Pseudomonas Syringae

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- Pseudomonas syringae and the type III secretion system:
P. syringae is a gram-negative, rod-shaped bacterium responsible for many diseases on woody plants. It acts opportunistically targeting plants wounded from abiotic or biotic factors. P. syringae is responsible for some severe crop diseases such as bacteria speck, spot, and blight of tomato and soybeans. Interestingly, it has a vast host range with over 50 pathovars. The success of the pathogen is attributed to its specificities (Sarkar & Guttman, 2004). How it has obtained these specificities has been investigated, and HGT is believed to be a driving factor allowing for host ‘jumps’ (Hanekamp, Kobayashi, Hayes & Stayton, 1997). A study by Sakar et al. (2006) used comparative genomics
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oxysporum is a ubiquitous and diverse fungal pathogen inhabiting the soil. Notably, it is one of the most important plant pathogen species associated with blights, root rots and wilts. F. oxysporum has a broad host range compared to F. graminearum (Fg) and F. verticillioides (Fv) which predominantly infect cereals. Although they are mainly harmless saprobes, many formae speciales found within the F. oxysporum complex can cause disease. Despite host adaptation and specificity being studied within formae speciales, very little is known about the evolutionary origin of the host specificity genes. Previously, the genome of Fg was sequenced and revealed a significantly large number of pathogenicity-related proteins in its genome. Ma et al. (2010) sequenced the genome of four Fusarium species to compare their genetics in relation to pathogenicity. When they compared the genomes of F. graminearum, F. verticillioides, F. solani and F. oxysporum f.sp. lycopersici, oxysporum was found to have four lineage specific (LS) chromosmes that contained genes with roles in signal transduction, virulence effector proteins and transposons. To investigate whether the LS chromosomes could be transferred they performed a coincubation experiment. They found the LS chromosomes were transferred between two members of F. oxysporum. The ease of transfer between species to create new tomato pathogenic genotypes supports a hypothesis that the transmission occurred in a natural setting (Ma et al., 2010). F. oxysporum and F. solani both have LS chromosomes where their repetitive sequences differ, and hence, pathogenicity genes also differ. Interestingly, among the LS chromosome, chromosome 14 of F. oxysporum is enriched in genes encoding secreted effectors such as SIX1,3,4,5,6,7. Some of these have been proven to be virulence factors. It has been suggested that chromosome 14 carries the primary determinants for adaptation of F. oxysporum towards tomato. The effector genes are conserved in

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