The rice RING E3 ligase, OsCTR1, inhibits trafficking to the chloroplasts of OsCP12 and OsRP1, and its overexpression confers drought tolerance in Arabidopsis

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Abstract
Plant growth under low water availability adversely affects many key processes with morphological, physiological, biochemical and molecular consequences. Here, we found that a rice gene, OsCTR1, encoding the RING Ub E3 ligase plays an important role in drought tolerance. OsCTR1 was highly expressed in response to dehydration treatment and defense‐related phytohormones, and its encoded protein was localized in both the chloroplasts and the cytosol. Intriguingly, the OsCTR1 protein was found predominantly targeted to the cytosol when rice protoplasts transfected with OsCTR1 were treated with abscisic acid (ABA). Several interacting partners were identified, which were mainly targeted to the chloroplasts, and interactions with OsCTR1 were confirmed by using biomolecular fluorescence complementation (BiFC). Interestingly, two chloroplast‐localized proteins (OsCP12 and OsRP1) interacted with OsCTR1 in the cytosol, and ubiquitination by OsCTR1 led to protein degradation via the Ub 26S proteasome. Heterogeneous overexpression of OsCTR1 in Arabidopsis exhibited hypersensitive phenotypes with respect to ABA‐responsive seed germination, seedling growth and stomatal closure. The ABA‐sensitive transgenic plants also showed improvement in their tolerance against severe water deficits. Taken together, our findings lend support to the hypothesis that the molecular functions of OsCTR1 are related to tolerance to water‐deficit stress via ABA‐dependent regulation and related systems.

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