Glutathione biosynthesis in Arabidopsis trichome cells

In Arabidopsis thaliana, trichome cells are specialized unicellular structu res 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 thei r capacity for glutathione (GSH) biosynthesis. First, we have analyzed by i n situ hybridization the tissue-specific expression of the genes Atcys-3A a nd sat5, which encode O-acetylserine(thio)lyase (OASTL) and serine acetyltr ansferase (SAT), respectively, as well as gsh1 and gsh2, which encode gamma -glutamylcysteine synthetase and glutathione synthetase, respectively. The four genes are highly expressed in leaf trichomes of Arabidopsis, and their mRNA accumulate to high levels. Second, we have directly measured cytoplas mic GSH concentration in intact cells by laser-scanning microscopy after la beling with monochlorobimane as a GSH-specific probe. From these measuremen ts, cytosolic GSH concentrations of 238 +/- 25, 80 +/- 2, and 144 +/- 19 mu M 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 highe r than the basement and epidermal cells. Third, after NaCl treatment, GSH b iosynthesis is markedly decreased in trichomes. Atcys-3A. sat5, gsh1, and g sh2 mRNA levels show a decrease in transcript abundance, and [GSH](cyt) is reduced to 47 +/- 5 mu M. These results suggest the important physiological significance of trichome cells related to GSH biosynthesis and their possi ble role as a sink during detoxification processes.