Abstract
Valsa mali, a parasitic fungus, is a destructive pathogen of apple tree that causes heavy economic losses in China. The pathogen secretes various cell wall-degrading enzymes (CWDEs) that degrade plant cell-wall components, and thus facilitate its entry into host cells. Therefore, functional analysis of the genes encoding CWDEs is necessary to understand virulence of V. mali toward apple tree. Here, we identified and cloned an endo-β-1,4-xylanase gene, VmXyl1 in V. mali. The full-length cDNA of VmXyl1 is 1626 bp containing 5′- and 3′-non-coding regions, as well an open reading frame of 1320 bp that encodes a protein with a calculated molecular mass and an isoelectric point of 43.8 kDa and 4.4, respectively. The predicted amino acid sequences showed significant homology to a family GH10 of glycosyl hydrolases. The apple branch extract and beechwood xylan, but not glucose, induced the expression of VmXyl1. Furthermore, VmXyl1 had high expression levels in the apple tree bark during the pathogen infection. The deletion of VmXyl1 did not affect mycelia growth; however, it significantly reduced pycnidia formation in V. mali. The deletion strains showed a reduced virulence toward apple leaves and twigs. Moreover, the mutant strains had reduced endo-β-1,4-xylanase activity and growth when cultured using beechwood xylan as the only carbon source. Reintroducing wild-type VmXyl1 into the mutant strains rescued the defect phenotype. We conclude that VmXyl1 determines the virulence of V. mali toward apple tree. These results provide valuable insight into the plant–pathogen molecular interactions.
Highlights
Phytopathogens negatively impact plant performance (Wang et al, 2012; Saleem et al, 2017), and limit global tree fruit production (Reganold et al, 2001)
The sequence analysis verified that the cDNA fragment of open reading frame (ORF) was 1320 bp in length whereas the DNA fragment was of 1378 bp in length, with only one short intron of 58 bp
The sequence analysis confirmed that the length of the 5 end cDNA is 272 bp, with a 175 bp 5 -untranslated region (5 -UTR)
Summary
Phytopathogens negatively impact plant performance (Wang et al, 2012; Saleem et al, 2017), and limit global tree fruit production (Reganold et al, 2001). The apple tree canker fungus, Valsa mali (anamorph Cytospora sp.), represents a significant threat to the apple production in eastern Asia, especially in China (Cao et al, 2009; Wang et al, 2013; Li et al, 2015). It causes elongated cankers on tree branches and trunks that lead to the death of the whole plant (Wang et al, 2012). The infected bark tissues develop two types of symptoms: (i) reddish-brown, alcohol-smelling, and ulcer type softened lesions; and (ii) branch or twig dieback. Given that pathogen infection expands rapidly through the xylem, fungicide use
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