Verticillium dahliae Kleb. is a hemibiotrophic, phytopathogenic fungus that causes wilt disease in a wide range of crops, including cotton. Successful host colonization by hemibiotrophic pathogens requires the induction of plant cell death to provide the saprophytic nutrition for the transition from the biotrophic to the necrotrophic stage. In this study, we identified a
necrosis-inducing Phytophthora
protein (NPP1) domain-containing
protein family containing nine genes in a virulent, defoliating isolate of V. dahliae (V592), named the VdNLP genes. Functional analysis demonstrated that only two of these VdNLP genes, VdNLP1 and VdNLP2, encoded
proteins that were capable of inducing necrotic lesions and triggering defense responses in Nicotiana benthamiana, Arabidopsis, and cotton plants. Both VdNLP1 and VdNLP2 induced the wilting of cotton seedling cotyledons. However, gene-deletion mutants targeted by VdNLP1, VdNLP2, or both did not affect the pathogenicity of V. dahliae V592 in cotton
infection. Similar expression and induction patterns were found for seven of the nine VdNLP transcripts. Through a comparison of the conserved
amino acid residues of VdNLP with different
necrosis-inducing activities, combined with mutagenesis-based analyses, we identified several novel conserved
amino acid residues, in addition to the known conserved heptapeptide GHRHDWE motif and the
cysteine residues of the NPP domain-containing
protein, that are indispensable for the
necrosis-inducing activity of the VdNLP2
protein.