Genetic dissection of powdery mildew resistance in interspecific half-sib grapevine families using SNP-based maps

Genotyping-by-sequencing-based high-resolution mapping reveals a single candidate gene for the grapevine veraison locus Ver1

Veraison marks the transition from berry growth to berry ripening and is a crucial phenological stage in grapevine (Vitis vinifera): the berries become soft and begin to accumulate sugars, aromatic substances, and, in red cultivars, anthocyanins for pigmentation, while the organic acid levels begin to decrease. These changes determine the potential quality of wine. However, rising global temperatures lead to earlier flowering and ripening, which strongly influence wine quality. Here, we combined genotyping-by-sequencing with a bioinformatics pipeline on ∼150 F1 genotypes derived from a cross between the early ripening variety "Calardis Musqué" and the late-ripening variety "Villard Blanc". Starting from 20,410 haplotype-based markers, we generated a high-density genetic map and performed a quantitative trait locus analysis based on phenotypic datasets evaluated over 20 yrs. Through locus-specific marker enrichment and recombinant screening of ∼1,000 additional genotypes, we refined the originally postulated 5-mb veraison locus, Ver1, on chromosome 16 to only 112 kb, allowing us to pinpoint the ethylene response factor VviERF027 (VCost.v3 gene ID: Vitvi16g00942, CRIBIv1 gene ID: VIT_16s0100g00400) as veraison candidate gene. Furthermore, the early veraison allele could be traced back to a clonal "Pinot" variant first mentioned in the seventeenth century. "Pinot Precoce Noir" passed this allele over "Madeleine Royale" to the maternal grandparent "Bacchus Weiss" and, ultimately, to the maternal parent "Calardis Musqué". Our findings are crucial for ripening time control, thereby improving wine quality, and for breeding grapevines adjusted to climate change scenarios that have a major impact on agro-ecosystems in altering crop plant phenology.

Novel loci associated with resistance to downy and powdery mildew in grapevine

Among the main challenges in current viticulture, there is the increasing demand for sustainability in the protection from fungal diseases, such as downy mildew (DM) and powdery mildew (PM). Breeding disease-resistant grapevine varieties is a key strategy for better managing fungicide inputs. This study explores the diversity of grapevine germplasm (cultivated and wild) from Caucasus and neighboring areas to identify genotypes resistant to DM and PM, based on 13 Simple Sequence Repeat (SSR) loci and phenotypical (artificial pathogen inoculation) analysis, and to identify loci associated with DM and PM resistance, via Genome-Wide Association Analysis (GWAS) on Single Nucleotide Polymorphism (SNP) profiles. SSR analysis revealed resistant alleles for 16 out of 88 genotypes. Phenotypic data identified seven DM and 31 PM resistant genotypes. GWAS identified two new loci associated with DM resistance, located on chromosome 15 and 16 (designated as Rpv36 and Rpv37), and two with PM resistance, located on chromosome 6 and 17 (designated as Ren14 and Ren15). The four novel loci identified genomic regions rich in genes related to biotic stress response, such as genes involved in pathogen recognition, signal transduction and resistance response. This study highlights potential candidate genes associated with resistance to DM and PM, providing valuable insights for breeding programs for resistant varieties. To optimize their utilization, further functional characterization studies are recommended.

Effect of ethyl methanesulfonate mediated mutation for enhancing morpho-physio-biochemical and yield contributing traits of fragrant rice

Background

Chemical mutagenesis has been successfully used for increasing genetic diversity in crop plants. More than 800 novel mutant types of rice (Oryza sativa L.) have been developed through the successful application of numerous mutagenic agents. Among a wide variety of chemical mutagens, ethyl-methane-sulfonate (EMS) is the alkylating agent that is most commonly employed in crop plants because it frequently induces nucleotide substitutions as detected in numerous genomes.

Methods

In this study, seeds of the widely consumed Basmati rice variety (Super Basmati, Oryza sativa L.) were treated with EMS at concentrations of 0.25%, 0.50%, 0.75%, 1.0%, and 1.25% to broaden its narrow genetic base.

Results

Sensitivity to a chemical mutagen such as ethyl methanesulfonate (EMS) was determined in the M1 generation. Results in M1 generation revealed that as the levels of applied EMS increased, there was a significant reduction in the germination percent, root length, shoot length, plant height, productive tillers, panicle length, sterile spikelet, total spikelet, and fertility percent as compared to the control under field conditions. All the aforementioned parameters decreased but there was an increase in EMS mutagens in an approximately linear fashion. Furthermore, there was no germination at 1.25% of EMS treatment for seed germination. A 50% germination was recorded between 0.50% and 0.75% EMS treatments. After germination, the subsequent parameters, viz. root length and shoot length had LD50 between 05.0% and 0.75% EMS dose levels. Significant variation was noticed in the photosynthetic and water related attributes of fragrant rice. The linear increase in the enzymatic attributes was noticed by the EMS mediated treatments. After the establishment of the plants in the M1 generation in the field, it was observed that LD50 for fertility percentage was at EMS 1.0% level, for the rice variety.

Conclusion

Hence, it is concluded that for creating genetic variability in the rice variety (Super Basmati), EMS doses from 0.5% to 0.75% are the most efficient, and effective.

Basic leucine zipper gene VvbZIP61 is expressed at a quantitative trait locus for high monoterpene content in grape berries

The widely appreciated muscat flavor of grapes and wine is mainly attributable to the monoterpenes that accumulate in ripe grape berries. To identify quantitative trait loci (QTL) for grape berry monoterpene content, an F1 mapping population was constructed by a cross between two grapevine genotypes, one with neutral aroma berries (cv. 'Beifeng') and the other with a pronounced muscat aroma (elite Vitis vinifera line '3-34'). A high-density genetic linkage map spanning 1563.7 cM was constructed using 3332 SNP markers that were assigned to 19 linkage groups. Monoterpenes were extracted from the berry of the F1 progeny, then identified and quantified by gas chromatography-mass spectrometry. Twelve stable QTLs associated with the amounts of 11 monoterpenes in berries were thus identified. In parallel, the levels of RNA in berries from 34 diverse cultivars were estimated by RNA sequencing and compared to the monoterpene content of the berries. The expression of five genes mapping to stable QTLs correlated well with the monoterpene content of berries. These genes, including the basic leucine zipper VvbZIP61 gene on chromosome 12, are therefore considered as potentially being involved in monoterpene metabolism. Overexpression of VvbZIP61 in Vitis amurensis callus through Agrobacterium-mediated transformation significantly increased the accumulation of several monoterpenes in the callus, including nerol, linalool, geranial, geraniol, β-myrcene, and D-limonene. It is hypothesized that VvbZIP61 expression acts to increase muscat flavor in grapes. These results advance our understanding of the genetic control of monoterpene biosynthesis in grapes and provide important information for the marker-assisted selection of aroma compounds in grape breeding.

QTL detection and candidate gene analysis of grape white rot resistance by interspecific grape (Vitis vinifera L. × Vitis davidii Foex.) crossing

Grape white rot, a devastating disease of grapevines caused by Coniella diplodiella (Speg.) Sacc., leads to significant yield losses in grape. Breeding grape cultivars resistant to white rot is essential to reduce the regular use of chemical treatments. In recent years, Chinese grape species have gained more attention for grape breeding due to their high tolerance to various biotic and abiotic factors along with changing climatic conditions. In this study, we employed whole-genome resequencing (WGR) to genotype the parents of 'Manicure Finger' (Vitis vinifera, female) and '0940' (Vitis davidii, male), along with 101 F₁ mapping population individuals, thereby constructing a linkage genetic map. The linkage map contained 9337 single-nucleotide polymorphism (SNP) markers with an average marker distance of 0.3 cM. After 3 years of phenotypic evaluation of the progeny for white rot resistance, we confirmed one stable quantitative trait locus (QTL) for white rot resistance on chromosome 3, explaining up to 17.9% of the phenotypic variation. For this locus, we used RNA-seq to detect candidate gene expression and identified PR1 as a candidate gene involved in white rot resistance. Finally, we demonstrated that recombinant PR1 protein could inhibit the growth of C. diplodiella and that overexpression of PR1 in susceptible V. vinifera increased grape resistance to the pathogen.

Candidate gene discovery of Botrytis cinerea resistance in grapevine based on QTL mapping and RNA-seq

Grape gray mold disease (Botrytis cinerea) is widespread during grape production especially in Vitis vinifera and causes enormous losses to the grape industry. In nature, the grapevine cultivar 'Beta ' (Vitis riparia × Vitis labrusca) showed high resistance to grape gray mold. Until now, the candidate genes and their mechanism of gray mold resistance were poorly understood. In this study, we firstly conducted quantitative trait locus (QTL) mapping for grape gray mold resistance based on two hybrid offspring populations that showed wide separation in gray mold resistance. Notably, two stable QTL related to gray mold resistance were detected and located on linkage groups LG2 and LG7. The phenotypic variance ranged from 6.86% to 13.70% on LG2 and 4.40% to 11.40% on LG7. Combined with RNA sequencing (RNA-seq), one structural gene VlEDR2 (Vitvi02g00982) and three transcription factors VlERF039 (Vitvi00g00859), VlNAC047 (Vitvi08g01843), and VlWRKY51 (Vitvi07g01847) that may be involved in VlEDR2 expression and grape gray mold resistance were selected. This discovery of candidate gray mold resistance genes will provide an important theoretical reference for grape gray mold resistance mechanisms, research, and gray mold-resistant grape cultivar breeding in the future.

A high-density integrated map for grapevine based on three mapping populations genotyped by the Vitis18K SNP chip

We present a high-density integrated map for grapevine, allowing refinement and improved understanding of the grapevine genome, while demonstrating the applicability of the Vitis18K SNP chip for linkage mapping. The improvement of grapevine through biotechnology requires identification of the molecular bases of target traits by studying marker-trait associations. The Vitis18K SNP chip provides a useful genotyping tool for genome-wide marker analysis. Most linkage maps are based on single mapping populations, but an integrated map can increase marker density and show order conservation. Here we present an integrated map based on three mapping populations. The parents consist of the well-known wine cultivars 'Cabernet Sauvignon', 'Corvina' and 'Rhine Riesling', the lesser-known wine variety 'Deckrot', and a table grape selection, G1-7720. Three high-density population maps with an average inter-locus gap ranging from 0.74 to 0.99 cM were developed. These maps show high correlations (0.9965-0.9971) with the reference assembly, containing only 93 markers with large order discrepancies compared to expected physical positions, of which a third is consistent across multiple populations. Moreover, the genetic data aid the further refinement of the grapevine genome assembly, by anchoring 104 yet unanchored scaffolds. From these population maps, an integrated map was constructed which includes 6697 molecular markers and reduces the inter-locus gap distance to 0.60 cM, resulting in the densest integrated map for grapevine thus far. A small number of discrepancies, mainly of short distance, involve 88 markers that remain conflictual across maps. The integrated map shows similar collinearity to the reference assembly (0.9974) as the single maps. This high-density map increases our understanding of the grapevine genome and provides a useful tool for its further characterization and the dissection of complex traits.

Traditional processing increases biological activities of Dendrobium offificinale Kimura et. Migo in Southeast Yunnan, China

The orchid Dendrobium officinale grows throughout southeast China and southeast Asian countries and is used to treat inflammation and diabetes in traditional Chinese medicine. Tie pi feng dou is a well-known traditional Chinese medicine made from the dried D. officinale stems. Processing alters the physicochemical properties of TPFD; however, it is unclear how processing affects the quality and medicinal value of this plant. Here, we analyzed and compared the chemical composition of fresh stems of D. officinale and TPFD and explored possible explanations for the enhanced medicinal efficacy of processed D. officinale stems using qualitative and quantitative methods. To identify the components of FSD and TPFD, we used ultra-high-performance liquid chromatography combined with mass spectrometry in negative and positive ion modes and interpreted the data using the Human Metabolome Database and multivariate statistical analysis. We detected 23,709 peaks and identified 2352 metabolites; 370 of these metabolites were differentially abundant between FSD and TPFD (245 more abundant in TPFD than in FSD, and 125 less abundant), including organooxygen compounds, prenol lipids, flavonoids, carboxylic acids and their derivatives, and fatty acyls. Of these, 43 chemical markers clearly distinguished between FSD and TPFD samples, as confirmed using orthogonal partial least squares discriminant analysis. A pharmacological activity analysis showed that, compared with FSD, TPFD had significantly higher levels of some metabolites with anti-inflammatory activity, consistent with its use to treat inflammation. In addition to revealing the basis of the medicinal efficacy of TPFD, this study supports the benefits of the traditional usage of D. officinale.

Phenotyping for QTL identification: A case study of resistance to Plasmopara viticola and Erysiphe necator in grapevine

Vitis vinifera is the most widely cultivated grapevine species. It is highly susceptible to Plasmopara viticola and Erysiphe necator, the causal agents of downy mildew (DM) and powdery mildew (PM), respectively. Current strategies to control DM and PM mainly rely on agrochemical applications that are potentially harmful to humans and the environment. Breeding for resistance to DM and PM in wine grape cultivars by introgressing resistance loci from wild Vitis spp. is a complementary and more sustainable solution to manage these two diseases. During the last two decades, 33 loci of resistance to P. viticola (Rpv) and 15 loci of resistance to E. necator (Ren and Run) have been identified. Phenotyping is salient for QTL characterization and understanding the genetic basis of resistant traits. However, phenotyping remains a major bottleneck for research on Rpv and Ren/Run loci and disease resistance evaluation. A thorough analysis of the literature on phenotyping methods used for DM and PM resistance evaluation highlighted phenotyping performed in the vineyard, greenhouse or laboratory with major sources of variation, such as environmental conditions, plant material (organ physiology and age), pathogen inoculum (genetic and origin), pathogen inoculation (natural or controlled), and disease assessment method (date, frequency, and method of scoring). All these factors affect resistance assessment and the quality of phenotyping data. We argue that the use of new technologies for disease symptom assessment, and the production and adoption of standardized experimental guidelines should enhance the accuracy and reliability of phenotyping data. This should contribute to a better replicability of resistance evaluation outputs, facilitate QTL identification, and contribute to streamline disease resistance breeding programs.

Identification of CaAN3 as a fruit-specific regulator of anthocyanin biosynthesis in pepper (Capsicum annuum)

The novel gene CaAN3 encodes an R2R3 MYB transcription factor that regulates fruit-specific anthocyanin accumulation. The key regulatory gene CaAN2 encodes an R2R3 MYB transcription factor that regulates anthocyanin biosynthesis in various tissues in pepper (Capsicum annuum). However, CaAN2 is not expressed in certain pepper accessions showing fruit-specific anthocyanin accumulation. In this study, we identified the novel locus CaAN3 as a regulator of fruit-specific anthocyanin biosynthesis, using an F₂ population derived from a hybrid cultivar with purple immature fruits and segregating for CaAN3. We extracted total RNA, assembled two RNA pools according to fruit color, and carried out bulked segregant RNA sequencing. We aligned the raw reads to the pepper reference genome Dempsey and identified 6,672 significant single nucleotide polymorphisms (SNPs) by calculating the Δ(SNP-index) between the two pools. We then conducted molecular mapping to delimit the target region of CaAN3 to the interval 184.6-186.4 Mbp on chromosome 10. We focused on Dem.v1.00043895, encoding an R2R3 MYB transcription factor, as the strongest candidate gene. Sequence analysis revealed four insertion/deletion polymorphisms in the promoter region of the green CaAN3 allele. We employed virus-induced gene silencing and transient overexpression assays to characterize the function of the candidate gene. When Dem.v1.00043895 was silenced in pepper, anthocyanin accumulation decreased in the pericarp, while the transient overexpression of Dem.v1.00043895 in Nicotiana benthamiana leaves resulted in the accumulation of anthocyanins around the infiltration sites. These results showed that Dem.v1.00043895 is CaAN3, an activator of anthocyanin biosynthesis in pepper fruits.

Powdery Mildew Resistance Genes in Vines: An Opportunity to Achieve a More Sustainable Viticulture

Grapevine (Vitis vinifera) is one of the main fruit crops worldwide. In 2020, the total surface area planted with vines was estimated at 7.3 million hectares. Diverse pathogens affect grapevine yield, fruit, and wine quality of which powdery mildew is the most important disease prior to harvest. Its causal agent is the biotrophic fungus Erysiphe necator, which generates a decrease in cluster weight, delays fruit ripening, and reduces photosynthetic and transpiration rates. In addition, powdery mildew induces metabolic reprogramming in its host, affecting primary metabolism. Most commercial grapevine cultivars are highly susceptible to powdery mildew; consequently, large quantities of fungicide are applied during the productive season. However, pesticides are associated with health problems, negative environmental impacts, and high costs for farmers. In paralleled, consumers are demanding more sustainable practices during food production. Therefore, new grapevine cultivars with genetic resistance to powdery mildew are needed for sustainable viticulture, while maintaining yield, fruit, and wine quality. Two main gene families confer resistance to powdery mildew in the Vitaceae, Run (Resistance to Uncinula necator) and Ren (Resistance to Erysiphe necator). This article reviews the powdery mildew resistance genes and loci and their use in grapevine breeding programs.

Detection of candidate gene LsACOS5 and development of InDel marker for male sterility by ddRAD-seq and resequencing analysis in lettuce

A new breeding method of F1 hybrid using male sterility would open an exciting frontier in lettuce breeding, a self-pollinating crop. Male sterility is a crucial trait in F1 hybrid breeding. It is essential to map the causative gene for using male sterility. The ms-S, male-sterile (MS) gene of 'CGN17397', was mapped to linkage group (LG) 8 by ddRAD-seq and narrowed down between two markers using two F2 populations. This region spans approximately 10.16 Mb, where 94 genes were annotated according to the lettuce reference genome sequence (version8 from 'Salinas'). The whole-genome sequencing of the MS lines 'CGN17397-MS' and male-fertile (MF) lines 'CGN17397-MF' revealed that only one gene differed in the area of Lsat_1_v5_gn_8_148221.1, a homolog of acyl-CoA synthetase5 (ACOS5), and was deleted in the MS lines. It was reported that ACOS5 was needed for pollen wall formation and that the null mutants of ACOS5 were entirely male sterility in some plants. Thus, I concluded that Lsat_1_v5_gn_8_148221.1 designated as LsACOS5 was a biologically plausible candidate gene for the ms-S locus. By using the structural polymorphism of LsACOS5, an InDel marker was developed to select the MS trait. The results obtained here provide valuable information for the genic male-sterility in lettuce.

Multi-trait genomic prediction using in-season physiological parameters increases prediction accuracy of complex traits in US wheat

BACKGROUND

Recently genomic selection (GS) has emerged as an important tool for plant breeders to select superior genotypes. Multi-trait (MT) prediction model provides an opportunity to improve the predictive ability of expensive and labor-intensive traits. In this study, we assessed the potential use of a MT genomic prediction model by incorporating two physiological traits (canopy temperature, CT and normalized difference vegetation index, NDVI) to predict 5 complex primary traits (harvest index, HI; grain yield, GY; grain number, GN; spike partitioning index, SPI; fruiting efiiciency, FE) using two cross-validation schemes CV1 and CV2.

RESULTS

In this study, we evaluated 236 wheat genotypes in two locations in 2 years. The wheat genotypes were genotyped with genotyping by sequencing approach which generated 27,466 SNPs. MT-CV2 (multi-trait cross validation 2) model improved predictive ability by 4.8 to 138.5% compared to ST-CV1(single-trait cross validation 1). However, the predictive ability of MT-CV1 was not significantly different compared to the ST-CV1 model.

CONCLUSIONS

The study showed that the genomic prediction of complex traits such as HI, GN, and GY can be improved when correlated secondary traits (cheaper and easier phenotyping) are used. MT genomic selection could accelerate breeding cycles and improve genetic gain for complex traits in wheat and other crops.

Multiparent-Derived, Marker-Assisted Introgression Lines of the Elite Indian Rice Cultivar, ‘Krishna Hamsa’ Show Resistance against Bacterial Blight and Blast and Tolerance to Drought

Major biotic stresses viz., bacterial blight (BB) and blast and brown plant hopper (BPH) coupled with abiotic stresses like drought stress, significantly affect rice yields. To address this, marker-assisted intercross (IC) breeding involving multiple donors was used to combine three BB resistance genes—xa5, xa13 and Xa21, two blast resistance genes—Pi9 and Pi54, two BPH resistance genes—Bph20 and Bph21, and four drought tolerant quantitative trait loci (QTL)—qDTY1.1, qDTY2.1, qDTY3.1 and qDTY12.1—in the genetic background of the elite Indian rice cultivar ‘Krishna Hamsa’. Three cycles of selective intercrossing followed by selfing coupled with foreground selection and phenotyping for the target traits resulted in the development of 196 introgression lines (ILs) with a myriad of gene/QTL combinations. Based on the phenotypic reaction, the ILs were classified into seven phenotypic classes of resistance/tolerance to the following: (1) BB, blast and drought—5 ILs; (2) BB and blast—10 ILs; (3) BB and drought—9 ILs; (4) blast and drought—42 ILs; (5) BB—3 ILs; (6) blast—84 ILs; and (7) drought—43 ILs; none of the ILs were resistant to BPH. Positive phenotypic response (resistance) was observed to both BB and blast in 2 ILs, BB in 9 ILs and blast in 64 ILs despite the absence of corresponding R genes. Inheritance of resistance to BB and/or blast in such ILs could be due to the unknown genes from other parents used in the breeding scheme. Negative phenotypic response (susceptibility) was observed in 67 ILs possessing BB-R genes, 9 ILs with blast-R genes and 9 ILs harboring QTLs for drought tolerance. Complex genic interactions and recombination events due to the involvement of multiple donors explain susceptibility in some of the marker positive ILs. The present investigation successfully demonstrates the possibility of rapid development of multiple stress-tolerant/resistant ILs in the elite cultivar background involving multiple donors through selective intercrossing and stringent phenotyping. The 196 ILs in seven phenotypic classes with myriad of gene/QTL combinations will serve as a useful genetic resource in combining multiple biotic and abiotic stress resistance in future breeding programs.

Candidate resistance genes to foliar phylloxera identified at Rdv3 of hybrid grape

The foliage of the native grape species Vitis riparia and certain cold-hardy hybrid grapes are particularly susceptible to the insect pest phylloxera, Daktulosphaira vitifoliae Fitch. A previous study using a cold-hardy hybrid grape biparental F1 population (N~125) detected the first quantitative trait locus (QTL) for foliar resistance on chromosome 14, designated as resistance to Daktulosphaira vitifoliae 3 (Rdv3). This locus spans a ~7-Mbp (10-20 cM) region and is too wide for effective marker-assisted selection or identification of candidate genes. Therefore, we fine mapped the QTL using a larger F1 population, GE1783 (N~1023), and genome-wide rhAmpSeq haplotype markers. Through three selective phenotyping experiments replicated in the greenhouse, we screened 184 potential recombinants of GE1783 using a 0 to 7 severity rating scale among other phylloxera severity traits. A 500-kb fine mapped region at 4.8 Mbp on chromosome 14 was identified. The tightly linked rhAmpSeq marker 14_4805213 and flanking markers can be used for future marker-assisted breeding. This region contains 36 candidate genes with predicted functions in disease resistance (R genes and Bonzai genes) and gall formation (bifunctional 3-dehydroquinate dehydratase/shikimate dehydrogenase). Disease resistance genes suggest a traditional R-gene-mediated resistance mechanism often accompanied by a hypersensitive response, which has been widely studied in the plant pathology field. A novel resistance mechanism, non-responsiveness to phylloxera gall formation is proposed as a function of the bifunctional dehydratase gene, which plays a role in gallic acid biosynthesis and is important in gall formation. This study has implications for improvement of foliar phylloxera resistance in cold-hardy hybrid germplasm and is a starting place to understand the mechanism of resistance in crops to gall-forming insects.

Extent of wild-to-crop interspecific introgression in grapevine (Vitis vinifera) as a consequence of resistance breeding and implications for the crop species definition

Over the past two centuries, introgression through repeated backcrossing has introduced disease resistance from wild grape species into the domesticated lineage Vitis vinifera subsp. sativa. Introgression lines are being cultivated over increasing vineyard surface areas, as their wines now rival in quality those obtained from preexisting varieties. There is, however, a lot of debate about whether and how wine laws defining commercial product categories, which are based on the classification of V. vinifera and interspecific hybrid grapes, should be revised to accommodate novel varieties that do not fit either category. Here, we developed a method of multilocus genotype analysis using short-read resequencing to identify haplotypic blocks of wild ancestry in introgression lines and quantify the physical length of chromosome segments free-of-introgression or with monoallelic and biallelic introgression. We used this genomic data to characterize species, hybrids and introgression lines and show that newly released resistant varieties contain 76.5-94.8% of V. vinifera DNA. We found that varietal wine ratings are not always commensurate with the percentage of V. vinifera ancestry and linkage drag of wild alleles around known resistance genes persists over at least 7.1-11.5 Mb, slowing down the recovery of the recurrent parental genome. This method also allowed us to identify the donor species of known resistance haplotypes, define the ancestry of wild genetic background in introgression lines with complex pedigrees, validate the ancestry of the historic varieties Concord and Norton, and unravel sample curation errors in public databases.

Insights into opium poppy (Papaver spp.) genetic diversity from genotyping-by-sequencing analysis

Opium poppy (Papaver somniferum) is one of the world’s oldest medicinal plants and a versatile model system to study secondary metabolism. However, our knowledge of its genetic diversity is limited, restricting utilization of the available germplasm for research and crop improvement. We used genotyping-by-sequencing to investigate the extent of genetic diversity and population structure in a collection of poppy germplasm consisting of 91 accessions originating in 30 countries of Europe, North Africa, America, and Asia. We identified five genetically distinct subpopulations using discriminate analysis of principal components and STRUCTURE analysis. Most accessions obtained from the same country were grouped together within subpopulations, likely a consequence of the restriction on movement of poppy germplasm. Alkaloid profiles of accessions were highly diverse, with morphine being dominant. Phylogenetic analysis identified genetic groups that were largely consistent with the subpopulations detected and that could be differentiated broadly based on traits such as number of branches and seed weight. These accessions and the associated genotypic data are valuable resources for further genetic diversity analysis, which could include definition of poppy core sets to facilitate genebank management and use of the diversity for genetic improvement of this valuable crop.

Mapping Major Disease Resistance Genes in Soybean by Genome-Wide Association Studies

Soybean is one of the most valuable agricultural crops in the world. Besides, this legume is constantly attacked by a wide range of pathogens (fungi, bacteria, viruses, and nematodes) compromising yield and increasing production costs. One of the major disease management strategies is the genetic resistance provided by single genes and quantitative trait loci (QTL). Identifying the genomic regions underlying the resistance against these pathogens on soybean is one of the first steps performed by molecular breeders. In the past, genetic mapping studies have been widely used to discover these genomic regions. However, over the last decade, advances in next-generation sequencing technologies and their subsequent cost decreasing led to the development of cost-effective approaches to high-throughput genotyping. Thus, genome-wide association studies applying thousands of SNPs in large sets composed of diverse soybean accessions have been successfully done. In this chapter, a comprehensive review of the majority of GWAS for soybean diseases published since this approach was developed is provided. Important diseases caused by Heterodera glycines, Phytophthora sojae, and Sclerotinia sclerotiorum have been the focus of the several GWAS. However, other bacterial and fungi diseases also have been targets of GWAS. As such, this GWAS summary can serve as a guide for future studies of these diseases. The protocol begins by describing several considerations about the pathogens and bringing different procedures of molecular characterization of them. Advice to choose the best isolate/race to maximize the discovery of multiple R genes or to directly map an effective R gene is provided. A summary of protocols, methods, and tools to phenotyping the soybean panel is given to several diseases. We also give details of options of DNA extraction protocols and genotyping methods, and we describe parameters of SNP quality to soybean data. Websites and their online tools to obtain genotypic and phenotypic data for thousands of soybean accessions are highlighted. Finally, we report several tricks and tips in Subheading 4, especially related to composing the soybean panel as well as generating and analyzing the phenotype data. We hope this protocol will be helpful to achieve GWAS success in identifying resistance genes on soybean.

Achieving maximum efficiency of Mungbean yellow mosaic India virus infection in mungbean by agroinoculation

Mungbean is one of the important food legumes in the Indian-sub-continent. Yellow mosaic disease, caused by Mungbean yellow mosaic virus and Mungbean yellow mosaic India virus (MYMIV) poses a severe threat to its production. Agroinoculation has been the most preferred way to test the function of genomic components of these viruses. However, the available inoculation methods are not as efficient as whitefly transmission, thereby limiting their usage for screening and biological studies. We hereby report an efficient and reproducible agroinoculation method for achieving maximum (100%) efficiency using tandem repeat infectious agro-constructs of DNA A and DNA B of MYMIV. The present study targeted wounding of various meristematic tissues of root, shoot, parts of germinating seeds and also non-meristematic tissue of stem to test the suitable tissue types for maximum infection. Among the various tissues selected for, the inoculation on the epicotyl region showed maximum infectivity. Further, to enhance the infectivity of MYMIV, different concentrations of acetosyringone, incubation time and Agrobacterium cell density were also standardized. The incubation of wounded sprouted seeds in 1.0 OD of agroculture containing repeat construct of MYMIV for 2-4 h without acetosyringone followed by sowing in soil showed maximum infection of MYMIV within 10-12 days on the first trifoliate leaf. This standardized method is reproducible and has potential to screen germplasm lines and will be useful in mungbean biological/virological studies and breeding programmes.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03088-w.

Analysis of the SC8-0071-014 and sc47-18 loci co-segregated with Ren1 gene in the genotypes of seedless grape varieties

Powdery mildew (Erysiphe necator) is one of the most common and economically significant diseases of grapes. The main method of controlling the disease is pesticide treatment. To reduce chemical treatments, it is necessary to select and introduce resistant varieties into production. DNA markers are currently actively used in the study of grape genetic resources. Seedless grape varieties are highly demanded by consumers. Ren1 is one of the known and mapped vine resistance genes to powdery mildew, inherited from V. vinifera; linked DNA markers for this resistance locus are known. A study of 34 seedless grape genotypes was carried out using DNA markers SC8-0071-014 and sc47-18 co-segregated with Ren1. In the studied sample of varieties, 12 types of alleles were identified in the sc47-18 locus and 9 types of alleles in the SC8-0071-014 locus. Target fragments, according to linked marker loci, indicating the presence of the Ren1 resistance gene, were identified in grape variety Lotus (Kriulyanskiy x Yangi Er).

High-Density Genetic Linkage Map Construction and White Rot Resistance Quantitative Trait Loci Mapping for Genus Vitis Based on Restriction Site-Associated DNA Sequencing

Grape white rot (Coniothyrium diplodiella) is a major fungal disease affecting grape yield and quality. Quantitative trait locus (QTL) analysis is an important method for studying important horticultural traits of grapevine. This study was conducted to construct a high-density map and conduct QTL mapping for grapevine white rot resistance. A mapping population with 177 genotypes was developed from interspecific hybridization of a white rot-resistant cultivar (Vitis vinifera × V. labrusca 'Zhuosexiang') and white rot-susceptible cultivar (V. vinifera 'Victoria'). Single-nucleotide polymorphism (SNP) markers were developed by restriction site-associated DNA sequencing. The female, male, and integrated maps contained 2,501, 4,110, and 6,249 SNP markers with average genetic distances of adjacent markers of 1.25, 0.77, and 0.50 cM, respectively. QTL mapping was conducted based on white rot resistance identification of 177 individuals in July and August of 2017 and 2018. Notably, one stable QTL related to white rot resistance was detected and located on linkage group LG14. The phenotypic variance ranged from 12.93 to 13.43%. An SNP marker (chr14_3929380), which cosegregated with white rot resistance, was discovered and shows potential for use in marker-assisted selection to generate new grapevine cultivars with resistance to white rot.

Fine Mapping of Leaf Trichome Density Revealed a 747-kb Region on Chromosome 1 in Cold-Hardy Hybrid Wine Grape Populations

Segregation for leaf trichome density was observed in a cold-hardy hybrid grape population GE1025 (N = ∼125, MN1264 × MN1246) that was previously used to detect a quantitative trait locus (QTL) underlying foliar phylloxera resistance on chromosome 14. Our hypothesis was that high trichome density was associated with resistance to phylloxera. Existing literature found trichome density QTL on chromosomes 1 and 15 using a hybrid grape population of "Horizon" × Illinois 547-1 and suggested a few candidate genes. To validate the reported QTL and our hypothesis, interval mapping was conducted in GE1025 with previous genotyping-by-sequencing (GBS) single nucleotide polymorphism (SNP) genotype data and phenotypic scores collected using a 0-6 trichome density scale at several leaf positions. Evaluations were done on replicated forced dormant cuttings in 2 years and on field-grown leaves in 1 year. There was no strong relationship between trichome density and phylloxera resistance except for a Pearson's correlation (r) of about -0.2 between a few trichome density traits and phylloxera severity traits at 2 and 3 weeks after infestation. Two genetic regions were repeatedly detected for multiple trichome density traits: from 10 to 20.7 Mbp (∼10 Mbp) on chromosome 1 for ribbon and simple density traits and from 2.4 to 8.9 Mbp on chromosome 10 for ribbon density traits, explaining 12.1-48.2 and 12.6-27.5% of phenotypic variation, respectively. To fine map, we genotyped a larger population, GE1783 (N = ∼1,023, MN1264 × MN1246), with conserved rhAmpSeq haplotype markers across multiple Vitis species and phenotyped 233 selected potential recombinants. Evaluations were conducted on field-grown leaves in a single year. The QTL for ribbon trichome density on adaxial vein and adaxial leaf and simple density on abaxial vein was fine mapped to 12.63-13.38 Mbp (747 kb) on chromosome 1. We found variations of MN1264 and MN1246 at candidate genes NAC transcription factor 29, EF-hand protein, and MYB140 in this region and three other surrounding candidate genes proposed previously. Even though no strong relationship between foliar phylloxera resistance and trichome density was found, this study validated and fine mapped a major QTL for trichome density using a cold-hardy hybrid grape population and shed light on a few candidate genes that have implications for different breeding programs.

Search for donors of powdery mildew resistance genes among seedless and table grape varieties

Powdery mildew (Erysiphe necator) is one of the most common and economically significant diseases of grapes. Currently, the main method of controlling the disease is pesticide treatment. Breeding of resistant varieties is necessary to reduce chemical treatments. Currently, a number of grape resistance genes to powdery mildew and DNA markers for identification the allelic status of these genes are known. In a study to determine the presence of resistance loci Ren3 and Ren9, 25 genotypes of table grape varieties were analyzed, including 18 seedless varieties. DNA markers GF15-42, ScORGF15-02 were used to identify Ren3 gene, and CenGen6 – to identify Ren9 gene. DNA of cultivars Regent and Seyve Villard 12-375, which have resistance alleles, were used as positive controls. As a result of DNA marker analysis, it was determined that genotypes of table varieties Viking, Kodryanka, Moldova, Nadezhda AZOS, Original and seedless varieties Pamyati Smirnova, Kishmish Zaporozhskiy and Kishmish 342 carry loci of resistance to powdery mildew Ren3 and Ren9.

Transcriptome-Derived Amplicon Sequencing Markers Elucidate the U.S. Podosphaera macularis Population Structure Across Feral and Commercial Plantings of Humulus lupulus

Obligately biotrophic plant pathogens pose challenges in population genetic studies due to their genomic complexities and elaborate culturing requirements with limited biomass. Hop powdery mildew (Podosphaera macularis) is an obligately biotrophic ascomycete that threatens sustainable hop production. P. macularis populations of the Pacific Northwest (PNW) United States differ from those of the Midwest and Northeastern United States, lacking one of two mating types needed for sexual recombination and harboring two strains that are differentially aggressive on the cultivar Cascade and able to overcome the Humulus lupulus R-gene R6 (V6), respectively. To develop a high-throughput marker platform for tracking the flow of genotypes across the United States and internationally, we used an existing transcriptome of diverse P. macularis isolates to design a multiplex of 54 amplicon sequencing markers, validated across a panel of 391 U.S. samples and 123 international samples. The results suggest that P. macularis from U.S. commercial hop yards form one population closely related to P. macularis of the United Kingdom, while P. macularis from U.S. feral hop locations grouped with P. macularis of Eastern Europe. Included in this multiplex was a marker that successfully tracked V6-virulence in 65 of 66 samples with a confirmed V6-phenotype. A new qPCR assay for high-throughput genotyping of P. macularis mating type generated the highest resolution distribution map of P. macularis mating type to date. Together, these genotyping strategies enable the high-throughput and inexpensive tracking of pathogen spread among geographical regions from single-colony samples and provide a roadmap to develop markers for other obligate biotrophs.

Confirmation and Fine Mapping of the Resistance Locus Ren9 from the Grapevine Cultivar 'Regent'

Grapevine (Vitis vinifera ssp. vinifera) is a major fruit crop with high economic importance. Due to its susceptibility towards fungal and oomycete pathogens such as Erysiphe necator and Plasmopara viticola, the causal agents of powdery and downy mildew (PM and DM, respectively), grapevine growers annually face a major challenge in coping with shortfalls of yield caused by these diseases. Here we report the confirmation of a genetic resource for grapevine resistance breeding against PM. During the delimitation process of Ren3 on chromosome 15 from the cultivar ‘Regent’, a second resistance-encoding region on chromosome 15 termed Ren9 was characterized. It mediates a trailing necrosis associated with the appressoria of E. necator and restricts pathogen growth. In this study, we confirm this QTL in a related mapping population of ‘Regent’ × ‘Cabernet Sauvignon’. The data show that this locus is located at the upper arm of chromosome 15 between markers GF15-58 (0.15 Mb) and GF15-53 (4 Mb). The efficiency of the resistance against one of the prominent European PM isolates (EU-B) is demonstrated. Based on fine-mapping and literature knowledge we propose two possible regions of interest and supply molecular markers to follow both regions in marker-assisted selection.

Innovative breeding technologies in lettuce for improved post-harvest quality

Societal awareness of healthy eating is increasing alongside the market for processed bagged salads, which remain as one of the strongest growing food sectors internationally, including most recently from indoor growing systems. Lettuce represents a significant proportion of this ready-to-eat salad market. However, such products typically have a short shelf life, with decay of post-harvest quality occurring through complex biochemical and physiological changes in leaves and resulting in spoilage, food waste and risks to health. We review the functional and quantitative genetic understanding of lettuce post-harvest quality, revealing that few findings have translated into improved cultivar development. We identify (i) phytonutrient status (for enhanced antioxidant and vitamin status, aroma and flavour) (ii) leaf biophysical, cell wall and water relations traits (for longer shelf life) (iii) leaf surface traits (for enhanced food safety and reduced spoilage) and (iv) chlorophyll, other pigments and developmental senescence traits (for appearance and colour), as key targets for future post-harvest breeding. Lettuce is well-placed for rapid future exploitation to address postharvest quality traits with extensive genomic resources including the recent release of the lettuce genome and the development of innovative breeding technologies. Although technologies such as CRISPR/Cas genome editing are paving the way for accelerated crop improvement, other equally important resources available for lettuce include extensive germplasm collections, bi-parental mapping and wide populations with genotyping for genomic selection strategies and extensive multiomic datasets for candidate gene discovery. We discuss current progress towards post-harvest quality breeding for lettuce and how such resources may be utilised for future crop improvement.

High-density genetic linkage map construction and cane cold hardiness QTL mapping for Vitis based on restriction site-associated DNA sequencing

Background

Cold hardiness is an important agronomic trait and can significantly affect grape production and quality. Until now, there are no reports focusing on cold hardiness quantitative trait loci (QTL) mapping. In this study, grapevine interspecific hybridisation was carried out with the maternal parent ‘Cabernet sauvignon’ and paternal parent ‘Zuoyouhong’. A total of 181 hybrid offspring and their parents were used as samples for restriction-site associated DNA sequencing (RAD). Grapevine cane phloem and xylem cold hardiness of the experimental material was detected using the low-temperature exotherm method in 2016, 2017 and 2018. QTL mapping was then conducted based on the integrated map.

Results

We constructed a high-density genetic linkage map with 16,076, 11,643, and 25,917 single-nucleotide polymorphism (SNP) markers anchored in the maternal, paternal, and integrated maps, respectively. The average genetic distances of adjacent markers in the maps were 0.65 cM, 0.77 cM, and 0.41 cM, respectively. Colinearity analysis was conducted by comparison with the grape reference genome and showed good performance. Six QTLs were identified based on the phenotypic data of 3 years and they were mapped on linkage group (LG) 2, LG3, and LG15. Based on QTL results, candidate genes which may be involved in grapevine cold hardiness were selected.

Conclusions

High-density linkage maps can facilitate grapevine fine QTL mapping, genome comparison, and sequence assembly. The cold hardiness QTL mapping and candidate gene discovery performed in this study provide an important reference for molecular-assisted selection in grapevine cold hardiness breeding.

Characterization of genetic determinants of the resistance to phylloxera, Daktulosphaira vitifoliae, and the dagger nematode Xiphinema index from muscadine background

BACKGROUND

Muscadine (Muscadinia rotundifolia) is known as a resistance source to many pests and diseases in grapevine. The genetics of its resistance to two major grapevine pests, the phylloxera D. vitifoliae and the dagger nematode X. index, vector of the Grapevine fanleaf virus (GFLV), was investigated in a backcross progeny between the F1 resistant hybrid material VRH8771 (Vitis-Muscadinia) derived from the muscadine R source 'NC184-4' and V. vinifera cv. 'Cabernet-Sauvignon' (CS).

RESULTS

In this pseudo-testcross, parental maps were constructed using simple-sequence repeats markers and single nucleotide polymorphism markers from a GBS approach. For the VRH8771 map, 2271 SNP and 135 SSR markers were assembled, resulting in 19 linkage groups (LG) and an average distance between markers of 0.98 cM. Phylloxera resistance was assessed by monitoring root nodosity number in an in planta experiment and larval development in a root in vitro assay. Nematode resistance was studied using 10-12 month long tests for the selection of durable resistance and rating criteria based on nematode reproduction factor and gall index. A major QTL for phylloxera larval development, explaining more than 70% of the total variance and co-localizing with a QTL for nodosity number, was identified on LG 7 and designated RDV6. Additional QTLs were detected on LG 3 (RDV7) and LG 10 (RDV8), depending on the in planta or in vitro experiments, suggesting that various loci may influence or modulate nodosity formation and larval development. Using a Bulked Segregant Analysis approach and a proportion test, markers clustered in three regions on LG 9, LG 10 and LG 18 were shown to be associated to the nematode resistant phenotype. QTL analysis confirmed the results and QTLs were thus designated respectively XiR2, XiR3 and XiR4, although a LOD-score below the significant threshold value was obtained for the QTL on LG 18.

CONCLUSIONS

Based on a high-resolution linkage map and a segregating grapevine backcross progeny, the first QTLs for resistance to D. vitifoliae and to X. index were identified from a muscadine source. All together these results open the way to the development of marker-assisted selection in grapevine rootstock breeding programs based on muscadine derived resistance to phylloxera and to X. index in order to delay GFLV transmission.

Haplotyping the Vitis collinear core genome with rhAmpSeq improves marker transferability in a diverse genus

Transferable DNA markers are essential for breeding and genetics. Grapevine (Vitis) breeders utilize disease resistance alleles from congeneric species ~20 million years divergent, but existing Vitis marker platforms have cross-species transfer rates as low as 2%. Here, we apply a marker strategy targeting the inferred Vitis core genome. Incorporating seven linked-read de novo assemblies and three existing assemblies, the Vitis collinear core genome is estimated to converge at 39.8 Mb (8.67% of the genome). Adding shotgun genome sequences from 40 accessions enables identification of conserved core PCR primer binding sites flanking polymorphic haplotypes with high information content. From these target regions, we develop 2,000 rhAmpSeq markers as a PCR multiplex and validate the panel in four biparental populations spanning the diversity of the Vitis genus, showing transferability increases to 91.9%. This marker development strategy should be widely applicable for genetic studies in many taxa, particularly those ~20 million years divergent.

Molecularly tagged genes and quantitative trait loci in cucumber with recommendations for QTL nomenclature

Cucumber, Cucumis sativus L. (2n = 2x = 14), is an important vegetable crop worldwide. It was the first specialty crop with a publicly available draft genome. Its relatively small, diploid genome, short life cycle, and self-compatible mating system offers advantages for genetic studies. In recent years, significant progress has been made in molecular mapping, and identification of genes and QTL responsible for key phenotypic traits, but a systematic review of the work is lacking. Here, we conducted an extensive literature review on mutants, genes and QTL that have been molecularly mapped or characterized in cucumber. We documented 81 simply inherited trait genes or major-effect QTL that have been cloned or fine mapped. For each gene, detailed information was compiled including chromosome locations, allelic variants and associated polymorphisms, predicted functions, and diagnostic markers that could be used for marker-assisted selection in cucumber breeding. We also documented 322 QTL for 42 quantitative traits, including 109 for disease resistances against seven pathogens. By alignment of these QTL on the latest version of cucumber draft genomes, consensus QTL across multiple studies were inferred, which provided insights into heritable correlations among different traits. Through collaborative efforts among public and private cucumber researchers, we identified 130 quantitative traits and developed a set of recommendations for QTL nomenclature in cucumber. This is the first attempt to systematically summarize, analyze and inventory cucumber mutants, cloned or mapped genes and QTL, which should be a useful resource for the cucurbit research community.

Evaluating and Mapping Grape Color Using Image-Based Phenotyping

Grape berry color is an economically important trait that is controlled by two major genes influencing anthocyanin synthesis in the skin. Color is often described qualitatively using six major categories; however, this is a subjective rating that often fails to describe variation within these six classes. To investigate minor genes influencing berry color, image analysis was used to quantify berry color using different color spaces. An image analysis pipeline was developed and utilized to quantify color in a segregating hybrid wine grape population across two years. Images were collected from grape clusters immediately after harvest and segmented by color to determine the red, green, and blue (RGB); hue, saturation, and intensity (HSI); and lightness, red-green, and blue-yellow values (L∗a∗b∗) of berries. QTL analysis identified known major QTL for color on chromosome 2 along with several previously unreported smaller-effect QTL on chromosomes 1, 5, 6, 7, 10, 15, 18, and 19. This study demonstrated the ability of an image analysis phenotyping system to characterize berry color and to more effectively capture variability within a population and identify genetic regions of interest.

Chalcone synthases (CHSs): the symbolic type III polyketide synthases

Present review provides a thorough insight on some significant aspects of CHSs over a period of about past three decades with a better outlook for future studies toward comprehending the structural and mechanistic intricacy of this symbolic enzyme. Polyketide synthases (PKSs) form a large family of iteratively acting multifunctional proteins that are involved in the biosynthesis of spectrum of natural products. They exhibit remarkable versatility in the structural configuration and functional organization with an incredible ability to generate different classes of compounds other than the characteristic secondary metabolite constituents. Architecturally, chalcone synthase (CHS) is considered to be the simplest representative of Type III PKSs. The enzyme is pivotal for phenylpropanoid biosynthesis and is also well known for catalyzing the initial step of the flavonoid/isoflavonoid pathway. Being the first Type III enzyme to be discovered, CHS has been subjected to ample investigations which, to a greater extent, have tried to understand its structural complexity and promiscuous functional behavior. In this context, we vehemently tried to collect the fragmented information entirely focussed on this symbolic enzyme from about past three-four decades. The aim of this review is to selectively summarize data on some of the fundamental aspects of CHSs viz, its history and distribution, localization, structure and analogs in non-plant hosts, promoter analyses, and role in defense, with an emphasis on mechanistic studies in different species and vis-à-vis mutation-led changes, and evolutionary significance which has been discussed in detail. The present review gives an insight with a better perspective for the scientific community for future studies devoted towards delimiting the mechanistic and structural basis of polyketide biosynthetic machinery vis-à-vis CHS.

Emergent Ascomycetes in Viticulture: An Interdisciplinary Overview

The reduction of pesticide usage is a current imperative and the implementation of sustainable viticulture is an urgent necessity. A potential solution, which is being increasingly adopted, is offered by the use of grapevine cultivars resistant to its main pathogenic threats. This, however, has contributed to changes in defense strategies resulting in the occurrence of secondary diseases, which were previously controlled. Concomitantly, the ongoing climate crisis is contributing to destabilizing the increasingly dynamic viticultural context. In this review, we explore the available knowledge on three Ascomycetes which are considered emergent and causal agents of powdery mildew, black rot and anthracnose. We also aim to provide a survey on methods for phenotyping disease symptoms in fields, greenhouse and lab conditions, and for disease control underlying the insurgence of pathogen resistance to fungicide. Thus, we discuss fungal genetic variability, highlighting the usage and development of molecular markers and barcoding, coupled with genome sequencing. Moreover, we extensively report on the current knowledge available on grapevine-ascomycete interactions, as well as the mechanisms developed by the host to counteract the attack. Indeed, to better understand these resistance mechanisms, it is relevant to identify pathogen effectors which are involved in the infection process and how grapevine resistance genes function and impact the downstream cascade. Dealing with such a wealth of information on both pathogens and the host, the horizon is now represented by multidisciplinary approaches, combining traditional and innovative methods of cultivation. This will support the translation from theory to practice, in an attempt to understand biology very deeply and manage the spread of these Ascomycetes.

Dissecting Resistance to Phytophthora cinnamomi in Interspecific Hybrid Chestnut Crosses Using Sequence-Based Genotyping and QTL Mapping

The soilborne oomycete Phytophthora cinnamomi-which causes root rot, trunk cankers, and stem lesions on an estimated 5,000 plant species worldwide-is a lethal pathogen of American chestnut (Castanea dentata) as well as many other woody plant species. P. cinnamomi is particularly damaging to chestnut and chinquapin trees (Castanea spp.) in the southern portion of its native range in the United States due to relatively mild climatic conditions that are conductive to disease development. Introduction of resistant genotypes is the most practical solution for disease management in forests because treatment with fungicides and eradication of the pathogen are neither practical nor economically feasible in natural ecosystems. Using backcross families derived from crosses of American chestnuts with two resistant Chinese chestnut cultivars Mahogany and Nanking, we constructed linkage maps and identified quantitative trait loci (QTLs) for resistance to P. cinnamomi that had been introgressed from these Chinese chestnut cultivars. In total, 957 plants representing five cohorts of three hybrid crosses were genotyped by sequencing and phenotyped by standardized inoculation and visual examination over a 6-year period from 2011 to 2016. Eight parental linkage maps comprising 7,715 markers were constructed, and 17 QTLs were identified on four linkage groups (LGs): LG_A, LG_C, LG_E, and LG_K. The most consistent QTLs were detected on LG_E in seedlings from crosses with both 'Mahogany' and 'Nanking' and LG_K in seedlings from 'Mahogany' crosses. Two consistent large and medium effect QTLs located ∼10 cM apart were present in the middle and at the lower end of LG_E; other QTLs were considered to have small effects. These results imply that the genetic architecture of resistance to P. cinnamomi in Chinese chestnut × American chestnut hybrid progeny may resemble the P. sojae-soybean pathosystem, with a few dominant QTLs along with quantitatively inherited partial resistance conferred by multiple small-effect QTLs.

A High-Throughput Phenotyping System Using Machine Vision to Quantify Severity of Grapevine Powdery Mildew

Powdery mildews present specific challenges to phenotyping systems that are based on imaging. Having previously developed low-throughput, quantitative microscopy approaches for phenotyping resistance to Erysiphe necator on thousands of grape leaf disk samples for genetic analysis, here we developed automated imaging and analysis methods for E. necator severity on leaf disks. By pairing a 46-megapixel CMOS sensor camera, a long-working distance lens providing 3.5× magnification, X-Y sample positioning, and Z-axis focusing movement, the system captured 78% of the area of a 1-cm diameter leaf disk in 3 to 10 focus-stacked images within 13.5 to 26 seconds. Each image pixel represented 1.44 μm² of the leaf disk. A convolutional neural network (CNN) based on GoogLeNet determined the presence or absence of E. necator hyphae in approximately 800 subimages per leaf disk as an assessment of severity, with a training validation accuracy of 94.3%. For an independent image set the CNN was in agreement with human experts for 89.3% to 91.7% of subimages. This live-imaging approach was nondestructive, and a repeated measures time course of infection showed differentiation among susceptible, moderate, and resistant samples. Processing over one thousand samples per day with good accuracy, the system can assess host resistance, chemical or biological efficacy, or other phenotypic responses of grapevine to E. necator. In addition, new CNNs could be readily developed for phenotyping within diverse pathosystems or for diverse traits amenable to leaf disk assays.

A novel high-density grapevine (Vitis vinifera L.) integrated linkage map using GBS in a half-diallel population

A half-diallel population involving five elite grapevine cultivars was generated and genotyped by GBS, and highly-informative segregation data was used to construct a high-density genetic map for Vitis vinifera L. Grapevine is one of the most relevant fruit crops in the world. Deeper genetic knowledge could assist modern grapevine breeding programs to develop new wine grape varieties able to face climate change effects. To assist in the rapid identification of markers for crop yield components, grape quality traits and adaptation potential, we generated a large Vitis vinifera L. population (N = 624) by crossing five red wine cultivars in a half-diallel scheme, which was subsequently sequenced by an efficient GBS procedure. A high number of fully informative genetic variants was detected using a novel mapping approach capable of reconstructing local haplotypes from adjacent biallelic SNPs, which were subsequently used to construct the densest consensus genetic map available for the cultivated grapevine to date. This 1378.3-cM map integrates 10 bi-parental consensus maps and orders 4437 markers in 3353 unique positions on 19 chromosomes. Markers are well distributed all along the grapevine reference genome, covering up to 98.8% of its genomic sequence. Additionally, a good agreement was observed between genetic and physical orders, adding confidence in the quality of this map. Collectively, our results pave the way for future genetic studies (such as fine QTL mapping) aimed to understand the complex relationship between genotypic and phenotypic variation in the cultivated grapevine. In addition, the method used (which efficiently delivers a high number of fully informative markers) could be of interest to other outbred organisms, notably perennial fruit crops.

Integrated physiological and genomic analysis reveals structural variations and expression patterns of candidate genes for colored- and green-leaf poplar

Colored-leaf plants are increasingly popular and have been attracting more and more attentions. However, the molecular mechanism of leaf coloration in plants has not been fully understood. In this study, a colored-leaf cultivar of Populus deltoides (Caihong poplar, CHP) and green-leaf cultivar of Populus deltoides L2025 were used to explore the mechanism of leaf coloration through physiological and the whole genome resequencing analysis. The content of anthocyanins, total Chl, and carotenoids in the leaves of CHP and L2025 were evaluated. The ratio of anthocyanins to total Chl in CHP was 25.0 times higher than that in L2025; this could be attributed to the red leaf color of CHP. Based on the whole genome resequencing analysis, 951,421 polymorphic SNPs and 221,907 indels were screened between CHP and L2025. Using qRT-PCR analysis, three structural genes (flavonol synthase 1 family protein, UDP-glucose flavonoid 3-O-glucosyltransferase 3′ and flavonoid 3-O-galactosyl transferase family protein) and six transcription factors (MYB-related protein Myb4, transcription factor GAMYB, PtrMYB179, transcription factor bHLH53, transcription factor bHLH3, VARICOSE family protein) may be involved in the anthocyanin synthesis pathway, which could be used as candidate genes to explore the molecular regulation mechanism of leaf coloration in Populus deltoids, and could be used in molecular breeding in the future.

FaRCa1: a major subgenome-specific locus conferring resistance to Colletotrichum acutatum in strawberry

Optimal strategies for genetic improvement in crops depend on accurate assessments of the genetic architecture of traits. The overall objective of the present study was to determine the genetic architecture of anthracnose fruit rot (AFR) resistance caused by the fungus Colletotrichum acutatum in the University of Florida strawberry (Fragaria × ananassa) breeding germplasm. In 2016-2017, 33 full-sib families resulting from crosses between parents with varying levels of AFR resistance were tested. In 2017-2018, six full-sib families resulting from putative heterozygous resistant parents and homozygous susceptible parents were tested. Additionally, a validation population consisting of 77 advanced selections and ten cultivars was tested in the second season. Inoculation was performed using a mixture of three local isolates of the C. acutatum species complex. Phenotypes were scored weekly, and genotyping was performed using the IStraw35 Affymetrix Axiom® SNP array. A pedigree-based QTL analysis was performed using FlexQTL™ software. A major resistance locus, which we name FaRCa1, was detected in both seasons with a peak located at 55-56 cM on LG 6B and explaining at least 50% of the phenotypic variation across trials and seasons. The resistant allele exhibited partial dominance in all trials. The FaRCa1 locus is distinct from the previously discovered Rca2 locus, which mapped to LG 7B. While Rca2 is effective against European isolates from pathogenicity group 2, FaRCa1 appears to confer resistance to isolates of pathogenicity group 1.

Modeling copy number variation in the genomic prediction of maize hybrids

Our study indicates that copy variants may play an essential role in the phenotypic variation of complex traits in maize hybrids. Moreover, predicting hybrid phenotypes by combining additive-dominance effects with copy variants has the potential to be a viable predictive model. Non-additive effects resulting from the actions of multiple loci may influence trait variation in single-cross hybrids. In addition, complementation of allelic variation could be a valuable contributor to hybrid genetic variation, especially when crossing inbred lines with higher contents of copy gains. With this in mind, we aimed (1) to study the association between copy number variation (CNV) and hybrid phenotype, and (2) to compare the predictive ability (PA) of additive and additive-dominance genomic best linear unbiased prediction model when combined with the effects of CNV in two datasets of maize hybrids (USP and HELIX). In the USP dataset, we observed a significant negative phenotypic correlation of low magnitude between copy number loss and plant height, revealing a tendency that more copy losses lead to lower plants. In the same set, when CNV was combined with the additive plus dominance effects, the PA significantly increased only for plant height under low nitrogen. In this case, CNV effects explicitly capture relatedness between individuals and add extra information to the model. In the HELIX dataset, we observed a pronounced difference in PA between additive (0.50) and additive-dominance (0.71) models for predicting grain yield, suggesting a significant contribution of dominance. We conclude that copy variants may play an essential role in the phenotypic variation of complex traits in maize hybrids, although the inclusion of CNVs into datasets does not return significant gains concerning PA.

The Major Origin of Seedless Grapes Is Associated with a Missense Mutation in the MADS-Box Gene VviAGL11

Seedlessness is greatly prized by consumers of fresh grapes. While stenospermocarpic seed abortion determined by the SEED DEVELOPMENT INHIBITOR (SDI) locus is the usual source of seedlessness in commercial grapevine (Vitis vinifera) cultivars, the underlying sdi mutation remains unknown. Here, we undertook an integrative approach to identify the causal mutation. Quantitative genetics and fine-mapping in two 'Crimson Seedless'-derived F1 mapping populations confirmed the major effect of the SDI locus and delimited the sdi mutation to a 323-kb region on chromosome 18. RNA-sequencing comparing seed traces of seedless and seeds of seeded F1 individuals identified processes triggered during sdi-determined seed abortion, including the activation of salicylic acid-dependent autoimmunity. The RNA-sequencing data set was investigated for candidate genes, and while no evidence for causal cis-acting regulatory mutations was detected, deleterious nucleotide changes in coding sequences of the seedless haplotype were predicted in two genes within the sdi fine-mapping interval. Targeted resequencing of the two genes in a collection of 124 grapevine cultivars showed that only the point variation causing the arginine-197-to-leucine substitution in the seed morphogenesis regulator gene AGAMOUS-LIKE11 (VviAGL11) was fully linked with stenospermocarpy. The concurrent postzygotic variation identified for this missense polymorphism and seedlessness phenotype in seeded somatic variants of the original stenospermocarpic cultivar supports a causal effect. We postulate that seed abortion caused by this amino acid substitution in VviAGL11 is the major cause of seedlessness in cultivated grapevine. This information can be exploited to boost seedless grape breeding.

SNP markers tightly linked to root knot nematode resistance in grapevine (Vitis cinerea) identified by a genotyping-by-sequencing approach followed by Sequenom MassARRAY validation

Plant parasitic nematodes, including root knot nematode Meloidogyne species, cause extensive damage to agriculture and horticultural crops. As Vitis vinifera cultivars are susceptible to root knot nematode parasitism, rootstocks resistant to these soil pests provide a sustainable approach to maintain grapevine production. Currently, most of the commercially available root knot nematode resistant rootstocks are highly vigorous and take up excess potassium, which reduces wine quality. As a result, there is a pressing need to breed new root knot nematode resistant rootstocks, which have no impact on wine quality. To develop molecular markers that predict root knot nematode resistance for marker assisted breeding, a genetic approach was employed to identify a root knot nematode resistance locus in grapevine. To this end, a Meloidogyne javanica resistant Vitis cinerea accession was crossed to a susceptible Vitis vinifera cultivar Riesling and results from screening the F1 individuals support a model that root knot nematode resistance, is conferred by a single dominant allele, referred as MELOIDOGYNE JAVANICA RESISTANCE1 (MJR1). Further, MJR1 resistance appears to be mediated by a hypersensitive response that occurs in the root apical meristem. Single nucleotide polymorphisms (SNPs) were identified using genotyping-by-sequencing and results from association and genetic mapping identified the MJR1 locus, which is located on chromosome 18 in the Vitis cinerea accession. Validation of the SNPs linked to the MJR1 locus using a Sequenom MassARRAY platform found that only 50% could be validated. The validated SNPs that flank and co-segregate with the MJR1 locus can be used for marker-assisted selection for Meloidogyne javanica resistance in grapevine.