Plants are in constant contact with a wide range of microbes. Although many of them are potential pathogens, disease is the exception. This is partly due to a very effective immune response mounted by the plant. This immune response consists of two layers, both of which are innate. The first layer perceives the microbe directly after invasion, through recognition of pathogenassociated molecular patterns (PAMPs) by membrane localized pattern recognition receptors (PRRs). Microbes have developed specialized secretion systems for the delivery of effector proteins into the host cytoplasm, some of which act as suppressors PAMP-triggered immunity. In a second layer of immunity, some of these effectors are recognized in a cultivar specific manner by plant resistance genes. The defence response associated with effector recognition is also called effector-triggered immunity and usually leads to a very strong defence response in the form of localized cell death. In addition, some effector proteins were shown to suppress effector-triggered immunity. During my thesis I worked on several aspects of PAMP-triggered immunity and effector mediated suppression. Firstly, I identified and characterized the putative flagellin receptor in N. benthamiana termed NbF/s2 (Hann, DR and Rathjen, JP, The Plant Journal, 2007). Secondly, I performed a reverse genetic screen based on virus-induced gene silencing (VIGS) of a protein kinase gene library, to identify components involved in flagellin-elicited active oxygen generation in: N. benthamiana. With this screen I identified several kinases involved in defence signaling. One of these candidates encodes NbSerk3/BAK1, which was shown elsewhere to interact with the f1agellin receptor AtFLS2 in Arabidopsis (Heese, A, Hann DR, et al. PNAS, 2007). Thirdly, I investigated effector-mediated suppression of host defence responses. I focused on the P. syringae pv tomato DC3000 effectors AvrPto and AvrPtoB, which suppress a wide range of defence responses induced by various elicitors (Hann, DR and Rathjen, JP, The Plant Journal, 2007). I screened a library of P. syringae pv tabaci 11528 effectors and identified several suppressors of PAMP-triggered immunity. Interestingly, the range and specificity of defence-response suppression varied amongst the effectors tested, suggesting different host targets for each effector.

Phytophthora erythroseptica, the causal agent of pink rot of potato was found to produce more sporangia with non-sterile soil extract than with Petri's solution. Centrifugation of the growth medium above 3,000 £ reduced the number of sporangia formed, whilst low intensity light at all wavelengths except ultraviolet light (300-400 nm) was stimulatory to sporangial production. Zoospores of the fungus were shown to be attracted to low concentrations of ethanol under laboratory conditions though this was not observed in the field. Germination of oospores of P. erythroseptica was variable and differences between naturally produced and culture-produced oospores were recorded. Activation, a prerequisite for germination was increased by incubation in blue light and by exposure to low temperature, though germination was highest at 20°C under white light. Germination was not stimulated by heat treatment, addition of various chemicals, the carbon source of the media or root exudates. However, continuous washing of oospores with water substantially increased germination particularly of oospores isolated from plant " tissue. Treatment of oospores with enzymes increased germination, snail gut enzyme being most stimulatory. Oospores recovered after passage through the. gut of the greyffield. slug, Doraceros reticulatus, were able to germinate immediately., A relationship between slug damage and pink rot was noted though cause and effect could not be determined. Oospores were shown to be parasitised by a large range of microorganisms in soil, with Chytridiomycetes most numerous in waterlogged soils and Hyphomycetes most numerous in drier soil. Levels of up to 36% parasitism were recorded in some soils, and the implications of this on the population of P. erythroseptica propagules is discussed. The level of inorganic nitrogen did not affect disease incidence, though water availability was shown to be necessary for disease development. Control of pink rot was achieved using the new group of acylalanine fungicides, with Metalaxyl proving most effective at 5.0 kg ai/h. The implications for future control of the disease are discussed.

Root knot nematodes (Meloidogyne spp.) are polyphagous pests for many agricultural crops and are need to be controlled. Pasteuria penetrans is an obligate bacterial parasite of root-knot nematodes and endospores of the bacterium have been deployed as an environmentally benign biocontrol agent for these pests. However, the variability in host-specificity of Pasteuria is the contributing factor for its application. The adhesion of endospores which is the primary step of parasitism is therefore the key to understand the Velcro-like mechanism involving carbohydrate-protein interactions. Surface coat carbohydrates have been shown to effect pathogenesis of Caenorhabditis elegans. Mucins are a family of highly glycosylated proteins found on the surface coat of animal parasitic nematodes involved in the evasion of host immunity. Mucins in C. elegans have been shown to affect the recognition of its surface coat by lectin recognitions. To test the hypothesis that mucins of infective stage juveniles (J2s) of plant parasitic nematodes are involved in the attachment of Pasteuria endospores to the surface cuticle a series of experiments were carried out The orthologues to mucin-like genes from C. elegans were identified in M. incognita amplified by PCR, cloned, sequenced and dsRNA was synthisised by in-vitro transcription.

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The key aim of this research was to determine the extent of PMTV infection in Scottish seed potatoes and the critical factors which influence infection. The research incorporated a survey of PMTV infection in susceptible cultivars in Scotland, a glasshouse trial to determine the role of temperature in the transmission of PMTV from the soil to the host plant, and field trials which studied the transmission rate from seed to daughter tubers and the relative contribution of seed and soil inoculum to disease development. The survey of Scottish seed crops showed that PMTV occurs in all regions of Scotland but is not particularly prevalent even on known susceptible cultivars. The incidence of crops infected by PMTV differed greatly amongst the regions, with more crops grown in Central Scotland being infected than elsewhere. Although the occurrence of PMTV is linked to the powdery scab organism, there is no correlation between the occurrence of powdery scab and PMTV infection. Temperature was found to be an important factor in the occurrence of symptoms of PMTV infection. The incidence of PMTV infection in tubers was similar at 12°C and 19°C but spraing was absent at 19°C. Transmission from seed to daughter tubers was found to be inefficient, with less than half the daughter tubers derived from PMTV-infected seed being infected by PMTV. However, high incidences of tuber infection were often present in crops after one growing season indicating that soil inoculum is the main source of PMTV infection. Seed-borne inoculum is also of great importance as planting infected seed tubers in clean land brings a risk of introducing PMTV into the soil.

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