pH in the plant endomembrane system-an import and export business

V-ATPase activity in the TGN/EE is required for exocytosis and recycling in Arabidopsis

In plants, vacuolar H(+)-ATPase (V-ATPase) activity acidifies both the trans-Golgi network/early endosome (TGN/EE) and the vacuole. This dual V-ATPase function has impeded our understanding of how the pH homeostasis within the plant TGN/EE controls exo- and endocytosis. Here, we show that the weak V-ATPase mutant deetiolated3 (det3) displayed a pH increase in the TGN/EE, but not in the vacuole, strongly impairing secretion and recycling of the brassinosteroid receptor and the cellulose synthase complexes to the plasma membrane, in contrast to mutants lacking tonoplast-localized V-ATPase activity only. The brassinosteroid insensitivity and the cellulose deficiency defects in det3 were tightly correlated with reduced Golgi and TGN/EE motility. Thus, our results provide strong evidence that acidification of the TGN/EE, but not of the vacuole, is indispensable for functional secretion and recycling in plants.

TRANSPARENT TESTA 13 is a tonoplast P3A -ATPase required for vacuolar deposition of proanthocyanidins in Arabidopsis thaliana seeds

Intracellular pH homeostasis is essential for all living cells. In plants, pH is usually maintained by three structurally distinct and differentially localized types of proton pump: P-type H(+) -ATPases in the plasma membrane, and multimeric vacuolar-type H(+) -ATPases (V-ATPases) and vacuolar H(+) -pyrophosphatases (H(+) -PPases) in endomembranes. Here, we show that reduced accumulation of proanthocyanidins (PAs) and hence the diminished brown seed coloration found in the Arabidopsis thaliana mutant transparent testa 13 (tt13) is caused by disruption of the gene encoding the P3A -ATPase AHA10. Identification of the gene encoded by the tt13 locus completes the molecular characterization of the classical set of transparent testa mutants. Cells of the tt13 seed coat endothelium do not contain PA-filled central vacuoles as observed in the wild-type. tt13 phenocopies tt12, a mutant that is defective in vacuolar import of the PA precursor epicatechin. Our data show that vacuolar loading with PA precursors depends on TT13. Consistent with the tt13 phenotype, but in contrast to other isoforms of P-type H(+) -ATPases, TT13 localizes to the tonoplast. PA accumulation in tt13 is partially restored by expression of the tonoplast localized H(+) -PPase VHP1. Our findings indicate that the P3A -ATPase TT13 functions as a proton pump in the tonoplast of seed coat endothelium cells, and generates the driving force for TT12-mediated transport of PA precursors to the vacuole.

Calcium sensor kinase activates potassium uptake systems in gland cells of Venus flytraps

The Darwin plant Dionaea muscipula is able to grow on mineral-poor soil, because it gains essential nutrients from captured animal prey. Given that no nutrients remain in the trap when it opens after the consumption of an animal meal, we here asked the question of how Dionaea sequesters prey-derived potassium. We show that prey capture triggers expression of a K(+) uptake system in the Venus flytrap. In search of K(+) transporters endowed with adequate properties for this role, we screened a Dionaea expressed sequence tag (EST) database and identified DmKT1 and DmHAK5 as candidates. On insect and touch hormone stimulation, the number of transcripts of these transporters increased in flytraps. After cRNA injection of K(+)-transporter genes into Xenopus oocytes, however, both putative K(+) transporters remained silent. Assuming that calcium sensor kinases are regulating Arabidopsis K(+) transporter 1 (AKT1), we coexpressed the putative K(+) transporters with a large set of kinases and identified the CBL9-CIPK23 pair as the major activating complex for both transporters in Dionaea K(+) uptake. DmKT1 was found to be a K(+)-selective channel of voltage-dependent high capacity and low affinity, whereas DmHAK5 was identified as the first, to our knowledge, proton-driven, high-affinity potassium transporter with weak selectivity. When the Venus flytrap is processing its prey, the gland cell membrane potential is maintained around -120 mV, and the apoplast is acidified to pH 3. These conditions in the green stomach formed by the closed flytrap allow DmKT1 and DmHAK5 to acquire prey-derived K(+), reducing its concentration from millimolar levels down to trace levels.

pH Regulation by NHX-Type Antiporters Is Required for Receptor-Mediated Protein Trafficking to the Vacuole in Arabidopsis

Protein trafficking requires proper ion and pH homeostasis of the endomembrane system. The NHX-type Na(+)/H(+) antiporters NHX5 and NHX6 localize to the Golgi, trans-Golgi network, and prevacuolar compartments and are required for growth and trafficking to the vacuole. In the nhx5 nhx6 T-DNA insertional knockouts, the precursors of the 2S albumin and 12S globulin storage proteins accumulated and were missorted to the apoplast. Immunoelectron microscopy revealed the presence of vesicle clusters containing storage protein precursors and vacuolar sorting receptors (VSRs). Isolation and identification of complexes of VSRs with unprocessed 12S globulin by 2D blue-native PAGE/SDS-PAGE indicated that the nhx5 nhx6 knockouts showed compromised receptor-cargo association. In vivo interaction studies using bimolecular fluorescence complementation between VSR2;1, aleurain, and 12S globulin suggested that nhx5 nhx6 knockouts showed a significant reduction of VSR binding to both cargoes. In vivo pH measurements indicated that the lumens of VSR compartments containing aleurain, as well as the trans-Golgi network and prevacuolar compartments, were significantly more acidic in nhx5 nhx6 knockouts. This work demonstrates the importance of NHX5 and NHX6 in maintaining endomembrane luminal pH and supports the notion that proper vacuolar trafficking and proteolytic processing of storage proteins require endomembrane pH homeostasis.

Interactions between plant endomembrane systems and the actin cytoskeleton

Membrane trafficking, organelle movement, and morphogenesis in plant cells are mainly controlled by the actin cytoskeleton. Not all proteins that regulate the cytoskeleton and membrane dynamics in animal systems have functional homologs in plants, especially for those proteins that form the bridge between the cytoskeleton and membrane; the membrane-actin adaptors. Their nature and function is only just beginning to be elucidated and this field has been greatly enhanced by the recent identification of the NETWORKED (NET) proteins, which act as membrane-actin adaptors. In this review, we will summarize the role of the actin cytoskeleton and its regulatory proteins in their interaction with endomembrane compartments and where they potentially act as platforms for cell signaling and the coordination of other subcellular events.