Mutations in the EDR1 Gene Alter the Response of Arabidopsis thaliana to Phytophthora infestans and the Bacterial PAMPs flg22 and elf18

Genetic approaches to dissect plant nonhost resistance mechanisms

Nonhost resistance (NHR) refers to the immunity of most tested genotypes of a plant species to most tested variants of a pathogen species. Thus, NHR is broad spectrum and durable in nature and constitutes a major safety barrier against invasion of a myriad of potentially pathogenic microbes in any plants including domesticated crops. Genetic study of NHR is generally more difficult compared to host resistance mainly because NHR is genetically more complicated and often lacks intraspecific polymorphisms. Nevertheless, substantial progress has been made towards the understanding of the molecular basis of NHR in the past two decades using various approaches. Not surprisingly, molecular mechanisms of NHR revealed so far encompasses pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity. In this review, we briefly discuss the inherent difficulty in genetic studies of NHR and summarize the main approaches that have been taken to identify genes contributing to NHR. We also discuss new enabling strategies for dissecting multilayered NHR in model plants with a focus on NHR against filamentous pathogens, especially biotrophic pathogens such as powdery mildew and rust fungi.

Arabidopsis non-host resistance PSS30 gene enhances broad-spectrum disease resistance in the soybean cultivar Williams 82

Non-host resistance (NHR), which protects all members of a plant species from non-adapted or non-host plant pathogens, is the most common form of plant immunity. NHR provides the most durable and robust form of broad-spectrum immunity against non-adaptive pathogens pathogenic to other crop species. In a mutant screen for loss of Arabidopsis (Arabidopsis thaliana) NHR against the soybean (Glycine max (L.) Merr.) pathogen Phytophthora sojae, the Phytophthora sojae-susceptible 30 (pss30) mutant was identified. The pss30 mutant is also susceptible to the soybean pathogen Fusarium virguliforme. PSS30 encodes a folate transporter, AtFOLT1, which was previously localized to chloroplasts and implicated in the transport of folate from the cytosol to plastids. We show that two Arabidopsis folate biosynthesis mutants with reduced folate levels exhibit a loss of non-host immunity against P. sojae. As compared to the wild-type Col-0 ecotype, the steady-state folate levels are reduced in the pss1, atfolt1 and two folate biosynthesis mutants, suggesting that folate is required for non-host immunity. Overexpression of AtFOLT1 enhances immunity of transgenic soybean lines against two serious soybean pathogens, the fungal pathogen F. virguliforme and the soybean cyst nematode (SCN) Heterodera glycines. Transgenic lines showing enhanced SCN resistance also showed increased levels of folate accumulation. This study thus suggests that folate contributes to non-host plant immunity and that overexpression of a non-host resistance gene could be a suitable strategy for generating broad-spectrum disease resistance in crop plants.

Genome editing for resistance against plant pests and pathogens

The conventional breeding of crops struggles to keep up with increasing food needs and ever-adapting pests and pathogens. Global climate changes have imposed another layer of complexity to biological systems, increasing the challenge to obtain improved crop cultivars. These dictate the development and application of novel technologies, like genome editing (GE), that assist targeted and fast breeding programs in crops, with enhanced resistance to pests and pathogens. GE does not require crossings, hence avoiding the introduction of undesirable traits through linkage in elite varieties, speeding up the whole breeding process. Additionally, GE technologies can improve plant protection by directly targeting plant susceptibility (S) genes or virulence factors of pests and pathogens, either through the direct edition of the pest genome or by adding the GE machinery to the plant genome or to microorganisms functioning as biocontrol agents (BCAs). Over the years, GE technology has been continuously evolving and more so with the development of CRISPR/Cas. Here we review the latest advancements of GE to improve plant protection, focusing on CRISPR/Cas-based genome edition of crops and pests and pathogens. We discuss how other technologies, such as host-induced gene silencing (HIGS) and the use of BCAs could benefit from CRISPR/Cas to accelerate the development of green strategies to promote a sustainable agriculture in the future.

What is the Molecular Basis of Nonhost Resistance?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.Nonhost resistance is typically considered the ability of a plant species to repel all attempts of a pathogen species to colonize it and reproduce on it. Based on this common definition, nonhost resistance is presumed to be very durable and, thus, of great interest for its potential use in agriculture. Despite considerable research efforts, the molecular basis of this type of plant immunity remains nebulous. We here stress the fact that "nonhost resistance" is a phenomenological rather than a mechanistic concept that comprises more facets than typically considered. We further argue that nonhost resistance essentially relies on the very same genes and pathways as other types of plant immunity, of which some may act as bottlenecks for particular pathogens on a given plant species or under certain conditions. Thus, in our view, the frequently used term "nonhost genes" is misleading and should be avoided. Depending on the plant-pathogen combination, nonhost resistance may involve the recognition of pathogen effectors by host immune sensor proteins, which might give rise to host shifts or host range expansions due to evolutionary-conditioned gains and losses in respective armories. Thus, the extent of nonhost resistance also defines pathogen host ranges. In some instances, immune-related genes can be transferred across plant species to boost defense, resulting in augmented disease resistance. We discuss future routes for deepening our understanding of nonhost resistance and argue that the confusing term "nonhost resistance" should be used more cautiously in the light of a holistic view of plant immunity.

Arabidopsis EDR1 Protein Kinase Regulates the Association of EDS1 and PAD4 to Inhibit Cell Death

ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4) are sequence-related lipase-like proteins that function as a complex to regulate defense responses in Arabidopsis by both salicylic acid-dependent and independent pathways. Here, we describe a gain-of-function mutation in PAD4 (S135F) that enhances resistance and cell death in response to infection by the powdery mildew pathogen Golovinomyces cichoracearum. The mutant PAD4 protein accumulates to wild-type levels in Arabidopsis cells, thus these phenotypes are unlikely to be due to PAD4 over accumulation. The phenotypes are similar to loss-of-function mutations in the protein kinase EDR1 (Enhanced Disease Resistance1), and previous work has shown that loss of PAD4 or EDS1 suppresses edr1-mediated phenotypes, placing these proteins downstream of EDR1. Here, we show that EDR1 directly associates with EDS1 and PAD4 and inhibits their interaction in yeast and plant cells. We propose a model whereby EDR1 negatively regulates defense responses by interfering with the heteromeric association of EDS1 and PAD4. Our data indicate that the S135F mutation likely alters an EDS1-independent function of PAD4, potentially shedding light on a yet-unknown PAD4 signaling function.

Genes involved in nonhost disease resistance as a key to engineer durable resistance in crops

Most potential pathogens fail to establish virulence for a plethora of plants found in nature. This intrinsic property to resist pathogen virulence displayed by organisms without triggering canonical resistance (R) genes has been termed nonhost resistance (NHR). While host resistance involves recognition of pathogen elicitors such as avirulence factors by bona fide R proteins, mechanism of NHR seems less obvious, often involving more than one gene. We can generally describe NHR in two steps: 1) pre-invasive resistance, either passive or active, which can restrict the pathogen from entering the host, and 2) post-invasive resistance, an active defense response that often results in hypersensitive response like programmed cell death and reactive oxygen species accumulation. While PAMP-triggered-immunity (PTI) is generally effective against nonhost pathogens, effector-triggered-immunity (ETI) can be effective against both host and nonhost pathogens. Prolonged interactions between adapted pathogens and their resistant host plants results in co-evolution, which can lead to new pathogen strains that can be virulent and cause disease on supposedly resistant host. In this context, engineering durable resistance by manipulating genes involved in NHR is an attractive approach for sustainable agriculture. Several genes involved in NHR have been characterized for their role in plant defense. In this review, we report genes involved in NHR identified to date and highlight a few examples where genes involved in NHR have been used to confer resistance in crop plants against economically important diseases.

Albugo-imposed changes to tryptophan-derived antimicrobial metabolite biosynthesis may contribute to suppression of non-host resistance to Phytophthora infestans in Arabidopsis thaliana

Background

Plants are exposed to diverse pathogens and pests, yet most plants are resistant to most plant pathogens. Non-host resistance describes the ability of all members of a plant species to successfully prevent colonization by any given member of a pathogen species. White blister rust caused by Albugo species can overcome non-host resistance and enable secondary infection and reproduction of usually non-virulent pathogens, including the potato late blight pathogen Phytophthora infestans on Arabidopsis thaliana. However, the molecular basis of host defense suppression in this complex plant–microbe interaction is unclear. Here, we investigate specific defense mechanisms in Arabidopsis that are suppressed by Albugo infection.

Results

Gene expression profiling revealed that two species of Albugo upregulate genes associated with tryptophan-derived antimicrobial metabolites in Arabidopsis. Albugo laibachii-infected tissue has altered levels of these metabolites, with lower indol-3-yl methylglucosinolate and higher camalexin accumulation than uninfected tissue. We investigated the contribution of these Albugo-imposed phenotypes to suppression of non-host resistance to P. infestans. Absence of tryptophan-derived antimicrobial compounds enables P. infestans colonization of Arabidopsis, although to a lesser extent than Albugo-infected tissue. A. laibachii also suppresses a subset of genes regulated by salicylic acid; however, salicylic acid plays only a minor role in non-host resistance to P. infestans.

Conclusions

Albugo sp. alter tryptophan-derived metabolites and suppress elements of the responses to salicylic acid in Arabidopsis. Albugo sp. imposed alterations in tryptophan-derived metabolites may play a role in Arabidopsis non-host resistance to P. infestans. Understanding the basis of non-host resistance to pathogens such as P. infestans could assist in development of strategies to elevate food security.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0360-z) contains supplementary material, which is available to authorized users.

Arabidopsis late blight: infection of a nonhost plant by Albugo laibachii enables full colonization by Phytophthora infestans

The oomycete pathogen Phytophthora infestans causes potato late blight, and as a potato and tomato specialist pathogen, is seemingly poorly adapted to infect plants outside the Solanaceae. Here, we report the unexpected finding that P. infestans can infect Arabidopsis thaliana when another oomycete pathogen, Albugo laibachii, has colonized the host plant. The behaviour and speed of P. infestans infection in Arabidopsis pre-infected with A. laibachii resemble P. infestans infection of susceptible potato plants. Transcriptional profiling of P. infestans genes during infection revealed a significant overlap in the sets of secreted-protein genes that are induced in P. infestans upon colonization of potato and susceptible Arabidopsis, suggesting major similarities in P. infestans gene expression dynamics on the two plant species. Furthermore, we found haustoria of A. laibachii and P. infestans within the same Arabidopsis cells. This Arabidopsis-A. laibachii-P. infestans tripartite interaction opens up various possibilities to dissect the molecular mechanisms of P. infestans infection and the processes occurring in co-infected Arabidopsis cells.