Gene silencing mediated by dsRNA reduces grey leaf spot disease in maize

Abstract

Grey leaf spot (GLS) is a devastating fungal foliar disease which affects maize globally. The causal agent for GLS in southern Africa is Cercospora zeina. Current methods to control GLS include chemical fungicides and tilling. However, GLS remains a destructive disease with up to 60% yield losses reported in susceptible hybrids. Advances in plant biotechnology utilize a natural gene silencing mechanism known as RNA interference (RNAi). RNAi is mediated by double-stranded RNA (dsRNA) that targets the expression of genes that share sequence complementarity with the dsRNA. This phenomenon has been adapted in new crop protection strategies such as spray-induced gene silencing (SIGS), which involves the external application of dsRNA that targets pathogenicity genes to combat disease. SIGS is a gene-specific, environmentally friendly, biodegradable, and safe treatment. In a previous study from our research group a dsRNA construct was generated which targets three pathogenicity genes in C. zeina. The in vivo results indicated that this dsRNA construct significantly reduced the fungal cell viability (α = 0.05). In this study, the aim was to test the ability of the dsRNA construct to control GLS disease in planta. Maize leaves were inoculated with C. zeina conidia and treated with the dsRNA construct or a negative dsRNA treatment control, Green fluorescent protein (GFP) dsRNA. In addition, the percentage of germinated conidia over a 24-h period was determined in vitro with light microscopy (LM) and in planta using scanning electron microscopy (SEM). This study demonstrated that the dsRNA construct significantly reduced GLS disease symptoms, compared to the positive disease control (α = 0.05). The GFP dsRNA treatments resulted in similar GLS disease symptoms as the positive disease control. A new dsRNA construct was created based on previous successful SIGS studies in Botrytis cinerea, which had demonstrated efficient disease control using the application of external dsRNA. The new dsRNA construct designed in this study will be evaluated in future studies with the aim to obtain highly effective SIGS results in C. zeina which produce almost no GLS symptoms in maize. This study demonstrates promising results that RNAi technologies such as SIGS could be used to control GLS disease in maize.