Functional characterization of the Arabidopsis thaliana Translocator-related protein AtTSPO

(eng) Early event in plant responses to water-related stresses include increased levels of active abscisic acid (ABA), which regulates the expression of ABA-responsive genes (up to 10% of the plant genome). Some of them are strictly ABA-dependent in that their expression is almost undetectable in absence of elevated levels of ABA. The Arabidopsis thaliana gene At2g47770 has the peculiarity of being rapidely upregulated under salt, osmotic stress or plant treatment by ABA, while its transcripts are undetectable in plant grown under normal physiological conditions. At2g47770 encodes a membrane-bound protein designated AtTSPO which belongs to the tryptophan-rich sensory protein/mitochondrial benzodiazepine receptor family, whose founding member are the mammalian TSPO1 (Translocator protein) and the bacterial TspO (Tryptophan-rich Sensory Protein). Plants TSPO are not yet characterized. In this study, we showed that AtTSPO is mainly detected in dry seeds (rich in ABA), but can be induced in vegetative tissues by osmotic, salt stress or exogenous ABA treatment. Using biochemical and microscopy approaches, we present evidence that AtTSPO is targeted to the endoplasmic reticulum and Golgi stacks in the plant cell. Constitutive expression of AtTSPO may be detrimental for the cell since it increases sensitivity to NaCl and ABA, and reduces greening of cultured Arabidopsis cells under light growing conditions. The ABA-dependent induction of AtTSPO is only transient. Downregulation of AtTSPO seems to be enhanced by boosting tetrapyrroles biosynthesis. A possible mechanism could be that AtTSPO degradation is triggered by tetrapyrroles binding and could help detoxifying the cell of highly photooxidative porphyrins during stress. Our results suggest that AtTSPO is a highly regulated protein, induced by abiotic stress to modulate, at least in part, transient intracellular ABA-dependent stress perception and/or signalling.