Plants detect conserved molecular patterns of pathogens via cell surface-localized receptors, such as the flagellin receptor kinase FLS2, that initiate effective plant immunity. Activated FLS2 is endocytosed, but the degree to which other receptor kinases exhibit similar spatiotemporal dynamics remains unclear. We show that internalization into a common endosomal pathway after ligand perception is a general phenomenon of the tested receptor kinases, including the danger peptide receptor PEPR1. FLS2 endocytosis is mediated by clathrin and is uncoupled from the regulation of acute pathogen-induced responses, but is involved in steady defenses and contributes to plant immunity against bacterial infection. We propose that clathrin-dependent internalization of ligand-activated receptor kinases into a common endosomal pathway facilitates the responses required for full plant immunity.
In the 21st century, the wheat stripe rust fungus has evolved to be the largest biotic limitation to global wheat production. New pathogen genotypes are more aggressive and able to infect previously resistant wheat varieties, leading to rapid pathogen migration across and between continents. We now know the full life cycle, microevolutionary relationships and past migration routes on a global scale. Current sequencing technologies have provided the first fungal draft genomes and simplified plant resistance gene cloning. Yet, we know nothing about the molecular and microevolutionary mechanisms that facilitate the infection process and cause new devastating pathogen races. These are the questions that need to be addressed by exploiting the synergies between novel 21st century biology tools and decades of dedicated pathology work.
Plants and animals detect the presence of potential pathogens through the perception of conserved microbial patterns by cell surface receptors. Certain solanaceous plants, including tomato, potato and pepper, detect flgII-28, a region of bacterial flagellin that is distinct from that perceived by the well-characterized FLAGELLIN-SENSING 2 receptor. Here we identify and characterize the receptor responsible for this recognition in tomato, called FLAGELLIN-SENSING 3. This receptor binds flgII-28 and enhances immune responses leading to a reduction in bacterial colonization of leaf tissues. Further characterization of FLS3 and its signalling pathway could provide new insights into the plant immune system and transfer of the receptor to other crop plants offers the potential of enhancing resistance to bacterial pathogens that have evolved to evade FLS2-mediated immunity.
This special issue is dedicated to these topics, featuring research articles as well as review papers. Early communication, from chemotaxis, recognition of the microbes by the plant, effective colonization, and the plant genes necessary for positive response to the microorganisms are the focus of nine of the papers. Biotic stress tolerance conferred by plant-associated microorganisms against insects and microbial pathogens is covered in three papers. Increased tolerance to abiotic factors including salinity and limited nutrients as well as overall increased growth and health are the topics of three of the papers. Many of the authors included future perspectives on how to move this important research field forward. Information gained from plant–microbe interaction studies in native habitats may be especially relevant since the host plant and microorganisms have co-evolved with opportunities by the plant to select over time the most beneficial symbionts. Understanding the requirements for recruitment, recognition, colonization, and response will be essential if this knowledge is to be applied to commercial agriculture. Determination of the mechanisms by which microbiota impart tolerance to biotic and abiotic stress will enable optimization for improved plant health and growth under the increased challenges resulting from climate change.
Elicitins are structurally conserved extracellular proteins in Phytophthora and Pythium oomycete pathogen species. They were first described in the late 1980s as abundant proteins in Phytophthora culture filtrates that have the capacity to elicit hypersensitive (HR) cell death and disease resistance in tobacco. Later, they became well-established as having features of microbe-associated molecular patterns (MAMPs) and to elicit defences in a variety of plant species. Research on elicitins culminated in the recent cloning of the elicitin response (ELR) cell surface receptor-like protein, from the wild potato Solanum microdontum, which mediates response to a broad range of elicitins. In this review, we provide an overview on elicitins and the plant responses they elicit. We summarize the state of the art by describing what we consider to be the nine most important features of elicitin biology.
Herbivore selection of plant hosts and plant responses to insect colonization have been subjects of intense investigations. A growing body of evidence suggests that for successful colonization to occur, (effector/virulence) proteins in insect saliva must modulate plant defense responses to the benefit of the insect. A range of insect saliva proteins that modulate plant defense responses have been identified, but there is no direct evidence that these proteins are delivered into specific plant tissues and enter plant cells. Aphids and other sap-sucking insects of the order Hemiptera use their specialized mouthparts (stylets) to probe plant mesophyll cells, until they reach the phloem cells for long-term feeding. Here we show by immunogold-labeling of ultrathin sections of aphid feeding sites that an immuno-suppressive aphid effector localizes in the cytoplasm of mesophyll cells near aphid stylets, but not in cells further away from aphid feeding sites. In contrast, another aphid effector protein localizes in the sheaths composed of gelling saliva that surround the aphid stylets. Thus, insects deliver effectors directly into plant tissue. Moreover, different aphid effectors locate extracellularly in the sheath saliva or are introduced into the cytoplasm of plant cells.
INVERNESS, Calif. — At the height of California’s fierce wildfire season, the Sierra Nevada and North Coast forests are choked with tens of millions of dead and dying trees, from gnarly oaks to elegant pines that are turning leafy chapels into tinderboxes of highly combustible debris.
Ground crews wielding chain saws, axes and wood chippers are braving the intense summer heat in the Sierra’s lower elevations, where most of the pine trees have died. The devastation and danger are greatest in the central and southern Sierra Nevada, where the estimated number of dead trees since 2010 is a staggering 66 million.
Scientists say rarely is one culprit to blame for the escalation in the state’s tree deaths, and the resulting fire hazard. Rather, destruction on such a broad scale is nearly always the result of a complex convergence of threats to forest ecosystems.
Chief among them is a severe, sustained drought in the Sierra Nevada that is stressing trees and disabling their natural defenses. Climate change is raising temperatures, making for warmer winters. No longer kept in check by winter’s freeze, bark beetle populations are growing. Separately, a nonnative, potent plant pathogen is thriving in the moist areas of the North Coast, introduced to California soil by global trade. Opportunistic fungi are standing by, ready to finish the kill.
Factor in human shortcomings — poor or absent forest management, a failure to clear out ignitable dead wood, the darker temptation of arson, unchecked carelessness — and you have a lethal recipe.
“It’s never just one thing that brings down trees,” said David Rizzo, the chairman of plant pathology at the University of California, Davis. “It’s always a combination. The first may weaken trees; the next stresses trees over time. Then comes a third, shutting down the trees’ immune and defense systems. Finally, the last may come along to disrupt nutrient systems. When all this happens at once, or in rapid succession, trees are no longer able to save themselves.”
Two of California’s prized forest regions are in failing health because such conditions have stacked the odds against them. In the Sierra Nevada, the losses of pines and other conifers are concentrated and widening.
Along the North Coast, a picturesque blink of a town called Inverness and the surrounding Marin County woodlands are “ground zero,” Dr. Rizzo said, for the mysterious plant pathogen that began infesting coast oaks probably as far back as the mid-1980s.
Dr. Rizzo, who closely studies Phytophthora ramorum, also known as sudden oak death, returns often to this cozy glen on a peninsula overlooking the clear blue water. Graduate students, research associates and others accompany him to this patch of forest serving as an outdoor classroom, laboratory and demonstration plot.
It took years of research — detective work, really — before experts like Dr. Rizzo and Matteo Garbelotto, a professor of environmental science policy and management at the University of California, Berkeley, discerned that the funguslike pathogen had infested coast oaks years before their showy demise.
Dr. Rizzo estimated that five million to 10 million coastal trees had died because of sudden oak death.
A related pathogen, Phytophthora infestans, was responsible for the Great Potato Famine in Ireland in the mid-1800s.
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.
Filamentous plant pathogens deliver effector proteins to host cells to promote infection. The Phytophthora infestans RXLR-type effector PexRD54 binds potato ATG8 via its ATG8-family interacting motif (AIM) and perturbs host selective autophagy. However, the structural basis of this interaction remains unknown. Here we define the crystal structure of PexRD54, which comprises a modular architecture including five tandem repeat domains, with the AIM sequence presented at the disordered C-terminus. To determine the interface between PexRD54 and ATG8, we solved the crystal structure of potato ATG8CL in complex with a peptide comprising the effectors AIM sequence, and established a model of the full-length PexRD54/ATG8CL complex using small angle X-ray scattering. Structure-informed deletion of the PexRD54 tandem domains reveals retention of ATG8CL binding in vitro and in planta. This study offers new insights into structure/function relationships of oomycete RXLR effectors and how these proteins engage with host cell targets to promote disease.
On 21 October 2013, the Italian phytosanitary service notified the European Commission (EC) that the plant pathogen Xylella fastidiosa had been detected in olive trees near Gallipoli, a tourist destination in Italy's southern region of Apulia (1). This xylem-limited bacterium is spread by insect vectors and causes disease in crops such as grapevines, citrus, coffee, and almond; various ornamentals; and trees such as oaks, elms, and sycamores. Because of the risks of X. fastidiosa being introduced, established, and spread throughout Europe, this species is a regulated quarantine pest. Yet, X. fastidiosa has been left unchecked and has marched northward, leaving destruction in its wake (see the photo) (2). The establishment of X. fastidiosa in Italy has been an agricultural, environmental, political, and cultural disaster.
The threat of X. fastidiosa to European and Mediterranean agriculture, forests, and ecosystems goes beyond specific crops such as grapevines or citrus. The current host range of this bacterium includes more than 300 plant species (3). Most of these species support some degree of pathogen multiplication without expressing symptoms. Susceptible hosts infected with X. fastidiosa often show disease symptoms only after months or years, although epidemics can spread fast and be devastating.
A phylogenetic study has shown that the genotype in Italy was likely introduced via contaminated plant material from Costa Rica (3). Several X. fastidiosa-infected coffee plants from Costa Rica have been intercepted at European ports since 2014, supporting this hypothesis (4). As a response, the EC in February 2014 approved European Union (EU) emergency measures aimed at preventing the introduction and spread of X. fastidiosa. Since May 2015, the import of coffee plants from Costa Rica and Honduras into the EU has been forbidden. Limiting the introduction of insect vectors is considered an easier task, but this is not possible for X. fastidiosa because any xylem-sap-sucking insect species can be a potential vector. Europe has few sharpshooter leafhopper species, the most important group of vectors in the Americas. However, various endemic spittlebug species (froghoppers) are also potential vectors of X. fastidiosa (3).
Trade is an important pathway in the introduction of plant pests and pathogens (5), and X. fastidiosa-infected plant material has likely been introduced via European ports on a regular basis. Given that biological and environmental conditions in Europe support X. fastidiosainfection, the question arises why the pathogen has not been reported previously. One possible explanation is that limited surveillance efforts missed previous introductions. Monitoring was one component of the EU emergency measures. After the French authorities started a systematic monitoring program for X. fastidiosa in 2014, they found 250 distinct infected areas in Corsica and several in the French Riviera. However, no disease epidemic has yet been noted in France, and the genotype of X. fastidiosa differs from that found in Italy.
Recognition of pathogen-derived molecules by pattern recognition receptors (PRRs) is a common feature of both animal and plant innate immune systems. In plants, PRR signalling is initiated at the cell surface by kinase complexes, resulting in the activation of immune responses that ward off microorganisms. However, the activation and amplitude of innate immune responses must be tightly controlled. In this Review, we summarize our knowledge of the early signalling events that follow PRR activation and describe the mechanisms that fine-tune immune signalling to maintain immune homeostasis. We also illustrate the mechanisms used by pathogens to inhibit innate immune signalling and discuss how the innate ability of plant cells to monitor the integrity of key immune components can lead to autoimmune phenotypes following genetic or pathogen-induced perturbations of these components.
Plants have evolved hundreds of nucleotide-binding and leucine-rich domain proteins (NLRs) as potential intracellular immune receptors, but the evolutionary mechanism leading to the ability to recognize specific pathogen effectors is elusive.
Here, we cloned Pvr4 (a Potyvirus resistance gene in Capsicum annuum) and Tsw (a Tomato spotted wilt virus resistance gene in Capsicum chinense) via a genome-based approach using independent segregating populations.
The genes both encode typical NLRs and are located at the same locus on pepper chromosome 10. Despite the fact that these two genes recognize completely different viral effectors, the genomic structures and coding sequences of the two genes are strikingly similar. Phylogenetic studies revealed that these two immune receptors diverged from a progenitor gene of a common ancestor.
Our results suggest that sequence variations caused by gene duplication and neofunctionalization may underlie the evolution of the ability to specifically recognize different effectors. These findings thereby provide insight into the divergent evolution of plant immune receptors.
Recent evidence suggests that the ubiquitin-proteasome system (UPS) is involved in several aspects of plant immunity and a range of plant pathogens subvert the UPS to enhance their virulence. Here we show that proteasome activity is strongly induced during basal defense in Arabidopsis. Mutant lines of the proteasome subunits RPT2a and RPN12a support increased bacterial growth of virulent Pseudomonas syringae pv. tomato DC3000 (Pst) and Pseudomonas syringae pv. maculicola ES4326. Both proteasome subunits are required for Pathogen-associated molecular patterns (PAMP)-triggered immunity (PTI) responses. Analysis of bacterial growth after a secondary infection of systemic leaves revealed that the establishment of systemic-acquired resistance (SAR) is impaired in proteasome mutants, suggesting that the proteasome also plays an important role in defense priming and SAR. In addition, we show that Pst inhibits proteasome activity in a type-III secretion dependent manner. A screen for type-III effector proteins from Pst for their ability to interfere with proteasome activity revealed HopM1, HopAO1, HopA1 and HopG1 as putative proteasome inhibitors. Biochemical characterization of HopM1 by mass-spectrometry indicates that HopM1 interacts with several E3 ubiquitin ligases and proteasome subunits. This supports the hypothesis that HopM1 associates with the proteasome leading to its inhibition. Thus, the proteasome is an essential component of PTI and SAR, which is targeted by multiple bacterial effectors.
Pseudomonas syringae pv. tomato DC3000 (PtoDC3000) is an extracellular model plant pathogen, yet its potential to produce secreted effectors that manipulate the apoplast has been under investigated. Here we identified 131 candidate small, secreted, non-annotated proteins from the PtoDC3000 genome, most of which are common to Pseudomonas species and potentially expressed during apoplastic colonization. We produced 43 of these proteins through a custom-made gateway-compatible expression system for extracellular bacterial proteins, and screened them for their ability to inhibit the secreted immune protease C14 of tomato using competitive activity-based protein profiling. This screen revealed C14-inhibiting protein-1 (Cip1), which contains motifs of the chagasin-like protease inhibitors. Cip1 mutants are less virulent on tomato, demonstrating the importance of this effector in apoplastic immunity. Cip1 also inhibits immune protease Pip1, which is known to suppress PtoDC3000 infection, but has a lower affinity for its close homolog Rcr3, explaining why this protein is not recognized in tomato plants carrying the Cf-2 resistance gene, which uses Rcr3 as a co-receptor to detect pathogen-derived protease inhibitors. Thus, this approach uncovered a protease inhibitor of P. syringae, indicating that also P. syringae secretes effectors that selectively target apoplastic host proteases of tomato, similar to tomato pathogenic fungi, oomycetes and nematodes.
The plant tumor disease known as crown gall was not called by that name until more recent times. Galls on plants were described by Malpighi (1679) who believed that these extraordinary growth are spontaneously produced. Agrobacterium was first isolated from tumors in 1897 by Fridiano Cavara in Napoli, Italy. After this bacterium was recognized to be the cause of crown gall disease, questions were raised on the mechanism by which it caused tumors on a variety of plants. Numerous very detailed studies led to the identification of Agrobacterium tumefaciens as the causal bacterium that cleverly transferred a genetic principle to plant host cells and integrated it into their chromosomes. Such studies have led to a variety of sophisticated mechanisms used by this organism to aid in its survival against competing microorganisms. Knowledge gained from these fundamental discoveries has opened many avenues for researchers to examine their primary organisms of study for similar mechanisms of pathogenesis in both plants and animals. These discoveries also advanced the genetic engineering of domesticated plants for improved food and fiber.
If I google “mealybugs,” the first pages of results deal almost entirely with ways of spotting and destroying them. There’s good reason for that: these small, sap-sucking bugs drain fluids from plants, spread diseases, foster the growth of molds, and cost millions in damage to crop growers every year. They destroy the things we eat, and so we destroy them.
But mealybugs are more than just symbols of both famine and pestilence. Over the last 15 years, a small team of scientists has shown that they are also symbols of interconnectedness. They are among the most spectacular examples of symbiosis—the phenomenon where different species live together in intimate association. And with every new discovery, their biology becomes even more elaborate and unbelievable.
The prelude to this story begins in the early 20th century, when an exceptionally productive zoologist named Paul Buchner started dissecting his way through the insect world. He showed that countless species are filled with microbes, many of which live inside their very cells. The mealybugs were no exception—they also contained inner bacteria, which seemed to be thickly embedded within “roundish or longish mucilaginous globules.” That is: big balls of mucus.
Buchner didn’t press the matter further. But when other scientists later analyzed these globules, they started getting very odd results. If the mealybugs swallowed antibiotics, the globules would rupture along with the bacteria inside them. Peculiar. The mucus also seemed to contain many of the elements of an actual cell. Also peculiar. And genetic studies showed that they contained DNA from two separate lineages of bacteria. That, at least, was explicable: Buchner’s globules probably contained two types of symbiotic bacteria rather than one.
In 2001, Carol von Dohlen tested this idea by studying the citrus mealybug—a tiny insect that looked like a lozenge dipped in icing sugar. Von Dohlen fashioned two fluorescent molecules—one red and one blue—that would each stick to DNA from one of the two bacteria. If the two microbes did indeed share the same living quarters, their respective glows should have blended into a sea of purple.
That is not what happened. Instead, von Dohlen saw red dots against a blue background. The red probe had stuck to the bacteria in the globules. But the blue probe was sticking to the globules themselves. These mucus-filled spheres weren’t enclosing two kinds of bacteria. They were bacteria.
Von Dohlen had discovered that the citrus mealybug is a living Russian doll—or perhaps a microbial turducken. The bacteria living in its cells have more bacteria living inside them. It contains multitudes, and its multitudes contain more multitudes. The bigger microbe was eventually named Tremblaya, and its inner companion was called Moranella. And then things got even weirder.
In 2011, von Dohlen teamed up with geneticist John McCutcheon to sequence the genomes of the two microbes. Both were very small, as is often the case with bacteria that find their way into insect cells. In the cozy confines of their hosts, these microbes can afford to lose genes that they would normally need for an independent existence. Tremblaya has even lost a group of supposedly indispensable genes that were there in the last common ancestor of all living things, and are found in everything from bacteria to bats. There should be twenty of them, and Tremblaya has none. It survives because the insect around it and the Moranella within it compensate for its genetic shortfall.
This convoluted set-up developed gradually. Tremblaya was first of the two partners to colonize mealybugs: it’s there in all the species from one particular lineage, and there are some mealybugs that carry it and it alone. Snug in a bug, it began jettisoning genes. In the citrus mealybug, Moranella joined the partnership. The duo became a trio, and Tremblaya continued its slide into genetic pauperdom. As long as any gene exists in one of the partners, the others can afford to lose it.
That’s abundantly clear when you look at genes for making nutrients. For example, it takes nine genes to make an essential amino acid called phenylalanine. But none of the three partners makes all nine. Tremblaya can build 1, 2, 5, 6, 7 and 8; Moranella can make 3, 4, and 5; and the mealybug alone makes the 9th. As I wrote in my new book, this reminds me of the Graeae of Greek mythology: the three sisters who share one eye and one tooth between them. They are still distinct entities, but they’re each like thirds of a single whole. They cooperate to make nutrients that they all rely upon, and none can survive without the other.
Now things get really strange. Other species of mealybugs are also Russian dolls, with the same bug-in-a-bug-in-a-bug set-up. One of them—the long-tailed mealybug—even seems to have two kinds of inner bacteria living inside its outer one.
No matter the mealybug species, the outer bacterium is always Tremblaya. But the inner bacterium varies considerably—it’s Moranella in the citrus mealybug, but different microbes in the other insects. The most obvious explanation for this pattern is that some ancestral mealybug became infected with its two nested microbes. As the insects diverged into different species, so did the microbes within them. For whatever reason, the outer one stayed the same, while the inner one changed into new forms—Moranella being just one of them. “It’s so weird and so uncommon to get these bacteria inside of each other that I thought it had to happen once,” says McCutcheon.
Parasitic plants in the Orobanchaceae cause serious agricultural problems worldwide. Parasitic plants develop a multicellular infectious organ called a haustorium after recognition of host-released signals. To understand the molecular events associated with host signal perception and haustorium development, we identified differentially regulated genes expressed during early haustorium development in the facultative parasite Phtheirospermum japonicumusing a de novo assembled transcriptome and a customized microarray. Among the genes that were upregulated during early haustorium development, we identified YUC3, which encodes a functional YUCCA (YUC) flavin monooxygenase involved in auxin biosynthesis. YUC3 was specifically expressed in the epidermal cells around the host contact site at an early time point in haustorium formation. The spatio-temporal expression patterns of YUC3 coincided with those of the auxin response marker DR5, suggesting generation of auxin response maxima at the haustorium apex. Roots transformed with YUC3 knockdown constructs formed haustoria less frequently than nontransgenic roots. Moreover, ectopic expression of YUC3 at the root epidermal cells induced the formation of haustorium-like structures in transgenic P. japonicum roots. Our results suggest that expression of the auxin biosynthesis gene YUC3 at the epidermal cells near the contact site plays a pivotal role in haustorium formation in the root parasitic plant P. japonicum.
For over 140 years, lichens have been regarded as a symbiosis between a single fungus, usually an ascomycete, and a photosynthesizing partner. Other fungi have long been known to occur as occasional parasites or endophytes, but the one lichen–one fungus paradigm has seldom been questioned. Here we show that many common lichens are composed of the known ascomycete, the photosynthesizing partner, and, unexpectedly, specific basidiomycete yeasts. These yeasts are embedded in the cortex, and their abundance correlates with previously unexplained variations in phenotype. Basidiomycete lineages maintain close associations with specific lichen species over large geographical distances and have been found on six continents. The structurally important lichen cortex, long treated as a zone of differentiated ascomycete cells, appears to consistently contain two unrelated fungi.
It’s the ancient story of plant evolution: photosynthetic algae moved to damp places on land, eventually evolving more complex architecture, and spreading across almost all terrestrial habitats. To cope with the drier conditions, plants developed roots to absorb water, and vascular tissue to transport it; a waxy cuticle coating their surfaces to prevent evaporation; and microscopic pores called stomata that open to allow carbon dioxide to diffuse in for photosynthesis but close to prevent excessive water loss.
How, then, does eelgrass (Zostera marina) fit in to this tale? It’s a monocot descended from the flowering plants, but it has turned its back on dry land and returned to the sea; a rare feat that only appears to have happened on three occasions. The recent sequencing of the eelgrass genome has revealed several interesting insights into the dramatic genetic changes that have allowed it to adapt to what lead author Professor Jeanine Olsen described as, “arguably the most extreme adaptation a terrestrial (and even a freshwater) species can undergo.”
Sayonara to stomata
If you live in the sea, conserving water isn’t your main concern. Eelgrass was known to lack stomata, but genetic comparisons to other species, including its freshwater relative Spirodela polyrhiza, revealed the first surprise of the study: eelgrass has lost not only its stomata but also the genes involved in their development and patterning. “The genes have just gone, so there’s no way back to land for seagrass,” said Olsen.
A difference in defense
When angiosperms are attacked by herbivores or pathogens, their defense response typically involves the release of volatile secondary metabolites through their stomata. How can eelgrass release these compounds without stomata? The answer is: it doesn’t. The genome study found that eelgrass is missing crucial genes involved in making ethylene (an important hormone release in times of stress), as well as those responsible for producing non-metabolic terpenoids, which act to repel pests.
Selective pressures of the marine environment differ greatly from those of terrestrial habitats, so different pathways may be involved. Second, eelgrass has a wide repertoire of pathogen resistance genes, which suggests that it is exposed to a very different set of pathogens that may not respond to typical immune responses. Third, volatile secondary metabolites are often involved in attracting pollinators; this is not believed to be necessary in eelgrass, where submarine pollination occurs using the water itself.
Soon after the gene-for-gene hypothesis was formulated in the 1940s, the search for the postulated gene products started. The hypothesis proposed that products of fungal avirulence (Avr) genes induce defense responses in plants, after recognition by matching resistance (R) gene-encoded proteins, often associated with a hypersensitive response (HR) effective against (obligate) biotrophic fungal pathogens. Here, I present a short overview of this research over the last 40 yr, which has led to new paradigms and terms, including nonspecific and race-specific elicitors, microbe-associated molecular patterns (MAMPs), MAMP-triggered immunity, effectors, effector-triggered susceptibility (ETS), effector-triggered immunity (ETI), biotrophic and necrotrophic effectors and the inverse gene-for-gene or matching allele hypothesis. The molecular arms race between plants and pathogenic fungi is now well understood, and biotrophic and necrotrophic effectors are exploited in plant disease resistance breeding, which presently occurs on science-based strategies, making agriculture more sustainable and less dependent on agrochemicals. Next-generation sequencing has accelerated the discovery of new types of effector, including secondary metabolite effectors and small RNAs derived from noncoding fungal genome sequences, which suppress basal plant defense responses. The identification of the biological functions of effectors remains a challenge that requires new technologies, including gene silencing and gene editing using CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats)-CRISPR-associated protein 9) technology.
Plants recognize pathogen-associated molecular patterns (PAMPs) via cell surface-localized pattern recognition receptors (PRRs), leading to PRR-triggered immunity (PTI). The Arabidopsis cytoplasmic kinase BIK1 is a downstream substrate of several PRR complexes. How plant PTI is negatively regulated is not fully understood. Here, we identify the protein phosphatase PP2C38 as a negative regulator of BIK1 activity and BIK1-mediated immunity. PP2C38 dynamically associates with BIK1, as well as with the PRRs FLS2 and EFR, but not with the co-receptor BAK1. PP2C38 regulates PAMP-induced BIK1 phosphorylation and impairs the phosphorylation of the NADPH oxidase RBOHD by BIK1, leading to reduced oxidative burst and stomatal immunity. Upon PAMP perception, PP2C38 is phosphorylated on serine 77 and dissociates from the FLS2/EFR-BIK1 complexes, enabling full BIK1 activation. Together with our recent work on the control of BIK1 turnover, this study reveals another important regulatory mechanism of this central immune component.
Many microbes interact with their hosts across a membrane interface, which is often distinct from existing membranes. Understanding how this interface acquires its identity has significant implications. In the symbiosis between legumes and rhizobia, the symbiosome encases the intracellular bacteria and receives host secretory proteins important for bacterial development. We show that the Medicago truncatula SYNTAXIN 132 (SYP132) gene undergoes alternative cleavage and polyadenylation during transcription, giving rise to two target-membrane soluble NSF attachment protein receptor (t-SNARE) isoforms. One of these isoforms, SYP132A, is induced during the symbiosis, is able to localize to the peribacteroid membrane, and is required for the maturation of symbiosomes into functional forms. The second isoform, SYP132C, has important functions unrelated to symbiosis. The SYP132A sequence is broadly found in flowering plants that form arbuscular mycorrhizal symbiosis, an ancestral mutualism between soil fungi and most land plants. SYP132A silencing severely inhibited arbuscule colonization, indicating that SYP132A is an ancient factor specifying plant–microbe interfaces.
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