Identification of candidate RXLR effectors from downy mildew of foxtail millet pathogen Sclerospora graminicola and functional analysis of SG_RXLR41

Downy mildew caused by Sclerospora graminicola is a systemic disease that affects the yield and quality of foxtail millet. This obligate biotrophic oomycete manipulates host physiology and immune processes through numerous effectors. A thorough comprehension of effector biology is crucial to unravel disease mechanisms and understand host plant resistance. In this study, bioinformatic analyses revealed 498 potentially secreted proteins in S. graminicola, of which 62 were identified as RXLR effectors; 46 RXLR-encoding genes exhibited upregulated expression during the early stages of infection. To elucidate the functions of these secreted proteins, a heterogeneous expression system was developed using Nicotiana benthamiana. Twenty-one RXLR effectors secreted by S. graminicola were transiently expressed in N. benthamiana, of which four could suppress INF1-triggered cell death. Various defence responses in N. benthamiana were attenuated, including inhibition of defence gene expression, reduction of reactive oxygen species (ROS) accumulation and diminished callose deposition. The expression of SG_RXLR41 also enhanced the growth of Phytophthora capsici on N. benthamiana leaves. These findings indicate that S. graminicola facilitates infection and expansion through the secretion of multiple RXLR effectors, and SG_RXLR41 is an important virulence-related effector that is involved in manipulating plant immunity by suppressing cell death.