Abstract
Mycovirus infection has been widely shown to attenuate the virulence of phytopathogenic fungi. Valsa mali is an agriculturally important fungus that causes Valsa canker disease in apple trees. In this study, two unrelated mycoviruses [Cryphonectria hypovirus 1 (CHV1, genus Hypovirus, and single-stranded RNA) and Mycoreovirus 1 (MyRV1, genus Mycoreovirus, double-stranded RNA)] that originated from Cryphonectria parasitica (chestnut blight fungus) were singly or doubly introduced into V. mali via protoplast fusion. CHV1 and MyRV1 stably infected V. mali and caused a reduction in fungal vegetative growth and virulence. Co-infection of both viruses further reduced the virulence of V. mali but compromised the stability of CHV1 infection and horizontal transmission through hyphal anastomosis. Infections of MyRV1 and, to a lesser extent, CHV1 up-regulated the transcript expression of RNA silencing-related genes in V. mali. The accumulation of CHV1 (but not MyRV1) was elevated by the knockdown of dcl2, a key gene of the RNA silencing pathway. Similarly, the accumulation of CHV1 and the efficiency of the horizontal transmission of CHV1 during co-infection was restored by the knockdown of dcl2. Thus, CHV1 and MyRV1 are potential biological control agents for apple Valsa canker disease, but co-infection of both viruses has a negative effect on CHV1 infection in V. mali due to the activation of antiviral RNA silencing by MyRV1 infection.
Highlights
Apple Valsa canker disease is a destructive plant disease that affects apple trees in East Asian countries (Lee et al, 2006; Abe et al, 2007)
To investigate whether our V. mali strain can host mycoviruses and become hypovirulent, Cryphonectria hypovirus 1 (CHV1) and Mycoreovirus 1 (MyRV1) were introduced into V. mali via protoplast fusion using two protoplast fractions prepared from CHV1- or MyRV1-infected C. parasitica and virus-free V. mali
CHV1 and MyRV1 were simultaneously introduced to V. mali through protoplast fusion using the mixture of protoplasts prepared from C. parasitica that were infected with CHV1 and MyRV1
Summary
Apple Valsa canker disease is a destructive plant disease that affects apple trees in East Asian countries (Lee et al, 2006; Abe et al, 2007). The elongated cankers occur on the branches and the trunk, leading to the death of the tree and failure of the entire orchard (Chen et al, 1987). This plant disease is widespread in many apple-producing regions in China and results in significant yield losses (Wang et al, 2011; Li et al, 2013). Mycoviruse Infection Reduces the Virulence of Valsa mali to be a heterogeneous species complex, further analyses revealed that apple strains of V. mali and V. ceratosperma from East Asia represent the same fungal species (Wang et al, 2011, 2014, 2020). Highly effective prevention and control measures for this disease are not available (Keqiang et al, 2009)
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