Pattern-recognition receptors are required for NLR-mediated plant immunity.

The plant immune system is fundamental for plant survival in natural ecosystems and for productivity in crop fields. Substantial evidence supports the prevailing notion that plants possess a two-tiered innate immune system, called pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI is triggered by microbial patterns via cell surface-localized pattern-recognition receptors (PRRs), whereas ETI is activated by pathogen effector proteins via predominantly intracellularly localized receptors called nucleotide-binding, leucine-rich repeat receptors (NLRs)1-4. PTI and ETI are initiated by distinct activation mechanisms and involve different early signalling cascades5,6. Here we show that Arabidopsis PRR and PRR co-receptor mutants-fls2 efr cerk1 and bak1 bkk1 cerk1 triple mutants-are markedly impaired in ETI responses when challenged with incompatible Pseudomonas syrinage bacteria. We further show that the production of reactive oxygen species by the NADPH oxidase RBOHD is a critical early signalling event connecting PRR- and NLR-mediated immunity, and that the receptor-like cytoplasmic kinase BIK1 is necessary for full activation of RBOHD, gene expression and bacterial resistance during ETI. Moreover, NLR signalling rapidly augments the transcript and/or protein levels of key PTI components. Our study supports a revised model in which potentiation of PTI is an indispensable component of ETI during bacterial infection. This revised model conceptually unites two major immune signalling cascades in plants and mechanistically explains some of the long-observed similarities in downstream defence outputs between PTI and ETI.

Ascorbate peroxidase 1 allows monitoring of cytosolic accumulation of effector-triggered reactive oxygen species using a luminol-based assay.

Biphasic production of reactive oxygen species (ROS) has been observed in plants treated with avirulent bacterial strains. The first transient peak corresponds to pattern-triggered immunity (PTI)-ROS, whereas the second long-lasting peak corresponds to effector-triggered immunity (ETI)-ROS. PTI-ROS are produced in the apoplast by plasma membrane-localized NADPH oxidases, and the recognition of an avirulent effector increases the PTI-ROS regulatory module, leading to ETI-ROS accumulation in the apoplast. However, how apoplastic ETI-ROS signaling is relayed to the cytosol is still unknown. Here, we found that in the absence of cytosolic ascorbate peroxidase 1 (APX1), the second phase of ETI-ROS accumulation was undetectable in Arabidopsis (Arabidopsis thaliana) using luminol-based assays. In addition to being a scavenger of cytosolic H2O2, we discovered that APX1 served as a catalyst in this chemiluminescence ROS assay by employing luminol as an electron donor. A horseradish peroxidase (HRP)-mimicking APX1 mutation (APX1W41F) further enhanced its catalytic activity toward luminol, whereas an HRP-dead APX1 mutation (APX1R38H) reduced its luminol oxidation activity. The cytosolic localization of APX1 implies that ETI-ROS might accumulate in the cytosol. When ROS were detected using a fluorescent dye, green fluorescence was observed in the cytosol 6 h after infiltration with an avirulent bacterial strain. Collectively, these results indicate that ETI-ROS eventually accumulate in the cytosol, and cytosolic APX1 catalyzes luminol oxidation and allows monitoring of the kinetics of ETI-ROS in the cytosol. Our study provides important insights into the spatial dynamics of ROS accumulation in plant immunity.

DGK5-mediated phosphatidic acid homeostasis interplays with reactive oxygen species in plant immune signaling.

Reactive oxygen species (ROS) and phosphatidic acid (PA) are important second messengers in plant immunity. PA binding to RBOHD, an NADPH oxidase responsible for ROS production, enhances RBOHD stability and promotes ROS production. Distinct phosphorylation of the lipid kinase DGK5 optimizes the PA burst in regulating ROS production.

Bacterial effectors manipulate plant abscisic acid signaling for creation of an aqueous apoplast.

Phytopathogens like Pseudomonas syringae induce "water soaking" in the apoplastic space of plant leaf tissue as a key virulence mechanism. Water soaking is commonly observed in diverse pathosystems, yet the underlying physiological basis remains largely elusive. Here, we show that one of the strong P. syringae water-soaking inducers, AvrE, alters the regulation of abscisic acid (ABA) to induce ABA signaling, stomatal closure, and, thus, water soaking. AvrE binds and inhibits the function of Arabidopsis type one protein phosphatases (TOPPs), which negatively regulate ABA by suppressing SnRK2s, a key node of the ABA signaling pathway. The topp12537 quintuple mutants display significantly enhanced water soaking after P. syringae inoculation, whereas the loss of the ABA pathway dampens P. syringae-induced water soaking and disease. Our study uncovers the hijacking of ABA signaling and stomatal closure by P. syringae effectors as key mechanisms of disease susceptibility.

Bacterial Infection and Hypersensitive Response Assays in Arabidopsis-Pseudomonas syringae Pathosystem.

Arabidopsis thaliana-Pseudomonas syringae pathosystem has been used as an important model system for studying plant-microbe interactions, leading to many milestones and breakthroughs in the understanding of plant immune system and pathogenesis mechanisms. Bacterial infection and plant disease assessment are key experiments in the studies of plant-pathogen interactions. The hypersensitive response (HR), which is characterized by rapid cell death and tissue collapse after inoculation with a high dose of bacteria, is a hallmark response of plant effector-triggered immunity (ETI), one layer of plant immunity triggered by recognition of pathogen-derived effector proteins. Here, we present a detailed protocol for bacterial disease and hypersensitive response assays applicable to studies of Pseudomonas syringae interaction with various plant species such as Arabidopsis, Nicotiana benthamiana, and tomato.

PTI-ETI crosstalk: an integrative view of plant immunity.

Plants resist attacks by pathogens via innate immune responses, which are initiated by cell surface-localized pattern-recognition receptors (PRRs) and intracellular nucleotide-binding domain leucine-rich repeat containing receptors (NLRs) leading to pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), respectively. Although the two classes of immune receptors involve different activation mechanisms and appear to require different early signalling components, PTI and ETI eventually converge into many similar downstream responses, albeit with distinct amplitudes and dynamics. Increasing evidence suggests the existence of intricate interactions between PRR-mediated and NLR-mediated signalling cascades as well as common signalling components shared by both. Future investigation of the mechanisms underlying signal collaboration between PRR-initiated and NLR-initiated immunity will enable a more complete understanding of the plant immune system. This review discusses recent advances in our understanding of the relationship between the two layers of plant innate immunity.

Genetic Structure and Relationship Analysis of an Association Population in Jute (Corchorus spp.) Evaluated by SSR Markers.

Population structure and relationship analysis is of great importance in the germplasm utilization and association mapping. Jute, comprised of white jute (C. capsularis L) and dark jute (C. olitorius L), is second to cotton in its commercial significance in the world. Here, we assessed the genetic structure and relationship in a panel of 159 jute accessions from 11 countries and regions using 63 SSRs. The structure analysis divided the 159 jute accessions from white and dark jute into Co and Cc group, further into Co1, Co2, Cc1 and Cc2 subgroups. Out of Cc1 subgroup, 81 accessions were from China and the remaining 10 accessions were from India (2), Japan (5), Thailand, Vietnam (2) and Pakistan (1). Out of Cc2 subgroup, 35 accessions were from China, and the remaining 3 accessions were from India, Pakistan and Thailand respectively. It can be inferred that the genetic background of these jute accessions was not always correlative with their geographical regions. Similar results were found in Co1 and Co2 subgroups. Analysis of molecular variance revealed 81% molecular variation between groups but it was low (19%) within subgroups, which further confirmed the genetic differentiation between the two groups. The genetic relationship analysis showed that the most diverse genotypes were Maliyeshengchangguo and Changguozhongyueyin in dark jute, BZ-2-2, Aidianyehuangma, Yangjuchiyuanguo, Zijinhuangma and Jute 179 in white jute, which could be used as the potential parents in breeding programs for jute improvement. These results would be very useful for association studies and breeding in jute.