Showing papers on "Biotic stress published in 1997"

Concepts of plant biotic stress. Some insights into the stress physiology of virus‐infected plants, from the perspective of photosynthesis

Abstract: The consequences of biotic stress have been poorly understood, partly because its application is difficult to control and partly because its physiological consequences are highly variable. Many plant viruses are recognised on the basis of leaf symptoms that depend on localised changes to chloroplast structure and function. This paper reviews recent progress in understanding early interactions between plant viruses and the photosynthetic apparatus, using chlorophyll fluorescence analysis of novel, defined algal-virus systems and using high resolution imaging of chlorophyll fluorescence and other photosynthetic processes in higher plant systems. We then consider the consequences of viral effects on photosynthetic functioning for whole plants and populations with an emphasis on the potential interactions with other environmental factors. Early responses indicated by increase in both non-photochemical quenching of fluorescence and increased reduction state of the primary electron transport acceptor QA suggest that, not surprisingly, both photoprotective and photoinhibitory processes contribute to the accelerated local demise of the photosynthetic apparatus and symptom development. In other cases, localised accumulations of carbohydrate and source-sink imbalance following infection may inhibit gene expression, leading to altered levels of chloroplast protein complexes and enzymes of photosynthetic metabolism coincident with symptom development. Recent experiments suggest that much of the variability in plant responses to biotic stress may result from interactions with other environmental factors, such as light intensity and nutrition. Experiments suggest that virus infections may have greater effects on fitness and competitive ability in low N, high light environments than in shaded, high nutrient conditions. Some ecological implications of these observations are discussed.

Role of phenylpropanoid compounds in plant responses to different stress factors

Abstract: Environmental stresses, such as pathogen attack, UV-irradiation, high light, wounding, nutrient deficiencies, temperature and herbicide treatment often increase the accumulation of phenylpropanoids (see Dixon and Paiva 1995) (Table 1). Phenylpropanoid compounds belong to plant secondary metabolites. They have been regarded as either metabolic wastes or substances with no role in fundamental life processes; they were also thought to be unusual compounds limited in distribution among plants. The last two decades brought, however, evidence that these compounds can play important roles in the control of many processes including plant developement, growth, xylogenesis and flowering (Jones 1984) and may protect plants e.g. from predators and from UV radiation or can act as free radical quenchers of stress-created singlet oxygen (Knox and Dodge 1985) and other active oxygen species (McKersie and Leshem 1994). Most of the so-called secondary products found in plants probably have functions associated with increasing survival, either by coping with unfavourable environmental conditions or by regulating some metabolic processes (Chalker-Scott and Fuchigam~ 1989).

Constitutive Traits Affecting Plant Performance Under Stress

Abstract: The final integrated plant response to drought stress in terms of total biomass or economic yield is conditioned by genes which are expressed constitutively and genes which are stress-responsive and stress-adaptive. Genes expressed constitutively may be stress-responsive, but they are not, by definition, stress adaptive. This brief review discusses the impact, whether positive or negative, of several major plant constitutive traits on plant production under drought stress. For most crop plants there is a genotype x drought stress interaction for yield which is expressed when stress is severe enough. Besides well-watered conditions, a high yield potential is expressed also under moderate levels of stress. However, a high yield potential may be negatively affecting yield under severe stress. The possible reasons for this negative association are discussed. Plant phenology has been widely implicated in plant production under drought stress. Phenology determines plant characters which may impact plant water-relations, crop assimilation and reproductive integrity under drought stress. The roles of plant size, vigor and potential growth rate in affecting productivity under stress are not well resolved. Some of the positive and negative implications of plant size towards plant productivity under drought stress are considered. Carbon assimilation, storage and utilization are expressed constitutively and their impact on stress adaptation is becoming evident, such as the case of plant reserve utilization. Finally, molecular techniques are now used to achieve the constitutive overexpression of stress adaptive genes. The few available examples and their implications are mentioned.

Accumulation of stress-proteins in intercellular spaces of barley leaves induced by biotic and abiotic factors

Abstract: Accumulation of the extracellular proteins localized in intercellular spaces of barley primary leaves was examined after inoculation with powdery mildew (Erysiphe graminis f. sp. hordei) as biotic stress factor and after abiotic stresses such as heat shock, low temperature and heavy metal (Mg,Zn, Cu, Al, Cd and Co) treatment using native polyacrylamide gel electrophoresis. Six to eight major pathogen-induced proteins (bands on native gel) have been identified. Their accumulation at host-parasite incompatibility was more expressive than at compatibility interaction. Elevated temperature did not induce pathogenesis-related (PR) proteins while low temperature induced three of them. Cu, Al, Cd and Co induced accumulation pattern of extracellular proteins was very similar to that in powdery mildew inoculated leaves. Mg and Zn had no effect on the induction of protein accumulation in the intercellular spaces of leaves. Induction of PR proteins by different stresses indicated their general function in the resistance of plants to changing environment.

Oak decline around the world

Abstract: Oak (Quercus spp.) decline is a malady related to the consequences of stress and successful attack of stressed trees by opportunistic (secondary) organisms (Wargo et al. 1983). It is a progressive process where trees decline in health for several years before they die. Houston (1981) developed a model of declines that is presented in Figure 1. So what is stress? It is pressure that brings ahout changes in a tree's physiology, form, or structure that predispose it to invasion by organisms that it ordinarily can resist. Stress can be biotic (defoliation by insects or fungi), abiotic (frost damage, defoliation by frost, drought, excess moisture, air pollution), or stand dynamics/life stage induced (low vigor due to competition or age). So what are secondary organisms? They are biotic agents, usually insects or fungi, that normally attack weakened trees and kill them but rarely can successfully attack healthy trees. These organisms play an important ecological role by killing trees that are weakened due to competition or other natural processes. Due to the numerous stress agents that can affect oak forests, oak decline can occur simultaneously in many different geographic areas, be triggered by entirely different or identical stress agents, and result in death of oaks from a wide variety of organisms.

Isolation and Identification of Arbuscular Mycorrhizal Fungi (AMF)

Abstract: Arbuscular mycorrhizal fungi (AMF) in the order Glomales colonize the root systems of the large majority of land plant species. They often have beneficial influence on growth and survival of their host due to better mineral nutrition and increased tolerance in abiotic and biotic stress situations. However, the functional and taxonomic characterization of individual AMF isolates is limited because of their obligate biotrophy. Morphological characteristics of the relatively large, asexual spores have been used for the identification and taxonomy of glomalean fungi. Difficulties in the identification of spores arise from the frequently poor state of spores isolated from the field and the few morphological characters available to discriminate species. Recent advances in biochemical and molecular techniques provide a promising tool to overcome these problems. In this study, AMF from different sites were characterised according to their morphological features as well as using isozymes as molecular markers.