Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem. However, sugar transport through the phloem can be affected by many environmental factors that alter source/sink relationships. In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic (water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants) and biotic (mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants) factors. Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency. Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted (e.g., by callose deposition) and under certain conditions, phloem transport is affected, earlier than photosynthesis. Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease. Biotic interactions (aphids, fungi, viruses…) also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides. The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted.

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Source-to-sink transport of sugar and regulation by environmental factors

A great deal of information is available in the literature on the effects of nutrition on disease development in plants and crops. However, much of this information is contradictory and although it is widely recognised that nutrition can influence disease in crops, limited progress has been made in the manipulation of crop nutrition to enhance disease control. Achieving this aim requires a sound understanding of the effects of fertilisation on nutrient levels and availability in crop tissues, and in turn, how the nutrient status of such tissues influences pathogen infection, colonisation and sporulation. Some of these details are known for a number of crop plants under controlled conditions, but very little of this type of information is available for crops under field conditions. This review focuses on nitrogen, sulphur, phosphorus, potassium and silicon, examines the availability of these nutrients in plant tissues to support pathogen growth and development, and reviews the effects of the different nutrients on disease development. The review also examines the potential for manipulating crop nutrition to enhance disease control in conventional and organic cropping systems.

Influence of nutrition on disease development caused by fungal pathogens: implications for plant disease control

Aggressiveness, the quantitative component of pathogenicity, and its role in the adaptation of plant pathogens are still insufficiently investigated. Using mainly examples of biotrophic and necrotrophic fungal pathogens of cereals and Phytophthora infestans on potato, the empirical knowledge on the nature of aggressiveness components and their evolution in response to host and environment is reviewed. Means of measuring aggressiveness components are considered, as well as the sources of environmental variance in these traits. The adaptive potential of aggressiveness components is evaluated by reviewing evidence for their heritability, as well as for constraints on their evolution, including differential interactions between host and pathogen genotypes and trade-offs between components of pathogenicity. Adaptations of pathogen aggressiveness components to host and environment are analysed, showing that: (i) selection for aggressiveness in pathogen populations can be mediated by climatic parameters; (ii) global population changes or remarkable population structures may be explained by variation in aggressiveness; and (iii) selection for quantitative traits can influence pathogen evolution in agricultural pathosystems and can result in differential adaptation to host cultivars, sometimes leading to erosion of quantitative resistance. Possible links with concepts in evolutionary ecology are suggested.

Aggressiveness and its role in the adaptation of plant pathogens

Modelling the impacts of pests and diseases on agricultural systems

In most fungal pathogen–plant systems, a high level of sugars in plant tissues enhances plant resistance. Several hypotheses have been proposed to explain the mechanisms of “high-sugar resistance”. Sugars constitute the primary substrate providing energy and structural material for defense responses in plants, while they may also act as signal molecules interacting with the hormonal signaling network regulating the plant immune system. Sugars enhance oxidative burst at early stages of infection, increasing lignification of cell walls, stimulate the synthesis of flavonoids and induce certain PR proteins. Some sugars act as priming agents inducing higher plant resistance to pathogens.

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The role of sugar signaling in plant defense responses against fungal pathogens

A model to predict Septoria tritici blotch (STB) and leaf rust effects on wheat growth was constructed and evaluated in two steps. At the leaf scale, Bastiaans' approach that predicts the relative photo synthesis of a wheat leaf infected with a single disease, was extended to the case of two diseases, one biotrophic and one necrotrophic by considering the leaf rust-STB complex. A glasshouse experiment with flag leaves inoculated either singly with one disease or with two diseases combined was per formed to check the leaf damage model. No inter action of the two diseases on photosynthesis loss was observed when they occurred simultaneously on the same leaf. In a second step, the single-leaf model was extended to the canopy scale to model the effects of the leaf rust-STB complex on the growth of a wheat crop. The model predicts the effects of disease on the growth of an affected crop relative to the growth of a healthy crop. The canopy model accounted for different contributions to photosyn thetic activity of leaf layers, derived from their posi tion in the canopy and their natural leaf senescence. Treatments differing in nitrogen fertilization, micro climatic conditions, and wheat cultivars were imple mented in a field experiment to evaluate the model. The model accurately estimated the effect of disease on crop growth for each cultivar, with differences from experimental values lower than 10%, which suggests that this model is well suited to aid an understanding of disease effects on plant growth. A reduction in green leaf area was the main effect of disease in these field experiments and STB accounted for more than 70% of the reduction in plant growth. Simulations suggested that the produc tion of rust spores may result in a loss of biomass from diseased crops and that stem photosynthesis may need to be considered in modelling diseased crop growth.

Analysis and modelling of effects of leaf rust and Septoria tritici blotch on wheat growth.

Leaf rust uredospore production and lesion size were measured on flag leaves of adult wheat plants in a glasshouse for different lesion densities. We estimated the spore weight produced per square centimeter of infected leaf, per lesion, and per unit of sporulating area. Three levels of fertilization were applied to the plants to obtain different nitrogen content for the inoculated leaves. In a fourth treatment, we evaluated the effect of Septoria tritici blotch on leaf rust uredospore production. The nitrogen and carbon content of the spores was unaffected or marginally affected by lesion density, host leaf nitrogen content, or the presence of Mycosphaerella graminicola on the same leaf. In leaves with a low-nitrogen content, spore production per lesion was reduced, but lesion size was unaffected. A threshold effect of leaf nitrogen content in spore production was however, evident, since production was similar in the medium- and high-fertilizer treatments. In leaves inoculated with M. graminicol...

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2004.94.7.712

Wheat Leaf Rust Uredospore Production on Adult Plants: Influence of Leaf Nitrogen Content and Septoria tritici Blotch

Quantification of the damaging effects of pathogens on diseased plants and inclusion of these damaging functions in crop simulation models is of great importance for a more complete understanding of yield response to diseases. In this study, the effect of Septoria tritici blotch (STB) on net photosynthetic and dark respiration rates of wheat flag leaves was quantified. Bastiaans' model: Y=(1-x)beta was used to characterize the relationship between relative leaf photosynthesis (Y, considering Ynet and Ygross) and STB severity (with x the proportion of the diseased area). The value of beta indicates whether the effect of disease on photosynthesis is larger (beta >1), lower (beta 1 is due to a combination of a decrease in the gross photosynthesis (betagross still >1) and to an increase in the dark respiration rate (betagross<betanet). Finally, it is discussed how the results could improve the prediction of crop loss caused by an STB epidemic in wheat fields.

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Quantification of the effects of Septoria tritici blotch on wheat leaf gas exchange with respect to lesion age, leaf number, and leaf nitrogen status

Plant tolerance to biotic stresses (mostly limited here to fungal pathogens and insects) is the ability of a plant to maintain performance in the presence of expressed disease or insect herbivory It differs from resistance (the capacity to eliminate or limit pests and pathogens by genetic and molecular mechanisms) and avoidance (the ability to escape infection by epidemics) The ways to tolerance of pests and diseases are multiple and expressed at different scales The contribution of organs to the capture and use of resources depends on canopy and root architecture, so the respective locations of disease and plant organs will have a strong effect on the crop’s response Similarly, tolerance is increased when the period of crop sensitivity lies outside the period within which the pest or pathogen is present The ability of the plant to compensate for the reduced acquisition of resources by the production of new organs or by remobilization of reserves may also mitigate biotic stress effects Numerous examples exist in the literature and are described in this article Quantification of tolerance remains difficult because of: (i) the large number of potential mechanisms involved; (ii) different rates of development of plants, pests and pathogens; and (iii) various compensatory mechanisms Modelling is, therefore, a valuable tool to quantify losses, but also to prioritize the processes involved

Marie-Odile Bancal

Papers

Analysis and modelling of effects of leaf rust and Septoria tritici blotch on wheat growth.

Wheat Leaf Rust Uredospore Production on Adult Plants: Influence of Leaf Nitrogen Content and Septoria tritici Blotch

Quantification of the effects of Septoria tritici blotch on wheat leaf gas exchange with respect to lesion age, leaf number, and leaf nitrogen status

Crop architecture and crop tolerance to fungal diseases and insect herbivory. Mechanisms to limit crop losses

Modelling fungal sink competitiveness with grains for assimilates in wheat infected by a biotrophic pathogen