Major Impacts of Widespread Structural Variation on Gene Expression and Crop Improvement in Tomato.

Recent years have seen a surge in plant genome sequencing projects and the comparison of multiple related individuals. The high degree of genomic variation observed led to the realization that single reference genomes do not represent the diversity within a species, and led to the expansion of the pan-genome concept. Pan-genomes represent the genomic diversity of a species and includes core genes, found in all individuals, as well as variable genes, which are absent in some individuals. Variable gene annotations often show similarities across plant species, with genes for biotic and abiotic stress commonly enriched within variable gene groups. Here we review the growth of pan-genomics in plants, explore the origins of gene presence and absence variation, and show how pan-genomes can support plant breeding and evolution studies.

/pdf/plant-pan-genomes-are-the-new-reference-4979sf46zn.pdf

Plant pan-genomes are the new reference

Since the early days of the genome era, the scientific community has relied on a single ‘reference’ genome for each species, which is used as the basis for a wide range of genetic analyses, including studies of variation within and across species. As sequencing costs have dropped, thousands of new genomes have been sequenced, and scientists have come to realize that a single reference genome is inadequate for many purposes. By sampling a diverse set of individuals, one can begin to assemble a pan-genome: a collection of all the DNA sequences that occur in a species. Here we review efforts to create pan-genomes for a range of species, from bacteria to humans, and we further consider the computational methods that have been proposed in order to capture, interpret and compare pan-genome data. As scientists continue to survey and catalogue the genomic variation across human populations and begin to assemble a human pan-genome, these efforts will increase our power to connect variation to human diversity, disease and beyond. Although single reference genomes are valuable resources, they do not capture genetic diversity among individuals. Sherman and Salzberg discuss the concept of ‘pan-genomes’, which are reference genomes that encompass the genetic variation within a given species. Focusing particularly on large eukaryotic pan-genomes, they describe the latest progress, the varied methodological approaches and computational challenges, as well as applications in fields such as agriculture and human disease.

Pan-genomics in the human genome era

Super-Pangenome by Integrating the Wild Side of a Species for Accelerated Crop Improvement

Phytohormones are integral to the regulation of fruit development and maturation. This review expands upon current understanding of the relationship between hormone signaling and fruit development, emphasizing fleshy fruit and highlighting recent work in the model crop tomato (Solanum lycopersicum) and additional species. Fruit development comprises fruit set initiation, growth, and maturation and ripening. Fruit set transpires after fertilization and is associated with auxin and gibberellic acid (GA) signaling. Interaction between auxin and GAs, as well as other phytohormones, is mediated by auxin-responsive Aux/IAA and ARF proteins. Fruit growth consists of cell division and expansion, the former shown to be influenced by auxin signaling. While regulation of cell expansion is less thoroughly understood, evidence indicates synergistic regulation via both auxin and GAs, with input from additional hormones. Fruit maturation, a transitional phase that precipitates ripening, occurs when auxin and GA levels subside with a concurrent rise in abscisic acid (ABA) and ethylene. During fruit ripening, ethylene plays a clear role in climacteric fruits, whereas non-climacteric ripening is generally associated with ABA. Recent evidence indicates varying requirements for both hormones within both ripening physiologies, suggesting rebalancing and specification of roles for common regulators rather than reliance upon one. Numerous recent discoveries pertaining to the molecular basis of hormonal activity and crosstalk are discussed, while we also note that many questions remain such as the molecular basis of additional hormonal activities, the role of epigenome changes, and how prior discoveries translate to the plethora of angiosperm species.

Phytohormones in fruit development and maturation

Modern tomatoes have narrow genetic diversity limiting their improvement potential. We present a tomato pan-genome constructed using genome sequences of 725 phylogenetically and geographically representative accessions, revealing 4,873 genes absent from the reference genome. Presence/absence variation analyses reveal substantial gene loss and intense negative selection of genes and promoters during tomato domestication and improvement. Lost or negatively selected genes are enriched for important traits, especially disease resistance. We identify a rare allele in the TomLoxC promoter selected against during domestication. Quantitative trait locus mapping and analysis of transgenic plants reveal a role for TomLoxC in apocarotenoid production, which contributes to desirable tomato flavor. In orange-stage fruit, accessions harboring both the rare and common TomLoxC alleles (heterozygotes) have higher TomLoxC expression than those homozygous for either and are resurgent in modern tomatoes. The tomato pan-genome adds depth and completeness to the reference genome, and is useful for future biological discovery and breeding.

https://www.nature.com/articles/s41588-019-0410-2.pdf?proof=t

The tomato pan-genome uncovers new genes and a rare allele regulating fruit flavor.

Headspace solid-phase microextraction (HS-SPME) coupled to gas chromatography–mass spectrometry (GC-MS) is widely employed for volatile analyses of plants, including mapping populations used in plant breeding research. Studies often employ a single internal surrogate standard, even when multiple analytes are measured, with the assumption that any relative changes in matrix effects among individuals would be similar for all compounds, i.e., matrix effects do not show Compound × Individual interactions. We tested this assumption using individuals from two plant populations: an interspecific grape (Vitis spp.) mapping population (n = 140) and a tomato (Solanum spp.) recombinant inbred line (RIL) population (n = 148). Individual plants from the two populations were spiked with a cocktail of internal standards (n = 6, 9, respectively) prior to HS-SPME-GC-MS. Variation in the relative responses of internal standards indicated that Compound × Individual interactions exist but were different between the two populations. For the grape population, relative responses among pairs of internal standards varied considerably among individuals, with a maximum of 249% relative standard deviation (RSD) for the pair of [U13C]hexanal and [U13C]hexanol. However, in the tomato population, relative responses of internal standard pairs varied much less, with pairwise RSDs ranging from 8% to 56%. The approach described in this paper could be used to evaluate the suitability of using surrogate standards for HS-SPME-GC-MS studies in other plant populations.

https://www.mdpi.com/1420-3049/23/10/2436/pdf

HS-SPME-GC-MS analyses of volatiles in plant populations-quantitating compound × individual matrix effects

Wild Vitis spp. and their interspecific hybrids are known to have high malate concentrations at sugar maturity as compared to domesticated Vitis vinifera, but it is unknown if differences in malate at harvest among species arise from differences in malate accumulation or degradation. Over two years, fruit from Vitis riparia and Vitis cinerea accessions along with commercial V. vinifera and interspecific hybrid cultivars were collected at multiple time points. In contrast to the well-known biphasic behavior of malate in V. vinifera (preveraison accumulation, postveraison degradation), we observed a range of behaviors for malate in wild species. On average, V. riparia accessions had malate per berry comparable to V. vinifera just prior to veraison, but degraded malate to a much lesser extent. V. cinerea accessions had lower malate prior to veraison than other species but showed an increase in malate from pre- to postveraison. Variation in postveraison malate behavior appears related to diminished malate degradation in the mesocarp of wild Vitis spp. Our results indicate that studies of malate behavior in Vitis spp. and their hybrids should include both pre- and postveraison time points.

Elizabeth A. Burzynski-Chang

Papers

The tomato pan-genome uncovers new genes and a rare allele regulating fruit flavor.

HS-SPME-GC-MS analyses of volatiles in plant populations-quantitating compound × individual matrix effects

Malate Content in Wild Vitis spp. Demonstrates a Range of Behaviors during Berry Maturation