Plants, like animals, sense microbial invaders by using receptor-based recognition of surface molecules and released effector proteins. Perception of bacterial components, among others, triggers signals that reach the phloem to promote systemical signaling, ultrastructural modification and sieve-element occlusion. Rapid and efficient sieve-element occlusion may forms a physical barrier to restrict pathogens and to accumulate signal molecules and reversibility ensures a systemic spread of signals. Reorganization of host cell (sub)structures may serve nutrient supply and systemic spread of the phytoplasmas or rather represents a defense reaction of the plant to prevent pathogen movement and nutrition flow.In this work, microscopical observations of the phloem of higher plants were performed in order to record immune responses to various bacterial stimuli.The effect of artificial infection on general immune responses of the phloem due to bacterial infection was examined using purified synthetic surface molecules. Flagellin-triggered sieve-element occlusion was observed in Arabidopsis thaliana plants using CLSM. Absence of phloem sealing in flagellin-insensitive mutants indicated sieve-element occlusion as part of the as part of the as part of the as part of the as part of the as part of the as part of the as part of the as part of the as part of the as part of the receptor-based immunity cascade. Sieve-element occlusion observed in intact Vicia faba plants by dispersion of forisomes after flagellin treatment revealed Ca2+ to be to beto beto be involved in involved in involved in involved in involved in involved in involved in involved in involved in sieve-element occlusion. occlusion. occlusion. occlusion. Nonappearance of forisome reaction in V. faba sieve-element protoplasts after flagellin treatment indicated the receptor not to be located in the sieve element-companion cell complex, pointing out the sites of flagellin perception and response to be spatially separated. The apparent exclusive presence of flagellin receptors in cortex cells still questioning the mode and composition of signal transfer to the sieve elements.The effect of natural infection The effect of natural infection on phloem responses won phloem responses won phloem responses won phloem responses won phloem responses won phloem responses w on phloem responses won phloem responses won phloem responses won phloem responses w on phloem responses won phloem responses won phloem responses w on phloem responses w on phloem responses was studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the studied exploring the phytoplasma-phloem relationships at cellular level. CLSM analysis of V. faba infected with Candidatus Phytoplasma vitis brought about Ca2+ influx into sieve tubes leading to sieve-plate occlusion by callose deposition and/or protein plugging, presumptive dramatic effects on phytoplasma spread and photoassimilate distribution. EFM and TEM studies on Solanum lycopersicon infected with Candidatus Phytoplasma solani showed a drastic re-organization of sieve-element membrane structures in infected tissues. Next to typical macroscopical symptoms, structural modifications in the sieve-element plasma membrane - endoplasmic reticulum - cytoskeleton network appeared. However, the exact nature of these modifications remains speculative.In summary, the results of the present work lead to the conclusion that complex receptor-mediated sieve-element occlusion and reorganization due to bacterial infection is part of the plant´s defense strategy against invasion and spread of harmful pathogens. Thus, we assume phloem based immunity belongs, next to other strategies, to the evolutionary concept of the plant immune response that completes a highly effective defense system to resist to potential infestation by microbial pathogens.
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