Frederica L. Theodoulou

Plant ABC proteins – a unified nomenclature and updated inventory

The ABC superfamily comprises both membrane-bound transporters and soluble proteins involved in a broad range of processes, many of which are of considerable agricultural, biotechnological and medical potential. Completion of the Arabidopsis and rice genome sequences has revealed a particularly large and diverse complement of plant ABC proteins in comparison with other organisms. Forward and reverse genetics, together with heterologous expression, have uncovered many novel roles for plant ABC proteins, but this progress has been accompanied by a confusing proliferation of names for plant ABC genes and their products. A consolidated nomenclature will provide much-needed clarity and a framework for future research.

Control of Ascorbate Synthesis by Respiration and Its Implications for Stress Responses

We show for the first time that respiration can control ascorbate (AA) synthesis in plants. Evidence for this control is provided by (a) the localization of l-galactono-1,4-lactone dehydrogenase (GalLDH), the terminal enzyme in AA biosynthesis, with mitochondrial complex I, and its regulation by

Homeostatic response to hypoxia is regulated by the N-end rule pathway in plants

Plants and animals are obligate aerobes, requiring oxygen for mitochondrial respiration and energy production. In plants, an unanticipated decline in oxygen availability (hypoxia), as caused by roots becoming waterlogged or foliage submergence, triggers changes in gene transcription and messenger RNA translation that promote anaerobic metabolism and thus sustain substrate-level ATP production. In contrast to animals, oxygen sensing has not been ascribed to a mechanism of gene regulation in response to oxygen deprivation in plants. Here we show that the N-end rule pathway of targeted proteolysis acts as a homeostatic sensor of severe low oxygen levels in Arabidopsis, through its regulation of key hypoxia-response transcription factors. We found that plants lacking components of the N-end rule pathway constitutively express core hypoxia-response genes and are more tolerant of hypoxic stress. We identify the hypoxia-associated ethylene response factor group VII transcription factors of Arabidopsis as substrates of this pathway. Regulation of these proteins by the N-end rule pathway occurs through a characteristic conserved motif at the amino terminus initiating with Met-Cys. Enhanced stability of one of these proteins, HRE2, under low oxygen conditions improves hypoxia survival and reveals a molecular mechanism for oxygen sensing in plants via the evolutionarily conserved N-end rule pathway. SUB1A-1, a major determinant of submergence tolerance in rice, was shown not to be a substrate for the N-end rule pathway despite containing the N-terminal motif, indicating that it is uncoupled from N-end rule pathway regulation, and that enhanced stability may relate to the superior tolerance of Sub1 rice varieties to multiple abiotic stresses.

The functions of inter- and intracellular glutathione transport systems in plants

Glutathione is one of the major redox buffers in most aerobic cells, and it has a broad spectrum of functions in plants. Recent discoveries implicate this thiol peptide in signalling and cellular homeostasis. Glutathione can sense intracellular redox status: perturbations of glutathione reduction state are transduced into changes in gene expression. This central role demands precise control of both the concentration and the reduction state of glutathione in different compartments. In addition to the regulation of glutathione biosynthesis and redox state, attention is now turning to the role of glutathione transporters.

Jasmonic Acid Levels Are Reduced in COMATOSE ATP-Binding Cassette Transporter Mutants. Implications for Transport of Jasmonate Precursors into Peroxisomes

We provide evidence that the peroxisomal ATP-binding cassette (ABC) transporter COMATOSE (CTS) is involved in the biosynthesis of jasmonic acid (JA) in Arabidopsis ( Arabidopsis thaliana ) leaves. Basal JA levels were greatly reduced but not completely abolished in two cts mutant alleles, and JA

Plant ABC transporters

The ATP binding cassette (ABC) superfamily is a large, ubiquitous and diverse group of proteins, most of which mediate transport across biological membranes. ABC transporters have been shown to function not only as ATP-dependent pumps, but also as ion channels and channel regulators. Whilst members of this gene family have been extensively characterised in mammalian and microbial systems, the study of plant ABC transporters is a relatively new field of investigation. Sequences of over 20 plant ABC proteins have been published and include homologues of P-glycoprotein, MRP, PDR5 and organellar transporters. At present, functions have been assigned to a small proportion of these genes and only the MRP subclass has been extensively characterised. This review aims to summarise literature relevant to the study of plant ABC transporters, to review methods of cloning, to discuss the utility of yeast and mammalian systems as models and to speculate on possible roles of uncharacterised ABC transporters in plants.

Nitric Oxide Sensing in Plants Is Mediated by Proteolytic Control of Group VII ERF Transcription Factors

Summary Nitric oxide (NO) is an important signaling compound in prokaryotes and eukaryotes. In plants, NO regulates critical developmental transitions and stress responses. Here, we identify a mechanism for NO sensing that coordinates responses throughout development based on targeted degradation of plant-specific transcriptional regulators, the group VII ethylene response factors (ERFs). We show that the N-end rule pathway of targeted proteolysis targets these proteins for destruction in the presence of NO, and we establish them as critical regulators of diverse NO-regulated processes, including seed germination, stomatal closure, and hypocotyl elongation. Furthermore, we define the molecular mechanism for NO control of germination and crosstalk with abscisic acid (ABA) signaling through ERF-regulated expression of ABSCISIC ACID INSENSITIVE5 (ABI5). Our work demonstrates how NO sensing is integrated across multiple physiological processes by direct modulation of transcription factor stability and identifies group VII ERFs as central hubs for the perception of gaseous signals in plants.

Cloning and functional characterization of a Brassica napus transporter that is able to transport nitrate and histidine.

A full-length cDNA for a membrane transporter was isolated from Brassica napus by its sequence homology to a previously cloned Arabidopsis low affinity nitrate transporter. The cDNA encodes a predicted protein of 589 amino acid residues with 12 putative transmembrane domains. The transporter belongs to a multigene family with members that have been identified in bacteria, fungi, plants, and animals and that are able to transport a range of different nitrogen-containing substrates, including amino acids, peptides, and nitrate. To identify the substrates of this plant gene, we have expressed the protein in Xenopus oocytes. The properties of the transporter are consistent with a proton cotransport mechanism for nitrate, and the voltage dependence of theK m for nitrate was determined. TheK m for nitrate was shown to increase from 4 to 14 mm as the membrane voltage became more negative from −40 to −180 mV. Oocytes expressing the gene could accumulate internal nitrate to concentrations higher than those measured in water-injected controls. A range of different substrate molecules for the transporter was tested, but of these, histidine gave the largest currents, although the affinity was in the millimolar range. The pH dependence of the activity of the transporter was different for the substrates, with histidine transport favored at alkaline and nitrate at acid external pH. Kinetic analysis of the mechanism of histidine transport suggests a cotransport of protons and the neutral form of the amino acid, with theK m for histidine decreasing at more negative membrane voltages. This gene is the first member of this family of transporters for which the transport of two very different types of substrate, nitrate and histidine, has been demonstrated.

The N-end rule pathway promotes seed germination and establishment through removal of ABA sensitivity in Arabidopsis

The N-end rule pathway targets protein degradation through the identity of the amino-terminal residue of specific protein substrates. Two components of this pathway in Arabidopsis thaliana, PROTEOLYSIS6 (PRT6) and arginyl-tRNA:protein arginyltransferase (ATE), were shown to regulate seed after-ripening, seedling sugar sensitivity, seedling lipid breakdown, and abscisic acid (ABA) sensitivity of germination. Sensitivity of prt6 mutant seeds to ABA inhibition of endosperm rupture reduced with after-ripening time, suggesting that seeds display a previously undescribed window of sensitivity to ABA. Reduced root growth of prt6 alleles and the ate1 ate2 double mutant was rescued by exogenous sucrose, and the breakdown of lipid bodies and seed-derived triacylglycerol was impaired in mutant seedlings, implicating the N-end rule pathway in control of seed oil mobilization. Epistasis analysis indicated that PRT6 control of germination and establishment, as exemplified by ABA and sugar sensitivity, as well as storage oil mobilization, occurs at least in part via transcription factors ABI3 and ABI5. The N-end rule pathway of protein turnover is therefore postulated to inactivate as-yet unidentified key component(s) of ABA signaling to influence the seed-to-seedling transition.

Peroxisomal ABC transporters.

Peroxisomes perform a range of different functions, dependent upon organism, tissue type, developmental stage or environmental conditions, many of which are connected with lipid metabolism. This review summarises recent research on ATP binding cassette (ABC) transporters of the peroxisomal membrane (ABC subfamily D) and their roles in plants, fungi and animals. Analysis of mutants has revealed that peroxisomal ABC transporters play key roles in specific metabolic and developmental functions in different organisms. A common function is import of substrates for β‐oxidation but much remains to be determined concerning transport substrates and mechanisms which appear to differ significantly between phyla.