Relationships between rust resistance genes at the M locus in flax

Overview and Management of the Most Common Eukaryotic Diseases of Flax (Linum usitatissimum)

Flax is an important crop cultivated for its seeds and fibers. It is widely grown in temperate regions, with an increase in cultivation areas for seed production (linseed) in the past 50 years and for fiber production (fiber flax) in the last decade. Among fiber-producing crops, fiber flax is the most valuable species. Linseed is the highest omega-3 oleaginous crop, and its consumption provides several benefits for animal and human health. However, flax production is impacted by various abiotic and biotic factors that affect yield and quality. Among biotic factors, eukaryotic diseases pose a significant threat to both seed production and fiber quality, which highlights the economic importance of controlling these diseases. This review focuses on the major eukaryotic diseases that affect flax in the field, describing the pathogens, their transmission modes and the associated plant symptoms. Moreover, this article aims to identify the challenges in disease management and provide future perspectives to overcome these biotic stresses in flax cultivation. By emphasizing the key diseases and their management, this review can aid in promoting sustainable and profitable flax production.

A rust-fungus Nudix hydrolase effector decaps mRNA in vitro and interferes with plant immune pathways

To infect plants, pathogenic fungi secrete small proteins called effectors. Here, we describe the catalytic activity and potential virulence function of the Nudix hydrolase effector AvrM14 from the flax rust fungus (Melampsora lini). We completed extensive in vitro assays to characterise the enzymatic activity of the AvrM14 effector. Additionally, we used in planta transient expression of wild-type and catalytically dead AvrM14 versions followed by biochemical assays, phenotypic analysis and RNA sequencing to unravel how the catalytic activity of AvrM14 impacts plant immunity. AvrM14 is an extremely selective enzyme capable of removing the protective 5' cap from mRNA transcripts in vitro. Homodimerisation of AvrM14 promoted biologically relevant mRNA cap cleavage in vitro and this activity was conserved in related effectors from other Melampsora spp. In planta expression of wild-type AvrM14, but not the catalytically dead version, suppressed immune-related reactive oxygen species production, altered the abundance of some circadian-rhythm-associated mRNA transcripts and reduced the hypersensitive cell-death response triggered by the flax disease resistance protein M1. To date, the decapping of host mRNA as a virulence strategy has not been described beyond viruses. Our results indicate that some fungal pathogens produce Nudix hydrolase effectors with in vitro mRNA-decapping activity capable of interfering with plant immunity.

Genetic Mapping and Molecular Characterization of a Broad-spectrum Phytophthora sojae Resistance Gene in Chinese Soybean

Phytophthora root rot (PRR) causes serious annual soybean yield losses worldwide. The most effective method to prevent PRR involves growing cultivars that possess genes conferring resistance to Phytophthora sojae (Rps). In this study, QTL-sequencing combined with genetic mapping was used to identify RpsX in soybean cultivar Xiu94-11 resistance to all P. sojae isolates tested, exhibiting broad-spectrum PRR resistance. Subsequent analysis revealed RpsX was located in the 242-kb genomic region spanning the RpsQ locus. However, a phylogenetic investigation indicated Xiu94-11 carrying RpsX is distantly related to the cultivars containing RpsQ, implying RpsX and RpsQ have different origins. An examination of candidate genes revealed RpsX and RpsQ share common nonsynonymous SNP and a 144-bp insertion in the Glyma.03g027200 sequence encoding a leucine-rich repeat (LRR) region. Glyma.03g027200 was considered to be the likely candidate gene of RpsQ and RpsX. Sequence analyses confirmed that the 144-bp insertion caused by an unequal exchange resulted in two additional LRR-encoding fragments in the candidate gene. A marker developed based on the 144-bp insertion was used to analyze the genetic population and germplasm, and proved to be useful for identifying the RpsX and RpsQ alleles. This study implies that the number of LRR units in the LRR domain may be important for PRR resistance in soybean.

Evolution of Disease Defense Genes and Their Regulators in Plants

Biotic stresses do damage to the growth and development of plants, and yield losses for some crops. Confronted with microbial infections, plants have evolved multiple defense mechanisms, which play important roles in the never-ending molecular arms race of plant–pathogen interactions. The complicated defense systems include pathogen-associated molecular patterns (PAMP) triggered immunity (PTI), effector triggered immunity (ETI), and the exosome-mediated cross-kingdom RNA interference (CKRI) system. Furthermore, plants have evolved a classical regulation system mediated by miRNAs to regulate these defense genes. Most of the genes/small RNAs or their regulators that involve in the defense pathways can have very rapid evolutionary rates in the longitudinal and horizontal co-evolution with pathogens. According to these internal defense mechanisms, some strategies such as molecular switch for the disease resistance genes, host-induced gene silencing (HIGS), and the new generation of RNA-based fungicides, have been developed to control multiple plant diseases. These broadly applicable new strategies by transgene or spraying ds/sRNA may lead to reduced application of pesticides and improved crop yield.

Genome analysis and avirulence gene cloning using a high-density RADseq linkage map of the flax rust fungus, Melampsora lini

BACKGROUND

Rust fungi are an important group of plant pathogens that cause devastating losses in agricultural, silvicultural and natural ecosystems. Plants can be protected from rust disease by resistance genes encoding receptors that trigger a highly effective defence response upon recognition of specific pathogen avirulence proteins. Identifying avirulence genes is crucial for understanding how virulence evolves in the field.

RESULTS

To facilitate avirulence gene cloning in the flax rust fungus, Melampsora lini, we constructed a high-density genetic linkage map using single nucleotide polymorphisms detected in restriction site-associated DNA sequencing (RADseq) data. The map comprises 13,412 RADseq markers in 27 linkage groups that together span 5860 cM and contain 2756 recombination bins. The marker sequences were used to anchor 68.9 % of the M. lini genome assembly onto the genetic map. The map and anchored assembly were then used to: 1) show that M. lini has a high overall meiotic recombination rate, but recombination distribution is uneven and large coldspots exist; 2) show that substantial genome rearrangements have occurred in spontaneous loss-of-avirulence mutants; and 3) identify the AvrL2 and AvrM14 avirulence genes by map-based cloning. AvrM14 is a dual-specificity avirulence gene that encodes a predicted nudix hydrolase. AvrL2 is located in the region of the M. lini genome with the lowest recombination rate and encodes a small, highly-charged proline-rich protein.

CONCLUSIONS

The M. lini high-density linkage map has greatly advanced our understanding of virulence mechanisms in this pathogen by providing novel insights into genome variability and enabling identification of two new avirulence genes.

Recombination Rate Heterogeneity within Arabidopsis Disease Resistance Genes

Meiotic crossover frequency varies extensively along chromosomes and is typically concentrated in hotspots. As recombination increases genetic diversity, hotspots are predicted to occur at immunity genes, where variation may be beneficial. A major component of plant immunity is recognition of pathogen Avirulence (Avr) effectors by resistance (R) genes that encode NBS-LRR domain proteins. Therefore, we sought to test whether NBS-LRR genes would overlap with meiotic crossover hotspots using experimental genetics in Arabidopsis thaliana. NBS-LRR genes tend to physically cluster in plant genomes; for example, in Arabidopsis most are located in large clusters on the south arms of chromosomes 1 and 5. We experimentally mapped 1,439 crossovers within these clusters and observed NBS-LRR gene associated hotspots, which were also detected as historical hotspots via analysis of linkage disequilibrium. However, we also observed NBS-LRR gene coldspots, which in some cases correlate with structural heterozygosity. To study recombination at the fine-scale we used high-throughput sequencing to analyze ~1,000 crossovers within the RESISTANCE TO ALBUGO CANDIDA1 (RAC1) R gene hotspot. This revealed elevated intragenic crossovers, overlapping nucleosome-occupied exons that encode the TIR, NBS and LRR domains. The highest RAC1 recombination frequency was promoter-proximal and overlapped CTT-repeat DNA sequence motifs, which have previously been associated with plant crossover hotspots. Additionally, we show a significant influence of natural genetic variation on NBS-LRR cluster recombination rates, using crosses between Arabidopsis ecotypes. In conclusion, we show that a subset of NBS-LRR genes are strong hotspots, whereas others are coldspots. This reveals a complex recombination landscape in Arabidopsis NBS-LRR genes, which we propose results from varying coevolutionary pressures exerted by host-pathogen relationships, and is influenced by structural heterozygosity.

Nonhost resistance to rust pathogens - a continuation of continua

The rust fungi (order: Pucciniales) are a group of widely distributed fungal plant pathogens, which can infect representatives of all vascular plant groups. Rust diseases significantly impact several crop species and considerable research focuses on understanding the basis of host specificity and nonhost resistance. Like many pathogens, rust fungi vary considerably in the number of hosts they can infect, such as wheat leaf rust (Puccinia triticina), which can only infect species in the genera Triticum and Aegilops, whereas Asian soybean rust (Phakopsora pachyrhizi) is known to infect over 95 species from over 42 genera. A greater understanding of the genetic basis determining host range has the potential to identify sources of durable resistance for agronomically important crops. Delimiting the boundary between host and nonhost has been complicated by the quantitative nature of phenotypes in the transition between these two states. Plant-pathogen interactions in this intermediate state are characterized either by (1) the majority of accessions of a species being resistant to the rust or (2) the rust only being able to partially complete key components of its life cycle. This leads to a continuum of disease phenotypes in the interaction with different plant species, observed as a range from compatibility (host) to complete immunity within a species (nonhost). In this review we will highlight how the quantitative nature of disease resistance in these intermediate interactions is caused by a continuum of defense barriers, which a pathogen needs to overcome for successfully establishing itself in the host. To illustrate continua as this underlying principle, we will discuss the advances that have been made in studying nonhost resistance towards rust pathogens, particularly cereal rust pathogens.

Population processes at multiple spatial scales maintain diversity and adaptation in the Linum marginale--Melampsora lini association

Host-pathogen coevolution is a major driver of species diversity, with an essential role in the generation and maintenance of genetic variation in host resistance and pathogen infectivity. Little is known about how resistance and infectivity are structured across multiple geographic scales and what eco-evolutionary processes drive these patterns. Across southern Australia, the wild flax Linum marginale is frequently attacked by its rust fungus Melampsora lini. Here, we compare the genetic and phenotypic structure of resistance and infectivity among population pairs from two regions where environmental differences associate with specific life histories and mating systems. We find that both host and pathogen populations are genetically distinct between these regions. The region with outcrossing hosts and pathogens that go through asexual cycles followed by sexual reproduction showed greater diversity of resistance and infectivity phenotypes, higher levels of resistance and less clumped within-population spatial distribution of resistance. However, in the region where asexual pathogens infect selfing hosts, pathogens were more infective and better adapted to sympatric hosts. Our findings largely agree with expectations based on the distinctly different host mating systems in the two regions, with a likely advantage for hosts undergoing recombination. For the pathogen in this system, sexual reproduction may primarily be a survival mechanism in the region where it is observed. While it appears to potentially have adverse effects on local adaptation in the short term, it may be necessary for longer-term coevolution with outcrossing hosts.

N-terminal motifs in some plant disease resistance proteins function in membrane attachment and contribute to disease resistance

To investigate the role of N-terminal domains of plant disease resistance proteins in membrane targeting, the N termini of a number of Arabidopsis and flax disease resistance proteins were fused to green fluorescent protein (GFP) and the fusion proteins localized in planta using confocal microscopy. The N termini of the Arabidopsis RPP1-WsB and RPS5 resistance proteins and the PBS1 protein, which is required for RPS5 resistance, targeted GFP to the plasma membrane, and mutation of predicted myristoylation and potential palmitoylation sites resulted in a shift to nucleocytosolic localization. The N-terminal domain of the membrane-attached Arabidopsis RPS2 resistance protein was targeted incompletely to the plasma membrane. In contrast, the N-terminal domains of the Arabidopsis RPP1-WsA and flax L6 and M resistance proteins, which carry predicted signal anchors, were targeted to the endomembrane system, RPP1-WsA to the endoplasmic reticulum and the Golgi apparatus, L6 to the Golgi apparatus, and M to the tonoplast. Full-length L6 was also targeted to the Golgi apparatus. Site-directed mutagenesis of six nonconserved amino acid residues in the signal anchor domains of L6 and M was used to change the localization of the L6 N-terminal fusion protein to that of M and vice versa, showing that these residues control the targeting specificity of the signal anchor. Replacement of the signal anchor domain of L6 by that of M did not affect L6 protein accumulation or resistance against flax rust expressing AvrL567 but removal of the signal anchor domain reduced L6 protein accumulation and L6 resistance, suggesting that membrane attachment is required to stabilize the L6 protein.

NLR functions in plant and animal immune systems: so far and yet so close

In plants and animals, the NLR family of receptors perceives non-self and modified-self molecules inside host cells and mediates innate immune responses to microbial pathogens. Despite their similar biological functions and protein architecture, animal NLRs are normally activated by conserved microbe- or damage-associated molecular patterns, whereas plant NLRs typically detect strain-specific pathogen effectors. Plant NLRs recognize either the effector structure or effector-mediated modifications of host proteins. The latter indirect mechanism for the perception of non-self, as well as the within-species diversification of plant NLRs, maximize the capacity to recognize non-self through the use of a finite number of innate immunoreceptors. We discuss recent insights into NLR activation, signal initiation through the homotypic association of N-terminal domains and subcellular receptor dynamics in plants and compare those with NLR functions in animals.

Co-evolutionary interactions between host resistance and pathogen effector genes in flax rust disease

Plant-pathogen co-evolutionary selection processes are continuous, complex and occur across many spatial and temporal scales. Comprehensive studies of the flax-flax rust pathosystem have led to the postulation of the gene-for-gene model, a genetic paradigm describing recognition events between host disease resistance proteins and pathogen effector proteins. The identification of directly interacting fungal effector proteins and plant disease resistance proteins in this pathosystem has facilitated the study of both the physical nature of these interactions and the evolutionary forces that have resulted in a molecular arms race between these organisms. The flax-flax rust pathosystem has also been detailed on the scale of interacting populations, and the integration of molecular- and population-scale datasets represents a unique opportunity to further our understanding of many poorly understood facets of host-pathogen dynamics. In this article, we discuss recent developments and insights in the flax-flax rust pathosystem and their implications for both long-term co-evolutionary dynamics in natural settings, as well as short-term co-evolutionary dynamics in agro-ecosystems.

Transformation of the flax rust fungus, Melampsora lini: selection via silencing of an avirulence gene

Rust fungi cause devastating diseases on many important food crops, with a damaging stem rust epidemic currently affecting wheat production in Africa and the Middle East. These parasitic fungi propagate exclusively on plants, precluding the use of many biotechnological tools available for other culturable fungi. In particular the lack of a stable transformation system has been an impediment to the genetic manipulation required for molecular analysis of rust pathogenicity. We have developed an Agrobacterium-mediated genetic transformation procedure for the model flax rust fungus Melampsora lini, which infects flax (Linum usitatissimum). Selection of transgenic rust lines is based on silencing of AvrL567, which encodes a rust effector protein that is recognised by the flax L6 immune receptor. The non-transgenic rust line is unable to infect flax plants expressing L6, while silenced transgenic lines are virulent on these plants, providing an effective selection system. This directly confirms that the cloned AvrL567 gene is responsible for flax rust virulence phenotypes, and demonstrates the utility of this system to probe rust gene function.

Recognition events and host-pathogen co-evolution in gene-for-gene resistance to flax rust

The outcome of infection of individual plants by pathogenic organisms is governed by complex interactions between the host and pathogen. These interactions are the result of long-term co-evolutionary processes involving selection and counterselection between plants and their pathogens. These processes are ongoing, and occur at many spatio-temporal scales, including genes and gene products, cellular interactions within host individuals, and the dynamics of host and pathogen populations. However, there are few systems in which host-pathogen interactions have been studied across these broad scales. In this review, we focus on research to elucidate the structure and function of plant resistance and pathogen virulence genes in the flax-flax rust interaction, and also highlight complementary co-evolutionary studies of a related wild plant-pathogen interaction. The confluence of these approaches is beginning to shed new light on host-pathogen molecular co-evolution in natural environments.