A vacuolar acid phosphatase (APase) that accumulates during phosphate (Pi) starvation of Arabidopsis (Arabidopsis thaliana) suspension cells was purified to homogeneity. The final preparation is a purple APase (PAP), as it exhibited a pink color in solution (A max 5 520 nm). It exists as a 100-kD homodimer composed of 55-kD glycosylated subunits that cross-reacted with an anti-(tomato intracellular PAP)-IgG. BLAST analysis of its 23-amino acid N-terminal sequence revealed that this PAP is encoded by At5g34850 (AtPAP26; one of 29 PAP genes in Arabidopsis) and that a 30-amino acid signal peptide is cleaved from the AtPAP26 preprotein during its translocation into the vacuole. AtPAP26 displays much stronger sequence similarity to orthologs from other plants than to other Arabidopsis PAPs. AtPAP26 exhibited optimal activity at pH 5.6 and broad substrate selectivity. The 5-fold increase in APase activity that occurred in Pi-deprived cells was paralleled by a similar increase in the amount of a 55-kD anti-(tomato PAP or AtPAP26)-IgG immunoreactive polypeptide and a .30-fold reduction in intracellular free Pi concentration. Semiquantitative reverse transcription-PCR indicated that Pi-sufficient, Pi-starved, and Pi-resupplied cells contain similar amounts of AtPAP26 transcripts. Thus, transcriptional controls appear to exert little influence on AtPAP26 levels, relative to translational and/or proteolytic controls. APase activity and AtPAP26 protein levels were also up-regulated in shoots and roots of Pi-deprived Arabidopsis seedlings. We hypothesize that AtPAP26 recycles Pi from intracellular P metabolites in Pi-starved Arabidopsis. As AtPAP26 also exhibited alkaline peroxidase activity, a potential additional role in the metabolism of reactive oxygen species is discussed.

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 ...

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Cellular homeostasis is essential for the physiology of eukaryotic cells. Eukaryotic cells, including plant cells, utilize two main pathways to adjust the level of cytoplasmic components, namely the proteasomal and the lysosomal/vacuolar pathways. Macroautophagy is a lysosomal/vacuolar pathway which, until recently, was thought to be non-specific and a bulk degradation process. However, selective autophagy which can be activated in the cell under various physiological conditions, involves the specific degradation of defined macromolecules or organelles by a conserved molecular mechanism. For this process to be efficient, the mechanisms underlying the recognition and selection of the cargo to be engulfed by the double membrane autophagosome are critical, and not yet well understood. Ubiquitin (poly-ubiquitin) conjugation to the target appears to be a conserved ligand mechanism in many types of selective autophagy, and defined receptors/adaptors recognizing and regulating the autophagosomal capture of the ubiquitylated target have been characterized. However, non-proteinaceous and non-ubiquitylated cargoes are also selectively degraded by this pathway. This ubiquitin-independent selective autophagic pathway also involves receptor and/or adaptor proteins linking the cargo to the autophagic machinery. Some of these receptor/adaptor proteins including accessory autophagy-related (Atg) and non-Atg proteins have been described in yeast and animal cells but not yet in plants. In this review we discuss the ubiquitin-independent cargo selection mechanisms in selective autophagy degradation of organelles and macromolecules and speculate on potential plant receptor/adaptor proteins.

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