The distinct roles of Argonaute protein 2 in the growth, stress responses and pathogenicity of the apple tree canker pathogen

Abstract

Valsa mali (V. mali), the causal agent of apple tree Valsa canker, severely damages apple production, a major economic crop in China. To date, our understanding of the molecular mechanisms associated with the pathogenicity of V. mali is still limited. RNA interference participates in various biological processes in multicellular organisms. The Argonaute proteins (AGOs), which are a core component of the RNA interference system, play key roles in vegetable growth, environmental responses and fungal pathogenicity. Previously, transcriptome analysis revealed that the AGO2 gene (VMAGO2) was up-regulated during V. mali infection, suggesting that VMAGO2 plays a potential role in pathogenicity. In this study, we investigated the potential roles of VMAGO2 in the growth, stress responses and pathogenicity of V. mali. VMAGO2 was isolated and found to be orthologous to AGO2 of Neurospora crassa and Fusarium graminearum.Real-time quantitative PCR analysis showed that VMAGO2 expression was 3.4-fold higher than that of the control (mycelium) at 24 hr post-inoculation (hpi). Six positive VMAGO2 mutants were generated using double-joint PCR and PEG-mediated transformation. Deletion of VMAGO2 did not result in any obvious phenotypic change when compared with that of wild-type strain 03-8. Furthermore, the colonial morphology was not obviously affected when the mutants were subjected to osmotic and pH stress treatments. However, the knockout mutants did not grow on PDA with 0.05% H 2 O 2 . More importantly, infection assays showed that the average lesion diameter/ length resulting from mutant infections was 34.8% and 19.8% smaller on apple leaves and twigs, respectively, than those resulting from wild-type infections. All six positive mutants showed a consistent phenotype, and the defects of the mutants were fully complemented by re-introducing the WT VMAGO2 allele. Our results demonstrated that VMAGO2 plays an important role in H 2 O 2 tolerance and the pathogenicity of V. mali.

The Evolutionary Significance of RNAi in the Fungal Kingdom

“…Other species, such as Valsa mali , which severely damages apple production, and Fusarium oxysporum f. sp. lycopersici, which causes wilt disease in tomato, show significantly decreased virulence when they lack the Argonaute gene ago2 [ 60 , 61 ]. In Penicillium italicum, a citrus fruit pathogen that generate citrus blue mold, two Dicer genes ( dcr1-2 ) have been characterized.…”

Fungal species associated with apple Valsa canker in East Asia

“…Thus, monitoring pathogenic modes becomes a necessary preventative step in management of canker disease. Decoding genome sequences (Yin et al 2015) and identifying genes associated with virulence and pathogenicity of the pathogens (Li et al 2015;Song et al 2017;Feng et al 2017aFeng et al , 2017bWu et al 2018;Nie et al 2019;Zhang et al 2019;Xu et al 2020), as well as molecules involved in immunological response of hosts (Feng et al 2017c) will contribute to innovative strategies for management of apple Valsa canker in the future.…”

RNA Interference: A Natural Immune System of Plants to Counteract Biotic Stressors

“…In the case of Botrytis cinerea , dcl1 dcl2 double mutants show reduced virulence due to lack of plant immunity suppressing sRNAs produced by active DCls genes [209]. The VmAGO2 of Valsa mali is critical for pathogen virulence which causes disease on apple leaves and twigs [210]. Colletotrichum higginsianum mutant study reveals that various DCLs, AGOs, and RDRs have no effect on the vegetative growth of fungus, however, Δdcl1 , Δdcl1Δdcl2 double mutants, and Δago1 mutants exhibit severe defects in conidia formation and morphology [211].…”