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

A highly diversified NLR cluster in melon contains homologs that confer powdery mildew and aphid resistance

Podosphaera xanthii is the main causal agent of powdery mildew (PM) on Cucurbitaceae. In Cucumis melo, the Pm-w resistance gene, which confers resistance to P. xanthii, is located on chromosome 5 in a cluster of nucleotide-binding leucine-rich repeat receptors (NLRs). We used positional cloning and transgenesis, to isolate the Pm-wWMR 29 gene encoding a coiled-coil NLR (CC-NLR). Pm-wWMR 29 conferred high level of resistance to race 1 of PM and intermediate level of resistance to race 3 of PM. Pm-wWMR 29 turned out to be a homolog of the Aphis gossypii resistance gene Vat-1PI 161375. We confirmed that Pm-wWMR 29 did not confer resistance to aphids, while Vat-1PI 161375 did not confer resistance to PM. We showed that both homologs were included in a highly diversified cluster of NLRs, the Vat cluster. Specific Vat-1PI 161375 and Pm-wWMR 29 markers were present in 10% to 13% of 678 accessions representative of wild and cultivated melon types worldwide. Phylogenic reconstruction of 34 protein homologs of Vat-1PI 161375 and Pm-wWMR 29 identified in 24 melon accessions revealed an ancestor with four R65aa-a specific motif in the LRR domain, evolved towards aphid and virus resistance, while an ancestor with five R65aa evolved towards PM resistance. The complexity of the cluster comprising the Vat/Pm-w genes and its diversity in melon suggest that Vat homologs may contribute to the recognition of a broad range of yet to be identified pests and pathogens.

Approaches to increase the validity of gene family identification using manual homology search tools

Identifying homologs is an important process in the analysis of genetic patterns underlying traits and evolutionary relationships among species. Analysis of gene families is often used to form and support hypotheses on genetic patterns such as gene presence, absence, or functional divergence which underlie traits examined in functional studies. These analyses often require precise identification of all members in a targeted gene family. Manual pipelines where homology search and orthology assignment tools are used separately are the most common approach for identifying small gene families where accurate identification of all members is important. The ability to curate sequences between steps in manual pipelines allows for simple and precise identification of all possible gene family members. However, the validity of such manual pipeline analyses is often decreased by inappropriate approaches to homology searches including too relaxed or stringent statistical thresholds, inappropriate query sequences, homology classification based on sequence similarity alone, and low-quality proteome or genome sequences. In this article, we propose several approaches to mitigate these issues and allow for precise identification of gene family members and support for hypotheses linking genetic patterns to functional traits.

The First Genome-Wide Mildew Locus O Genes Characterization in the Lamiaceae Plant Family

Powdery mildew (PM) is a widespread plant disease that causes significant economic losses in thousands crops of temperate climates, including Lamiaceae species. Multiple scientific studies describe a peculiar form of PM-resistance associated at the inactivation of specific members of the Mildew Locus O (MLO) gene family, referred to as mlo-resistance. The characterization of Lamiaceae MLO genes, at the genomic level, would be a first step toward their potential use in breeding programs. We carried out a genome-wide characterization of the MLO gene family in 11 Lamiaceae species, providing a manual curated catalog of 324 MLO proteins. Evolutionary history and phylogenetic relationships were studied through maximum likelihood analysis and motif patter reconstruction. Our approach highlighted seven different clades diversified starting from an ancestral MLO domain pattern organized in 18 highly conserved motifs. In addition, 74 Lamiaceae putative PM susceptibility genes, clustering in clade V, were identified. Finally, we performed a codon-based evolutionary analysis, revealing a general high level of purifying selection in the eleven Lamiaceae MLO gene families, and the occurrence of few regions under diversifying selection in candidate susceptibility factors. The results of this work may help to address further biological questions concerning MLOs involved in PM susceptibility. In follow-up studies, it could be investigated whether the silencing or loss-of-function mutations in one or more of these candidate genes may lead to PM resistance.

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.