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.     

The pepper extracellular peroxidase CaPO2 is required for salt, drought and oxidative stress tolerance as well as resistance to fungal pathogens

In plants, biotic and abiotic stresses regulate the expression and activity of various peroxidase isoforms. Capsicum annuum EXTRACELLULAR PEROXIDASE 2 (CaPO2) was previously shown to play a role in local and systemic reactive oxygen species bursts and disease resistance during bacterial pathogen infection. Here, we report CaPO2 expression patterns and functions during conditions of biotic and abiotic stress. In pepper plants, CaPO2 expression was strongly induced by abscisic acid, but not by defense-related plant hormones such as salicylic acid, ethylene and jasmonic acid. CaPO2 was also strongly induced by abiotic and biotic stress treatments, including drought, cold, high salinity and infection by the hemibiotrophic fungal pathogen Colletotrichum coccodes. Loss-of-function of CaPO2 in virus-induced gene silenced pepper plants led to increased susceptibility to salt- and osmotic-induced stress. In contrast, CaPO2 overexpression in transgenic Arabidopsis thaliana plants conferred enhanced tolerance to high salt, drought, and oxidative stress, while also enhancing resistance to infection by the necrotrophic fungal pathogen Alternaria brassicicola. Taken together, these results provide evidence for the involvement of pepper extracellular peroxidase CaPO2 in plant defense responses to various abiotic stresses and plant fungal pathogens.     

植物中逆境反应相关的WRKY转录因子研究进展

WRKY转录因子是植物体内一类比较大的转录因子家族,它在植物的生长发育以及抗逆境反应中起着非常重要的作用。本文综述了WRKY转录因子在植物应对冻害、干旱、盐害等非生物胁迫与病原菌、虫害等生物胁迫反应中的重要调控功能,并概括了WRKY转录因子在调控这些逆境反应中的机制。     

Effects of jasmonic acid, ethylene, and salicylic acid signaling on the rhizosphere bacterial community of Arabidopsis thaliana

Systemically induced resistance is a promising strategy to control plant diseases, as it affects numerous pathogens. However, since induced resistance reduces one or both growth and activity of plant pathogens, the indigenous microflora may also be affected by an enhanced defensive state of the plant. The aim of this study was to elucidate how much the bacterial rhizosphere microflora of Arabidopsis is affected by induced systemic resistance (ISR) or systemic acquired resistance (SAR). Therefore, the bacterial microflora of wild-type plants and plants affected in their defense signaling was compared. Additionally, ISR was induced by application of methyl jasmonate and SAR by treatment with salicylic acid or benzothiadiazole. As a comparative model, we also used wild type and ethylene-insensitive tobacco. Some of the Arabidopsis genotypes affected in defense signaling showed altered numbers of culturable bacteria in their rhizospheres; however, effects were dependent on soil type. Effects of plant genotype on rhizosphere bacterial community structure could not be related to plant defense because chemical activation of ISR or SAR had no significant effects on density and structure of the rhizosphere bacterial community. These findings support the notion that control of plant diseases by elicitation of systemic resistance will not significantly affect the resident soil bacterial microflora.     

Induced systemic resistance (ISR) in plants: mechanism of action

Plants possess a range of active defense apparatuses that can be actively expressed in response to biotic stresses (pathogens and parasites) of various scales (ranging from microscopic viruses to phytophagous insect). The timing of this defense response is critical and reflects on the difference between coping and succumbing to such biotic challenge of necrotizing pathogens/parasites. If defense mechanisms are triggered by a stimulus prior to infection by a plant pathogen, disease can be reduced. Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are two forms of induced resistance wherein plant defenses are preconditioned by prior infection or treatment that results in resistance against subsequent challenge by a pathogen or parasite. Selected strains of plant growth-promoting rhizobacteria (PGPR) suppress diseases by antagonism between the bacteria and soil-borne pathogens as well as by inducing a systemic resistance in plant against both root and foliar pathogens. Rhizobacteria mediated ISR resembles that of pathogen induced SAR in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Several rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface whereas other rhizobacteria trigger different signaling pathway independent of SA. The existence of SA-independent ISR pathway has been studied in Arabidopsis thaliana, which is dependent on jasmonic acid (JA) and ethylene signaling. Specific Pseudomonas strains induce systemic resistance in viz., carnation, cucumber, radish, tobacco, and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways besides extended protection to a broader spectrum of pathogens than ISR/SAR alone. Beside Pseudomonas strains, ISR is conducted by Bacillus spp. wherein published results show that several specific strains of species B. amyloliquifaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B.sphaericus elicit significant reduction in the incidence or severity of various diseases on a diversity of hosts.     

硅对植物抗虫性的影响及其机制

硅不是植物必需营养元素,但硅在提高植物对一系列非生物和生物胁迫的抗性方面都具有重要作用。综述了硅对植物抗虫性的影响及其机制。在多数植物中,增施硅肥可增强其抗虫性;所增强的抗性与硅肥种类和施用方式之间存在关系。植物组织中沉积的硅可增加其硬度和耐磨度,降低植物可消化性,从而增强植物组成性防御,包括延缓昆虫生长发育、降低繁殖力、减轻植物受害程度;植物体内的硅含量以及硅沉积的位点和排列方式影响组成性防御作用的强度。此外,硅可以调节植物诱导性防御,包括直接防御和间接防御,直接防御涉及增加有毒物质含量、产生局部过敏反应或系统获得抗性、产生有毒化合物和防御蛋白,从而延缓昆虫发育;间接防御主要通过释放挥发性化合物吸引植食性昆虫的捕食性和寄生性天敌而导致植食性昆虫种群下降。     

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.     

Enhanced Botrytis cinerea Resistance of Arabidopsis Plants Grown in Compost May Be Explained by Increased Expression of Defense-Related Genes,as Revealed by Microarray Analysis

Composts are the products obtained after the aerobic degradation of different types of organic matter waste and can be used as substrates or substrate/soil amendments for plant cultivation. There is a small but increasing number of reports that suggest that foliar diseases may be reduced when using compost, rather than standard substrates, as growing medium. The purpose of this study was to examine the gene expression alteration produced by the compost to gain knowledge of the mechanisms involved in compost-induced systemic resistance. A compost from olive marc and olive tree leaves was able to induce resistance against Botrytis cinerea in Arabidopsis, unlike the standard substrate, perlite. Microarray analyses revealed that 178 genes were differently expressed, with a fold change cut-off of 1, of which 155 were up-regulated and 23 were down-regulated in compost-grown, as against perlite-grown plants. A functional enrichment study of up-regulated genes revealed that 38 Gene Ontology terms were significantly enriched. Response to stress, biotic stimulus, other organism, bacterium, fungus, chemical and abiotic stimulus, SA and ABA stimulus, oxidative stress, water, temperature and cold were significantly enriched, as were immune and defense responses, systemic acquired resistance, secondary metabolic process and oxireductase activity. Interestingly, PR1 expression, which was equally enhanced by growing the plants in compost and by B. cinerea inoculation, was further boosted in compost-grown pathogen-inoculated plants. Compost triggered a plant response that shares similarities with both systemic acquired resistance and ABA-dependent/independent abiotic stress responses.     

Regulation of Salicylic Acid and N-Hydroxy-Pipecolic Acid in Systemic Acquired Resistance

In plants, salicylic acid (SA) is a central immune signal that is involved in both local and systemic acquired resistance (SAR). In addition to SA, several other chemical signals are also involved in SAR and these include N-hydroxy-pipecolic acid (NHP), a newly discovered plant metabolite that plays a crucial role in SAR. Recent discoveries have led to a better understanding of the biosynthesis of SA and NHP and their signaling during plant defense responses. Here, I review the recent progress in role of SA and NHP in SAR. In addition, I discuss how these signals cooperate with other SAR-inducing chemicals to regulate SAR.     

SAR increases fitness of Arabidopsis thaliana in the presence of natural bacterial pathogens

Given the substantial costs of plant defenses against pathogens, there should be corresponding benefits that prevent resistance from being lost in natural plant populations. Here, we present evidence that systemic acquired resistance (SAR) benefits plants attacked by pathogenic bacteria in nature. In a large field experiment, we found that Arabidopsis thaliana treated with salicylic acid exhibited reduced titers of bacteria in their leaves and elevated fitness relative to controls. Most common members of the culturable bacterial community suffered this decrease, consistent with the role of SAR as a broad spectrum defense. We found no evidence of negative interactions between SAR and jasmonate-dependent resistance. Plants treated with jasmonic acid received significantly lower insect damage to their siliques, but exhibited no differences in bacterial growth or fitness relative to controls. Collectively, these data suggest a likely role of pathogenic bacteria in the maintenance of SAR, but not jasmonate-dependent resistance, in nature.     

Light regulation and daytime dependency of inducible plant defenses in Arabidopsis: phytochrome signaling controls systemic acquired resistance rather than local defense

We have examined molecular and physiological principles underlying the light dependency of defense activation in Arabidopsis (Arabidopsis thaliana) plants challenged with the bacterial pathogen Pseudomonas syringae. Within a fixed light/dark cycle, plant defense responses and disease resistance significantly depend on the time of day when pathogen contact takes place. Morning and midday inoculations result in higher salicylic acid accumulation, faster expression of pathogenesis-related genes, and a more pronounced hypersensitive response than inoculations in the evening or at night. Rather than to the plants' circadian rhythm, this increased plant defense capability upon day inoculations is attributable to the availability of a prolonged light period during the early plant-pathogen interaction. Moreover, pathogen responses of Arabidopsis double mutants affected in light perception, i.e. cryptochrome1cryptochrome2 (cry1cry2), phototropin1phototropin2 (phot1phot2), and phytochromeAphytochromeB (phyAphyB) were assessed. Induction of defense responses by either avirulent or virulent P. syringae at inoculation sites is relatively robust in leaves of photoreceptor mutants, indicating little cross talk between local defense and light signaling. In addition, the blue-light receptor mutants cry1cry2 and phot1phot2 are both capable of establishing a full systemic acquired resistance (SAR) response. Induction of SAR and salicylic-acid-dependent systemic defense reactions, however, are compromised in phyAphyB mutants. Phytochrome regulation of SAR involves the essential SAR component FLAVIN-DEPENDENT MONOOXYGENASE1. Our findings highlight the importance of phytochrome photoperception during systemic rather than local resistance induction. The phytochrome system seems to accommodate the supply of light energy to the energetically costly increase in whole plant resistance.     

微生物诱导的植物系统抗性

综述了由植物病原菌和非病原性的根际促生菌诱导产生的两种植物系统抗性:系统获得性抗性(SAR)和系统诱导抗性(ISR),比较了两类系统抗性的诱导、信号分子和机理的异同点,阐述了信号分子水杨酸在系统获得性抗性诱导过程中的作用及茉莉酸和乙烯在系统诱导抗性产生过程中的作用。     

Systemic Acquired Resistance

Upon infection with necrotizing pathogens many plants develop an enhanced resistance to further pathogen attack also in the uninoculated organs. This type of enhanced resistance is referred to as systemic acquired resistance (SAR). In the SAR state, plants are primed (sensitized) to more quickly and more effectively activate defense responses the second time they encounter pathogen attack. Since SAR depends on the ability to access past experience, acquired disease resistance is a paradigm for the existence of a form of “plant memory”. Although the phenomenon has been known since the beginning of the 20th century, major progress in the understanding of SAR was made over the past sixteen years. This review covers the current knowledge of molecular, biochemical and physiological mechanisms that are associated with SAR.Key Words: Arabidopsis, benzothiadiazole, defense response potentiation, 2,6-dichloroisonicotinic acid, elicitor, MAP kinase, parsley cell culture, priming, salicylic acid, sensitization     

Plant innate immunity: An updated insight into defense mechanism

Plants are invaded by an array of pathogens of which only a few succeed in causing disease. The attack by others is countered by a sophisticated immune system possessed by the plants. The plant immune system is broadly divided into two, viz. microbial-associated molecular-patterns-triggered immunity (MTI) and effector-triggered immunity (ETI). MTI confers basal resistance, while ETI confers durable resistance, often resulting in hypersensitive response. Plants also possess systemic acquired resistance (SAR), which provides long-term defense against a broad-spectrum of pathogens. Salicylic-acid-mediated systemic acquired immunity provokes the defense response throughout the plant system during pathogen infection at a particular site. Trans-generational immune priming allows the plant to heritably shield their progeny towards pathogens previously encountered. Plants circumvent the viral infection through RNA interference phenomena by utilizing small RNAs. This review summarizes the molecular mechanisms of plant immune system, and the latest breakthroughs reported in plant defense. We discuss the plant–pathogen interactions and integrated defense responses in the context of presenting an integral understanding in plant molecular immunity.     

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.     

Differential effectiveness of salicylate-dependent and jasmonate/ethylene-dependent induced resistance in Arabidopsis

Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are each involved in the regulation of basal resistance against different pathogens. These three signals play important roles in induced resistance as well. SA is a key regulator of pathogen-induced systemic acquired resistance (SAR), whereas JA and ET are required for rhizobacteria-mediated induced systemic resistance (ISR). Both types of induced resistance are effective against a broad spectrum of pathogens. In this study, we compared the spectrum of effectiveness of SAR and ISR using an oomycete, a fungal, a bacterial, and a viral pathogen. In noninduced Arabidopsis plants, these pathogens are primarily resisted through either SA-dependent basal resistance (Peronospora parasitica and Turnip crinkle virus [TCV]), JA/ET-dependent basal resistance responses (Alternaria brassicicola), or a combination of SA-, JA-, and ET-dependent defenses (Xanthomonas campestris pv. armoraciae). Activation of ISR resulted in a significant level of protection against A. brassicicola, whereas SAR was ineffective against this pathogen. Conversely, activation of SAR resulted in a high level of protection against P. parasitica and TCV, whereas ISR conferred only weak and no protection against P. parasitica and TCV, respectively. Induction of SAR and ISR was equally effective against X. campestris pv. armoraciae. These results indicate that SAR is effective against pathogens that in noninduced plants are resisted through SA-dependent defenses, whereas ISR is effective against pathogens that in noninduced plants are resisted through JA/ET-dependent defenses. This suggests that SAR and ISR constitute a reinforcement of extant SA- or JA/ET-dependent basal defense responses, respectively.     

生防菌诱导植物系统抗性及其生化和细胞学机制

生防菌通常可利用竞争、抗生、寄生和交叉保护等直接的拮抗机制抑制植物病害;同时某些生防菌还能促进植物生长,诱导植物对真菌、细菌和病毒引起的病害乃至对线虫和昆虫为害的抗性,称为诱导系统抗性(ISR).ISR具有非特异性、广谱性和系统性,其在表型上与病原菌侵染激发的系统获得抗性(SAR)相似,具有同样的效率;但在寄主植物上不发生过敏性坏死反应(HR),无可见症状,为发展和改善更加安全而环境友好的植物保护策略开辟了新的思路.本文总结了生防真菌和细菌诱导系统抗性及其激发子和信号转导途径等方面的研究进展,重点阐述了寄主防御反应的生化和细胞学机制,并对ISR在植物病害生物防治中的应用前景进行了展望.