From landrace to modern hybrid broccoli: the genomic and morphological domestication syndrome within a diverse B. oleracea collection

Worldwide, broccoli (Brassica oleracea var. italica) is among the most economically important, nutritionally rich, and widely-grown vegetable crops. To explore the genomic basis of the dramatic changes in broccoli morphology in the last century, we evaluated 109 broccoli or broccoli/cauliflower intermediates for 24 horticultural traits. Genotype-by-sequencing markers were used to determine four subpopulations within italica: Calabrese broccoli landraces and hybrids, sprouting broccoli, and violet cauliflower, and to evaluate between and within group relatedness and diversity. While overall horticultural quality and harvest index of improved hybrid broccoli germplasm has increased by year of cultivar release, this improvement has been accompanied by a considerable reduction in allelic diversity when compared to the larger pool of germplasm. Two landraces are the most likely founding source of modern broccoli hybrids, and within these modern hybrids, we identified 13 reduction-in-diversity genomic regions, 53 selective sweeps, and 30 (>1 Mbp) runs of homozygosity. Landrace accessions collected in southern Italy contained 4.8-fold greater unique alleles per accessions compared to modern hybrids and provide a valuable resource in subsequent improvement efforts. This work broadens the understanding of broccoli germplasm, informs conservation efforts, and enables breeding for complex quality traits and regionally adapted cultivars.

The novel gene BrMYB2, located on chromosome A07, with a short intron 1 controls the purple-head trait of Chinese cabbage (Brassica rapa L.)

Anthocyanins are important secondary metabolites in plants, but information on anthocyanin biosynthesis mechanisms in Chinese cabbage is limited. The new purple head Chinese cabbage cultivar 11S91 was analyzed, and an R2R3-MYB regulatory gene BrMYB2, located on chromosome A07, controlling the dominant purple-head trait was isolated. High expression of BrMYB2 generated a large accumulation of anthocyanins in 11S91, accompanied by highly upregulated BrTT8, BrF3'H, BrDFR1, BrANS1, BrUGTs, BrATs, and BrGSTs. 11S91 inherited the purple locus from purple trait donor 95T2-5, and they shared consensus CDSs and gDNAs with those of BrMYB2 (cBrMYB2 and gBrMYB2). Two SNPs in cBrMYB2 in 11S91 did not cause loss of function; in addition to several SNPs at both ends of intron 1, a large deletion had occurred in intron 1 of gBrMYB2 in 11S91. Genetic transformation of Arabidopsis showed that gBrMYB2 overexpression lines presented deeper purple color and higher expression than did the cBrMYB2 and cBrmyb2 lines, whereas gBrmyb2 with a long intron 1 did not cause the purple phenotype. We first show that BrMYB2 promotes anthocyanin biosynthesis under the control of the short intron 1 of gBrMYB2 in purple head Chinese cabbage, and gBrmyb2 with a long intron 1 represses anthocyanin production in white head Chinese cabbage. This evidence provides a new understanding of anthocyanin biosynthesis and purple germplasm generation in Brassica vegetables.

Exploiting the diversity of tomato: the development of a phenotypically and genetically detailed germplasm collection

A collection of 163 accessions, including Solanum pimpinellifolium, Solanum lycopersicum var. cerasiforme and Solanum lycopersicum var. lycopersicum, was selected to represent the genetic and morphological variability of tomato at its centers of origin and domestication: Andean regions of Peru and Ecuador and Mesoamerica. The collection is enriched with S. lycopersicum var. cerasiforme from the Amazonian region that has not been analyzed previously nor used extensively. The collection has been morphologically characterized showing diversity for fruit, flower and vegetative traits. Their genomes were sequenced in the Varitome project and are publicly available (solgenomics.net/projects/varitome). The identified SNPs have been annotated with respect to their impact and a total number of 37,974 out of 19,364,146 SNPs have been described as high impact by the SnpEeff analysis. GWAS has shown associations for different traits, demonstrating the potential of this collection for this kind of analysis. We have not only identified known QTLs and genes, but also new regions associated with traits such as fruit color, number of flowers per inflorescence or inflorescence architecture. To speed up and facilitate the use of this information, F2 populations were constructed by crossing the whole collection with three different parents. This F2 collection is useful for testing SNPs identified by GWAs, selection sweeps or any other candidate gene. All data is available on Solanaceae Genomics Network and the accession and F2 seeds are freely available at COMAV and at TGRC genebanks. All these resources together make this collection a good candidate for genetic studies.

Utilization of Ogura CMS germplasm with the clubroot resistance gene by fertility restoration and cytoplasm replacement in Brassica oleracea L

Clubroot disease, a major plant root disease caused by Plasmodiophora brassicae, has become one of the most destructive diseases among cultivated cruciferous vegetables. However, clubroot-resistant Brassica oleracea materials are rare. A few clubroot-resistant cabbage varieties are available on the market, but all are Ogura cytoplasmic male sterile (CMS) types. Therefore, in this study, to reutilize the clubroot-resistant Ogura CMS germplasm of cabbage, a new fertility-restored Ogura CMS material, 16Q2-11, was used as a bridge to transfer the clubroot resistance (CR) gene from the Ogura CMS cytoplasm to the normal cytoplasm by a two-step method (a fertility restoration and cytoplasm replacement method). In the first cross for fertility restoration of Ogura CMS clubroot-resistant cabbage (FRCRC), 16Q2-11 was used as a restorer to cross with Ogura CMS materials containing the CR gene CRb2. Eleven Rfo-positive progenies were generated, of which four contained CRb2: F8-514, F8-620, F8-732 and F8-839. After inoculation with race 4 of P. brassicae, these four CRb2-positive individuals showed resistance. Furthermore, F8-514 and F8-839 were then used as male parents in the second cross of FRCRC to cross with cabbage inbred lines, resulting in the successful introgression of the CRb2 gene into the inbred lines. All offspring produced from this step of cross, which had a normal cytoplasm, showed a high resistance to race 4 of P. brassicae and could be utilized for the breeding of clubroot-resistant cabbage varieties in the future. This is the first time that the Ogura CMS restorer has been used to restore the fertility of Ogura CMS clubroot-resistant cabbages, which could improve germplasm diversity in cabbage and provide a reference method for using CMS germplasm in Brassica crops.

Construction of an anchoring SSR marker genetic linkage map and detection of a sex-linked region in two dioecious populations of red bayberry

Red bayberry (Morella rubra) is an evergreen fruit tree found in southern China whose whole-genome sequence has recently been published. We updated the linkage map of the species by adding 118 SSR markers and the female-specific marker MrFT2_BD-SEX. The integrated map included eight linkage groups and spanned 491 cM. Eleven sex-associated markers were identified, six of which were located in linkage group 8, in agreement with the previously reported location of the sex-determining region. The MrFT2_BD-SEX marker was genotyped in 203 cultivated accessions. Among the females of the accessions, we found two female-specific alleles, designated W-b (151 bp) and W-d (129 bp). We previously found that 'Dongkui', a female cultivar, could produce viable pollen (we refer to such plants 'Dongkui-male') and serve as the paternal parent in crosses. The genotypes of the MrFT2_BD-SEX marker were W-b/Z in 'Biqi' and W-d/Z in 'Dongkui-male'. The progeny of a cross between these parents produced a 3:1 female (W-) to male (ZZ) ratio and the expected 1:1:1:1 ratio of W-b/W-d: W-b/Z: W-d/Z: Z/Z. In addition, the flowering and fruiting phenotypes of all the F1 progeny fit their genotypes. Our results confirm the existence of ZW sex determination and show that the female phenotype is controlled by a single dominant locus (W) in a small genomic region (59 kb and less than 3.3 cM). Furthermore, we have produced a homozygous "super female" (WW) that should produce all-female offspring in the F2 generation, providing a foundation for commercial use and presenting great potential for use in modern breeding programs.

A roadmap for research in octoploid strawberry

The cultivated strawberry (Fragaria × ananassa) is an allo-octoploid species, originating nearly 300 years ago from wild progenitors from the Americas. Since that time the strawberry has become the most widely cultivated fruit crop in the world, universally appealing due to its sensory qualities and health benefits. The recent publication of the first high-quality chromosome-scale octoploid strawberry genome (cv. Camarosa) is enabling rapid advances in genetics, stimulating scientific debate and provoking new research questions. In this forward-looking review we propose avenues of research toward new biological insights and applications to agriculture. Among these are the origins of the genome, characterization of genetic variants, and big data approaches to breeding. Key areas of research in molecular biology will include the control of flowering, fruit development, fruit quality, and plant-pathogen interactions. In order to realize this potential as a global community, investments in genome resources must be continually augmented.

A CIN-like TCP transcription factor (LsTCP4) having retrotransposon insertion associates with a shift from Salinas type to Empire type in crisphead lettuce (Lactuca sativa L.)

To improve several agronomic traits in crisphead lettuce (Lactuca sativa L.) under high-temperature growth conditions, we investigated the correlation among those traits in multiple cultivars and performed genetic mapping of their causal genes. In a field cultivation test of Empire type (serrated leaf) and Salinas type (wavy leaf) cultivars, Empire type cultivars showed increased tipburn susceptibility and late bolting compared with Salinas type cultivars. We attempted genetic mapping of leaf shape and bolting time by ddRAD-seq using the F2 population derived from a cross between 'VI185' (Empire type) and 'ShinanoGreen' (Salinas type). These analyses suggested that both traits are controlled by a single locus in LG5. Genotyping of 51 commercial lettuce cultivars with a tightly linked marker (LG5_v8_252.743Mbp) at this locus showed an association between its genotype and the serrated leaf phenotype. By further fine mapping and transcriptome analysis, a gene encoding putative CIN-like TCP transcription factor was determined to be a candidate gene at this locus and was designated as LsTCP4. An insertion of retrotransposable element was found in the allele of 'VI185', and its transcript level in the leaves was lower than that in 'ShinanoGreen'. Because shapes of leaf epidermal cells in 'VI185' were similar to those in the TCP family mutant of Arabidopsis thaliana, the leaf shape phenotype was likely caused by reduced expression of LsTCP4. Furthermore, because it is known that the TCP family protein also controls flowering time via interaction with FT in A. thaliana, it was highly possible that LsTCP4 gave pleiotropic effects on both leaf shape and bolting time in lettuce.

Genome-wide analysis of expression quantitative trait loci (eQTLs) reveals the regulatory architecture of gene expression variation in the storage roots of sweet potato

Dissecting the genetic regulation of gene expression is critical for understanding phenotypic variation and species evolution. However, our understanding of the transcriptional variability in sweet potato remains limited. Here, we analyzed two publicly available datasets to explore the landscape of transcriptomic variations and its genetic basis in the storage roots of sweet potato. The comprehensive analysis identified a total of 724,438 high-confidence single nucleotide polymorphisms (SNPs) and 26,026 expressed genes. Expression quantitative trait locus (eQTL) analysis revealed 4408 eQTLs regulating the expression of 3646 genes, including 2261 local eQTLs and 2147 distant eQTLs. Two distant eQTL hotspots were found with target genes significantly enriched in specific functional classifications. By combining the information from regulatory network analyses, eQTLs and association mapping, we found that IbMYB1-2 acts as a master regulator and is the major gene responsible for the activation of anthocyanin biosynthesis in the storage roots of sweet potato. Our study provides the first insight into the genetic architecture of genome-wide expression variation in sweet potato and can be used to investigate the potential effects of genetic variants on key agronomic traits in sweet potato.

High frequency DNA rearrangement at qγ27 creates a novel allele for Quality Protein Maize breeding

Copy number variation (CNV) is a major source of genetic variation and often contributes to phenotypic variation in maize. The duplication at the 27-kDa γ-zein locus (qγ27) is essential to convert soft endosperm into hard endosperm in quality protein maize (QPM). This duplication is unstable and generally produces CNV at this locus. We conducted genetic experiments designed to directly measure DNA rearrangement frequencies occurring in males and females of different genetic backgrounds. The average frequency with which the duplication rearranges to single copies is 1.27 × 10-3 and varies among different lines. A triplication of γ27 gene was screened and showed a better potential than the duplication for the future QPM breeding. Our results highlight a novel approach to directly determine the frequency of DNA rearrangements, in this case resulting in CNV at the qγ27 locus. Furthermore, this provides a highly effective way to test suitable parents in QPM breeding.

Selection footprints reflect genomic changes associated with breeding efforts in 56 cucumber inbred lines

Cucumber selective breeding over recent decades has dramatically increased productivity and quality, but the genomic characterizations and changes associated with this breeding history remain unclear. Here, we analyzed the genome resequencing data of 56 artificially selected cucumber inbred lines that exhibit various phenotypes to detect trait-associated sequence variations that reflect breeding improvement. We found that the 56 cucumber lines could be assigned to group 1 and group 2, and the two groups formed a distinctive genetic structure due to the breeding history involving hybridization and selection. Differentially selected regions were identified between group 1 and group 2, with implications for genomic-selection breeding signatures. These regions included known quantitative trait loci or genes that were reported to be associated with agronomic traits. Our results advance knowledge of cucumber genomics, and the 56 selected inbred lines could be good germplasm resources for breeding.

Pan-plastome approach empowers the assessment of genetic variation in cultivated Capsicum species

Pepper species (Capsicum spp.) are widely used as food, spice, decoration, and medicine. Despite the recent old-world culinary impact, more than 50 commercially recognized pod types have been recorded worldwide from three taxonomic complexes (A, B, and P). The current study aimed to apply a pan-plastome approach to resolve the plastomic boundaries among those complexes and identify effective loci for the taxonomical resolution and molecular identification of the studied species/varieties. High-resolution pan-plastomes of five species and two varieties were assembled and compared from 321 accessions. Phyloplastomic and network analyses clarified the taxonomic position of the studied species/varieties and revealed a pronounced number of accessions to be the rare and endemic species, C. galapagoense, that were mistakenly labeled as C. annuum var. glabriusculum among others. Similarly, some NCBI-deposited plastomes were clustered differently from their labels. The rpl23-trnI intergenic spacer contained a 44 bp tandem repeat that, in addition to other InDels, was capable of discriminating the investigated Capsicum species/varieties. The rps16-trnQ/rbcL-accD/ycf3-trnS gene set was determined to be sufficiently polymorphic to retrieve the complete phyloplastomic signal among the studied Capsicum spp. The pan-plastome approach was shown to be useful in resolving the taxonomical complexes, settling the incomplete lineage sorting conflict and developing a molecular marker set for Capsicum spp. identification.

A QTL associated with leaf trichome traits has a major influence on the abundance of the predatory mite Typhlodromus pyri in a hybrid grapevine population

The abundance of predatory phytoseiid mites, Typhlodromus pyri, important biological control agents of spider mite pests in numerous crops, is positively influenced by the density of leaf trichomes and tuft-form domatia in vein axils. Identification of the genetic regions controlling both trophic levels could facilitate the improvement of predatory mite habitat in breeding programs. The abundance of T. pyri and non-glandular trichomes was measured in a segregating F₁ family derived from the cross of the complex Vitis hybrid, 'Horizon', with Illinois 547-1 (V. rupestris B38 × V. cinerea B9), finding positive correlation among traits. High density genetic maps were used to localize one major quantitative trait locus (QTL) on chromosome 1 of Illinois 547-1 associated with both predatory mite abundance and leaf trichomes. This QTL explained 23% of the variation in phytoseiid abundance and similar amounts of variance in domatia rating (21%), domatia size (16%), leaf bristle density (37% in veins and 33% in blades), and leaf hair density (20% in veins and 15% in blades). Another nine QTL distributed among chromosomes 1, 2, 5, 8, and 15 were associated solely with trichome density, and explained 7-17% of the phenotypic variation. Combined, our results provide evidence of the genetic architecture of non-glandular trichomes in Vitis, with a major locus influencing trichome densities, domatia size and predatory mite abundance. This information is relevant for breeding grapevines with a more favorable habitat for biological control agents.

Combining computer vision and deep learning to enable ultra-scale aerial phenotyping and precision agriculture: A case study of lettuce production

Aerial imagery is regularly used by crop researchers, growers and farmers to monitor crops during the growing season. To extract meaningful information from large-scale aerial images collected from the field, high-throughput phenotypic analysis solutions are required, which not only produce high-quality measures of key crop traits, but also support professionals to make prompt and reliable crop management decisions. Here, we report AirSurf, an automated and open-source analytic platform that combines modern computer vision, up-to-date machine learning, and modular software engineering in order to measure yield-related phenotypes from ultra-large aerial imagery. To quantify millions of in-field lettuces acquired by fixed-wing light aircrafts equipped with normalised difference vegetation index (NDVI) sensors, we customised AirSurf by combining computer vision algorithms and a deep-learning classifier trained with over 100,000 labelled lettuce signals. The tailored platform, AirSurf-Lettuce, is capable of scoring and categorising iceberg lettuces with high accuracy (>98%). Furthermore, novel analysis functions have been developed to map lettuce size distribution across the field, based on which associated global positioning system (GPS) tagged harvest regions have been identified to enable growers and farmers to conduct precision agricultural practises in order to improve the actual yield as well as crop marketability before the harvest.

Genome editing for horticultural crop improvement

Horticultural crops provide humans with many valuable products. The improvement of the yield and quality of horticultural crops has been receiving increasing research attention. Given the development and advantages of genome-editing technologies, research that uses genome editing to improve horticultural crops has substantially increased in recent years. Here, we briefly review the different genome-editing systems used in horticultural research with a focus on clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9)-mediated genome editing. We also summarize recent progress in the application of genome editing for horticultural crop improvement. The combination of rapidly advancing genome-editing technology with breeding will greatly increase horticultural crop production and quality.

A point mutation resulting in a 13 bp deletion in the coding sequence of Cldf leads to a GA-deficient dwarf phenotype in watermelon

The dwarf architecture is an important and valuable agronomic trait in watermelon breeding and has the potential to increase fruit yield and reduce labor cost in crop cultivation. However, the molecular basis for dwarfism in watermelon remains largely unknown. In this study, a recessive dwarf allele (designated as Cldf (Citrullus lanatus dwarfism)) was fine mapped in a 32.88 kb region on chromosome 09 using F₂ segregation populations derived from reciprocal crossing of a normal line M08 and a dwarf line N21. Gene annotation of the corresponding region revealed that the Cla015407 gene encoding a gibberellin 3β-hydroxylase functions as the best possible candidate gene for Cldf. Sequence analysis showed that the fourth polymorphism site (a G to A point mutation) at the 3' AG splice receptor site of the intron leads to a 13 bp deletion in the coding sequence of Cldf in dwarf line N21 and thus results in a truncated protein lacking the conserved domain for binding 2-oxoglutarate. In addition, the dwarf phenotype of Cldf could be rescued by exogenous GA₃ application. Phylogenetic analysis suggested that the small multigene family GA3ox (GA3 oxidase) in cucurbit species may originate from three ancient lineages in Cucurbitaceae. All these data support the conclusion that Cldf is a GA-deficient mutant, which together with the cosegregated marker can be used for breeding new dwarf cultivars.

Quantitative trait loci controlling Phytophthora cactorum resistance in the cultivated octoploid strawberry (Fragaria × ananassa)

The cultivated strawberry, Fragaria × ananassa  (Fragaria spp.) is the most economically important global soft fruit. Phytophthora cactorum, a water-borne oomycete causes economic losses in strawberry production globally. A bi-parental cross of octoploid cultivated strawberry segregating for resistance to P. cactorum, the causative agent of crown rot disease, was screened using artificial inoculation. Multiple putative resistance quantitative trait loci (QTL) were identified and mapped. Three major effect QTL (FaRPc6C, FaRPc6D and FaRPc7D) explained 37% of the variation observed. There were no epistatic interactions detected between the three major QTLs. Testing a subset of the mapping population progeny against a range of P. cactorum isolates revealed no significant interaction (p = 0.0593). However, some lines showed higher susceptibility than predicted, indicating that additional undetected factors may affect the expression of some quantitative resistance loci. Using historic crown rot disease score data from strawberry accessions, a preliminary genome-wide association study (GWAS) of 114 individuals revealed an additional locus associated with resistance to P. cactorum. Mining of the Fragaria vesca Hawaii 4 v1.1 genome revealed candidate resistance genes in the QTL regions.

MicroRNA396-mediated alteration in plant development and salinity stress response in creeping bentgrass

The conserved microRNA396 (miR396) is involved in plant growth, development, and abiotic stress response in multiple plant species through regulating its targets, Growth Regulating Factor (GRF) transcription factor genes. However, the role of miR396 has not yet been characterized in perennial monocot species. In addition, the molecular mechanism of miR396-mediated abiotic stress response remains unclear. To elucidate the role of miR396 in perennial monocot species, we generated transgenic creeping bentgrass (Agrostis stolonifera) overexpressing Osa-miR396c, a rice miRNA396 gene. Transgenic plants exhibited altered development, including less shoot and root biomass, shorter internodes, smaller leaf area, fewer leaf veins, and epidermis cells per unit area than those of WT controls. In addition, transgenics showed enhanced salt tolerance associated with improved water retention, increased chlorophyll content, cell membrane integrity, and Na⁺ exclusion during high salinity exposure. Four potential targets of miR396 were identified in creeping bentgrass and up-regulated in response to salt stress. RNA-seq analysis indicates that miR396-mediated salt stress tolerance requires the coordination of stress-related functional proteins (antioxidant enzymes and Na⁺/H⁺ antiporter) and regulatory proteins (transcription factors and protein kinases). This study establishes a miR396-associated molecular pathway to connect the upstream regulatory and downstream functional elements, and provides insight into the miRNA-mediated regulatory networks.

Cgr1, a ripe rot resistance QTL in Vitis amurensis 'Shuang Hong' grapevine

Ripe rot is a serious grapevine disease in Vitis L. and Muscadinia (Planch.) Small. However, resistance to this disease has been reported in some oriental Vitis species. To identify resistance-related Quantitative Trait Loci (QTLs) from the Chinese grape species V. amurensis, an F₁ population of V. vinifera 'Cabernet Sauvignon' × V. amurensis 'Shuang Hong' was used to map the ripe rot resistance loci expected in 'Shuang Hong' grape. A total of 7598 single nucleotide polymorphisms (SNPs) between the parents were identified in our previous study, and 934 SNPs were selected for genetic map construction. These SNPs are distributed across the 19 chromosomes covering a total of 1665.31 cM in length, with an average of 1.81 cM between markers. Ripe rot resistance phenotypes among the hybrids were evaluated in vitro using excised leaves for three consecutive years from 2016 to 2018; a continuous variation was found among the F₁ hybrids, and the Pearson correlation coefficients of the phenotypes scored in all three years were significant at the 0.01 level. Notably, the first QTL reported for resistance to grape ripe rot disease, named Cgr1, was identified on chromosome 14 of 'Shuang Hong' grapevine. Cgr1 could explain up to 19.90% of the phenotypic variance. In addition, a SNP named 'np19345' was identified as a molecular marker closely linked to the peak of Cgr1 and has the potential to be developed as a marker for the Cgr1 resistance haplotype.

Auxin response factor 6A regulates photosynthesis, sugar accumulation, and fruit development in tomato

Auxin response factors (ARFs) are involved in auxin-mediated transcriptional regulation in plants. In this study, we performed functional characterization of SlARF6A in tomato. SlARF6A is located in the nucleus and exhibits transcriptional activator activity. Overexpression of SlARF6A increased chlorophyll contents in the fruits and leaves of tomato plants, whereas downregulation of SlARF6A decreased chlorophyll contents compared with those of wild-type (WT) plants. Analysis of chloroplasts using transmission electron microscopy indicated increased sizes of chloroplasts in SlARF6A-overexpressing plants and decreased numbers of chloroplasts in SlARF6A-downregulated plants. Overexpression of SlARF6A increased the photosynthesis rate and accumulation of starch and soluble sugars, whereas knockdown of SlARF6A resulted in opposite phenotypes in tomato leaves and fruits. RNA-sequence analysis showed that regulation of SlARF6A expression altered the expression of genes involved in chlorophyll metabolism, photosynthesis and sugar metabolism. SlARF6A directly bound to the promoters of SlGLK1, CAB, and RbcS genes and positively regulated the expression of these genes. Overexpression of SlARF6A also inhibited fruit ripening and ethylene production, whereas downregulation of SlARF6A increased fruit ripening and ethylene production. SlARF6A directly bound to the SAMS1 promoter and negatively regulated SAMS1 expression. Taken together, these results expand our understanding of ARFs with regard to photosynthesis, sugar accumulation and fruit development and provide a potential target for genetic engineering to improve fruit nutrition in horticulture crops.