) of the remaining crop types examined in this study. On average, linkage disequilibrium decayed to background levels within 600 kilobase (kb), allowing for sufficient coverage of the genome for GWAS using the physical positions of the 8273 SNPs polymorphic among the landraces. Although other relationships varied, the previous placement of collard with the cabbage family was confirmed through phylogenetic analysis and principal coordinates analysis (PCoA).
The glucosinolate make-up of the edible parts of some Brassica oleracea L. crops has been investigated previously, but the leafy-green collard (B. oleracea var. viridis) remains relatively unstudied on this topic. Due to this lack of information, a collection of US landraces was examined for glucosinolate content of leaves. The specific objectives of this examination were to compare levels of certain glucosinolates among the conserved collard landraces, identify any individuals with a distinct glucosinolate profile and determine the potential of collard as a target for chemoprotective-based plant breeding. During winter 2010/2011, 81 collard landraces, four other viridis and four collard cultivars were evaluated in the field and harvested leaves were assayed for glucosinolates. In a subsequent study, 19 selected landraces plus the cultivars were included in a repeat trial in 2012/2013. Eighteen collard landraces contained relatively high levels of glucoraphanin in leaves in both years, and three
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