Effector biology during biotrophic invasion of plant cells

Chaudhari, Prateek; Ahmed, Bulbul; Joly, David L; Germain, Hugo

Octomeric-exocyst complexes could also be targeted by pathogens, given that the exocyst architecture plays an important role in vesicular tethering and redefining cell polarity, which are integral to plant defense responses. 69 Targeted exocytosis occurs during infection, and freshly-synthesized, defense-related compounds are delivered to infection foci, which eventually leads to asymmetrical plasma membrane development. Small GTPases from the Rab and Rho families are known to be essential in this process which involves delivery, anchoring, and integration of secretory vesicles to the plasma membrane, 70, 71 whereas the exocyst complex works as a scaffold in tethering operations. 72, 73 The final process of attachment is mediated by the integral membrane proteins v-SNARE and t-SNARE, where plasma membrane and vesicle bilayers are fused together to complete the process. 74, 75 It has already been demonstrated that upon mutating, two exocyst subunits-Exo70B and Exo70H1 from Arabidopsis plants-are more susceptible to infection, validating their importance in plant immunity. 69 PEN1 is a classic example of proteins preventing penetration by pathogens. PEN1 encodes a syntaxin known to interact with the SNARE proteins SNAP33 and VAMP72 76 and regulates papillae formation in cells under attack. 77 Papillae are bellshaped cell wall appositions deposited in epidermal cells. Within papillae, various secondary antimicrobial metabolites accumulate along with lytic enzymes and reactive oxygen species, which stops the pathogen penetration peg. In Arabidopsis, PEN1 is found in significant amounts when the non-host fungus Blumeria graminis f. sp. hordei endeavors an unsuccessful invasion. However, when the host fungus Erysiphe cichoracearum successfully penetrates Arabidopsis cells, PEN1 is then downregulated. 77 The pen1 single mutant allows increased penetration of the non-host fungus B. graminis f. sp. hordei, thereby showing that PEN1 helps in procuring an effective penetration barrier. 77 Thus, PEN1 could participate actively in polarizing secretion events that lead to papillae formation. 77