SUMMARYThe Arabidopsis gene At2g47770 encodes a membrane-bound protein designated AtTSPO (Arabidopsis thaliana TSPO-related). AtTSPO is related to the bacterial outer membrane tryptophan-rich sensory protein (TspO) and the mammalian mitochondrial 18-kDa translocator protein (18 kDa TSPO), members of the group of TspO/MBR domain-containing membrane proteins. In this study we show that AtTSPO is mainly detected in dry seeds, but can be induced in vegetative tissues by osmotic or salt stress or abscisic acid (ABA) treatment, corroborating available transcriptome data. Using subcellular fractionation, immunocytochemistry and fluorescent protein tagging approaches we present evidence that AtTSPO is targeted to the secretory pathway in plants. Induced or constitutively expressed AtTSPO can be detected in the endoplasmic reticulum and the Golgi stacks of plant cells. AtTSPO tagged with fluorescent protein in transgenic plants (Arabidopsis and tobacco) was mainly detected in the Golgi stacks of leaf epidermal cells. Constitutive expression of AtTSPO resulted in increased sensitivity to NaCl, but not to osmotic stress, and in reduced greening of cultured Arabidopsis cells under light growing conditions. Transgenic Arabidopsis plants overexpressing AtTSPO were more sensitive to ABA-induced growth inhibition, indicating that constitutive expression of AtTSPO may enhance ABA sensitivity. AtTSPO is rapidly downregulated during seed imbibition, and the ABA-dependent induction in plant is transient. Downregulation of AtTSPO seems to be boosted by treatment with aminolevulinic acid. Taken together, these 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.

The Arabidopsis thaliana multi-stress regulator TSPO is transiently induced by abiotic stresses. The final destination of this polytopic membrane protein is the Golgi apparatus, where its accumulation is strictly regulated, and TSPO is downregulated through a selective autophagic pathway. TSPO-related proteins regulate the physiology of the cell by generating functional protein complexes. A split-ubiquitin screen for potential TSPO interacting partners uncovered a plasma membrane aquaporin, PIP2;7. Pull-down assays and fluorescence imaging approaches revealed that TSPO physically interacts with PIP2;7 at the endoplasmic reticulum and Golgi membranes in planta. Intriguingly, constitutive expression of fluorescently tagged PIP2;7 in TSPO-overexpressing transgenic lines resulted in patchy distribution of the fluorescence, reminiscent of the pattern of constitutively expressed yellow fluorescent protein-TSPO in Arabidopsis. Mutational stabilization of TSPO or pharmacological inhibition of the autophagic pathway affected concomitantly the detected levels of PIP2;7, suggesting that the complex containing both proteins is degraded through the autophagic pathway. Coexpression of TSPO and PIP2;7 resulted in decreased levels of PIP2;7 in the plasma membrane and abolished the membrane water permeability mediated by transgenic PIP2;7. Taken together, these data support a physiological role for TSPO in regulating the cell-surface expression of PIP2;7 during abiotic stress conditions through protein-protein interaction and demonstrate an aquaporin regulatory mechanism involving TSPO. INTRODUCTIONEnvironmental stresses such as drought, salinity, or cold are common limiting factors for plant growth and development. These stresses impose osmotic and oxidative stresses at the cellular level, and a critical function of the phytohormone abscisic acid (ABA) is to mediate the plant response to these insults during vegetative growth (Finkelstein et al., 2002;Nambara and Marion-Poll, 2005;Yamaguchi-Shinozaki and Shinozaki, 2006). The increase in active ABA levels in plant cells during water-related stress regulates the expression of ABA-responsive genes by interacting with cytosolic and/or organelle-bound receptors and downstream effectors modulating the activity of defined transcriptional regulators (Fujii and Zhu, 2009;Ma et al., 2009;Park et al., 2009;Wu et al., 2009;Shang et al., 2010). It is thought that up to 10% of the Arabidopsis thaliana transcriptome is responsive to ABA signaling . Extensive studies of stress and ABA-induced gene expression during vegetative growth revealed two waves of response: an early transient response peaking at ;3 h and a late sustained response from 10 h onward (reviewed in Finkelstein, 2013). Characteristically, the so-called "early" genes encode regulatory proteins, such as transcription factors, protein kinases, and phosphatases, and a set of proteins of unknown function Fujita et al., 2006). The "late" genes are presumed to contribute to plant adaptation to the stress and encode proteins such as ...

RNA editing is converting hundreds of cytosines into uridines during organelle gene expression of land plants. The pentatricopeptide repeat (PPR) proteins are at the core of this posttranscriptional RNA modification. Even if a PPR protein defines the editing site, a DYW domain of the same or another PPR protein is believed to catalyze the deamination. To give insight into the organelle RNA editosome, we performed tandem affinity purification of the plastidial CHLOROPLAST BIOGENESIS 19 (CLB19) PPR editing factor. Two PPR proteins, dually targeted to mitochondria and chloroplasts, were identified as potential partners of CLB19. These two proteins, a P-type PPR and a member of a small PPR-DYW subfamily, were shown to interact in yeast. Insertional mutations resulted in embryo lethality that could be rescued by embryo-specific complementation. A transcriptome analysis of these complemented plants showed major editing defects in both organelles with a very high PPR type specificity, indicating that the two proteins are core members of E+-type PPR editosomes.

The lack of trehalose accumulation in most plant species has been partly attributed to the presence of an active trehalase. Although trehalose synthesis enzymes are thought to be cytosolic, and previous studies have indicated that trehalase activity is extracellular, the exact location of the enzyme has not yet been established in plant cell. We present evidence that the yet uncharacterised full-length Arabidopsis trehalase is a plasma membrane-bound protein, probably anchored to the membrane through a predicted N-terminal membrane spanning domain. The full-length AtTRE1, when expressed in yeast can functionally substitute for the extracellularly active trehalase Ath1p, by sustaining the growth of an ath1 null mutant strain on trehalose and at pH 4.8. We further demonstrate that At-TRE1 expressed in yeast is plasma membrane-bound as in plant cell. In light of these findings, the regulation of plant cell endogenous trehalose by trehalase is discussed.

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For the first time, the behaviour of tobacco cell suspensions submitted to four porphyrins was described. The potential killer effect of these photosensitizers on tobacco cells was evaluated. Biological results were correlated with photophysical properties and the reactive oxygen species production capacity of tested compounds. Surprisingly, the anionic free-base porphyrin showed the strongest phototoxic effect.

Antimicrobial photodynamic treatment (APDT) based on the use of a photosensitizer to produce reactive oxygen species (ROS) that induce cell death could be envisaged to fight against plant pathogens. For setting this strategy, we want to study how plants themselves respond to photodynamic treatment. In previous work we showed that tomato plantlets were able to resist photoactivated tetra (N-methylpyridyl) porphyrin (CP) or the zinc metalated form (CP-Zn). To enlarge our plant expertise related to exogenous porphyrins treatment and to further defend this approach, we studied how a weed like Arabidopsis thaliana responded to exogenous supply of anionic and cationic porphyrins. Both types of photosensitizers had no negative effect on seed germination and did not hamper the development etiolated Arabidopsis plantlet under dark conditions. Thus, post-emergence effects of porphyrin photoactivation on the development of 14 day-old in vitro Arabidopsis plantlet under light were observed. CP-Zn was the most efficient photosensitizer to kill Arabidopsis plantlets while anionic tetra (4-sulfonatophenyl) porphyrin only delayed their growth and development. Indeed only 7% of plantlets could be rescued after CP-Zn treatment. Furthermore, non-enzymatic and enzymatic defense components involved in detoxification of ROS generated by CP-Zn under illumination were downregulated or stable with the exception of sevenfold increase in proline content. As previously demonstrated in the literature for microbial agents and in the present work for Arabidopsis, CP-Zn was efficient enough to eradicate unwanted vegetation and plant pathogens without at the same time killing plants of agronomic interest such as tomato plantlets.