Haitao Cui

Effector-Triggered Immunity: From Pathogen Perception to Robust Defense

In plant innate immunity, individual cells have the capacity to sense and respond to pathogen attack. Intracellular recognition mechanisms have evolved to intercept perturbations by pathogen virulence factors (effectors) early in host infection and convert it to rapid defense. One key to resistance success is a polymorphic family of intracellular nucleotide-binding/leucine-rich-repeat (NLR) receptors that detect effector interference in different parts of the cell. Effector-activated NLRs connect, in various ways, to a conserved basal resistance network in order to transcriptionally boost defense programs. Effector-triggered immunity displays remarkable robustness against pathogen disturbance, in part by employing compensatory mechanisms within the defense network. Also, the mobility of some NLRs and coordination of resistance pathways across cell compartments provides flexibility to fine-tune immune outputs. Furthermore, a number of NLRs function close to the nuclear chromatin by balancing actions of defense-repressing and defense-activating transcription factors to program cells dynamically for effective disease resistance.

ETHYLENE INSENSITIVE3 and ETHYLENE INSENSITIVE3-LIKE1 Repress SALICYLIC ACID INDUCTION DEFICIENT2 Expression to Negatively Regulate Plant Innate Immunity in Arabidopsis

Pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) trigger plant immunity that forms the first line inducible defenses in plants. The regulatory mechanism of MAMP-triggered immunity, however, is poorly understood. Here, we show that Arabidopsis thaliana transcription factors ETHYLENE INSENSITIVE3 (EIN3) and ETHYLENE INSENSITIVE3-LIKE1 (EIL1), previously known to mediate ethylene signaling, also negatively regulate PAMP-triggered immunity. Plants lacking EIN3 and EIL1 display enhanced PAMP defenses and heightened resistance to Pseudomonas syringae bacteria. Conversely, plants overaccumulating EIN3 are compromised in PAMP defenses and exhibit enhanced disease susceptibility to Pseudomonas syringae. Microarray analysis revealed that EIN3 and EIL1 negatively control PAMP response genes. Further analyses indicated that SALICYLIC ACID INDUCTION DEFICIENT2 (SID2), which encodes isochorismate synthase required for pathogen-induced biosynthesis of salicylic acid (SA), is a key target of EIN3 and EIL1. Consistent with this, the ein3-1 eil1-1 double mutant constitutively accumulates SA in the absence of pathogen attack, and a mutation in SID2 restores normal susceptibility in the ein3 eil1 double mutant. EIN3 can specifically bind SID2 promoter sequence in vitro and in vivo. Taken together, our data provide evidence that EIN3/EIL1 directly target SID2 to downregulate PAMP defenses.

Pseudomonas syringae Effector Protein AvrB Perturbs Arabidopsis Hormone Signaling by Activating MAP Kinase 4

Pathogenic microbes often modulate phytohormone physiology in the host to their advantage. We previously showed that the Pseudomonas syringae effector protein AvrB perturbs hormone signaling, as exemplified by upregulated expression of jasmonic acid response genes, and enhances plant susceptibility. Here we show that these effects of AvrB require the Arabidopsis mitogen-activated protein kinase MAP kinase 4 (MPK4), HSP90 chaperone components, and the AvrB-interacting protein, RIN4. AvrB interacts with MPK4 and the HSP90 chaperone, and AvrB induces MPK4 activation in a manner promoted by HSP90; RIN4 likely acts downstream of MPK4. These findings link Arabidopsis proteins MPK4, HSP90, and RIN4 into a pathway that P. syringae AvrB activates for the benefit of the bacterium, perturbing hormone signaling and enhancing plant susceptibility.

RAR1, a central player in plant immunity, is targeted by Pseudomonas syringae effector AvrB

Pathogenic bacterial effectors suppress pathogen-associated molecular pattern (PAMP)-triggered host immunity, thereby promoting parasitism. In the presence of cognate resistance genes, it is proposed that plants detect the virulence activity of bacterial effectors and trigger a defense response, referred to here as effector-triggered immunity (ETI). However, the link between effector virulence and ETI at the molecular level is unknown. Here, we show that the Pseudomonas syringae effector AvrB suppresses PAMP-triggered immunity (PTI) through RAR1, a cochaperone of HSP90 required for ETI. AvrB expressed in plants lacking the cognate resistance gene RPM1 suppresses cell wall defense induced by the flagellar peptide flg22, a well known PAMP, and promotes the growth of nonpathogenic bacteria in a RAR1-dependent manner. rar1 mutants display enhanced cell wall defense in response to flg22, indicating that RAR1 negatively regulates PTI. Furthermore, coimmunoprecipitation experiments indicated that RAR1 and AvrB interact in the plant. The results demonstrate that RAR1 molecularly links PTI, effector virulence, and ETI. The study supports that both pathogen virulence and plant disease resistance have evolved around PTI.

Plant immunity: a lesson from pathogenic bacterial effector proteins

Phytopathogenic bacteria inject an array of effector proteins into host cells to alter host physiology and assist the infection process. Some of these effectors can also trigger disease resistance as a result of recognition in the plant cell by cytoplasmic immune receptors. In addition to effector‐triggered immunity, plants immunity can be triggered upon the detection of Pathogen/Microbe‐Associated Molecular Patterns by surface‐localized immune receptors. Recent progress indicates that many bacterial effector proteins use a variety of biochemical properties to directly attack key components of PAMP‐triggered immunity and effector‐triggered immunity, providing new insights into the molecular basis of plant innate immunity. Emerging evidence indicate that the evolution of disease resistance in plants is intimately linked to the mechanism by which bacterial effectors promote parasitism. This review focuses on how these studies have conceptually advanced our understanding of plant–pathogen interactions.

Network of GRAS Transcription Factors Involved in the Control of Arbuscule Development in Lotus japonicus

A network of transcription factors regulates arbuscular mycorrhizal symbiosis in Lotus. Arbuscular mycorrhizal (AM) fungi, in symbiosis with plants, facilitate acquisition of nutrients from the soil to their host. After penetration, intracellular hyphae form fine-branched structures in cortical cells termed arbuscules, representing the major site where bidirectional nutrient exchange takes place between the host plant and fungus. Transcriptional mechanisms underlying this cellular reprogramming are still poorly understood. GRAS proteins are an important family of transcriptional regulators in plants, named after the first three members: GIBBERELLIC ACID-INSENSITIVE, REPRESSOR of GAI, and SCARECROW. Here, we show that among 45 transcription factors up-regulated in mycorrhizal roots of the legume Lotus japonicus, expression of a unique GRAS protein particularly increases in arbuscule-containing cells under low phosphate conditions and displays a phylogenetic pattern characteristic of symbiotic genes. Allelic rad1 mutants display a strongly reduced number of arbuscules, which undergo accelerated degeneration. In further studies, two RAD1-interacting proteins were identified. One of them is the closest homolog of Medicago truncatula, REDUCED ARBUSCULAR MYCORRHIZATION1 (RAM1), which was reported to regulate a glycerol-3-phosphate acyl transferase that promotes cutin biosynthesis to enhance hyphopodia formation. As in M. truncatula, the L. japonicus ram1 mutant lines show compromised AM colonization and stunted arbuscules. Our findings provide, to our knowledge, new insight into the transcriptional program underlying the host’s response to AM colonization and propose a function of GRAS transcription factors including RAD1 and RAM1 during arbuscule development.

Effector-Triggered and Pathogen-Associated Molecular Pattern-Triggered Immunity Differentially Contribute to Basal Resistance to Pseudomonas syringae

Pathogens induce pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) in plants. PAMPs are microbial molecules recognized by host plants as nonself signals, whereas pathogen effectors are evolved to aid in parasitism but are sometimes recognized by specific intracellular resistance proteins. In the absence of detectable ETI determining classical incompatible interactions, basal resistance exists during compatible and nonhost interactions. What triggers the basal resistance has remained elusive. Here, we provide evidence that ETI contributes to basal resistance during both compatible and nonhost Arabidopsis-Pseudomonas syringae interactions. Mutations in RAR1 and NDR1, two genes required for ETI, compromise basal resistance in both compatible and nonhost interactions. Complete nonhost resistance to P. syringae pv. tabaci required a functional type III secretion system. PTI appears to play a greater role in nonhost resistance than basal resistance during compatible interactions, because abrogation of PTI compromises basal resistance during nonhost but not compatible interactions. Strikingly, simultaneous abrogation of ETI and flagellin-induced PTI rendered plants completely susceptible to the nonadapted bacterium P. syringae pv. tabaci, indicating that ETI and PTI act synergistically during nonhost resistance. Thus, both nonhost resistance and basal resistance to virulent bacteria can be unified under PTI and ETI.

A core function of EDS1 with PAD4 is to protect the salicylic acid defense sector in Arabidopsis immunity

Plant defenses induced by salicylic acid (SA) are vital for resistance against biotrophic pathogens. In basal and receptor-triggered immunity, SA accumulation is promoted by Enhanced Disease Susceptibility1 with its co-regulator Phytoalexin Deficient4 (EDS1/PAD4). Current models position EDS1/PAD4 upstream of SA but their functional relationship remains unclear. In a genetic and transcriptomic analysis of Arabidopsis autoimmunity caused by constitutive or conditional EDS1/PAD4 overexpression, intrinsic EDS1/PAD4 signaling properties and their relation to SA were uncovered. A core EDS1/PAD4 pathway works in parallel with SA in basal and effector-triggered bacterial immunity. It protects against disabled SA-regulated gene expression and pathogen resistance, and is distinct from a known SA-compensatory route involving MAPK signaling. Results help to explain previously identified EDS1/PAD4 regulated SA-dependent and SA-independent gene expression sectors. Plants have evolved an alternative route for preserving SA-regulated defenses against pathogen or genetic perturbations. In a proposed signaling framework, EDS1 with PAD4, besides promoting SA biosynthesis, maintains important SA-related resistance programs, thereby increasing robustness of the innate immune system.

Ctf4-related protein recruits LHP1-PRC2 to maintain H3K27me3 levels in dividing cells in Arabidopsis thaliana

Significance Plant development depends on the Polycomb Group (PcG) pathway, which ensures an epigenetic memory of gene repression. PcG proteins form complexes that decorate histones of target genes with signature modifications. Decorated histones are diluted during DNA replication, therefore compensatory mechanisms must ensure their renewal during cell division. We show that a protein related to a yeast replication factor plays an important role in maintaining a PcG-associated chromatin modification in dividing plant cells. Polycomb Repressive Complex (PRC) 2 catalyzes the H3K27me3 modification that warrants inheritance of a repressive chromatin structure during cell division, thereby assuring stable target gene repression in differentiated cells. It is still under investigation how H3K27me3 is passed on from maternal to filial strands during DNA replication; however, cell division can reinforce H3K27me3 coverage at target regions. To identify novel factors involved in the Polycomb pathway in plants, we performed a forward genetic screen for enhancers of the like heterochromatin protein 1 (lhp1) mutant, which shows relatively mild phenotypic alterations compared with other plant PRC mutants. We mapped enhancer of lhp1 (eol) 1 to a gene related to yeast Chromosome transmission fidelity 4 (Ctf4) based on phylogenetic analysis, structural similarities, physical interaction with the CMG helicase component SLD5, and an expression pattern confined to actively dividing cells. A combination of eol1 with the curly leaf (clf) allele, carrying a mutation in the catalytic core of PRC2, strongly enhanced the clf phenotype; furthermore, H3K27me3 coverage at target genes was strongly reduced in eol1 clf double mutants compared with clf single mutants. EOL1 physically interacted with CLF, its partially redundant paralog SWINGER (SWN), and LHP1. We propose that EOL1 interacts with LHP1–PRC2 complexes during replication and thereby participates in maintaining the H3K27me3 mark at target genes.

Characterization of the interaction between Oidium heveae and Arabidopsis thaliana.

Oidium heveae, an obligate biotrophic pathogen of rubber trees (Hevea brasiliensis), causes significant yield losses of rubber worldwide. However, the molecular mechanisms underlying the interplay between O. heveae and rubber trees remain largely unknown. In this study, we isolated an O. heveae strain, named HN1106, from cultivated H. brasiliensis in Hainan, China. We found that O. heveae HN1106 triggers the hypersensitive response in a manner that depends on the effector-triggered immunity proteins EDS1 (Enhanced Disease Susceptibility 1) and PAD4 (Phytoalexin Deficient 4) and on salicylic acid (SA) in the model plant Arabidopsis thaliana. However, SA-independent resistance also appears to limit O. heveae infection of Arabidopsis, because the pathogen does not produce conidiospores on npr1 (nonexpressor of pr1), sid2 (SA induction deficient 2) and NahG plants, which show disruptions in SA signalling. Furthermore, we found that the callose synthase PMR4 (Powdery Mildew Resistant 4) prevents O. heveae HN1106 penetration into leaves in the early stages of infection. To elucidate the potential mechanism of resistance of Arabidopsis to O. heveae HN1106, we inoculated 47 different Arabidopsis accessions with the pathogen, and analysed the plant disease symptoms and O. heveae HN1106 hyphal growth and conidiospore formation on the leaves. We found that the accession Lag2-2 showed significant susceptibility to O. heveae HN1106. Overall, this study provides a basis for future research aimed at combatting powdery mildew caused by O. heveae in rubber trees.