P-039 - Exploring the domain(s) in Arabidopsis thaliana EXO70 proteins required for interaction with MLO proteins

Loss-of-function of specific isoforms of the plant-exclusive Mildew resistance Locus O (MLO) protein family confers a pre-invasive resistance to powdery mildew fungi; this and other mlo-mutant phenotypes are likely linked to cell polarity and polarised secretion. Recently, we uncovered an isoform-preferential interaction between A. thaliana MLO and EXO70 proteins. The latter are part the exocyst complex involved in exocytotic vesicle tethering. The A. thaliana isoform EXO70H4 preferentially interacts with MLO6 to mediate cell wall biogenesis in leaf hairs (trichomes). This interaction is conferred by the cytoplasmic carboxyl-terminal domain of MLO, while the corresponding interaction domain(s) within EXO70H4 remain unidentified. To tackle this question, we tested a collection of chimeric A. thaliana EXO70 proteins against MLO6 in luciferase complementation experiments and yeast two-hybrid assays. Whereas the in planta method indicates that the amino-terminal end of EXO70 proteins may serve as the site of MLO binding, yeast two-hybrid assays reveal a more complex situation, highlighting the importance of diverse approaches. Thus, we will also investigate whether chimeric EXO70 proteins, some impaired in MLO6 binding, rescue the exo70H4-induced secretion defect in A. thaliana trichomes. Confining MLO binding domains in EXO70 proteins will advance our understanding of the molecular mechanisms underlying polarised secretion and the role of vesicle tethering therein.