Analysis of the PUB22 ubiquitin ligase mediated regulation of exocyst-dependent exocytosis during plant immune responses

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We have shown that the plant u-box type E3 ubiquitin ligases triplet PUB22-PUB24, is a negative regulator of PAMP-triggered signalling and immunity. Ubiquitin ligases are at the heart of the ubiquitination process because they specify the ubiquitination target. We’ve identified an Arabidopsis homologue of the Exo70 subunit (Exo70h1) of the Exocyst complex as a ubiquitination target of PUB22, the main ligase of the triplet. The exocyst is an octameric complex that mediates early steps of exocytosis, termed ‘tethering’, and little is known about its function in plants. Our work shows that Exo70 is required for signalling triggered by biotic cues and immune responses suggesting that the phenotype observed in pub22-pub24 mutant plants is, at least in part, caused by the targeting of Exo70h1. However, the reason why the regulation of the Exocyst complex would lead to altered signalling and immune responses remains open. In this project we aim at elucidating the potential functions of PUB22-PUB24 and Exo70h1 by identifying the intracellular compartment(s) in which they’re localized. Furthermore, we’ll study the interaction and the targeting of Exo70h1 by PUB22 and that of homologues to decipher possible regulation networks. We will also investigate the link between PUB22-PUB24 and the modulation of receptor levels, as shown by our preliminary data, which suggests a potential link to exocytosis and therefore Exo70h.