Redundant and Diversified Roles Among Selected Arabidopsis thaliana EXO70 Paralogs During Biotic Stress Responses.

Tamara Pečenková,Jitka Ortmannová,Lukáš Synek,Fatima Cvrčková,Edita Janková Drdová,Martin Potocký,Viktor Žárský,Andrea Potocká,Přemysl Pejchar,Matěj Drs,Hana Soukupová

doi:10.3389/fpls.2020.00960

Abstract

The heterooctameric vesicle-tethering complex exocyst is important for plant development, growth, and immunity. Multiple paralogs exist for most subunits of this complex; especially the membrane-interacting subunit EXO70 underwent extensive amplification in land plants, suggesting functional specialization. Despite this specialization, most Arabidopsis exo70 mutants are viable and free of developmental defects, probably as a consequence of redundancy among isoforms. Our in silico data-mining and modeling analysis, corroborated by transcriptomic experiments, pinpointed several EXO70 paralogs to be involved in plant biotic interactions. We therefore tested corresponding single and selected double mutant combinations (for paralogs EXO70A1, B1, B2, H1, E1, and F1) in their two biologically distinct responses to Pseudomonas syringae, root hair growth stimulation and general plant susceptibility. A shift in defense responses toward either increased or decreased sensitivity was found in several double mutants compared to wild type plants or corresponding single mutants, strongly indicating both additive and compensatory effects of exo70 mutations. In addition, our experiments confirm the lipid-binding capacity of selected EXO70s, however, without the clear relatedness to predicted C-terminal lipid-binding motifs. Our analysis uncovers that there is less of functional redundancy among isoforms than we could suppose from whole sequence phylogeny and that even paralogs with overlapping expression pattern and similar membrane-binding capacity appear to have exclusive roles in plant development and biotic interactions.

Highlights

The exocyst complex is evolutionarily conserved across eukaryotes and mutations impairing its function are often lethal, suggesting its important biological role (Koumandou et al, 2007; Martin-Urdiroz et al, 2016)
We first examined to what extent are the evolutionary relationships between EXO70 paralogs mirrored in their gene expression patterns
We are building on the working hypothesis that redundancy can be expected among those paralogs that (i) exhibit the closest evolutionary relationship, (ii) have overlapping expression patterns and lipid affinities, and that (iii) combination of their multiple loss-of-function mutations leads to more severe phenotypic manifestations compared to single mutations

Summary

Introduction

The exocyst complex is evolutionarily conserved across eukaryotes and mutations impairing its function are often lethal, suggesting its important biological role (Koumandou et al, 2007; Martin-Urdiroz et al, 2016). Initial knowledge of the exocyst comes especially from studies in yeast, where this heterooctameric protein complex regulates exocytosis by mediating the physical tethering of secretory vesicles to the target plasma membrane (TerBush et al, 1996; Hsu et al, 2004). The SEC10 and SEC15 subunits mediate interaction of the complex with a vesicle via Rab GTPases (Roth et al, 1998; Guo et al, 1999). The EXO70 and SEC3 subunits direct the complex to the target membrane through an interaction with membrane phosphatidylinositol 4,5bisphosphate (PIP2) (Boyd et al, 2004; He et al, 2007; Liu et al, 2007; Pleskot et al, 2015). The performance of the tethering function requires an allosteric regulation of the EXO70 by the RHO GTPases (Wu et al, 2010; Rossi et al, 2020)

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