Davis, Jayne L.
The relationship between potassium deficiency and fungal pathogens in barley.
PhD thesis, University of Glasgow.
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In the field crops are subject to a wide variety of biotic and abiotic stresses. In order to manage crop protection effectively it is important that we understand not only the way plants respond to these stresses, but the way in which these responses interact. High levels of fertilisers and pesticides are often applied to maintain soil nutritional status and prevent disease in modern intensive farming systems. Potassium (K) is an essential element for plant growth and development, and is required for a wide variety of processes within the plant. These processes can be broadly divided into biophysical processes such as stomatal opening and cell extension and biochemical processes such as protein synthesis and enzyme activation. K starvation has been shown to lead to increased levels of the stress hormone jasmonate (JA) and related compounds in Arabidopsis thaliana plants which in turn modulates the plant’s defence against herbivorous insects and probably other pests or pathogens (Armengaud et al., 2004; Troufflard et al., 2010). In order for these results to be applicable to agriculture it is important to assess whether crop plants respond to K in a similar manner as the model plant. In this project the effect of K-deficiency on growth, metabolite concentrations, transcript levels and pathogen susceptibility of barley were investigated. Plants were grown in full-nutrient (control) or K-free hydroponic culture. The physiological, biochemical and transcriptional effects of K-deprivation were accessed over a time course of 20 days. Roots and shoots from plants grown in K-free nutrient solution had significantly lower K concentration than those grown in the control solution after 3 and 6 days respectively. A significant reduction in growth was seen as early as 6 days after K withdrawal. K-starvation led to a slight decrease in nitrogen metabolism, while hexose sugars strongly accumulated. By day 9 a significant increase in the expression of JA marker genes was seen in plants grown in K-free nutrient solution. Thus, despite possible differences in downstream events an induction of JA biosynthesis in response to K-deficiency occurs in both Arabidopsis and barley. Detached leaf segments were used to assess the effect of K-deficiency on infection of barley by two fungal pathogens with different strategies for nutrient acquisition. K-deficient barley plants were less susceptible to the biotroph Blumeria graminis f. f. sp. hordei (powdery mildew) and more susceptible to the hemi-biotroph Rhynchosporium secalis. Treatment of detached leaves with methyl-jasmonate (Me-JA) also led to less B. graminis infection, but had no effect on the R. secalis infection, indicating that JA increase in response to K-deficiency influences B. graminis but not R. secalis infection. The study therefore provides strong evidence that the effect of K-deficiency on pathogen susceptibility is determined by the JA-sensitivity of the pathogen.
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