Investigation of orthologous pathogen recognition gene-rich regions in solanaceous species

Genome-wide characterization of the NLR gene family in tomato (Solanum lycopersicum) and their relatedness to disease resistance

Nucleotide-binding leucine-rich-repeat receptors (NLR), the largest group of genes associated with plant disease resistance (R), have attracted attention due to their crucial role in protecting plants from pathogens. Genome-wide studies of NLRs have revealed conserved domains in the annotated tomato genome. The 321 NLR genes identified in the tomato genome have been randomly mapped to 12 chromosomes. Phylogenetic analysis and classification of NLRs have revealed that 211 genes share full-length domains categorized into three major clades (CNL, TNL, and RNL); the remaining 110 NLRs share partial domains and are classified in CN, TN, and N according to their motifs and gene structures. The cis-regulatory elements of NLRs exhibit the maximum number of these elements and are involved in response to biotic and abiotic stresses, pathogen recognition, and resistance. Analysis of the phylogenetic relationship between tomato NLRs and orthologs in other species has shown conservation among Solanaceae members and variation with A. thaliana. Synteny and Ka/Ks analyses of Solanum lycopersicum and Solanum tuberosum orthologs have underscored the importance of NLR conservation and diversification from ancestral species millions of years ago. RNA-seq data and qPCR analysis of early and late blight diseases in tomatoes revealed consistent NLR expression patterns, including upregulation in infected compared to control plants (with some exceptions), suggesting the role of NLRs as key regulators in early blight resistance. Moreover, the expression levels of NLRs associated with late blight resistance (Solyc04g007060 [NRC4] and Solyc10g008240 [RIB12]) suggested that they regulate S. lycopersicum resistance to P. infestans. These findings provide important fundamental knowledge for understanding NLR evolution and diversity and will empower the broader characterization of disease resistance genes for pyramiding through speed cloning to develop disease-tolerant varieties.

NB-LRR Lineage-Specific Equipment Is Sorted Out by Sequence Pattern Adaptation and Domain Segment Shuffling

The nucleotide-binding and leucine-rich repeat (NB-LRR) genes, also known as resistance (R)-genes, play an important role in the activation of immune responses. In recent years, large-scale studies have been performed to highlight the diversification of plant NB-LRR repertories. It is well known that, to provide new functionalities, NB-LRR sequences are subject to duplication, domain fusions and acquisition and other kinds of mutations. Although some mechanisms that govern NB-LRR protein domain adaptations have been uncovered, to retrace the plant-lineage-specific evolution routes of R protein structure, a multi-genome comparative analysis was performed. This study allowed us to define groups of genes sharing homology relationships across different species. It is worth noting that the most populated groups contained well-characterized R proteins. The arsenal profile of such groups was investigated in five botanical families, including important crop species, to underline specific adaptation signatures. In addition, the dissection of 70 NB domains of well-characterized R-genes revealed the NB core motifs from which the three main R protein classes have been diversified. The structural remodeling of domain segments shaped the specific NB-LRR repertoires observed in each plant species. This analysis provided new evolutionary and functional insights on NB protein domain shuffling. Taken together, such findings improved our understanding of the molecular adaptive selection mechanisms occurring at plant R loci.

Multilevel evolution shapes the function of NB-LRR encoding genes in plant innate immunity

A sophisticated innate immune system based on diverse pathogen receptor genes (PRGs) evolved in the history of plant life. To reconstruct the direction and magnitude of evolutionary trajectories of a given gene family, it is critical to detect the ancestral signatures. The rearrangement of functional domains made up the diversification found in PRG repertoires. Structural rearrangement of ancient domains mediated the NB-LRR evolutionary path from an initial set of modular proteins. Events such as domain acquisition, sequence modification and temporary or stable associations are prominent among rapidly evolving innate immune receptors. Over time PRGs are continuously shaped by different forces to find their optimal arrangement along the genome. The immune system is controlled by a robust regulatory system that works at different scales. It is important to understand how the PRG interaction network can be adjusted to meet specific needs. The high plasticity of the innate immune system is based on a sophisticated functional architecture and multi-level control. Due to the complexity of interacting with diverse pathogens, multiple defense lines have been organized into interconnected groups. Genomic architecture, gene expression regulation and functional arrangement of PRGs allow the deployment of an appropriate innate immunity response.

Prediction of NB-LRR resistance genes based on full-length sequence homology

The activation of plant immunity is mediated by resistance (R)-gene receptors, also known as nucleotide-binding leucine-rich repeat (NB-LRR) genes, which in turn trigger the authentic defense response. R-gene identification is a crucial goal for both classic and modern plant breeding strategies for disease resistance. The conventional method identifies NB-LRR genes using a protein motif/domain-based search (PDS) within an automatically predicted gene set of the respective genome assembly. PDS proved to be imprecise since repeat masking prior to automatic genome annotation unwittingly prevented comprehensive NB-LRR gene detection. Furthermore, R-genes have diversified in a species-specific manner, so that NB-LRR gene identification cannot be universally standardized. Here, we present the full-length Homology-based R-gene Prediction (HRP) method for the comprehensive identification and annotation of a genome's R-gene repertoire. Our method has substantially addressed the complex genomic organization of tomato (Solanum lycopersicum) NB-LRR gene loci, proving to be more performant than the well-established RenSeq approach. HRP efficiency was also tested on three differently assembled and annotated Beta sp. genomes. Indeed, HRP identified up to 45% more full-length NB-LRR genes compared to previous approaches. HRP also turned out to be a more refined strategy for R-gene allele mining, testified by the identification of hitherto undiscovered Fom-2 homologs in five Cucurbita sp. genomes. In summary, our high-performance method for full-length NB-LRR gene discovery will propel the identification of novel R-genes towards development of improved cultivars.

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.

The Tomato Interspecific NB-LRR Gene Arsenal and Its Impact on Breeding Strategies

Tomato (Solanum lycopersicum L.) is a model system for studying the molecular basis of resistance in plants. The investigation of evolutionary dynamics of tomato resistance (R)-loci provides unique opportunities for identifying factors that promote or constrain genome evolution. Nucleotide-binding domain and leucine-rich repeat (NB-LRR) receptors belong to one of the most plastic and diversified families. The vast amount of genomic data available for Solanaceae and wild tomato relatives provides unprecedented insights into the patterns and mechanisms of evolution of NB-LRR genes. Comparative analysis remarked a reshuffling of R-islands on chromosomes and a high degree of adaptive diversification in key R-loci induced by species-specific pathogen pressure. Unveiling NB-LRR natural variation in tomato and in other Solanaceae species offers the opportunity to effectively exploit genetic diversity in genomic-driven breeding programs with the aim of identifying and introducing new resistances in tomato cultivars. Within this motivating context, we reviewed the repertoire of NB-LRR genes available for tomato improvement with a special focus on signatures of adaptive processes. This issue is still relevant and not thoroughly investigated. We believe that the discovery of mechanisms involved in the generation of a gene with new resistance functions will bring great benefits to future breeding strategies.

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.

A chromosome-anchored eggplant genome sequence reveals key events in Solanaceae evolution

With approximately 450 species, spiny Solanum species constitute the largest monophyletic group in the Solanaceae family, but a high-quality genome assembly from this group is presently missing. We obtained a chromosome-anchored genome assembly of eggplant (Solanum melongena), containing 34,916 genes, confirming that the diploid gene number in the Solanaceae is around 35,000. Comparative genomic studies with tomato (S. lycopersicum), potato (S. tuberosum) and pepper (Capsicum annuum) highlighted the rapid evolution of miRNA:mRNA regulatory pairs and R-type defense genes in the Solanaceae, and provided a genomic basis for the lack of steroidal glycoalkaloid compounds in the Capsicum genus. Using parsimony methods, we reconstructed the putative chromosomal complements of the key founders of the main Solanaceae clades and the rearrangements that led to the karyotypes of extant species and their ancestors. From 10% to 15% of the genes present in the four genomes were syntenic paralogs (ohnologs) generated by the pre-γ, γ and T paleopolyploidy events, and were enriched in transcription factors. Our data suggest that the basic gene network controlling fruit ripening is conserved in different Solanaceae clades, and that climacteric fruit ripening involves a differential regulation of relatively few components of this network, including CNR and ethylene biosynthetic genes.

Transgressive segregation reveals mechanisms of Arabidopsis immunity to Brassica-infecting races of white rust (Albugo candida)

Most plants resist most plant pathogens. Barley resists wheat-infecting powdery mildew races (and vice versa), and both barley and wheat resist potato late blight. Such “nonhost” resistance could result because the pathogen fails to suppress defense or triggers innate immunity due to failure to evade detection. Albugo candida causes white rust on most Brassicaceae, and we investigated Arabidopsis NHR to Brassica-infecting races. Transgressive segregation for resistance in Arabidopsis recombinant inbred lines revealed genes encoding nucleotide-binding, leucine-rich repeat (NLR) immune receptors. Some of these NLR-encoding genes confer resistance to white rust in Brassica sp. This genetic method thus provides a route to reveal resistance genes for crops, widening the pool from which such genes might be obtained.

Arabidopsis thaliana accessions are universally resistant at the adult leaf stage to white rust (Albugo candida) races that infect the crop species Brassica juncea and Brassica oleracea. We used transgressive segregation in recombinant inbred lines to test if this apparent species-wide (nonhost) resistance in A. thaliana is due to natural pyramiding of multiple Resistance (R) genes. We screened 593 inbred lines from an Arabidopsis multiparent advanced generation intercross (MAGIC) mapping population, derived from 19 resistant parental accessions, and identified two transgressive segregants that are susceptible to the pathogen. These were crossed to each MAGIC parent, and analysis of resulting F₂ progeny followed by positional cloning showed that resistance to an isolate of A. candida race 2 (Ac2V) can be explained in each accession by at least one of four genes encoding nucleotide-binding, leucine-rich repeat (NLR) immune receptors. An additional gene was identified that confers resistance to an isolate of A. candida race 9 (AcBoT) that infects B. oleracea. Thus, effector-triggered immunity conferred by distinct NLR-encoding genes in multiple A. thaliana accessions provides species-wide resistance to these crop pathogens.

Genome Sequencing of Ancient Plant Remains: Findings, Uses and Potential Applications for the Study and Improvement of Modern Crops

The advent of new sequencing technologies is revolutionizing the studies of ancient DNA (aDNA). In the last 30 years, DNA extracted from the ancient remains of several plant species has been explored in small-scale studies, contributing to understand the adaptation, and migration patterns of important crops. More recently, NGS technologies applied on aDNA have opened up new avenues of research, allowing investigation of the domestication process on the whole-genome scale. Genomic approaches based on genome-wide and targeted sequencing have been shown to provide important information on crop evolution and on the history of agriculture. Huge amounts of next-generation sequencing (NGS) data offer various solutions to overcome problems related to the origin of the material, such as degradation, fragmentation of polynucleotides, and external contamination. Recent advances made in several crop domestication studies have boosted interest in this research area. Remains of any nature are potential candidates for aDNA recovery and almost all the analyses that can be made on fresh DNA can also be performed on aDNA. The analysis performed on aDNA can shed light on many phylogenetic questions concerning evolution, domestication, and improvement of plant species. It is a powerful instrument to reconstruct patterns of crop adaptation and migration. Information gathered can also be used in many fields of modern agriculture such as classical breeding, genome editing, pest management, and product promotion. Whilst unlocking the hidden genome of ancient crops offers great potential, the onus is now on the research community to use such information to gain new insight into agriculture.