Molecular and ecological plant defense responses along an elevational gradient in a boreal ecosystem

Plants have the capacity to alter their phenotype in response to environmental factors, such as herbivory, a phenomenon called phenotypic plasticity. However, little is known on how plant responses to herbivory are modulated by environmental variation along ecological gradients. To investigate this question, we used bilberry (Vaccinium myrtillus L.) plants and an experimental treatment to induce plant defenses (i.e., application of methyl jasmonate; MeJA), to observe ecological responses and gene expression changes along an elevational gradient in a boreal system in western Norway. The gradient included optimal growing conditions for bilberry in this region (ca. 500 m a.s.l.), and the plant's range limits at high (ca. 900 m a.s.l.) and low (100 m a.s.l.) elevations. Across all altitudinal sites, MeJA‐treated plants allocated more resources to herbivory resistance while reducing growth and reproduction than control plants, but this response was more pronounced at the lowest elevation. High‐elevation plants growing under less herbivory pressure but more resource‐limiting conditions exhibited consistently high expression levels of defense genes in both MeJA‐treated and untreated plants at all times, suggesting a constant state of “alert.” These results suggest that plant defense responses at both the molecular and ecological levels are modulated by the combination of climate and herbivory pressure, such that plants under different environmental conditions differentially direct the resources available to specific antiherbivore strategies. Our findings are important for understanding the complex impact of future climate changes on plant–herbivore interactions, as this is a major driver of ecosystem functioning and biodiversity.     

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.     

Callose: the plant cell wall polysaccharide with multiple biological functions

Callose (β-1,3-glucan) is a linear plant polysaccharide that plays an important role in different stages of individual development as well as in defense against unfavourable environmental factors. In plants, it is synthesized by callose synthases, and degraded by β-1,3-glucanases. This review summarizes the current knowledge on structure and function of callose in plant tissue as well as its importance under stress conditions. Despite the considerable progress in clarifying the role of this polysaccharide in plants that has been achieved during the last period, many questions regarding its synthesis or involvement in defense responses still remain to be solved. A more in-depth understanding of callose function in plants will require integration of different experimental approaches from the field of chemistry, cell biology, genetics as well as systemic biology.     

A quantitative evaluation of major plant defense hypotheses, nature versus nurture, and chemistry versus ants

Variation in plant secondary metabolite content can arise due to environmental and genetic variables. Because these metabolites are important in modifying a plant’s interaction with the environment, many studies have examined patterns of variation in plant secondary metabolites. Investigations of chemical defenses are often linked to questions about the efficacies of plant defenses and hypotheses on their evolution in different plant guilds. We performed a series of meta-analyses to examine the importance of environmental and genetic sources of variation in secondary metabolites as well as the antiherbivore properties of different classes of defense. We found both environmental and genetic variation affect secondary metabolite production, supporting continued study of the carbon-nutrient balance and growth-differentiation balance hypotheses. Defenses in woody plants are more affected by genetic variation, and herbaceous plant defenses are more influenced by environmental variation. Plant defenses in agricultural and natural systems show similar responses to manipulations, as do plants in laboratory, greenhouse, or field studies. What does such variation mean to herbivores? A comparison of biotic, physical, and chemical defenses revealed the most effective defensive strategy for a plant is biotic mutualisms with ants. Fast-growing plants are most often defended with qualitative defenses and slow-growing plants with quantitative defenses, as the plant apparency and resource availability hypotheses predict. However, we found the resource availability hypothesis provides the best explanation for the evolution of plant defenses, but the fact that there is considerable genetic and environmental variation in defenses indicates herbivores can affect plant chemistry in ecological and evolutionary time.     

Costs and benefits of jasmonic acid induced responses in soybean

In response to herbivory, plants have evolved defense strategies to reduce herbivore preference and performance. A strategy whereby defenses are induced only upon herbivory can mitigate costs of defense when herbivores are scarce. Although costs and benefits of induced responses are generally assumed, empirical evidence for many species is lacking. Soybean (Glycine max L. Merr.) has emerged as a model species with which to address questions about induced responses. To our knowledge, this is the first study to examine the fitness costs and benefits of jasmonic acid-induced responses by soybean in the absence and presence of soybean loopers (Chrysodeix includens Walker) (Lepidoptera: Noctuidae). In a greenhouse experiment we demonstrated that soybean induction was costly. Induced plants produced 10.1% fewer seeds that were 9.0% lighter, and had 19.2% lower germination rates than noninduced plants. However, induction provided only modest benefits to soybeans. In a choice experiment, soybean loopers significantly preferred leaves from noninduced plants, consuming 62% more tissue than from induced plants. Soybean loopers that fed on plants that were previously subjected to treatment with jasmonic acid matured at the same rate and to the same size as those that fed on control plants. However, at high conspecific density, soybean looper survivorship was reduced by 44% on previously induced relative to control plants. Reduced soybean looper preference and survivorship did not translate into fitness benefits for soybeans. Our findings support theoretical predictions of costly induced defenses and highlight the importance of considering the environmental context in studies of plant defense.     

中国东部海岛植物功能性状及其影响因子

研究海岛植物功能性状及其影响因子,有助于理解植物在海岛环境中的适应策略。以中国东部近海7个海岛典型植物群落为研究对象,测定群落内每种植物的6个功能性状(胸径、株高、平均单叶面积、比叶面积、小枝密度和小枝含水率),比较植物功能性状在不同气候带间的差异,探讨功能性状对气候因子(年均温、年降水和年平均风速)和土壤因子(土壤含水率、土壤总碳、土壤总氮和土壤总磷)的响应关系,并分析影响这些性状的主要驱动因子。研究结果表明:(1)所测功能性状中除比叶面积外,其他性状在不同气候带间差异显著,但常绿和落叶植物的功能性状在不同气候带间的差异不同,表明不同生活型植物对环境的响应程度和适应能力不同。(2)海岛植物功能性状对气候和土壤因子均有不同程度的响应,气候因子是影响中国东部海岛植物功能性状的重要因子,其中水热条件(年均温和年降水)是主导因子,在水热资源丰富的南方海岛,植物具有较大的胸径和小枝含水率、较小的叶片和小枝密度,北方海岛反之。此外,海岛植物的小枝性状(小枝密度和小枝含水率)受年平均风速的影响显著。该研究结果可为海岛植被恢复重建中物种的选择及植被布局规划提供一定的参考。     

Genotypic Tannin Levels in Populus tremula Impact the Way Nitrogen Enrichment Affects Growth and Allocation Responses for Some Traits and Not for Others

Plant intraspecific variability has been proposed as a key mechanism by which plants adapt to environmental change. In boreal forests where nitrogen availability is strongly limited, nitrogen addition happens indirectly through atmospheric N deposition and directly through industrial forest fertilization. These anthropogenic inputs of N have numerous environmental consequences, including shifts in plant species composition and reductions in plant species diversity. However, we know less about how genetic differences within plant populations determine how species respond to eutrophication in boreal forests. According to plant defense theories, nitrogen addition will cause plants to shift carbon allocation more towards growth and less to chemical defense, potentially enhancing vulnerability to antagonists. Aspens are keystone species in boreal forests that produce condensed tannins to serve as chemical defense. We conducted an experiment using ten Populus tremula genotypes from the Swedish Aspen Collection that express extreme levels of baseline investment into foliar condensed tannins. We investigated whether investment into growth and phenolic defense compounds in young plants varied in response to two nitrogen addition levels, corresponding to atmospheric N deposition and industrial forest fertilization. Nitrogen addition generally caused growth to increase, and tannin levels to decrease; however, individualistic responses among genotypes were found for height growth, biomass of specific tissues, root:shoot ratios, and tissue lignin and N concentrations. A genotype’s baseline ability to produce and store condensed tannins also influenced plant responses to N, although this effect was relatively minor. High-tannin genotypes tended to grow less biomass under low nitrogen levels and more at the highest fertilization level. Thus, the ability in aspen to produce foliar tannins is likely associated with a steeper reaction norm of growth responses, which suggests a higher plasticity to nitrogen addition, and potentially an advantage when adapting to higher concentrations of soil nitrogen.     

一氧化氮与激发子诱导的植物抗病防卫反应

来源于真菌或植物细胞壁的激发子可以诱导植物的抗性反应。一系列的信号分子,如一氧化氮、活性氧、茉莉酸、水杨酸、乙烯等都参与了激发子诱导的植物抗性反应。它们在介导激发子刺激诱发胞内抗性反应的过程中起着重要的作用。本文介绍了激发子的种类,并简述了激发了受体以及植物细胞对激发子刺激的感受与传递;重点介绍了一氧化氮在激发子诱导植物抗性反应过程中的作用,以及它与其他信号分子之间相互关系的研究进展。     

Hormonal interactions during root tropic growth: hydrotropism versus gravitropism

Terrestrial plants have evolved remarkable morphological plasticity that enables them to adapt to their surroundings. One of the most important traits that plants have acquired is the ability to sense environmental cues and use them as a basis for governing their growth orientation. The directional growth of plant organs relative to the direction of environmental stimuli is a tropism. The Cholodny–Went theory proposes that auxin plays a key role in several tropisms. Recent molecular genetic studies have strongly supported this hypothesis for gravitropism. However, the molecular mechanisms of other tropisms are far less clear. Hydrotropism is the response of roots to a moisture gradient. Since its re-discovery in 1985, root hydrotropism has been shown to be common among higher plant species. Additionally, in some species, gravitropism interferes with hydrotropism, suggesting that both shared and divergent mechanisms mediating the two tropisms exist. This hypothesis has been supported by recent studies, which provide an understanding of how roots sense multiple environmental cues and exhibit different tropic responses. In this review, we focus on the overlapping and unique mechanisms of the hormonal regulation underlying gravitropism and hydrotropism in roots.     

Competition among plants: Concepts, individual-based modelling approaches, and a proposal for a future research strategy

Competition is a key process in plant populations and communities. We thus need, if we are to predict the responses of ecological systems to environmental change, a comprehensive and mechanistic understanding of plant competition. Considering competition, however, only at the population level is not sufficient because plant individuals usually are different, interact locally, and can adapt their behaviour to the current state of themselves and of their biotic and abiotic environment. Therefore, simulation models that are individual-based and spatially explicit are increasingly used for studying competition in plant systems. Many different individual-based modelling approaches exist to represent competition, but it is not clear how good they are in reflecting essential aspects of plant competition. We therefore first summarize current concepts and theories addressing plant competition. Then, we review individual-based approaches for modelling competition among plants. We distinguish between approaches that are used for more than 10 years and more recent ones. We identify three major gaps that need to be addressed more in the future: the effects of plants on their local environment, adaptive behaviour, and below-ground competition. To fill these gaps, the representation of plants and their interactions have to be more mechanistic than most existing approaches. Developing such new approaches is a challenge because they are likely to be more complex and to require more detailed knowledge and data on individual-level processes underlying competition. We thus need a more integrated research strategy for the future, where empirical and theoretical ecologists as well as computer scientists work together on formulating, implementing, parameterization, testing, comparing, and selecting the new approaches.     

RAB GTPases and SNAREs at the trans-Golgi network in plants

Membrane traffic is a fundamental cellular system to exchange proteins and membrane lipids among single membrane-bound organelles or between an organelle and the plasma membrane in order to keep integrity of the endomembrane system. RAB GTPases and SNARE proteins, the key regulators of membrane traffic, are conserved broadly among eukaryotic species. However, genome-wide analyses showed that organization of RABs and SNAREs that regulate the post-Golgi transport pathways is greatly diversified in plants compared to other model eukaryotes. Furthermore, some organelles acquired unique properties in plant lineages. Like in other eukaryotic systems, the trans-Golgi network of plants coordinates secretion and vacuolar transport; however, uniquely in plants, it also acts as a platform for endocytic transport and recycling. In this review, we focus on RAB GTPases and SNAREs that function at the TGN, and summarize how these regulators perform to control different transport pathways at the plant TGN. We also highlight the current knowledge of RABs and SNAREs’ role in regulation of plant development and plant responses to environmental stimuli.

     

Transgenic expression of a fungal endo-polygalacturonase increases plant resistance to pathogens and reduces auxin sensitivity

Polygalacturonases (PGs), enzymes that hydrolyze the homogalacturonan of the plant cell wall, are virulence factors of several phytopathogenic fungi and bacteria. On the other hand, PGs may activate defense responses by releasing oligogalacturonides (OGs) perceived by the plant cell as host-associated molecular patterns. Tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana) plants expressing a fungal PG (PG plants) have a reduced content of homogalacturonan. Here, we show that PG plants are more resistant to microbial pathogens and have constitutively activated defense responses. Interestingly, either in tobacco PG or wild-type plants treated with OGs, resistance to fungal infection is suppressed by exogenous auxin, whereas sensitivity to auxin of PG plants is reduced in different bioassays. The altered plant defense responses and auxin sensitivity in PG plants may reflect an increased accumulation of OGs and subsequent antagonism of auxin action. Alternatively, it may be a consequence of perturbations of cellular physiology and elevated defense status as a result of altered cell wall architecture.     

Molecular communications between plant heat shock responses and disease resistance

As sessile, plants are continuously exposed to potential dangers including various abiotic stresses and pathogen attack. Although most studies focus on plant responses under an ideal condition to a specific stimulus, plants in nature must cope with a variety of stimuli at the same time. This indicates that it is critical for plants to fine-control distinct signaling pathways temporally and spatially for simultaneous and effective responses to various stresses. Global warming is currently a big issue threatening the future of humans. Reponses to high temperature affect many physiological processes in plants including growth and disease resistance, resulting in decrease of crop yield. Although plant heat stress and defense responses share important mediators such as calcium ions and heat shock proteins, it is thought that high temperature generally suppresses plant immunity. We therefore specifically discuss on interactions between plant heat and defense responses in this review hopefully for an integrated understanding of these responses in plants.     

Cloning and functional characterization of a fructan 1-exohydrolase (1-FEH) in the cold tolerant Patagonian species <Emphasis Type="Italic">Bromus pictus</Emphasis>

Protein phosphorylation/dephosphorylation plays critical roles in stress responses in plants. This report presents a comparative characterization of the serine/threonine PP2A catalytic subunit family in Solanum tuberosum (potato) and S. lycopersicum (tomato), two important food crops of the Solanaceae family, based on the sequence analysis and expression profiles in response to environmental stress. Sequence homology analysis revealed six isoforms in potato and five in tomato clustered into two subfamilies (I and II). The data presented in this work show that the expression of different PP2Ac genes is regulated in response to environmental stresses in potato and tomato plants and suggest that, in general, mainly members of the subfamily I are involved in stress responses in both species. However, the differences found in the expression profiles between potato and tomato suggest divergent roles of PP2A in the plant defense mechanisms against stress in these closely related species.     

Effects of windspeed on the growth and biomass allocation of white mustard Sinapis alba L.

Summary We examined how different wind speeds and interactions between plant age and wind affect growth and biomass allocation of Sinapis alba L. (white mustard). Physiological and growth measurements were made on individuals of white mustard grown in controlled-environment wind tunnels at windspeeds of 0.3, 2.2 and 6.0 ms^–1 for 42 days. Plants were harvested at four different dates. Increasing wind speed slightly increased transpiration and stomatal conductance. We did not observe a significant decline in the photosynthetic rate per unit of leaf area. Number of leaves, stem length, leaf area and dry weights of total biomass and plant parts were significantly lower in plants exposed at high wind speed conditions. There were no significant differences in the unit leaf rate nor relative growth rates, although these were always lower in plants grown at high wind speed. Allocation and architectural parameters were also examined. After 42 days of exposure to wind, plants showed higher leaf area ratio, root and leaf weight ratios and root/shoot ratio than those grown at control treatment. Only specific leaf area declined significantly with wind speed, but stem and reproductive parts also decreased. The responses of plants to each wind speed treatment depended on the age of the plant for most of the variables. It is suggested that wind operates in logarithmic manner, with relatively small or no effect at lower wind speeds and a much greater effect at higher speeds. Since there is no evidence of a significant reduction in photosynthetic rate of Sinapis with increasing wind speed it is suggested that the effect of wind on plant growth was due to mechanical effects leading to changes in allocation and developmental patterns.     

Brassinosteroid-Mediated Stress Responses

Brassinosteroids (BRs) are a group of naturally occurring plant steroidal compounds with wide-ranging biological activity that offer the unique possibility of increasing crop yields through both changing plant metabolism and protecting plants from environmental stresses. In recent years, genetic and biochemical studies have established an essential role for BRs in plant development, and on this basis BRs have been given the stature of a phytohormone. A remarkable feature of BRs is their potential to increase resistance in plants to a wide spectrum of stresses, such as low and high temperatures, drought, high salt, and pathogen attack. Despite this, only a few studies aimed at understanding the mechanism by which BRs promote stress resistance have been undertaken. Studies of the BR signaling pathway and BR gene-regulating properties indicate that there is cross-talk between BRs and other hormones, including those with established roles in plant defense responses such as abscisic acid, jasmonic acid, and ethylene. Recent studies aimed at understanding how BRs modulate stress responses suggest that complex molecular changes underlie BR-induced stress tolerance in plants. Analyses of these changes should generate exciting results in the future and clarify whether the ability of BRs to increase plant resistance to a range of stresses lies in the complex interactions of BRs with other hormones. Future studies should also elucidate if BRI1, an essential component of the BR receptor, directly participates in stress response signaling through interactions with ligands and proteins involved in plant defense responses.     

Catalase-deficient tobacco plants: tools for in planta studies on the role of hydrogen peroxide

Adequate responses to environmental changes are crucial for plant growth and survival. However, the molecular and biochemical mechanisms involved are poorly understood and the signaling networks remain elusive. The accumulation of active oxygen species (AOS) is a central theme during plant responses to both biotic and abiotic stresses. In both situations, AOS can play two divergent roles: either exacerbating damage or activating multiple defense responses, thereby acting as signal molecules. Such a dual function was first described in pathogenesis, but also recently has been demonstrated during several abiotic stress responses. To allow for these different roles, cellular levels of AOS must be tightly controlled. This control can be attained through a diverse battery of oxidant scavengers. Perturbation of this scavenging capacity can lead to dramatic imbalances of AOS concentrations, leading to a modified redox status. Here, we summarize mainly the work done on plants that are deficient in catalase activity. These plants not only revealed the importance of catalase in coping with environmental stress but also provided us with a powerful tool to investigate the (multiple) roles of H2O2 in an intact plant system.     

Interspecific variation in the defensive responses of obligate plant-ants: experimental tests and consequences for herbivory

The aggressive behavior of ants that protect plants from herbivores in exchange for rewards such as shelter or food is thought to be an important form of biotic defense against herbivory, particularly in tropical systems. To date, however, no one has compared the defensive responses of different ant taxa associated with the same plant species, and attempted to relate these differences to longer-term efficacy of ant defense. We used experimental cues associated with herbivory—physical damage and extracts of chemical volatiles from leaf tissue—to compare the aggressive responses of two ant species obligately associated with the Amazonian myrmecophyte Tococa bullifera (Melastomataceae). We also conducted a colony removal experiment to quantify the level of resistance from herbivores provided to plants by each ant species. Our experiments demonstrate that some cues eliciting a strong response from one ant species elicited no response by the other. For cues that do elicit responses, the magnitude of these responses can vary interspecifically. These patterns were consistent with the level of resistance provided from herbivores to plants. The colony removal experiment showed that both ant species defend plants from herbivores: however, herbivory was higher on plants colonized by the less aggressive ant species. Our results add to the growing body of literature indicating defensive ant responses are stimulated by cues associated with herbivory. However, they also suggest the local and regional variation in the composition of potential partner taxa could influence the ecology and evolution of defensive mutualisms in ways that have previously remained unexplored.     

From seed to seed: the role of photoreceptors in Arabidopsis development

As sessile organisms, plants have evolved a multitude of developmental responses to cope with the ever-changing environmental conditions that challenge the plant throughout its life cycle. Of the many environmental cues that regulate plant development, light is probably the most important. From determining the developmental pattern of the emerging seedling, to influencing the organization of organelles to best maximize energy available for photosynthesis, light has dramatic effects on development during all stages of plant life. In plants, three classes of photoreceptors that mediate light perception have been characterized at the molecular level. The phytochromes recognize light in the red portion of the spectrum, while cryptochromes and phototropins perceive blue and UVA light. In this review, we discuss the different aspects of development that are regulated by these photoreceptors in the model plant species Arabidopsis thaliana and how the phytochromes, cryptochromes, and phototropins bring about changes in development seen in the growing plant.