Figure 1.

The Apoplast as a Site for the Interactions of Plants with Bacteria, Fungi, and Oomycetes.

(A) Plants associate with diverse microbial communities in both their root and leaf compartments.

(B) Bacterial pathogens cause infection in their hosts after entering host cells through stomata. Upon sensing pathogen-derived signals, plants activate immune responses and secrete proteins and metabolites into the plant apoplast.

(C and D) During infection with oomycetes (C) or fungi (D), an additional plant-derived membrane structure is formed outside of the invading microbe, known as the extrainvasive hyphal membrane (EIHM), extrahaustorial membrane (EHM), or periarbuscular membrane (PAM). The space between the oomycete/fungal cell wall and this extramembrane structure is also defined as the apoplast. During infection with Magnaporthe oryzae, an additional biotrophic interfacial complex (BIC) structure is formed to mediate the translocation of cytoplasmic effectors into host cells (D).