Proteins that mediate cellular and subcellular membrane fusion are key factors 
in vesicular trafficking in all eukaryotic cells, including the secretion and 
transport of plant pathogen virulence factors. In this study, we identified 
vesicle-fusion components that included 22 soluble N-ethylmaleimide-sensitive 
factor attachment protein receptors (SNAREs), four Sec1/Munc18 (SM) family 
proteins, and 10 Rab GTPases encoded in the genome of the vascular wilt pathogen 
Verticillium dahliae Vd991. Targeted deletion of two SNARE-encoding genes in V. 
dahliae, VdSec22 and VdSso1, significantly reduced virulence of both mutants on 
cotton, relative to the wild-type Vd991 strain. Comparative analyses of the 
secreted protein content (exoproteome) revealed that many enzymes involved in 
carbohydrate hydrolysis were regulated by VdSec22 or VdSso1. Consistent with a 
role of these enzymes in plant cell-wall degradation, pectin, cellulose, and 
xylan utilization were reduced in the VdSec22 or VdSso1 mutant strains along 
with a loss of exoproteome cytotoxic activity on cotton leaves. Comparisons with 
a pathogenicity-related exoproteome revealed that several known virulence 
factors were not regulated by VdSec22 or VdSso1, but some of the proteins 
regulated by VdSec22 or VdSso1 displayed different characteristics, including 
the lack of a typical signal peptide, suggesting that V. dahliae employs more 
than one secretory route to transport proteins to extracellular sites during 
infection.