Insect eggs suppress plant defence against chewing herbivores

Plants activate direct and indirect defences in response to insect egg deposition. However, whether eggs can manipulate plant defence is unknown. In Arabidopsis thaliana, oviposition by the butterfly Pieris brassicae triggers cellular and molecular changes that are similar to the changes caused by biotrophic pathogens. In the present study, we found that the plant defence signal salicylic acid (SA) accumulates at the site of oviposition. This is unexpected, as the SA pathway controls defence against fungal and bacterial pathogens and negatively interacts with the jasmonic acid (JA) pathway, which is crucial for the defence against herbivores. Application of P. brassicae or Spodoptera littoralis egg extract onto leaves reduced the induction of insect‐responsive genes after challenge with caterpillars, suggesting that egg‐derived elicitors suppress plant defence. Consequently, larval growth of the generalist herbivore S. littoralis, but not of the specialist P. brassicae, was significantly higher on plants treated with egg extract than on control plants. In contrast, suppression of gene induction and enhanced S. littoralis performance were not seen in the SA‐deficient mutant sid2‐1, indicating that it is SA that mediates this phenomenon. These data reveal an intriguing facet of the cross‐talk between SA and JA signalling pathways, and suggest that insects have evolved a way to suppress the induction of defence genes by laying eggs that release elicitors. We show here that egg‐induced SA accumulation negatively interferes with the JA pathway, and provides an advantage for generalist herbivores.     

AtCPK1 calcium‐dependent protein kinase mediates pathogen resistance in Arabidopsis

In mammals, lipid bodies play a key role during pathological and infectious diseases. However, our knowledge on the function of plant lipid bodies, apart from their role as the major site of lipid storage in seed tissues, remains limited. Here, we provide evidence that a calcium‐dependent protein kinase (CPK) mediates pathogen resistance in Arabidopsis. AtCPK1 expression is rapidly induced by fungal elicitors. Loss‐of‐function mutants of AtCPK1 exhibit higher susceptibility to pathogen infection compared to wild‐type plants. Conversely, over‐expression of AtCPK1 leads to accumulation of salicylic acid (SA) and constitutive expression of SA‐regulated defence and disease resistance genes, which, in turn, results in broad‐spectrum protection against pathogen infection. Expression studies in mutants affected in SA‐mediated defence responses revealed an interlocked feedback loop governing AtCPK1 expression and components of the SA‐dependent signalling pathway. Moreover, we demonstrate the dual localization of AtCPK1 in lipid bodies and peroxisomes. Overall, our findings identify AtCPK1 as a component of the innate immune system of Arabidopsis plants.     

Receptor quality control in the endoplasmic reticulum for plant innate immunity

Pattern recognition receptors in eukaryotes initiate defence responses on detection of microbe‐associated molecular patterns shared by many microbe species. The Leu‐rich repeat receptor‐like kinases FLS2 and EFR recognize the bacterial epitopes flg22 and elf18, derived from flagellin and elongation factor‐Tu, respectively. We describe Arabidopsis ‘priority in sweet life’ (psl) mutants that show de‐repressed anthocyanin accumulation in the presence of elf18. EFR accumulation and signalling, but not of FLS2, are impaired in psl1, psl2, and stt3a plants. PSL1 and PSL2, respectively, encode calreticulin3 (CRT3) and UDP‐glucose:glycoprotein glycosyltransferase that act in concert with STT3A‐containing oligosaccharyltransferase complex in an N‐glycosylation pathway in the endoplasmic reticulum. However, EFR‐signalling function is impaired in weak psl1 alleles despite its normal accumulation, thereby uncoupling EFR abundance control from quality control. Furthermore, salicylic acid‐induced, but EFR‐independent defence is weakened in psl2 and stt3a plants, indicating the existence of another client protein than EFR for this immune response. Our findings suggest a critical and selective function of N‐glycosylation for different layers of plant immunity, likely through quality control of membrane‐localized regulators.     

Do strigolactones contribute to plant defence?

Strigolactones are multifunctional molecules involved in several processes outside and within the plant. As signalling molecules in the rhizosphere, they favour the establishment of arbuscular mycorrhizal symbiosis, but they also act as host detection cues for root parasitic plants. As phytohormones, they are involved in the regulation of plant architecture, adventitious rooting, secondary growth and reproductive development, and novel roles are emerging continuously. In the present study, the possible involvement of strigolactones in plant defence responses was investigated. For this purpose, the resistance/susceptibility of the strigolactone‐deficient tomato mutant Slccd8 against the foliar fungal pathogens Botrytis cinerea and Alternaria alternata was assessed. Slccd8 was more susceptible to both pathogens, pointing to a new role for strigolactones in plant defence. A reduction in the content of the defence‐related hormones jasmonic acid, salicylic acid and abscisic acid was detected by high‐performance liquid chromatography coupled to tandem mass spectrometry in the Slccd8 mutant, suggesting that hormone homeostasis is altered in the mutant. Moreover, the expression level of the jasmonate‐dependent gene PinII, involved in the resistance of tomato to B. cinerea, was lower than in the corresponding wild‐type. We propose here that strigolactones play a role in the regulation of plant defences through their interaction with other defence‐related hormones, especially with the jasmonic acid signalling pathway.     

The synthetic cationic lipid diC14 activates a sector of the Arabidopsis defence network requiring endogenous signalling components

Natural and synthetic elicitors have contributed significantly to the study of plant immunity. Pathogen‐derived proteins and carbohydrates that bind to immune receptors, allow the fine dissection of certain defence pathways. Lipids of a different nature that act as defence elicitors, have also been studied, but their specific effects have been less well characterized, and their receptors have not been identified. In animal cells, nanoliposomes of the synthetic cationic lipid 3‐tetradecylamino‐tert‐butyl‐N‐tetradecylpropionamidine (diC14) activate the TLR4‐dependent immune cascade. Here, we have investigated whether this lipid induces Arabidopsis defence responses. At the local level, diC14 activated early and late defence gene markers (FRK1, WRKY29, ICS1 and PR1), acting in a dose‐dependent manner. This lipid induced the salicylic acid (SA)‐dependent, but not jasmonic acid (JA)‐dependent, pathway and protected plants against Pseudomonas syringae pv. tomato (Pst), but not Botrytis cinerea. diC14 was not toxic to plant or pathogen, and potentiated pathogen‐induced callose deposition. At the systemic level, diC14 induced PR1 expression and conferred resistance against Pst. diC14‐induced defence responses required the signalling protein EDS1, but not NDR1. Curiously, the lipid‐induced defence gene expression was lower in the fls2/efr/cerk1 triple mutant, but still unchanged in the single mutants. The amidine headgroup and chain length were important for its activity. Given the robustness of the responses triggered by diC14, its specific action on a defence pathway and the requirement for well‐known defence components, this synthetic lipid is emerging as a useful tool to investigate the initial events involved in plant innate immunity.     

A role for β‐sitosterol to stigmasterol conversion in plant–pathogen interactions

Upon inoculation with pathogenic microbes, plants induce an array of metabolic changes that potentially contribute to induced resistance or even enhance susceptibility. When analysing leaf lipid composition during the Arabidopsis thaliana–Pseudomonas syringae interaction, we found that accumulation of the phytosterol stigmasterol is a significant plant metabolic process that occurs upon bacterial leaf infection. Stigmasterol is synthesized from β‐sitosterol by the cytochrome P450 CYP710A1 via C22 desaturation. Arabidopsis cyp710A1 mutant lines impaired in pathogen‐inducible expression of the C22 desaturase and concomitant stigmasterol accumulation are more resistant to both avirulent and virulent P. syringae strains than wild‐type plants, and exogenous application of stigmasterol attenuates this resistance phenotype. These data indicate that induced sterol desaturation in wild‐type plants favours pathogen multiplication and plant susceptibility. Stigmasterol formation is triggered through perception of pathogen‐associated molecular patterns such as flagellin and lipopolysaccharides, and through production of reactive oxygen species, but does not depend on the salicylic acid, jasmonic acid or ethylene defence pathways. Isolated microsomal and plasma membrane preparations exhibited a similar increase in the stigmasterol/β‐sitosterol ratio as whole‐leaf extracts after leaf inoculation with P. syringae, indicating that the stigmasterol produced is incorporated into plant membranes. The increased contents of stigmasterol in leaves after pathogen attack do not influence salicylic acid‐mediated defence signalling but attenuate pathogen‐induced expression of the defence regulator flavin‐dependent monooxygenase 1. P. syringae thus promotes plant disease susceptibility through stimulation of sterol C22 desaturation in leaves, which increases the stigmasterol to β‐sitosterol ratio in plant membranes.     

Chemical activators: A novel and sustainable management approach for Meloidogyne incognita (Kofoid and White) Chitwood in Chamomilla recutita L

Use of chemical activator for the management of root-knot disease in medicinal and aromatic plants can become an attractive alternative to traditionally used nematicides. Large numbers of chemical molecules are present in the plants and are involved in the induction of different types of proteins. The purpose of our research study is to explore the possibilities of resistance factors that are inherent in the plant by treating them with few chemical activators to activate against root-knot nematode infection. Efforts were made to achieve a satisfactory suppression of root-knot nematode, Meloidogyne incognita, a serious menace to successful cultivation of chamomile, Matricaria recutita L. Rauch (syn. Matricaria chamomilla L. Fain. Asteraceae) causing root-knot disease through use of different chemical activators. Here we examined selected groups of resistance activator viz. Isonicotinamide, 2-chloronicotinic acid, 5-nitrosalicylic acid, 4-chlorosalicylic acid, DL-2 aminobutyric acid, 2-aminobutyric acid, O-acetylsalicylic acid, 4-amino salicylic acid, salicylic acid and these were used as soil drench using 3 week old seedlings transplanted to root-knot nematode infested pots. Maximum reduction in root-knot severity and nematode population occurred with 4-chlorosalicylic acid, O-acetyl salicylic acid, 2-chloronicotinic acid and gave significant flower yield advantages. Present experiment suggests a strong possibility of these activators in integrated management for protection against plant parasitic nematodes.     

Cyclic lipopeptides from Bacillus subtilis activate distinct patterns of defence responses in grapevine

Non‐self‐recognition of microorganisms partly relies on the perception of microbe‐associated molecular patterns (MAMPs) and leads to the activation of an innate immune response. Bacillus subtilis produces three main families of cyclic lipopeptides (LPs), namely surfactins, iturins and fengycins. Although LPs are involved in induced systemic resistance (ISR) activation, little is known about defence responses induced by these molecules and their involvement in local resistance to fungi. Here, we showed that purified surfactin, mycosubtilin (iturin family) and plipastatin (fengycin family) are perceived by grapevine plant cells. Although surfactin and mycosubtilin stimulated grapevine innate immune responses, they differentially activated early signalling pathways and defence gene expression. By contrast, plipastatin perception by grapevine cells only resulted in early signalling activation. Gene expression analysis suggested that mycosubtilin activated salicylic acid (SA) and jasmonic acid (JA) signalling pathways, whereas surfactin mainly induced an SA‐regulated response. Although mycosubtilin and plipastatin displayed direct antifungal activity, only surfactin and mycosubtilin treatments resulted in a local long‐lasting enhanced tolerance to the necrotrophic fungus Botrytis cinerea in grapevine leaves. Moreover, challenge with specific strains overproducing surfactin and mycosubtilin led to a slightly enhanced stimulation of the defence response compared with the LP‐non‐producing strain of B. subtilis. Altogether, our results provide the first comprehensive view of the involvement of LPs from B. subtilis in grapevine plant defence and local resistance against the necrotrophic pathogen Bo. cinerea. Moreover, this work is the first to highlight the ability of mycosubtilin to trigger an immune response in plants.     

The Elongator complex‐associated protein DRL1 plays a positive role in immune responses against necrotrophic fungal pathogens in Arabidopsis

DEFORMED ROOT AND LEAVES1 (DRL1) is an Arabidopsis homologue of the yeast TOXIN TARGET4 (TOT4)/KILLER TOXIN‐INSENSITIVE12 (KTI12) protein that is physically associated with the RNA polymerase II‐interacting protein complex named Elongator. Mutations in DRL1 and Elongator lead to similar morphological and molecular phenotypes, suggesting that DRL1 and Elongator may functionally overlap in Arabidopsis. We have shown previously that Elongator plays an important role in both salicylic acid (SA)‐ and jasmonic acid (JA)/ethylene (ET)‐mediated defence responses. Here, we tested whether DRL1 also plays a similar role as Elongator in plant immune responses. Our results show that, although DRL1 partially contributes to SA‐induced cytotoxicity, it does not play a significant role in SA‐mediated expression of PATHOGENESIS‐RELATED genes and resistance to the virulent bacterial pathogen Pseudomonas syringae pv. maculicola ES4326. In contrast, DRL1 is required for JA/ET‐ and necrotrophic fungal pathogen Botrytis cinerea‐induced defence gene expression and for resistance to B. cinerea and Alternaria brassicicola. Furthermore, unlike the TOT4/KTI12 gene which, when overexpressed in yeast, confers zymocin resistance, a phenotype of the tot4/kti12 mutant, overexpression of DRL1 does not change B. cinerea‐induced defence gene expression and resistance to this pathogen. Finally, DRL1 contains an N‐terminal P‐loop and a C‐terminal calmodulin (CaM)‐binding domain and is a CaM‐binding protein. We demonstrate that both the P‐loop and the CaM‐binding domain are essential for the function of DRL1 in B. cinerea‐induced expression of PDF1.2 and ORA59, and in resistance to B. cinerea, suggesting that the function of DRL1 in plant immunity may be regulated by ATP/GTP and CaM binding.     

Signalling requirements for Erwinia amylovora‐induced disease resistance,callose deposition and cell growth in the non‐host Arabidopsis thaliana

Erwinia amylovora is the causal agent of the fire blight disease in some plants of the Rosaceae family. The non‐host plant Arabidopsis serves as a powerful system for the dissection of mechanisms of resistance to E. amylovora. Although not yet known to mount gene‐for‐gene resistance to E. amylovora, we found that Arabidopsis activated strong defence signalling mediated by salicylic acid (SA), with kinetics and amplitude similar to that induced by the recognition of the bacterial effector avrRpm1 by the resistance protein RPM1. Genetic analysis further revealed that SA signalling, but not signalling mediated by ethylene (ET) and jasmonic acid (JA), is required for E. amylovora resistance. Erwinia amylovora induces massive callose deposition on infected leaves, which is independent of SA, ET and JA signalling and is necessary for E. amylovora resistance in Arabidopsis. We also observed tumour‐like growths on E. amylovora‐infected Arabidopsis leaves, which contain enlarged mesophyll cells with increased DNA content and are probably a result of endoreplication. The formation of such growths is largely independent of SA signalling and some E. amylovora effectors. Together, our data reveal signalling requirements for E. amylovora‐induced disease resistance, callose deposition and cell fate change in the non‐host plant Arabidopsis. Knowledge from this study could facilitate a better understanding of the mechanisms of host defence against E. amylovora and eventually improve host resistance to the pathogen.     

A novel Arabidopsis–oomycete pathosystem: differential interactions with Phytophthora capsici reveal a role for camalexin,indole glucosinolates and salicylic acid in defence

Phytophthora capsici causes devastating diseases on a broad range of plant species. To better understand the interaction with its host plants, knowledge obtained from a model pathosystem can be instrumental. Here, we describe the interaction between P. capsici and Arabidopsis and the exploitation of this novel pathosystem to assign metabolic pathways involved in defence against P. capsici. Inoculation assays on Arabidopsis accessions with different P. capsici isolates revealed interaction specificity among accession‐isolate combinations. In a compatible interaction, appressorium‐mediated penetration was followed by the formation of invasive hyphae, haustoria and sporangia in leaves and roots. In contrast, in an incompatible interaction, P. capsici infection elicited callose deposition, accumulation of active oxygen species and cell death, resulting in early pathogen encasement in leaves. Moreover, Arabidopsis mutants with defects in salicylic acid signalling, camalexin or indole glucosinolates biosynthesis pathways displayed severely compromised resistance to P. capsici. It is anticipated that this model pathosystem will facilitate the genetic dissection of complex traits responsible for resistance against P. capsici.