Catabolism of polyamines

Summary. Owing to the establishment of cells and transgenic animals which either lack or over-express acetylCoA:spermidine N¹-acetyltransferase a major progress was made in our understanding of the role of polyamine acetylation. Cloning of polyamine oxidases of mammalian cell origin revealed the existence of several enzymes with different substrate and molecular properties. One appears to be identical with the polyamine oxidase that was postulated to catalyse the conversion of spermidine to putrescine within the interconversion cycle. The other oxidases are presumably spermine oxidases, because they prefer free spermine to its acetyl derivatives as substrate. Transgenic mice and cells which lack spermine synthase revealed that spermine is not of vital importance for the mammalian organism, but its transformation into spermidine is a vitally important reaction, since in the absence of active polyamine oxidase, spermine accumulates in blood and causes lethal toxic effects.Numerous metabolites of putrescine, spermidine and spermine, which are presumably the result of diamine oxidase-catalysed oxidative deaminations, are known as normal constituents of organs of vertebrates and of urine. Reasons for the apparent contradiction that spermine is in vitro a poor substrate of diamine oxidase, but is readily transformed into N⁸-(2-carboxyethyl)spermidine in vivo, will need clarification.Several attempts were made to establish diamine oxidase as a regulatory enzyme of polyamine metabolism. However, diamine oxidase has a slow turnover. This, together with the efficacy of the homeostatic regulation of the polyamines via the interconversion reactions and by transport pathways renders a role of diamine oxidase in the regulation of polyamine concentrations unlikely. 4-Aminobutyric acid, the product of putrescine catabolism has been reported to have antiproliferative properties. Since ornithine decarboxylase and diamine oxidase activities are frequently elevated in tumours, it may be hypothesised that diamine oxidase converts excessive putrescine into 4-aminobutyric acid and thus restricts tumour growth and prevents malignant transformation. This function of diamine oxidase is to be considered as part of a general defence function, of which the prevention of histamine and cadaverine accumulation from the gastrointestinal tract is a well-known aspect.     

Cell wall degrading enzymes,inhibitory proteins,and oligosaccharides participate in the molecular dialogue between plants and pathogens

The wall interface between plants and pathogens plays an important role in the outcome of their interactions. Studying the degradation of plant pectic polysaccharides by microbial pectinases, and of microbial β-glucans by plant glucanases has shown that these polymers are a source of oligosaccharides which elicit defence responses in plants. The extent of degradation appears to be controlled by the presence of inhibitory proteins which counteract enzyme hydrolysis. Thus, plant cell walls participate in the molecular dialogue established between plants and pathogens.     

Antibiotic activities of peptides, hydrogen peroxide and peroxynitrite in plant defence.

Genes encoding plant antibiotic peptides show expression patterns that are consistent with a defence role. Transgenic over-expression of defence peptide genes is potentially useful to engineer resistance of plants to relevant pathogens. Pathogen mutants that are sensitive to plant peptides in vitro have been obtained and a decrease of their virulence in planta has been observed, which is consistent with their hypothetical defence role. A similar approach has been followed to elucidate the potential direct anti-microbial role of hydrogen peroxide. Additionally, a scavenger of peroxynitrite has been used to investigate its involvement in plant defence.     

Apoplastic polyamine oxidation plays different roles in local responses of tobacco to infection by the necrotrophic fungus Sclerotinia sclerotiorum and the biotrophic bacterium Pseudomonas viridiflava

The role of polyamine (PA) metabolism in tobacco (Nicotiana tabacum) defense against pathogens with contrasting pathogenic strategies was evaluated. Infection by the necrotrophic fungus Sclerotinia sclerotiorum resulted in increased arginine decarboxylase expression and activity in host tissues, as well as putrescine and spermine accumulation in leaf apoplast. Enhancement of leaf PA levels, either by using transgenic plants or infiltration with exogenous PAs, led to increased necrosis due to infection by S. sclerotiorum. Specific inhibition of diamine and PA oxidases attenuated the PA-induced enhancement of leaf necrosis during fungal infection. When tobacco responses to infection by the biotrophic bacterium Pseudomonas viridiflava were investigated, an increase of apoplastic spermine levels was detected. Enhancement of host PA levels by the above-described experimental approaches strongly decreased in planta bacterial growth, an effect that was blocked by a PA oxidase inhibitor. It can be concluded that accumulation and further oxidation of free PAs in the leaf apoplast of tobacco plants occurs in a similar, although not identical way during tobacco defense against infection by microorganisms with contrasting pathogenesis strategies. This response affects the pathogen's ability to colonize host tissues and results are detrimental for plant defense against necrotrophic pathogens that feed on necrotic tissue; on the contrary, this response plays a beneficial role in defense against biotrophic pathogens that depend on living tissue for successful host colonization. Thus, apoplastic PAs play important roles in plant-pathogen interactions, and modulation of host PA levels, particularly in the leaf apoplast, may lead to significant changes in host susceptibility to different kinds of pathogens.     

Clonal integration beyond resource sharing: implications for defence signalling and disease transmission in clonal plant networks

Resource sharing between ramets of clonal plants is a well-known phenomenon, which allows stoloniferous and rhizomatous species to internally translocate water, mineral nutrients and carbohydrates from sites of high supply to sites of high demand. The mechanisms and implications of resource integration in clonal plants have extensively been studied in the past. Vascular ramet connections are likely to provide an excellent means to share substances other than resources, such as systemic defence signals and pathogens. The aim of this paper is to propose the idea that physical ramet connections of clonal plants can be used (1) to transmit signals, which enable members of clonal plant networks to share information about their biotic and abiotic environments, and (2) to facilitate the internal distribution of systemic pathogens in clonal plant networks and populations. We will focus on possible mechanisms as well as on potential ecological and evolutionary implications of clonal integration beyond resource sharing. More specifically, we will explore the role of physiological integration in clonal plant networks for the systemic transmission of direct and indirect defence signals after localized herbivore attack. We propose that sharing defence induction signals among ramets may be the basis for an efficient early warning system, and it may allow for effective indirect defence signalling to herbivore enemies through a systemic release of volatiles from entire clonal fragments. In addition, we will examine the role of clonal integration for the internal spread of systemic pathogens and pathogen defence signals within clonal plants. Clonal plants may use developmental mechanisms such as increased flowering and clone fragmentation, but also specific biochemical defence strategies to fight pathogens. We propose that clonal plant networks can act as stores and vectors of diseases in plant populations and communities and that clonal life histories favour the evolution of pathogens with a low virulence.     

Molecular and physiological stages of priming: how plants prepare for environmental challenges

Being sessile organisms, plants must respond to various challenges in the environment. The priming process consists of three clear stages. The first stage includes all the cellular changes in the absence of the challenge so-called pre-challenge priming stage. These changes are expected to be rather subtle, affecting the preparation of the plant to properly manage subsequent responses to pathogens with no major fitness costs. Most of the research that has been conducted at this stage has been dedicated to the study of changes in gene expression and protein phosphorylation. However, the metabolic changes that occur during the pre-challenge priming stage are poorly understood. The second stage affects the early to late stages of the defence response, which occurs after the interaction with a pathogen has been established. Most studies involving priming are dedicated to the molecular events that take place during this stage. Most studies have shown that defence priming is strongly hormonally regulated; however, there is also evidence of the involvement of phenolic derivative compounds and many other secondary metabolites, leading to stronger and faster plant responses. The third priming phase ranges from long lasting defence priming to trans-generational acquired resistance. Long-term metabolic transitions, that occur in the offspring of primed plants, remain to be elucidated. Here we review existing information in the literature that relates to the metabolic changes that occur during all three defence priming stages and highlight the metabolic transitions that are associated with the stimulation of priming and the characteristics of the pathogens whenever possible.     

Natural variation in priming of basal resistance: from evolutionary origin to agricultural exploitation

Biotic stress has a major impact on the process of natural selection in plants. As plants have evolved under variable environmental conditions, they have acquired a diverse spectrum of defensive strategies against pathogens and herbivores. Genetic variation in the expression of plant defence offers valuable insights into the evolution of these strategies. The 'zigzag' model, which describes an ongoing arms race between inducible plant defences and their suppression by pathogens, is now a commonly accepted model of plant defence evolution. This review explores additional strategies by which plants have evolved to cope with biotic stress under different selective circumstances. Apart from interactions with plant-beneficial micro-organisms that can antagonize pathogens directly, plants have the ability to prime their immune system in response to selected environmental signals. This defence priming offers disease protection that is effective against a broad spectrum of virulent pathogens, as long as the augmented defence reaction is expressed before the invading pathogen has the opportunity to suppress host defences. Furthermore, priming has been shown to be a cost-efficient defence strategy under relatively hostile environmental conditions. Accordingly, it is possible that selected plant varieties have evolved a constitutively primed immune system to adapt to levels of disease pressure. Here, we examine this hypothesis further by evaluating the evidence for natural variation in the responsiveness of basal defence mechanisms, and discuss how this genetic variation can be exploited in breeding programmes to provide sustainable crop protection against pests and diseases.     

Switching on plant genes by external chemical signals

During the past decade there has been rapidly increasing interest in the role of plant volatiles in insect-plant interactions and the induction of plant defence systems by both pathogens and herbivores. Scientists are striving to link the proximate studies elucidating pathways and genes with the ultimate adaptive studies that attempt to explain their ecological role. However, we still do not know whether plants 'talk' to one another by employing 'phytopheromones'.     

Subtilisin‐like proteases in plant defence: the past,the present and beyond

Subtilisin‐like proteases (or subtilases) are a very diverse family of serine peptidases present in many organisms, but mostly in plants. With a broad spectrum of biological functions, ranging from protein turnover and plant development to interactions with the environment, subtilases have been gaining increasing attention with regard to their involvement in plant defence responses against the most diverse pathogens. Over the last 5 years, the number of published studies associating plant subtilases with pathogen resistance and plant immunity has increased tremendously. In addition, the observation of subtilases and serine protease inhibitors secreted by pathogens has also gained prominence. In this review, we focus on the active participation of subtilases in the interactions established by plants with the environment, highlighting their role in plant–pathogen communication.