Atcys-3A from Arabidopsis encodes the cytosolic isoform of O-acetylserine(thiol)lyase that catalyzes the last step of cysteine biosynthesis. The Atcys-3A transcript is present in different organs of mature plants, being more abundant in roots and declining to 40-50% in rosette leaves and flowers. In situ hybridization studies have shown a high Atcys-3A signal in root tissues, mainly localized to the cortex and xylem parenchyma. In a flower before anthesis, the transcript is detected exclusively in anthers and sepals and evenly distributed throughout the receptacle of the flower. An unexpected observation from these studies is the highest expression of Atcys-3A mRNA found in trichomes of either leaf or stem. The presence of high levels of the transcript is observed very early in trichome cell development. This is the first report describing the cellular localization of any plant O-acetylserine(thiol)lyase mRNA. The high level of Atcys-3A expression in trichomes raises new aspects to the biological function of trichomes, related to sulfate metabolism.