Deciphering the biological processes underlying tomato biomass production and composition

Deeper genomic insights into tomato CLE genes repertoire identify new active peptides

Background

In eukaryotes, cell-to-cell communication relies on the activity of small signaling peptides. In plant genomes, many hundreds of genes encode for such short peptide signals. However, only few of them are functionally characterized and due to the small gene size and high sequence variability, the comprehensive identification of such peptide-encoded genes is challenging. The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-RELATED (CLE) gene family encodes for short peptides that have a role in plant meristem maintenance, vascular patterning and responses to environment. The full repertoire of CLE genes and the role of CLE signaling in tomato (Solanum lycopersicum)- one of the most important crop plants- has not yet been fully studied.

Results

By using a combined approach, we performed a genome-wide identification of CLE genes using the current tomato genome version SL 4.0. We identified 52 SlCLE genes, including 37 new non annotated before. By analyzing publicly available RNAseq datasets we could confirm the expression of 28 new SlCLE genes. We found that SlCLEs are often expressed in a tissue-, organ- or condition-specific manner. Our analysis shows an interesting gene diversification within the SlCLE family that seems to be a result of gene duplication events. Finally, we could show a biological activity of selected SlCLE peptides in the root growth arrest that was SlCLV2-dependent.

Conclusions

Our improved combined approach revealed 37 new SlCLE genes. These findings are crucial for better understanding of the CLE signaling in tomato. Our phylogenetic analysis pinpoints the closest homologs of Arabidopsis CLE genes in tomato genome and can give a hint about the function of newly identified SlCLEs. The strategy described here can be used to identify more precisely additional short genes in plant genomes. Finally, our work suggests that the mechanism of root-active CLE peptide perception is conserved between Arabidopsis and tomato. In conclusion, our work paves the way to further research on the CLE-dependent circuits modulating tomato development and physiological responses.

Unlocking the molecular basis of wheat straw composition and morphological traits through multi-locus GWAS

BACKGROUND

Rapid reductions in emissions from fossil fuel burning are needed to curb global climate change. Biofuel production from crop residues can contribute to reducing the energy crisis and environmental deterioration. Wheat is a renewable source for biofuels owing to the low cost and high availability of its residues. Thus, identifying candidate genes controlling these traits is pivotal for efficient biofuel production. Here, six multi-locus genome-wide association (ML-GWAS) models were applied using 185 tetraploid wheat accessions to detect quantitative trait nucleotides (QTNs) for fifteen traits associated with biomass composition.

RESULTS

Among the 470 QTNs, only 72 identified by at least two models were considered as reliable. Among these latter, 16 also showed a significant effect on the corresponding trait (p.value < 0.05). Candidate genes survey carried out within 4 Mb flanking the QTNs, revealed putative biological functions associated with lipid transfer and metabolism, cell wall modifications, cell cycle, and photosynthesis. Four genes encoded as Cellulose Synthase (CeSa), Anaphase promoting complex (APC/C), Glucoronoxylan 4-O Methyltransferase (GXM) and HYPONASTIC LEAVES1 (HYL1) might be responsible for an increase in cellulose, and natural and acid detergent fiber (NDF and ADF) content in tetraploid wheat. In addition, the SNP marker RFL_Contig3228_2154 associated with the variation in stem solidness (Q.Scsb-3B) was validated through two molecular methods (High resolution melting; HRM and RNase H²-dependent PCR; rhAMP).

CONCLUSIONS

The study provides new insights into the genetic basis of biomass composition traits on tetraploid wheat. The application of six ML-GWAS models on a panel of diverse wheat genotypes represents an efficient approach to dissect complex traits with low heritability such as wheat straw composition. The discovery of genes/genomic regions associated with biomass production and straw quality parameters is expected to accelerate the development of high-yielding wheat varieties useful for biofuel production.

Large-scale gene gains and losses molded the NLR defense arsenal during the Cucurbita evolution

Genome-wide annotation reveals that the gene birth-death process of the Cucurbita R family is associated with a species-specific diversification of TNL and CNL protein classes. The Cucurbitaceae family includes nearly 1000 plant species known universally as cucurbits. Cucurbita genus includes many economically important worldwide crops vulnerable to more than 200 pathogens. Therefore, the identification of pathogen-recognition genes is of utmost importance for this genus. The major class of plant-resistance (R) genes encodes nucleotide-binding site and leucine-rich repeat (NLR) proteins, and is divided into three sub-classes namely, TIR-NB-LRR (TNL), CC-NB-LRR (CNL) and RPW8-NB-LRR (RNL). Although the characterization of the NLR gene family has been carried out in important Cucurbita species, this information is still linked to the availability of sequenced genomes. In this study, we analyzed 40 de novo transcriptomes and 5 genome assemblies, which were explored to investigate the Cucurbita expressed-NLR (eNLR) and NLR repertoires using an ad hoc gene annotation approach. Over 1850 NLR-encoding genes were identified, finely characterized and compared to 96 well-characterized plant R-genes. The maximum likelihood analyses revealed an unusual diversification of CNL/TNL genes and a strong RNL conservation. Indeed, several gene gain and loss events have shaped the Cucurbita NLR family. Finally, to provide a first validation step Cucurbita, eNLRs were explored by real-time PCR analysis. The NLR repertories of the 12 Cucurbita species presented in this paper will be useful to discover novel R-genes.

Tomato transcriptomic response to Tuta absoluta infestation

BACKGROUND

The South America pinworm, Tuta absoluta, is a destructive pest of tomato that causes important losses worldwide. Breeding of resistant/tolerant tomato cultivars could be an effective strategy for T. absoluta management but, despite the economic importance of tomato, very limited information is available about its response to this treat. To elucidate the defense mechanisms to herbivore feeding a comparative analysis was performed between a tolerant and susceptible cultivated tomato at both morphological and transcriptome level to highlight constitutive leaf barriers, molecular and biochemical mechanisms to counter the effect of T. absoluta attack.

RESULTS

The tolerant genotype showed an enhanced constitutive barrier possibly as result of the higher density of trichomes and increased inducible reactions upon mild infestation thanks to the activation/repression of key transcription factors regulating genes involved in cuticle formation and cell wall strength as well as of antinutritive enzymes, and genes involved in the production of chemical toxins and bioactive secondary metabolites.

CONCLUSIONS

Overall, our findings suggest that tomato resilience to the South America pinworm is achieved by a combined strategy between constitutive and induced defense system. A well-orchestrated modulation of plant transcription regulation could ensure a trade-off between defense needs and fitness costs. Our finding can be further exploited for developing T. absoluta tolerant cultivars, acting as important component of integrated pest management strategy for more sustainable production.

Genomic Analyses of Phenotypic Differences Between Native and Invasive Populations of Diffuse Knapweed (Centaurea diffusa)

Invasive species represent excellent opportunities to study the evolutionary potential of traits important to success in novel environments. Although some ecologically important traits have been identified in invasive species, little is typically known about the genetic mechanisms that underlie invasion success in non-model species. Here, we use a genome-wide association (GWAS) approach to identify the genetic basis of trait variation in the non-model, invasive, diffuse knapweed [Centaurea diffusa Lam. (Asteraceae)]. To assist with this analysis, we have assembled the first draft genome reference and fully annotated plastome assembly for this species, and one of the first from this large, weedy, genus, which is of major ecological and economic importance. We collected phenotype data from 372 individuals from four native and four invasive populations of C. diffusa grown in a common environment. Using these individuals, we produced reduced-representation genotype-by-sequencing (GBS) libraries and identified 7,058 SNPs. We identify two SNPs associated with leaf width in these populations, a trait which significantly varies between native and invasive populations. In this rosette forming species, increased leaf width is a major component of increased biomass, a common trait in invasive plants correlated with increased fitness. Finally, we use annotations from Arabidopsis thaliana to identify 98 candidate genes that are near the associated SNPs and highlight several good candidates for leaf width variation.

Accelerating Tomato Breeding by Exploiting Genomic Selection Approaches

Genomic selection (GS) is a predictive approach that was built up to increase the rate of genetic gain per unit of time and reduce the generation interval by utilizing genome-wide markers in breeding programs. It has emerged as a valuable method for improving complex traits that are controlled by many genes with small effects. GS enables the prediction of the breeding value of candidate genotypes for selection. In this work, we address important issues related to GS and its implementation in the plant context with special emphasis on tomato breeding. Genomic constraints and critical parameters affecting the accuracy of prediction such as the number of markers, statistical model, phenotyping and complexity of trait, training population size and composition should be carefully evaluated. The comparison of GS approaches for facilitating the selection of tomato superior genotypes during breeding programs is also discussed. GS applied to tomato breeding has already been shown to be feasible. We illustrated how GS can improve the rate of gain in elite line selection, and descendent and backcross schemes. The GS schemes have begun to be delineated and computer science can provide support for future selection strategies. A new promising breeding framework is beginning to emerge for optimizing tomato improvement procedures.