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As a nontolerant plant to a large number of toxic compounds, Arabidopsis thaliana is a suitable model to study regulation of genes involved in response to heavy metals. Using a cDNA-microarray approach, we identified some ABC transporters that are differentially regulated after cadmium treatments, making them putative candidates for being involved in Cd sequestration and redistribution in plants. Regarding yeast and fission yeast, in which Cd is able to form complexes either with glutathione (GSH) or phytochelatins (PC) subsequently transported into vacuoles via ABC transporters, it is also very likely that some plant ABC transporters are able to transport GS2–Cd or PC–Cd complexes into subcellular compartments or outside of the cell. The characterization of such transporters is of great interest for developing molecular biology approaches in phytoremediation. Keywords: Arabidopsis thaliana, cDNA microarray, ABC transporters

Plants are a particularly rich source of ABC proteins. The genome of the model plant Arabidopsis thaliana is capable of encoding a total of 129 ABC proteins, which fall into 12 or more subfamilies. This gene count outstrips those for the human genome and any other animal genome sequenced to date. The human genome is estimated to encode a mere 48 ABC proteins while those of the nematode worm, Caenorhabditis elegans , and the fruit fly, Drosophila melanogaster , encode only 58 and 51, respectively. Of the total ABC protein open reading frames (ORFs) identified in Arabidopsis , at least 103 probably encode membrane proteins–proteins possessing membrane spans contiguous with one or two nucleotide-binding domains. Arabidopsis allocates nearly twice as many of its transporter genes to ABC transporters as C. elegans and Drosophila. This chapter uses the Arabidopsis ABC protein superfamily as a model for plants in general. Arabidopsis is the first plant whose ORFs encoding ABC proteins have been systematically inventoried in their entirety. The nomenclature adopted is one based on the acronyms of the prototypes, when available, of a particular ABC protein subfamily defined from another source. Searches of the earlier literature and comparisons with the ABC proteins found in organisms other than plants are thereby simplified and expedited. A nomenclature similar to that applied to the ABC protein-encoding ORFs of yeast is favored.

Herbicide metabolism and detoxification in plants is generally a multiphase process involving functionalisation (most frequently by oxidation or hydrolysis), conjugation (to glutathione, glucose, or an organic acid), processing of the conjugate (by degradation, secondary conjugation) and compartmentation (vacuolar or extracellular deposition). Many enzymes involved in the metabolism of herbicides in plants such as cytochrome P450-dependent monooxygenases, glutathione S-transferases, glucosyl and malonyl transferases have been investigated in the past. Transfer into crops of genes for herbicide-metabolising enzymes from bacteria or mammals has been successfully employed to increase herbicide tolerance. Recent research has revealed that both glutathione and glucose conjugates of herbicides are exported from the cytosol to the central vacuole of plant cells. Accumulation of glutathione conjugates in the vacuole is mediated by transporters in the vacuolar membrane that are directly energised by ATP. Evidence exists that glucose conjugates of herbicides are transported into the vacuole by ATP-stimulated transporters that are distinct from those involved in glutathione conjugate transport. While herbicide glutathionation and glucosylation takes place in the cytosol, subsequent compartmentation of these conjugates and possible further metabolism in the vacuole may be crucial for sutained detoxification. Herbicide safeners enhance herbicide oxidation and conjugation, as well as the activity of vacuolar transport ATPases involved in herbicide conjugate export. Thus, the various processes involved in cellular detoxification of herbicides in plants constitute integrated pathways that are apparently subject to coordinated regulation.

Apoplastic and vacuolar forms of alpha-mannosidase from barley leaves were purified (the latter ca 1000-fold). Isoenzymes were separated during purification. They consisted of four subunits with an apparent M(r) of 53, 46, 40 and 24 x 10(3) in varying proportions. In two-dimensional electropherograms, the M(r) 46 and 40 x 10(3) polypeptides were separated into two isomeric forms of different isoelectric point. The native proteins focused in a pH gradient at different isoelectric points. Their M(r)S ranged between 290 and 270 x 10(3) as judged from size exclusion chromatography and electrophoretic mobility. The compartmentation of the isoenzymes revealed tissue- and age-specificity.

The vacuoles of mesophyll cells are capable of storing amino acids which are mobilized on demand. An amino-acid carrier was characterized by studying the efflux of vacuolar amino acids from isolated mesophyll vacuoles using the silicon oil layer centrifugation technique. Efflux was slow in the absence of ATP. It was stimulated by a factor of more than 5 by the addition of ATP. A similar stimulation was brought about by adenylyl imidodiphosphate ( Ado PP [NH] P ), suggesting that efflux activation by ATP did not involve ATP hydrolysis. GTP, CTP, UTP and ADP were not effective, or were much less effective than ATP, in activating the amino-acid transport system. Neutral amino acids inhibited ATP-stimulated amino-acid efflux by an allosteric mechanism. External leucine, valine, phenylalanine and methionine were more effective in inhibiting efflux than alanine, arginine, lysine or glutamic acid when present outside the vacuole. Protein-modifying agents such as N -ethylmaleimide and p -chloromercuriphenylsulfonic acid activated amino-acid efflux from the vacuoles, whereas proteinase treatment led to an inhibition. Stimulation by ATP of amino-acid export from the vacuole and inhibition by specific amino acids may be part of a mechanism to stabilize cytoplasmic amino-acid levels during protein synthesis.