Signal Transduction Pathways in Mycorrhizal Associations: Comparisons with the Rhizobium-Legume Symbiosis

A number of genera of soil fungi interact with plant roots to establish symbiotic associations whereby phosphate acquired by the fungus is exchanged for fixed carbon from the plant. Recent progress in investigating these associations, designated as mycorrhizae (sing., mycorrhiza), has led to the identification of specific steps in the establishment of the symbiosis in which the fungus and the plant interact in response to various molecular signals. Some of these signals are conserved with those of the Rhizobium-legume nitrogen-fixing symbiosis, suggesting that the two plant-microbe interactions share a common signal transduction pathway. Nevertheless, only legume hosts nodulate in response to Rhizobium, whereas the vast majority of flowering plants establish mycorrhizal associations. The key questions for the future are: what are the signal molecules produced by mycorrhizal fungi and how are they perceived by the plant? Copyright 1998 Academic Press.     

Detection of and response to signals involved in host-microbe interactions by plant-associated bacteria.

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.     

植物的环境信号分子茉莉酸及其生物学功能

茉莉酸信号分子参与植物生长发育众多生理过程的调控,尤其是作为环境信号分子能有效地介导植物对生物及非生物胁迫的防御反应。迄今已知具有信号分子生理功能的至少包括茉莉酸(jasmonic acid,JA)以及茉莉酸甲酯(methyl jasmonate,Me JA)和茉莉酸-异亮氨酸复合物(jasmonoyl-isoleucine,JA-Ile)等茉莉酸衍生物,统称为茉莉酸类化合物(jasmonates,JAs)。从环境信号分子角度介绍了茉莉酸信号的启动(环境信号感知与转导、茉莉酸类化合物合成)、传递(局部传递、维管束传输、空气传播)和生物学功能(茉莉酸信号受体、调控的转录因子、参与的生物学过程)。     

Molecular mechanisms involved in bacterial speck disease resistance of tomato

An important recent advance in the field of plant-microbe interactions has been the cloning of genes that confer resistance to specific viruses, bacteria, fungi or nematodes. Disease resistance (R) genes encode proteins with predicted structural motifs consistent with them having roles in signal recognition and transduction. The future challenge is to understand how R gene products specifically perceive defence-eliciting signals from the pathogen and transduce those signals to pathways that lead to the activation of plant defence responses. In tomatoes, the Pto kinase (product of the Pto R gene) confers resistance to strains of the bacterial speck pathogen, Pseudomonas syringae pv. tomato, that carry the corresponding avirulence gene avrPto. Resistance to bacterial speck disease is initiated by a mechanism involving the physical interaction of the Pto kinase and the AvrPto protein. This recognition event initiates signalling events that lead to defence responses including an oxidative burst, the hypersensitive response and expression of pathogenesis-related genes. Pto-interacting (Pti) proteins have been identified that appear to act downstream of the Pto kinase and our current studies are directed at elucidating the roles of these components.     

Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.     

Unravelling rhizosphere-microbial interactions: opportunities and limitations

The rhizosphere is a biologically active zone of the soil around plant roots that contains soil-borne microbes including bacteria and fungi. Plant-microbe interactions in the rhizosphere can be beneficial to the plant, the microbes or to neither of them. One of the major difficulties that plant biologists and microbiologists face when studying these interactions is that many groups of microbes that inhabit this zone are not cultivable in the laboratory. Recent developments in molecular biology methods are shedding some light on rhizospheric microbial diversity. This review discusses recent findings and future challenges in the study of plant-microbe interactions in the rhizosphere.     

Signaling complexes: junctions on the intracellular information super highway.

An enigmatic yet fundamental principle of signal transduction is that parallel signaling pathways assembled from a common repertoire of enzymes are able to propagate diverse physiological responses. A key feature of such a mechanism is that separate signaling pathways are organized into localized transduction units, each tailored to respond optimally to a particular signal. Protein-protein interactions maintained by anchoring, adapter and scaffolding proteins provide the molecular glue that holds these signal transduction units together. A major objective of the signaling community is to ascertain how signals flow through compartmentalized transduction units that contain transmembrane receptors, protein kinases, phosphatases and their substrates.     

Regulation of the type III secretion system in phytopathogenic bacteria

The type III secretion system (TTSS) is a specialized protein secretion machinery used by numerous gram-negative bacterial pathogens of animals and plants to deliver effector proteins directly into the host cells. In plant-pathogenic bacteria, genes encoding the TTSS were discovered as hypersensitive response and pathogenicity (hrp) genes, because mutation of these genes typically disrupts the bacterial ability to cause diseases on host plants and to elicit hypersensitive response on nonhost plants. The hrp genes and the type III effector genes (collectively called TTSS genes hereafter) are repressed in nutrient-rich media but induced when bacteria are infiltrated into plants or incubated in nutrient-deficient inducing media. Multiple regulatory components have been identified in the plant-pathogenic bacteria regulating TTSS genes under various conditions. In Ralstonia solanacearum, several signal transduction components essential for the induction of TTSS genes in plants are dispensable for the induction in inducing medium. In addition to the inducing signals, recent studies indicated the presence of negative signals in the plant regulating the Pseudomonas syringae TTSS genes. Thus, the levels of TTSS gene expression in plants likely are determined by the interactions of multiple signal transduction pathways. Studies of the hrp regulons indicated that TTSS genes are coordinately regulated with a number of non-TTSS genes.     

抗病原菌植物基因工程进展

植物病原菌给农林生产带来巨大的损失,植物基因工程在培育抗病原菌植物方面是传统育种技术的补充和发展,短短几年,在抗细菌和抗真菌植物基因工程方面出现了一些全新的成功策略,这些范例都是针对病原菌的生理结构、致病机理及与植物的相互关系。本文概括论述了这些策略的基本思路并对其局限性加以探讨。随着植物病理学、植物分子生物学和病原菌分子生物学的研究进展,新的抗性策略将会出现。     

Interactions and intersections of plant signaling pathways.

Plant signal transduction is a rapidly expanding field of research, and during the last decade a wealth of insight into how plants perceive and transmit signals as part of normal development and in response to environmental cues has been and is continuing to be unraveled. Although ?signaling cascades are often viewed as linear chains of events it is now becoming increasingly apparent, through the use of cell biological, molecular and genetic approaches, that plant signal transduction involves extensive cross-talk between different pathways. The numerous interactions and intersections which take place are potentially important to modulate and balance the various inputs from different signaling cascades so that plants can integrate all this information to execute the proper developmental responses.     

植物抗寒及其基因表达研究进展

植物经过逐渐降低的温度从而提高抗寒能力 ,这个过程被人们称为低温驯化。植物低温驯化过程是一个复杂的生理、生化和能量代谢变化过程 ,这些变化主要包括膜系统的稳定性、可溶性蛋白的积累和小分子渗透物质 ,比如脯氨酸、糖等 ,这些变化中的一些是植物抗寒必需的 ,而另外一些变化不是必需的。主要对冷害和低温生理生化变化、低温诱导表达基因的功能和作用、低温驯化的调节机制及其信号转导方面进行了综述。通过差别筛选 c DNA文库的方法已经鉴定了许多低温诱导表达、进而提高植物抗寒能力的基因 ,其中有脱水素、COR基因和 CBF1转录因子等。低温信号的感受、转导和调节表达是低温驯化的关键环节 ,低温信号的转导过程与干旱胁迫之间具有一定的交叉 ,这为利用 ABA等来提高植物抗寒能力成为可能 ,相信不久的将来人们可以通过提高植物抗寒能力从而增加经济产量成为现实。     

Cooperation of plants and microorganisms: getting closer to the genetic construction of sustainable agro-systems

The molecular research into two types of beneficial plant-microbe symbioses is reviewed: nutritional (with N(2)-fixing bacteria or mycorrhizal fungi) and defensive (with endo- and epiphytic microbes suppressing pathogens and phytophagans). These symbioses are based on the signaling interactions that result in the development of novel tissue/cellular structures and of extended metabolic capacities in the partners, which greatly improve the adaptive potential of plants due to a decrease in their sensitivity to biotic and abiotic stresses. The molecular, genetic and ecological knowledge on plant-microbe interactions provides a strategy for the organization of sustainable crop production based on substituting the agrochemicals (mineral fertilizers, pesticides) by microbial inoculants. An improvement of plant-microbe symbioses should involve the coordinated modifications in the partners' genotypes resulting in highly complementary combinations. These modifications should be based on the broad utilization of genetic resources from natural symbiotic systems aimed at: (i) increased competitiveness of the introduced (effective) with respect to local (ineffective) microbial strains, and (ii) overcoming the limiting steps in the metabolic machineries of the symbiotic systems.     

Plant sphingolipids: structural diversity,biosynthesis, first genes and functions

In mammals and Saccharomyces cerevisiae, sphingolipids have been a subject of intensive research triggered by the interest in their structural diversity and in mammalian pathophysiology as well as in the availability of yeast mutants and suppressor strains. More recently, sphingolipids have attracted additional interest, because they are emerging as an important class of messenger molecules linked to many different cellular functions. In plants, sphingolipids show structural features differing from those found in animals and fungi, and much less is known about their biosynthesis and function. This review focuses on the sphingolipid modifications found in plants and on recent advances in the functional characterization of genes gaining new insight into plant sphingolipid biosynthesis. Recent studies indicate that plant sphingolipids may be also involved in signal transduction, membrane stability, host-pathogen interactions and stress responses.     

Protein kinases in elicitor signal transduction in plant cells

Plants have the ability to respond to pathogen invasion by specific defense reactions. Components of mammalian signal transduction chains have been identified in plants, and several lines of evidence have implicated such components in elicitor signal transmission in defense responses. In particular, it has been assumed that elicitor signals are transduced via a protein kinase cascade, although the identity of the protein kinases and the function of the phosphorylated proteins remain to be determined. The purpose of this review is to discuss the roles of protein kinases in elicitor signal transduction pathways in plant cells based on recent progress in this field.     

Integration of C/N-nutrient and multiple environmental signals into the ABA signaling cascade

Due to their immobility, plants have developed sophisticated mechanisms to robustly monitor and appropriately respond to dynamic changes in nutrient availability. Carbon (C) and nitrogen (N) are especially important in regulating plant metabolism and development, thereby affecting crop productivity. In addition to their independent utilization, the ratio of C to N metabolites in the cell, referred to as the “C/N balance”, is important for the regulation of plant growth, although molecular mechanisms mediating C/N signaling remain unclear. Recently ABI1, a protein phosphatase type 2C (PP2C), was shown to be a regulator of C/N response in Arabidopsis plants. ABI1 functions as a negative regulator of abscisic acid (ABA) signal transduction. ABA is versatile phytohormone that regulates multiple aspects of plant growth and adaptation to environmental stress. This review highlights the regulation of the C/N response mediated by a non-canonical ABA signaling pathway that is independent of ABA biosynthesis, as well as recent findings on the direct crosstalk between multiple cellular signals and the ABA signaling cascade.