Priming for stress resistance: from the lab to the field

Upon treatment with necrotizing pathogens, many plants develop an enhanced capacity for activating defense responses to biotic and abiotic stress--a process called priming. The primed state can also be induced by colonization of plant roots with beneficial micro-organisms or by treatment of plants with various natural and synthetic compounds. Priming is thought to be the mechanism by which plants can show induced resistance against ostensibly virulent pathogens after a conditioning treatment. Although the phenomenon has been known for years, it has been appreciated just recently that priming for enhanced defense responses can result from plant-plant communication in nature and that priming can also boost the resistance of crops to biotic and abiotic stresses in the field.     

Primed plants do not forget

In their struggle for life, plants can employ sophisticated strategies to defend themselves against potentially harmful pathogens and insects. One mechanism by which plants can increase their level of resistance is by intensifying the responsiveness of their immune system upon recognition of selected signals from their environment. This so-called priming of defence can provide long-lasting resistance, which is based on a faster and/or stronger defence reaction upon pathogen or pest attack. Priming can target various layers of induced defence that are active during different stages of the plant–attacker interaction. Recent discoveries have extended our knowledge about the mechanistic basis of defence priming and suggest that a primed defence state can be inherited epi-genetically from defence-expressing plants. In this review, we provide an overview of the latest insights about defence priming, ranging from early responses controlled by adjustments in hormone-dependent signalling pathways and availability of signal transduction proteins, to longer lasting mechanisms that involve possible regulation chromatin modification or DNA methylation.     

Steady state dendritic cells present parenchymal self-antigen and contribute to, but are not essential for, tolerization of naive and Th1 effector CD4 cells

Bone marrow-derived APC are critical for both priming effector/memory T cell responses to pathogens and inducing peripheral tolerance in self-reactive T cells. In particular, dendritic cells (DC) can acquire peripheral self-Ags under steady state conditions and are thought to present them to cognate T cells in a default tolerogenic manner, whereas exposure to pathogen-associated inflammatory mediators during the acquisition of pathogen-derived Ags appears to reprogram DCs to prime effector and memory T cell function. Recent studies have confirmed the critical role of DCs in priming CD8 cell effector responses to certain pathogens, although the necessity of steady state DCs in programming T cell tolerance to peripheral self-Ags has not been directly tested. In the current study, the role of steady state DCs in programming self-reactive CD4 cell peripheral tolerance was assessed by combining the CD11c-diphtheria toxin receptor transgenic system, in which DC can be depleted via treatment with diphtheria toxin, with a TCR-transgenic adoptive transfer system in which either naive or Th1 effector CD4 cells are induced to undergo tolerization after exposure to cognate parenchymally derived self-Ag. Although steady state DCs present parenchymal self-Ag and contribute to the tolerization of cognate naive and Th1 effector CD4 cells, they are not essential, indicating the involvement of a non-DC tolerogenic APC population(s). Tolerogenic APCs, however, do not require the cooperation of CD4(+)CD25(+) regulatory T cells. Similarly, DC were required for maximal priming of naive CD4 cells to vaccinia viral-Ag, but priming could still occur in the absence of DC.     

Priming of inducible defenses protects Norway spruce against tree-killing bark beetles

Plants can form an immunological memory known as defense priming, whereby exposure to a priming stimulus enables quicker or stronger response to subsequent attack by pests and pathogens. Such priming of inducible defenses provides increased protection and reduces allocation costs of defense. Defense priming has been widely studied for short-lived model plants such as Arabidopsis, but little is known about this phenomenon in long-lived plants like spruce. We compared the effects of pretreatment with sublethal fungal inoculations or application of the phytohormone methyl jasmonate (MeJA) on the resistance of 48-year-old Norway spruce (Picea abies) trees to mass attack by a tree-killing bark beetle beginning 35 days later. Bark beetles heavily infested and killed untreated trees but largely avoided fungus-inoculated trees and MeJA-treated trees. Quantification of defensive terpenes at the time of bark beetle attack showed fungal inoculation induced 91-fold higher terpene concentrations compared with untreated trees, whereas application of MeJA did not significantly increase terpenes. These results indicate that resistance in fungus-inoculated trees is a result of direct induction of defenses, whereas resistance in MeJA-treated trees is due to defense priming. This work extends our knowledge of defense priming from model plants to an ecologically important tree species.     

Molecular and physiological stages of priming: how plants prepare for environmental challenges

Being sessile organisms, plants must respond to various challenges in the environment. The priming process consists of three clear stages. The first stage includes all the cellular changes in the absence of the challenge so-called pre-challenge priming stage. These changes are expected to be rather subtle, affecting the preparation of the plant to properly manage subsequent responses to pathogens with no major fitness costs. Most of the research that has been conducted at this stage has been dedicated to the study of changes in gene expression and protein phosphorylation. However, the metabolic changes that occur during the pre-challenge priming stage are poorly understood. The second stage affects the early to late stages of the defence response, which occurs after the interaction with a pathogen has been established. Most studies involving priming are dedicated to the molecular events that take place during this stage. Most studies have shown that defence priming is strongly hormonally regulated; however, there is also evidence of the involvement of phenolic derivative compounds and many other secondary metabolites, leading to stronger and faster plant responses. The third priming phase ranges from long lasting defence priming to trans-generational acquired resistance. Long-term metabolic transitions, that occur in the offspring of primed plants, remain to be elucidated. Here we review existing information in the literature that relates to the metabolic changes that occur during all three defence priming stages and highlight the metabolic transitions that are associated with the stimulation of priming and the characteristics of the pathogens whenever possible.     

Seed priming for abiotic stress tolerance: an overview

Plants are exposed to any number of potentially adverse environmental conditions such as water deficit, high salinity, extreme temperature, submergence, etc. These abiotic stresses adversely affect the plant growth and productivity. Nowadays various strategies are employed to generate plants that can withstand these stresses. In recent years, seed priming has been developed as an indispensable method to produce tolerant plants against various stresses. Seed priming is the induction of a particular physiological state in plants by the treatment of natural and synthetic compounds to the seeds before germination. In plant defense, priming is defined as a physiological process by which a plant prepares to respond to imminent abiotic stress more quickly or aggressively. Moreover, plants raised from primed seeds showed sturdy and quick cellular defense response against abiotic stresses. Priming for enhanced resistance to abiotic stress obviously is operating via various pathways involved in different metabolic processes. The seedlings emerging from primed seeds showed early and uniform germination. Moreover, the overall growth of plants is enhanced due to the seed-priming treatments. The main objective of this review is to provide an overview of various crops in which seed priming is practiced and about various seed-priming methods and its effects.     

Priming in plant-pathogen interactions

Plants can acquire enhanced resistance to pathogens after treatment with necrotizing attackers, nonpathogenic root-colonizing pseudomonads, salicylic acid, beta-aminobutyric acid and many other natural or synthetic compounds. The induced resistance is often associated with an enhanced capacity to mobilize infection-induced cellular defence responses - a process called 'priming'. Although the phenomenon has been known for years, most progress in our understanding of priming has been made only recently. These studies show that priming often depends on the induced disease resistance key regulator NPR1 (also known as NIM1 or SAI1) and that priming has a major effect on the regulation of cellular plant defence responses.     

Transgenerational priming of immunity: maternal exposure to a bacterial antigen enhances offspring humoral immunity

Young vertebrates have limited capacity to synthesize antibodies and are dependent on the protection of maternally transmitted antibodies for humoral disease resistance early in life. However, mothers may enhance fitness by priming their offspring's immune systems to elevate disease resistance. Transgenerational induced defences have been documented in plants and invertebrates, but maternal priming of offspring immunity in vertebrates has been essentially neglected. To test the ability of mothers to stimulate the immune systems of offspring, we manipulated maternal and offspring antigen exposure in a wild population of birds, pied flycatchers (Ficedula hypoleuca). We show that immunization of the mother before egg laying apparently stimulates a transgenerational defence against pathogens by elevating endogenous offspring antibody production. If the disease environments encountered by mothers and offspring are similar, this transgenerational immune priming may allow young to better cope with the local pathogen fauna.     

Ethylene as a modulator of disease resistance in plants

The role of ethylene in the hormonal regulation of plant development has been well established. In addition, it has been implicated in biotic stress, both as a virulence factor of fungal and bacterial pathogens and as a signaling compound in disease resistance. This apparent discrepancy has stimulated research on the effects of various types of pathogens on mutant and transgenic plants that are impaired in ethylene production or perception. It has become clear that ethylene differentially affects resistance against pathogens with different lifestyles and plays an important role in mediating different types of induced resistance.     

Induction of resistance against pathogens by β-aminobutyric acid

Among the many types of plant stressors, pathogen attack, mainly fungi and bacteria can cause particularly severe damage both to individual plants and, on a wider scale, to agricultural productivity. The magnitude of these pathogen-induced problems has stimulated rapid progress in green biotechnology research into plant defense mechanisms. Plants can develop local and systemic wide-spectrum resistance induced by their exposure to virulent (systemic acquired resistance—SAR) or non-pathogenic microbes and various chemical elicitors (induced systemic resistance—ISR). β-Aminobutyric acid (BABA), non-protein amino acid, is though to be important component of the signaling pathway regulating ISR response in plants. After treatment with BABA or various chemicals, after infection by a necrotizing pathogen, colonization of the roots by beneficial microbes many plants establish a unique physiological state that is called the “primed” state of the plant. This review will focus on the recent knowledge about the role of BABA in the induction of ISR against pathogens mainly against fungi.     

How do plants defend themselves against pathogens-Biochemical mechanisms and genetic interventions

In agro-ecosystem, plant pathogens hamper food quality, crop yield, and global food security. Manipulation of naturally occurring defense mechanisms in host plants is an effective and sustainable approach for plant disease management. Various natural compounds, ranging from cell wall components to metabolic enzymes have been reported to protect plants from infection by pathogens and hence provide specific resistance to hosts against pathogens, termed as induced resistance. It involves various biochemical components, that play an important role in molecular and cellular signaling events occurring either before (elicitation) or after pathogen infection. The induction of reactive oxygen species, activation of defensive machinery of plants comprising of enzymatic and non-enzymatic antioxidative components, secondary metabolites, pathogenesis-related protein expression (e.g. chitinases and glucanases), phytoalexin production, modification in cell wall composition, melatonin production, carotenoids accumulation, and altered activity of polyamines are major induced changes in host plants during pathogen infection. Hence, the altered concentration of biochemical components in host plants restricts disease development. Such biochemical or metabolic markers can be harnessed for the development of “pathogen-proof” plants. Effective utilization of the key metabolites-based metabolic markers can pave the path for candidate gene identification. This present review discusses the valuable information for understanding the biochemical response mechanism of plants to cope with pathogens and genomics-metabolomics-based sustainable development of pathogen proof cultivars along with knowledge gaps and future perspectives to enhance sustainable agricultural production.     

Natural variation in priming of basal resistance: from evolutionary origin to agricultural exploitation

Biotic stress has a major impact on the process of natural selection in plants. As plants have evolved under variable environmental conditions, they have acquired a diverse spectrum of defensive strategies against pathogens and herbivores. Genetic variation in the expression of plant defence offers valuable insights into the evolution of these strategies. The 'zigzag' model, which describes an ongoing arms race between inducible plant defences and their suppression by pathogens, is now a commonly accepted model of plant defence evolution. This review explores additional strategies by which plants have evolved to cope with biotic stress under different selective circumstances. Apart from interactions with plant-beneficial micro-organisms that can antagonize pathogens directly, plants have the ability to prime their immune system in response to selected environmental signals. This defence priming offers disease protection that is effective against a broad spectrum of virulent pathogens, as long as the augmented defence reaction is expressed before the invading pathogen has the opportunity to suppress host defences. Furthermore, priming has been shown to be a cost-efficient defence strategy under relatively hostile environmental conditions. Accordingly, it is possible that selected plant varieties have evolved a constitutively primed immune system to adapt to levels of disease pressure. Here, we examine this hypothesis further by evaluating the evidence for natural variation in the responsiveness of basal defence mechanisms, and discuss how this genetic variation can be exploited in breeding programmes to provide sustainable crop protection against pests and diseases.     

Preparing plants for improved cold tolerance by priming

Cold is a major stressor, which limits plant growth and development in many parts of the world, especially in the temperate climate zones. A large number of experimental studies has demonstrated that not only acclimation and entrainment but also the experience of single short stress events of various abiotic or biotic kinds (priming stress) can improve the tolerance of plants to chilling temperatures. This process, called priming, depends on a stress “memory”. It does not change cold sensitivity per se but beneficially modifies the response to cold and can last for days, months, or even longer. Elicitor factors and antagonists accumulate due to increased biosynthesis or decreased degradation either during or after the priming stimulus. Comparison of priming studies investigating improved tolerance to chilling temperatures highlighted key regulatory functions of ROS/RNS and antioxidant enzymes, plant hormones, especially jasmonates, salicylates, and abscisic acid, and signalling metabolites, such as β‐ and γ‐aminobutyric acid (BABA and GABA) and melatonin. We conclude that these elicitors and antagonists modify local and systemic cold tolerance by integration into cold‐induced signalling cascades.     

Targeting novel chemical and constitutive primed metabolites against Plectosphaerella cucumerina

Priming is a physiological state for protection of plants against a broad range of pathogens, and is achieved through stimulation of the plant immune system. Various stimuli, such as beneficial microbes and chemical induction, activate defense priming. In the present study, we demonstrate that impairment of the high‐affinity nitrate transporter 2.1 (encoded by NRT2.1) enables Arabidopsis to respond more quickly and strongly to Plectosphaerella cucumerina attack, leading to enhanced resistance. The Arabidopsis thaliana mutant lin1 (affected in NRT2.1) is a priming mutant that displays constitutive resistance to this necrotroph, with no associated developmental or growth costs. Chemically induced priming by β–aminobutyric acid treatment, the constitutive priming mutant ocp3 and the constitutive priming present in the lin1 mutant result in a common metabolic profile within the same plant–pathogen interactions. The defense priming significantly affects sugar metabolism, cell‐wall remodeling and shikimic acid derivatives levels, and results in specific changes in the amino acid profile and three specific branches of Trp metabolism, particularly accumulation of indole acetic acid, indole‐3–carboxaldehyde and camalexin, but not the indolic glucosinolates. Metabolomic analysis facilitated identification of three metabolites in the priming fingerprint: galacturonic acid, indole‐3–carboxylic acid and hypoxanthine. Treatment of plants with the latter two metabolites by soil drenching induced resistance against P. cucumerina, demonstrating that these compounds are key components of defense priming against this necrotrophic fungus. Here we demonstrate that indole‐3–carboxylic acid induces resistance by promoting papillae deposition and H₂O₂ production, and that this is independent of PR1, VSP2 and PDF1.2 priming.     

Mitogen-Activated Protein Kinases 3 and 6 Are Required for Full Priming of Stress Responses in Arabidopsis thaliana

In plants and animals, induced resistance (IR) to biotic and abiotic stress is associated with priming of cells for faster and stronger activation of defense responses. It has been hypothesized that cell priming involves accumulation of latent signaling components that are not used until challenge exposure to stress. However, the identity of such signaling components has remained elusive. Here, we show that during development of chemically induced resistance in Arabidopsis thaliana, priming is associated with accumulation of mRNA and inactive proteins of mitogen-activated protein kinases (MPKs), MPK3 and MPK6. Upon challenge exposure to biotic or abiotic stress, these two enzymes were more strongly activated in primed plants than in nonprimed plants. This elevated activation was linked to enhanced defense gene expression and development of IR. Strong elicitation of stress-induced MPK3 and MPK6 activity is also seen in the constitutive priming mutant edr1, while activity was attenuated in the priming-deficient npr1 mutant. Moreover, priming of defense gene expression and IR were lost or reduced in mpk3 or mpk6 mutants. Our findings argue that prestress deposition of the signaling components MPK3 and MPK6 is a critical step in priming plants for full induction of defense responses during IR.     

Molecular aspects of defence priming

Plants can be primed for more rapid and robust activation of defence to biotic or abiotic stress. Priming follows perception of molecular patterns of microbes or plants, recognition of pathogen-derived effectors or colonisation by beneficial microbes. However the process can also be induced by treatment with some natural or synthetic compounds and wounding. The primed mobilization of defence is often associated with development of immunity and stress tolerance. Although the phenomenon has been known for decades, the molecular basis of priming is poorly understood. Here, I summarize recent progress made in unravelling molecular aspects of defence priming that is the accumulation of dormant mitogen-activated protein kinases, chromatin modifications and alterations of primary metabolism.     

Benzothiadiazole-induced priming for potentiated responses to pathogen infection, wounding, and infiltration of water into leaves requires the NPR1/NIM1 gene in Arabidopsis

Systemic acquired resistance (SAR) is a plant defense state that is induced, for example, after previous pathogen infection or by chemicals that mimic natural signaling compounds. SAR is associated with the ability to induce cellular defense responses more rapidly and to a greater degree than in noninduced plants, a process called "priming." Arabidopsis plants were treated with the synthetic SAR inducer benzothiadiazole (BTH) before stimulating two prominent cellular defense responses, namely Phe AMMONIA-LYASE (PAL) gene activation and callose deposition. Although BTH itself was essentially inactive at the immediate induction of these two responses, the pretreatment with BTH greatly augmented the subsequent PAL gene expression induced by Pseudomonas syringae pv. tomato infection, wounding, or infiltrating the leaves with water. The BTH pretreatment also enhanced the production of callose, which was induced by wounding or infiltrating the leaves with water. It is interesting that the potentiation by BTH pretreatment of PAL gene activation and callose deposition was not seen in the Arabidopsis nonexpresser of PR genes 1/noninducible immunity 1 mutant, which is compromised in SAR. In a converse manner, augmented PAL gene activation and enhanced callose biosynthesis were found, without BTH pretreatment, in the Arabidopsis constitutive expresser of pathogenesis-related genes (cpr)1 and constitutive expresser of pathogenesis-related genes 5 mutants, in which SAR is constitutive. Moreover, priming for potentiated defense gene activation was also found in pathogen-induced SAR. In sum, the results suggest that priming is an important cellular mechanism in acquired disease resistance of plants that requires the nonexpresser of PR genes 1/noninducible immunity 1 gene.