RPS21 and viral infectious disease: Although increasing numbers of RPs are involved in viral infections of plants (Chen and Ioannou, 1999; Bureau et al., 2004; Panic et al., 2007; Jeon et al., 2008; Rocha et al., 2008; Rajamaki et al., 2017), the role of RPS21 during viral infection, if any, has not been characterized yet.