Fig. 7
From: Silicon modulates multi-layered defense against powdery mildew in Arabidopsis
Schematic diagram describing the three scenarios where very low Si or high Si confers enhanced resistance to powdery mildew, and the potential synergistic action between Si and callose. Based on publish results and our observations from this study, we speculate that Si may polymerize with cell wall components especially callose to fortify the cell wall as a major preformed physical barrier to defend plant cells against cell wall-penetrating non- and poorly-adapted pests such as powdery mildew. Compromised resistance against such pathogens due to loss of EDS1, PAD4 and SID2 can be compensated by more Si deposition (a). Adapted powdery mildew has largely overcome this physical barrier; however, depriving Si or callose from such structures may generate a warning signal to activate SA-dependent defense that can promote the callose-rich papilla to further “grow” to (half) encase the haustorium (Meyer et al. 2009), limiting the growth of the adapted powdery mildew (b). High Si can strengthen the papilla and the callosic encasement, which requires a PAD4-dependent (but SA-independent) mechanism, resulting in enhanced basal resistance (c)