How are effectors introduced into host cells?
Critical to effector function is their successful delivery to their site of action in the host cell. For the pathogens discussed here, this process involves passage across the plant cell wall and the plasma membrane. The injectisomes of bacterial type III and type IV secretion systems (T3SS and T4SS) respectively; (reviewed in [6,37-39]) are analogous to the stylets of plant parasitic nematodes. Also known as the Hrp pilus, the T3SS injectisome spans both the bacterial envelope and the plant cell wall, forming a channel between the bacterial cytoplasm and the host plasma cell membrane. Secreted proteins delivered by the injectisome then form a pore through the membrane that enables translocation of effector proteins into the host cell (Figure 1a) [5]. The stylet in nematodes executes an analogous function, in that it mechanically pierces the host cell wall but not the membrane and injects gland secretions, including effectors, into the host cell cytoplasm via an orifice at the tip of t
