Glutathione biosynthesis inArabidopsistrichome cells

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Abstract
InArabidopsis thaliana, trichome cells are specialized unicellular structures with uncertain functions. Based on earlier observations that one of the genes involved in cysteine biosynthesis (Atcys-3A) is highly expressed in trichomes, we have extended our studies in trichome cells to determine their capacity for glutathione (GSH) biosynthesis. First, we have analyzed byin situhybridization the tissue-specific expression of the genesAtcys-3Aand sat5, which encodeO-acetylserine(thio)lyase (OASTL) and serine acetyltransferase (SAT), respectively, as well asgsh1andgsh2, which encode γ-glutamylcysteine synthetase and glutathione synthetase, respectively. The four genes are highly expressed in leaf trichomes ofArabidopsis, and their mRNA accumulate to high levels. Second, we have directly measured cytoplasmic GSH concentration in intact cells by laser-scanning microscopy after labeling with monochlorobimane as a GSH-specific probe. From these measurements, cytosolic GSH concentrations of 238 ± 25, 80 ± 2, and 144 ± 19 μM were estimated for trichome, basement, and epidermal cells, respectively. Taking into account the volume of the cells measured using stereological techniques, the trichomes have a total GSH content more than 300-fold higher than the basement and epidermal cells. Third, after NaCl treatment, GSH biosynthesis is markedly decreased in trichomes.Atcys-3A,sat5, gsh1, andgsh2mRNA levels show a decrease in transcript abundance, and [GSH]cytis reduced to 47 ± 5 μM. These results suggest the important physiological significance of trichome cells related to GSH biosynthesis and their possible role as a sink during detoxification processes.