Two-component elements mediate interactions between cytokinin and salicylic acid in plant immunity

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant-pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant-pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA-dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA-dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses against pathogens.     

The impact of cytokinin on jasmonate-salicylate antagonism in Arabidopsis immunity against infection with Pst DC3000

Cytokinin has long been shown to be an essential modulator of growth and development in plants. However, its implications in plant immunity have only recently been realized. The interaction between jasmonate and salicylate pathways is regarded as a central backbone of plant immune defense. However, the effect of cytokinin on the jasmonate and salicylate mediated balance in plant immunity is still not known. Here, we analyze the impact of cytokinin on the jasmonate-salicylate antagonism in Arabidopsis immunity regarding infection with hemibiotrophic pathogen Pseudomonas syringae pv tomato DC3000 (Pst DC3000). Systems biology analysis of a refined hormone immune pathway model provides insights into the impact of cytokinin on the balance between jasmonate and salicylic acid pathways in Arabidopsis. Targeted experiments validate model simulations monitoring bacterial growth in wild type plants as well as in jasmonate pathway mutants. An integrated analysis shows that CK promotes the SA pathway of plant immunity and does not promote JA-mediated Arabidopsis susceptibility against infection with Pst DC3000. Finally, we discuss these results in the context of an emerging model of auxin-cytokinin antagonism in plant immunity.     

植物激素信号之间的相互作用

植物激素问的相互作用对植物的正常发育来说非常重要。不同植物激素之间存在相互协同、对抗和因果等关系,以精细调控植物的发育和对环境的反应等,植物激素信号之间的相互作用已成为植物细胞中不同信号间相互作用机制研究的模式系统。现对不同植物激素在生物合成、代谢、运输和信号转导途径等层次上的相互作用进行综述,并对这一领域的研究进行了总结和展望。     

Thirty years of resistance: Zig-zag through the plant immune system

Understanding the plant immune system is crucial for using genetics to protect crops from diseases. Plants resist pathogens via a two-tiered innate immune detection-and-response system. The first plant Resistance (R) gene was cloned in 1992 . Since then, many cell-surface pattern recognition receptors (PRRs) have been identified, and R genes that encode intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) have been cloned. Here, we provide a list of characterized PRRs and NLRs. In addition to immune receptors, many components of immune signaling networks were discovered over the last 30 years. We review the signaling pathways, physiological responses, and molecular regulation of both PRR- and NLR-mediated immunity. Recent studies have reinforced the importance of interactions between the two immune systems. We provide an overview of interactions between PRR- and NLR-mediated immunity, highlighting challenges and perspectives for future research.

A review of major research advances in plant immunity during the last three decades and individual characterized immune receptors, their immune signaling pathways, and interactions between immune systems     

Making sense of hormone crosstalk during plant immune responses

In response to biotic stress, crosstalk between plant hormonal signaling pathways prioritizes defense over other cellular functions. Some plant pathogens take advantage of this regulatory system by mimicking hormones that interfere with host immune responses to promote virulence. Here we discuss the various roles that crosstalk may play in response to pathogens with different infection strategies.     

韧皮部取食昆虫诱导的植物防御反应

刺吸式昆虫与寄主植物之间具有特殊的生物互作关系。本文对刺吸式昆虫取食韧皮部诱导的植物防御反应类型、 防御物质变化、 信号途径以及植物反应转录组学研究等方面进行综述。韧皮部取食昆虫取食诱导的植物防御反应机制主要包括： (1)改变自身的营养状况; (2)产生有毒的次生化合物; (3)产生防御蛋白。防御反应与植物水杨酸、 茉莉酸、 乙烯等信号分子密切相关。研究表明, 刺吸式昆虫取食诱导的植物防御反应主要引发以水杨酸为主的信号途径, 但相关分子互作机制还有待明确。日益丰富的基因组资源和不断发展的分子生物学技术为揭示植物防御反应中信号分子的作用机制、 找出植物内生抗性的特异因子以及阐明诱导防御机制奠定了基础。了解刺吸式昆虫取食诱导的植物防御反应, 为深入理解植物-昆虫间协同进化关系提供了依据, 为害虫治理和抗虫植物的培育提供了新的思路。     

Sending mixed messages: auxin-cytokinin crosstalk in roots

Despite their relatively simple appearance, roots are incredibly complex organs that are highly adapted to differing environments. Many aspects of root development are co-ordinated by subtle spatial differences in the concentrations of the phytohormones auxin and cytokinin. Events from the formation of a root during embryogenesis to the determination of the network of lateral roots are controlled by interactions between these hormones. Recently, interactions have been defined where auxin signaling promotes the expression of cytokinin signaling inhibitors, cytokinin signaling promotes the expression of auxin signaling inhibitors and finally where cytokinin signaling regulates the complex network of auxin transport proteins to position zones of high auxin signaling. We are witnessing a period of discovery in which we are beginning to understand how these hormonal pathways communicate to regulate root formation.     

Identification and function of MicroRNAs encoded by herpesviruses

MicroRNAs (miRNAs) play important roles in eukaryotes,plants and some viruses.It is increasingly clear that miRNAs-encoded by viruses can affect the viral life cycle and host physiology.Viral miRNAs could repress the innate and adaptive host immunity,modulate cellular signaling pathways,and regulate the expression of cellular and viral genes.These functions facilitate viral acute and persistent infections,and have profound effects on the host cell survival and disease progression.Here,we discuss the miRNAs encoded by herpesviruses,and their regulatory roles involved in virus-host interactions.     

Temperature-regulation of plant architecture

As sessile organisms, plants have evolved great plasticity to adapt to their surrounding environment. Temperature signals regulate the timing of multiple developmental processes and have dramatic effects on plant architecture and biomass. The modulation of plant architecture by temperature is of increasing relevance with regard to crop productivity and global climate change. Unlike many other organisms, the mechanisms through which plants sense changes in ambient temperature remain elusive. Multiple studies have identified crosstalk between ambient temperature sensing, light signaling, cold acclimation and pathogen response pathways. The regulation of plant architecture by temperature appears to involve the complex integration of multiple hormone signaling networks. Gibberellin (GA), Salicylic Acid (SA) and cytokinin have been implicated in the regulation of plant growth during chilling, whilst a predominant role for auxin is observed at high temperatures. This mini-review summarizes current knowledge of plant growth regulation by temperature and crosstalk with other abiotic and biotic stress signaling pathways.Key words: temperature, architecture, elongation, growth, hormone, auxin, gibberellin, salicylic acid, biomass     

Plant-bacterial pathogen interactions mediated by type III effectors

Effectors secreted by the bacterial type III system play a central role in the interaction between Gram-negative bacterial pathogens and their host plants. Recent advances in the effector studies have helped cementing several key concepts concerning bacterial pathogenesis, plant immunity, and plant-pathogen co-evolution. Type III effectors use a variety of biochemical mechanisms to target specific host proteins or DNA for pathogenesis. The identifications of their host targets led to the identification of novel components of plant innate immune system. Key modules of plant immune signaling pathways such as immune receptor complexes and MAPK cascades have emerged as a major battle ground for host-pathogen adaptation. These modules are attacked by multiple type III effectors, and some components of these modules have evolved to actively sense the effectors and trigger immunity.     

Salicylic acid and its function in plant immunity

The small phenolic compound salicylic acid (SA) plays an important regulatory role in multiple physiological processes including plant immune response. Significant progress has been made during the past two decades in understanding the SA-mediated defense signaling network. Characterization of a number of genes functioning in SA biosynthesis, conjugation, accumulation, signaling, and crosstalk with other hormones such as jasmonic acid, ethylene, abscisic acid, auxin, gibberellic acid, cytokinin, brassinosteroid, and peptide hormones has sketched the finely tuned immune response network. Full understanding of the mechanism of plant immunity will need to take advantage of fast developing genomics tools and bioinformatics techniques. However, elucidating genetic components involved in these pathways by conventional genetics, biochemistry, and molecular biology approaches will continue to be a major task of the community. High-throughput method for SA quantification holds the potential for isolating additional mutants related to SA-mediated defense signaling.     

Cytokinin affects circadian-clock oscillation in a phytochrome B- and Arabidopsis response regulator 4-dependent manner

In higher plants, many developmental processes, such as photomorphogenesis and flowering, are coregulated by light and the phytohormone cytokinin. Interactions between light and cytokinin pathways are presumably mediated by common signaling intermediates. However, the molecular mechanism of these interactions remains unclear. Here, we report that cytokinin specifically induces the expression of the Arabidopsis circadian oscillator genes LATE ELONGATED HYPOCOTYL ( LHY ) and CIRCADIAN CLOCK-ASSOCIATED 1 ( CCA1 ) but represses the expression of TIMING OF CAB EXPRESSION 1 in a light-dependent manner. Consistent with these observations, cytokinin causes a shifted phase of the circadian clock. Mutant studies showed that the altered clock oscillation modulated by cytokinin is dependent on phytochrome B ( PHYB ) and Arabidopsis RESPONSE REGULATOR 4 ( ARR4 ). Whereas overexpression of LHY or CCA1 renders plants slightly more sensitive to cytokinin, phyB and a lhy/cca1 double mutant are less sensitive to the hormone. These results suggest that cytokinin affects the circadian clock oscillation in a PHYB - and ARR4 -dependent manner and that cytokinin signaling is also regulated by light-signaling components, including PHYB , LHY and CCA1 . Therefore, phyB, ARR4 and the circadian oscillator may function as signaling intermediates to integrate light and cytokinin pathways.     

Modulation of signaling pathways by RNA virus capsid proteins

Capsid proteins are structural components of virus particles. They are nucleic acid-binding proteins whose main recognized function is to package viral genomes into protective structures called nucleocapsids. Research over the last 10 years indicates that in addition to their role as genome guardians, viral capsid proteins modulate host cell signaling networks. Disruption or alteration of intracellular signaling pathways by viral capsids may benefit replication of the virus by affecting innate immunity and in some cases, may underlie disease progression. In this review, we describe how the capsid proteins from medically relevant RNA viruses interact with host cell signaling pathways.     

蛋白磷酸化修饰在植物-病原微生物互作中的作用研究进展

蛋白磷酸化修饰是植物细胞信号调控的普遍机制。植物-病原微生物互作过程中, 关键调控蛋白的磷酸化状态影响免疫信号的激活。多种病原微生物通过干扰宿主蛋白的磷酸化状态攻击免疫系统, 以提高致病性。该文对植物免疫调控过程中关键元件的磷酸化修饰及其在免疫信号中的调控作用进行了综述。研究植物-病原菌互作过程中关键蛋白的磷酸化修饰, 有助于深入探讨植物-病原微生物互作的分子机理。该文将为寻找广谱抗病的新途径提供理论依据。