Plant Genome

The large genome and polyploidy of wheat (Triticum aestivum L.) makes it difficult to identify desirable genetic changes based on phenotypic screening due to gene redundancy. Forward genetics is, therefore, more difficult in wheat than in diploid plants. A modified TILLING (Targeting Induced Local Lesions IN Genomes) method including the harvest of five heads per M₁ plant, storage of M₂ seeds, using unlabeled primers and agarose gels for mutation detection, and crossing of useful mutants for desired grain quality was explored in this report. A soft wheat cultivar, QAL2000, and a hard wheat cultivar, Ventura, were mutagenized with ethyl methanesulfonate (EMS). Screening of the waxy genes Wx‐A1 and Wx‐D1 in 2348 EMS‐treated M₂ plants allowed identification of 121 mutants, including silent, missense, and knockout (truncation) mutations. A complete waxy wheat was successfully bred in 18 mo by crossing two truncation mutants (Wx‐A1‐truncation and Wx‐D1‐truncation; Wx‐B1 is naturally null in both mutants). Screening of two puroindoline genes (Pina and Pinb) in QAL2000 identified 19 mutants. A hard grain variant of a soft cultivar was identified due to a mutation in Pinb caused by a premature stop codon. Background mutations were observed and further self‐fertilization or crossing with a wild type was performed to eliminate deleterious mutations. With the rapid accumulation of wheat genomics information, many potential target genes of interest can be screened for mutations in these TILLING populations.

Single nucleotide polymorphisms (SNPs) are ideal molecular markers due to their higher abundance. Although several types of genotyping platforms for assaying large number of SNPs are available, in cases such as marker‐assisted selection, where few markers are required for genotyping a set of potential lines, high‐throughput SNP genotyping platforms (e.g., iScan or Infinium) may not be cost effective. In this scenario, GoldenGate assays based on VeraCode technology using Illumina BeadXpress seems to be the most cost‐effective platform. The objective of this study was to develop cost‐effective SNP genotyping platforms in chickpea (Cicer arietinum L.) and pigeonpea (Cajanus cajan L.). Two sets of SNPs, one each for chickpea (96 SNPs) and pigeonpea (48 SNPs), were developed and tested by genotyping 288 diverse genotypes from respective reference sets. The SNPs selected for the oligo pool assays had high transferability to crop wild relative species. The mean polymorphism information content value of assayed SNP markers was 0.31 and 0.32 in chickpea and pigeonpea, respectively. No unique pattern was observed in the chickpea reference set whereas two major groups were observed in the case of the pigeonpea reference set. The Illumina BeadXpress platform assays developed for chickpea and pigeonpea are highly informative and cost effective for undertaking genetic studies in these legume species.

Hybrid breeding in barley (Hordeum vulgare L.) offers great opportunities to accelerate the rate of genetic improvement and to boost yield stability. A crucial requirement consists of the efficient selection of superior hybrid combinations. We used comprehensive phenotypic and genomic data from a commercial breeding program with the goal of examining the potential to predict the hybrid performances. The phenotypic data were comprised of replicated grain yield trials for 385 two‐way and 408 three‐way hybrids evaluated in up to 47 environments. The parental lines were genotyped using a 3k single nucleotide polymorphism (SNP) array based on an Illumina Infinium assay. We implemented ridge regression best linear unbiased prediction modeling for additive and dominance effects and evaluated the prediction ability using five‐fold cross validations. The prediction ability of hybrid performances based on general combining ability (GCA) effects was moderate, amounting to 0.56 and 0.48 for two‐ and three‐way hybrids, respectively. The potential of GCA‐based hybrid prediction requires that both parental components have been evaluated in a hybrid background. This is not necessary for genomic prediction for which we also observed moderate cross‐validated prediction abilities of 0.51 and 0.58 for two‐ and three‐way hybrids, respectively. This exemplifies the potential of genomic prediction in hybrid barley. Interestingly, prediction ability using the two‐way hybrids as training population and the three‐way hybrids as test population or vice versa was low, presumably, because of the different genetic makeup of the parental source populations. Consequently, further research is needed to optimize genomic prediction approaches combining different source populations in barley.

There is tremendous interest in using association mapping to identify genes responsible for quantitative variation of complex traits with agricultural and evolutionary importance. Recent advances in genomic technology, impetus to exploit natural diversity, and development of robust statistical analysis methods make association mapping appealing and affordable to plant research programs. Association mapping identifies quantitative trait loci (QTLs) by examining the marker‐trait associations that can be attributed to the strength of linkage disequilibrium between markers and functional polymorphisms across a set of diverse germplasm. General understanding of association mapping has increased significantly since its debut in plants. We have seen a more concerted effort in assembling various association‐mapping populations and initiating experiments through either candidate‐gene or genome‐wide approaches in different plant species. In this review, we describe the current status of association mapping in plants and outline opportunities and challenges in complex trait dissection and genomics‐assisted crop improvement.

A major problem for the pepper (Capsicum annuum) industry is the root rot disease caused by Phytophthora capsici (Pc), to which all commercial varieties suffer yield losses despite good management practices and available landraces with high levels of resistance. A high‐density map with 3887 markers was generated in a set of recombinant inbred lines (RIL) derived from the highly resistant Capsicum annuum accession Criollo de Morelos‐334 and Early Jalapeño. These lines have been systematically screened for Pc resistance against a set of isolates collected from Mexico, New Mexico, New Jersey, California, Michigan and Tennessee. Quantitative trait loci (QTL) associated with effective resistance across isolates have been identified and validated with SNP markers across additional segregating populations. By leveraging transcriptomic and genomic information, we describe CaDMR1, a homoserine kinase (HSK), as a candidate gene responsible for the major QTL on chromosome P5 for resistance to Pc. SNP markers for the resistant allele were validated to facilitate gene pyramiding schemes for recurrent selection in pepper.