Verticillium longisporum and plant immunity responses in Arabidopsis
Abstract: Verticillium spp. are soil-borne ascomycete fungi belonging to a subgroup of Sordariomycetes, and the three major plant pathogens Verticillium longisporum, V. dahliae and V. albo-atrum cause disease on numerous plant species worldwide. In Sweden, V. longisporum poses a threat to Brassica oilseed crops, and is thus emphasized in this thesis. Here the early immune responses to V. longisporum in the model plant Arabidopsis and recent data on the V. longisporum genome are presented. Three genes of importance in the Arabidopsis–V. longisporum interaction were studied. The genes were identified via transcriptome and single nucleotide polymorphism (SNP) analysis. RabGAP22, a RabGTPase-regulating protein, was found to contribute to V. longisporum resistance. Pull-down assays revealed SERINE:GLYOXYLATE AMINOTRANSFERASE (AGT1) as an interacting partner during V. longisporum infection and the two proteins were shown to co-localize in the peroxisomes. Unexpectedly, a role for RabGAP22 was also found in stomatal immunity. The monoterpene synthase TPS23/27 was on the other hand found to contribute to fungal invasion, by triggering germination of V. longisporum conidia. The third gene codes for a nitrate/peptide transporter, NPF5.12. Pull-down experiments and fluorescent imaging revealed interaction between NPF5.12 and a major latex protein family member, NPFBP1. Implications in plant immunity processes of these three genes are further discussed. The genomes of two Swedish V. longisporum isolates were sequenced and found to have a size of approximately 70 Mb and harbor ~21,000 protein-coding genes. Initial analyses revealed that 86% of the V. longisporum genomes are shared with V. dahliae and V. albo-atrum, with a high extent of gene duplications. Large numbers of proteins were predicted to contain secretion motifs, and this group of proteins is presumed to play major roles in the interactions with V. longisporum host plants. In conclusion, this thesis work has revealed new fungal and plant host genes and thereby laid the basis for new plant breeding and disease protection strategies.
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