Suppression of plant defence response by a mycorrhiza helper bacterium

The aim of the present study was to determine whether the mycorrhiza helper bacterium Streptomyces sp. AcH 505 could serve as a biocontrol agent against Heterobasidion root and butt rot. Bacterial influence on mycelial growth of Heterobasidion sp. isolates, on the colonization of wood discs and Norway spruce (Picea abies) roots was determined. The effect of AcH 505 on plant photosynthesis, peroxidase activity and gene expression, and needle infections were investigated. AcH 505 was antagonistic to 11 of 12 tested fungal Heterobasidion isolates. The antagonism resulted in a suppression of fungal colonization of Norway spruce roots and wood discs. Mycelial growth rate of the 12th strain, Heterobasidion abietinum 331 was not affected by AcH 505, and colonization of roots by this fungal strain was promoted by AcH 505. Bacterial inoculation led to decreased peroxidase activities and gene expression levels in roots. AcH 505 promotes plant root colonization by Heterobasidion strains that are tolerant to antifungal metabolites produced by the bacterium. This may result from unknown bacterial factors that suppress the plant defence response.     

Novel Rosaceae plant elicitor peptides as sustainable tools to control Xanthomonas arboricola pv. pruni in Prunus spp.

Fruit crops are regarded as important health promoters and constitute a major part of global agricultural production, and Rosaceae species are of high economic impact. Their culture is threatened by bacterial diseases, whose control is based on preventative treatments using compounds of limited efficacy and negative environmental impact. One of the most economically relevant examples is the pathogen Xanthomonas arboricola pv. pruni (Xap) affecting Prunus spp. The plant immune response against pathogens can be triggered and amplified by plant elicitor peptides (Peps), perceived by specific receptors (PEPRs). Although they have been described in various angiosperms, scarce information is available on Rosaceae species. Here, we identified the Pep precursor (PROPEP), Pep and PEPR orthologues of 10 Rosaceae species and confirmed the presence of the Pep/PEPR system in this family. We showed the perception and elicitor activity of Rosaceae Peps using the Prunus–Xap pathosystem as proof‐of‐concept. Treatment with nanomolar doses of Peps induced the corresponding PROPEP and a set of defence‐related genes in Prunus leaves, and enhanced resistance against Xap. Peps from the same species had the highest efficiencies. Rosaceae Peps could potentially be used to develop natural, targeted and environmentally friendly strategies to enhance the resistance of Prunus species against biotic attackers.     

Early events in the elicitation of plant defence

Plants successfully use inducible defence mechanisms to combat potential pathogens. Elicitors are signaling compounds that stimulate any of such defence responses. Recent progress in the isolation of pure elicitors has made possible investigations on elicitor-binding proteins which might function as receptors in signal transduction pathways that ultimately activate the defences. The elicitor-binding sites studied so far show a high degree of ligand specificity, as do the candidate binding proteins identified for some of the ligands. Following elicitor perception, a number of rapid reactions are detectable in plant cells, including enhanced ion fluxes across the plasma membrane, formation of reactive oxygen intermediates, changes in protein phosphorylation, and lipid oxidation. Intriguing questions arising from these observations are whether the elicitor-binding proteins constitute receptors in plant defence signaling and whether any of the rapid events participate in signal transduction during defence activation. Received: 17 November 1997 / Accepted: 22 April 1998     

Importance of plant‐ and microbe‐driven metabolic pathways for plant defence

Expression of plant phenotypes can depend on both plant genomes and interactions between plants and the microbes living in, on and near their roots. We understand a growing number of the mechanistic links between plant genotypes and phenotypes, such as defence against herbivory (see brief review in Hubbard et al., 2019), yet the links between root microbiomes and the comprehensive swathe of plant phenotypes they affect (Friesen et al., 2011) remain less clear. In this issue of Molecular Ecology, Hubbard et al. (2019) follow microbe‐ and plant‐driven changes in plant defence against hervibory from molecular underpinnings to ecological consequences, contrasting both the metabolites affected and the magnitude of defensive impact. Naively, we might expect plant genomes to drive more variation in phenotype than the root microbiome, but Hubbard et al. (2019) find the opposite, implying profound consequences for plant trait evolution and ecological interactions.     

Calcium and the plant cytoskeleton

Abstract. The regulation of the plant cytoskeleton by the concentration of free ionized Ca²⁺ in the cytosol is discussed. The effect is illustrated by considering the role of Ca²⁺ in regulating cytoplasmic streaming in Physarum and characean algae, and flagellar beating in Chlamydomonas . Examples are given of cytoskeletal processes in other plant cells that may be Ca²⁺ -regulated, together with an appraisal of some technical difficulties that currently limit their study.     

植物钙感受器及其介导的逆境信号途径

钙信号是植物生长发育和逆境响应的重要调控因子, 是植物生理与逆境生物学研究领域中的热点之一。当植物细胞受到外界逆境刺激时, 其胞内会产生具有时空特异性的Ca2+信号变化, 这种变化首先被胞内钙感受器所感知并解码, 再由钙感受器互作蛋白将信号传递到下游, 从而激活下游早期响应基因的表达或相关离子通道的活性, 最终产生特异性逆境响应。植物细胞通过感知胞内钙信号的变化如何识别来自外界不同性质或不同强度的刺激, 是近几年植物生物学家所关注的科学问题。文章主要总结了近几年在植物钙感受器研究领域中的最新进展, 包括钙依赖蛋白激酶(CDPKs)、钙调素(CaMs)、类钙调素蛋白(CMLs)、类钙调磷酸酶B蛋白(CBLs)及其互作蛋白激酶(CIPKs)等的结构、功能及其介导的逆境信号途径, 并提供新的见解和展望。     

Perception of insect feeding by plants

The recognition of phytophagous insects by plants induces a set of very specific responses aimed at deterring tissue consumption and reprogramming metabolism and development of the plant to tolerate the herbivore. The recognition of insects by plants requires the plant’s ability to perceive chemical cues generated by the insects and to distinguish a particular pattern of tissue disruption. Relatively little is known about the molecular basis of insect perception by plants and the signalling mechanisms directly associated with this perception. Importantly, the insect feeding behaviour (piercing‐sucking versus chewing) is a decisive determinant of the plant’s defence response, and the mechanisms used to perceive insects from different feeding guilds may be distinct. During insect feeding, components of the saliva of chewing or piercing‐sucking insects come into contact with plant cells, and elicitors or effectors present in this insect‐derived fluid are perceived by plant cells to initiate the activation of specific signalling cascades. Although receptor–ligand interactions controlling insect perception have yet not been molecularly described, a significant number of regulatory components acting downstream of receptors and involved in the activation of defence responses against insects has been reported. Some of these regulators mediate changes in the phytohormone network, while others directly control gene expression or the redox state of the cell. These processes are central in the orchestration of plant defence responses against insects.     

The phytotoxin syringomycin elicits Ca2+-dependent callose synthesis in suspension-cultured cells of Catharanthus roseus

Deposition of the 1,3-β-glucan callose onto the cell wall represents one of the defence reactions of plants against pathogens. This process can be induced in suspension cells of Catharanthus roseus by subtoxic concentrations of the bacterial phytotoxin syringomycin and is associated with a slight increase in Ca²⁺ uptake and some K⁺ release. Under these conditions callose formation can be prevented by complexing external Ca²⁺, indicating that some Ca²⁺ uptake is essential as a signal. However, higher syringomycin concentrations elicit increased Ca²⁺ uptake without increasing callose formation, although the potential for callose synthesis is not exhausted – as shown using digitonin as an additional elicitor. These results suggest that, superimposed on Ca²⁺, another, yet unknown signal is also involved in the regulation of callose synthesis.     

Calcium as a plant nutrient

Abstract. Calcium occurs as a relatively large, divalent ion which readily enters the apoplast and is bound in exchangeable form in cell walls and on the exterior surface of the plasmalemma. It occurs in only very low concentrations in the cytoplasm and chloroplasts and appears to have a limited role as an enzymatic cofactor. The soil solution usually provides an adequate supply of Ca to plants. The well known physiological disorders resulting from localized Ca deficiencies within the plant are thus attributable to poor Ca distribution rather than restriction in uptake. Calcium is moved largely in the xylem and only to a very limited extent in the phloem. The phloem/xylem ratio of the solute input can be particularly critical in organs which are naturally low in Ca, such as fruits and young leaves. Factors which influence the distribution of Ca such as humidity, root pressure, phytohormone activity, can also affect the occurrence of these disorders.     

Calcium in plant senescence and fruit ripening

Abstract. Calcium has long been associated with regulation of the ripening process of fruit and post-harvest storage life. Specifically, maintenance of relatively high calcium concentrations in fruit tissue results in a slower rate of ripening, as seen in lower respiration rates, reduced ethylene production, and slower softening of fruit flesh. There are also some specific fruit disorders such as bitter pit, which can be prevented if sufficient calcium is present. Senescence of other plant tissues such as leaves and flowers has also been shown to be retarded by the application of calcium.
Work leading to the above information is reviewed and discussed in the context of what is currently known of cellular regulation of calcium in plants. The major sites for the action of calcium in senescence and ripening are suggested to be in membrane structure and function, and in cell wall structure. Although high external concentrations of calcium are an advantage in reducing the rate of senescence or ripening, it is emphasized that normal cell function requires the maintenance of low concentrations of free calcium in the cell cytosol. It is suggested that one possible feature of senescence is a breakdown in such cellular regulation.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

Trade-offs in non-native plant herbivore defences enhance performance

Non-native plants are typically released from specialist enemies but continue to be attacked by generalists, albeit at lower intensities. This reduced herbivory may lead to less investment in constitutive defences and greater investment in induced defences, potentially reducing defence costs. We compared herbivory on 27 non-native and 59 native species in the field and conducted bioassays and chemical analyses on 12 pairs of non-native and native congeners. Non-natives suffered less damage and had weaker constitutive defences, but stronger induced defences than natives. For non-natives, the strength of constitutive defences was correlated with the intensity of herbivory experienced, whereas induced defences showed the reverse. Investment in induced defences correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. To our knowledge, these are the first linkages reported among trade-offs in plant defences related to the intensity of herbivory, allocation to constitutive versus induced defences, and growth.     

The role of G-proteins in plant immunity

Heterotrimeric G-proteins play an important regulatory role in multiple physiological processes, including the plant immune response, and substantial progress has been made in elucidating the G-protein-mediated defense-signaling network. This mini-review discusses the importance of G-proteins in plant immunity. We also provide an overview of how G-proteins affect plant cell death and stomatal movement. Our recent studies demonstrated that G-proteins are involved in signal transduction and induction of stomatal closure and defense responses. We also discuss future directions for G-protein signaling studies involving plant immunity.     

白念珠菌钙稳态系统及钙信号途径的研究进展

白念珠菌是临床重要的条件致病菌,其胞内的钙稳态及钙信号途径与宿主侵染、压力应答等诸多生理过程紧密相关。研究该菌的钙稳态系统及钙信号调控网络,对明确白念珠菌的侵染机制与耐药机理,以及开发具有新靶点的抗真菌药物具有重要意义。本文对这些内容的研究进展进行了综述。