Salicylic acid: an old hormone up to new tricks

Salicylic acid (SA) acts as a signalling molecule in plant defence against biotrophic and hemibiotrophic phytopathogens. The biosynthesis of SA on pathogen detection is essential for local and systemic acquired resistance, as well as the accumulation of pathogenesis‐related (PR) proteins. SA biosynthesis can occur via several different substrates, but is predominantly accomplished by isochorismate synthase (ICS1) following pathogen recognition. The roles of BTB domain‐containing proteins, NPR1, NPR3 and NPR4, in SA binding and signal transduction have been re‐examined recently and are elaborated upon in this review. The pathogen‐mediated manipulation of SA‐dependent defences, as well as the crosstalk between the SA signalling pathway, other plant hormones and defence signals, is also discussed in consideration of recent research. Furthermore, the recent links established between SA, pathogen‐triggered endoplasmic reticulum stress and the unfolded protein response are highlighted.     

Controlled levels of salicylic acid are required for optimal photosynthesis and redox homeostasis

Sudden exposure of plants to high light (HL) leads to metabolic and physiological disruption of the photosynthetic cells. Changes in ROS content, adjustment of photosynthetic processes and the antioxidant pools and, ultimately, gene induction are essential components for a successful acclimation to the new light conditions. The influence of salicylic acid (SA) on plant growth, short-term acclimation to HL, and on the redox homeostasis of Arabidopsis thaliana leaves was assessed here. The dwarf phenotype displayed by mutants with high SA content (cpr1-1, cpr5-1, cpr6-1, and dnd1-1) was less pronounced when these plants were grown in HL, suggesting that the inhibitory effect of SA on growth was partly overcome at higher light intensities. Moreover, higher SA content affected energy conversion processes in low light, but did not impair short-term acclimation to HL. On the other hand, mutants with low foliar SA content (NahG and sid2-2) were impaired in acclimation to transient exposure to HL and thus predisposed to oxidative stress. Low and high SA levels were strictly correlated to a lower and higher foliar H(2)O(2) content, respectively. Furthermore high SA was also associated with higher GSH contents, suggesting a tight correlation between SA, H(2)O(2) and GSH contents in plants. These observations implied an essential role of SA in the acclimation processes and in regulating the redox homeostasis of the cell. Implications for the role of SA in pathogen defence signalling are also discussed.     

Protein kinase‐mediated signalling in priming: Immune signal initiation,propagation, and establishment of long‐term pathogen resistance in plants

“Priming” in plant phytopathology describes a phenomenon where the “experience” of primary infection by microbial pathogens leads to enhanced and beneficial protection of the plant against secondary infection. The plant is able to establish an immune memory, a state of systemic acquired resistance (SAR), in which the information of “having been attacked” is integrated with the action of “being prepared to defend when it happens again.” Accordingly, primed plants are often characterized by faster and stronger activation of immune reactions that ultimately result in a reduction of pathogen spread and growth. Prerequisites for SAR are (a) the initiation of immune signalling subsequent to pathogen recognition, (b) a rapid defence signal propagation from a primary infected local site to uninfected distal parts of the plant, and (c) a switch into an immune signal‐dependent establishment and subsequent long‐lasting maintenance of phytohormone salicylic acid‐based systemic immunity. Here, we provide a summary on protein kinases that contribute to these three conceptual aspects of “priming” in plant phytopathology, complemented by data addressing the role of protein kinases crucial for immune signal initiation also for signal propagation and SAR.     

Induction of systemic disease resistance in Nicotiana benthamiana by the cyclodipeptides cyclo (l‐Pro‐l‐Pro) and cyclo (d‐Pro‐d‐Pro)

Cyclodipeptides, formed from two amino acids by cyclodehydration, are produced naturally by many organisms, and are known to possess a large number of biological activities. In this study, we found that cyclo (l ‐Pro‐l ‐Pro) and cyclo (d ‐Pro‐d ‐Pro) (where Pro is proline) could induce defence responses and systemic resistance in Nicotiana benthamiana. Treatment with the two cyclodipeptides led to a reduction in disease severity by Phytophthora nicotianae and Tobacco mosaic virus (TMV) infections compared with controls. Both cyclopeptides triggered stomatal closure, induced reactive oxygen species production and stimulated cytosolic calcium ion and nitric oxide production in guard cells. In addition, the application of cyclodipeptides significantly up‐regulated the expression of the plant defence gene PR‐1a and the PR‐1a protein, and increased cellular salicylic acid (SA) levels. These results suggest that the SA‐dependent defence pathway is involved in cyclodipeptide‐mediated pathogen resistance in N. benthamiana. We report the systemic resistance induced by cyclodipeptides, which sheds light on the potential of cyclodipeptides for the control of plant diseases.     

New insights into the regulation of plant immunity by amino acid metabolic pathways

Besides defence pathways regulated by classical stress hormones, distinct amino acid metabolic pathways constitute integral parts of the plant immune system. Mutations in several genes involved in Asp‐derived amino acid biosynthetic pathways can have profound impact on plant resistance to specific pathogen types. For instance, amino acid imbalances associated with homoserine or threonine accumulation elevate plant immunity to oomycete pathogens but not to pathogenic fungi or bacteria. The catabolism of Lys produces the immune signal pipecolic acid (Pip), a cyclic, non‐protein amino acid. Pip amplifies plant defence responses and acts as a critical regulator of plant systemic acquired resistance, defence priming and local resistance to bacterial pathogens. Asp‐derived pyridine nucleotides influence both pre‐ and post‐invasion immunity, and the catabolism of branched chain amino acids appears to affect plant resistance to distinct pathogen classes by modulating crosstalk of salicylic acid‐ and jasmonic acid‐regulated defence pathways. It also emerges that, besides polyamine oxidation and NADPH oxidase, Pro metabolism is involved in the oxidative burst and the hypersensitive response associated with avirulent pathogen recognition. Moreover, the acylation of amino acids can control plant resistance to pathogens and pests by the formation of protective plant metabolites or by the modulation of plant hormone activity.     

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.     

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.     

Rapid systemic signaling during abiotic and biotic stresses: is the ROS wave master of all trades?

Rapidly communicating the perception of an abiotic stress event, wounding or pathogen infection, from its initial site of occurrence to the entire plant, i.e. rapid systemic signaling, is essential for successful plant acclimation and defense. Recent studies highlighted an important role for several rapid whole‐plant systemic signals in mediating plant acclimation and defense during different abiotic and biotic stresses. These include calcium, reactive oxygen species (ROS), hydraulic and electric waves. Although the role of some of these signals in inducing and coordinating whole‐plant systemic responses was demonstrated, many questions related to their mode of action, routes of propagation and integration remain unanswered. In addition, it is unclear how these signals convey specificity to the systemic response, and how are they integrated under conditions of stress combination. Here we highlight many of these questions, as well as provide a proposed model for systemic signal integration, focusing on the ROS wave.     

Local and systemic regulation of PSII efficiency in triticale infected by the hemibiotrophic pathogen Microdochium nivale

Microdochium nivale is a fungal pathogen that causes yield losses of cereals during winter. Cold hardening under light conditions induces genotype‐dependent resistance of a plant to infection. We aim to show how photosystem II (PSII) regulation contributes to plant resistance. Using mapping population of triticale doubled haploid lines, three M. nivale strains and different infection assays, we demonstrate that plants that maintain a higher maximum quantum efficiency of PSII show less leaf damage upon infection. The fungus can establish necrotrophic or biotrophic interactions with susceptible or resistant genotypes, respectively. It is suggested that local inhibition of photosynthesis during the infection of sensitive genotypes is not balanced by a supply of energy from the tissue surrounding the infected cells as efficiently as in resistant genotypes. Thus, defence is limited, which in turn results in extensive necrotic damage. Quantitative trait loci regions, involved in the control of both PSII functioning and resistance, were located on chromosomes 4 and 6, similar to a wide range of PSII‐ and resistance‐related genes. A meta‐analysis of microarray experiments showed that the expression of genes involved in the repair and de novo assembly of PSII was maintained at a stable level. However, to establish a favourable energy balance for defence, genes encoding PSII proteins resistant to oxidative degradation were downregulated to compensate for the upregulation of defence‐related pathways. Finally, we demonstrate that the structural and functional integrity of the plant is a factor required to meet the energy demand of infected cells, photosynthesis‐dependent systemic signalling and defence responses.     

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.     

Salicylic acid, active oxygen species and systemic acquired resistance in plants

Infection of plants, particularly by a necrotizing pathogen, usually induces a long-lasting, broad-based, systemic resistance to secondary pathogen attack. Many studies implicate salicylic acid as an essential signal in the development of such systemic acquired resistance in several plant species. Salicylic acid appears to mediate plant defence by binding to and inhibiting catalase, thus increasing the concentration of H(2)O(2) and other active oxygen species. Active oxygen species may then act as second messengers that induce plant defence gene expression, analogous to their activation of gene expression in mammalian cells.     

Salicylic acid and photosynthesis: signalling and effects

Salicylic acid (SA) is a well-known signalling molecule playing a role in local and systemic acquired resistance against pathogens as well as in acclimation to certain abiotic stressors. As a stress-related signalling compound, it may directly or indirectly affect various physiological processes, including photosynthesis. The effects of exogenously applied SA on plant physiological processes under optimal environmental conditions are controversial. Several studies suggest that SA may have a positive effect on germination or plant growth in various plant species. However, SA may also act as a stress factor, having a negative influence on various physiological processes. Its mode of action depends greatly on several factors, such as the plant species, the environmental conditions (light, temperature, etc.) and the concentration. Exogenous SA may also alleviate the damaging effects of various stress factors, and this protection may also be manifested as higher photosynthetic capacity. Unfavourable environmental conditions have also been shown to increase the endogenous SA level in plants. Recent results strongly suggest that controlled SA levels are important in plants for optimal photosynthetic performance and for acclimation to changing environmental stimuli. The present review discusses the effects of exogenous and endogenous SA on the photosynthetic processes under optimal and stress conditions.