CYSTEINE, GAMMA-GLUTAMYLCYSTEINE, AND GLUTATHIONE LEVELS IN MAIZE SEEDLINGS - DISTRIBUTION AND TRANSLOCATION IN NORMAL AND CADMIUM-EXPOSED PLANTS

The levels of cysteine (Cys), gamma-glutamylcysteine (gamma-EC), and glutathione (GSH) were measured in the endosperms, scutella, roots, and shoots of maize (Zea mays L.) seedlings. GSH was the major thiol in roots, shoots, and scutellum, Cys predominated in endosperms. The endosperm, scutellum, and functional phloem translocation were required for maintenance of GSH pools in roots and shoots of 6-day-old seedlings. Exposure of roots to 3 micromolar Cd, besides causing a decline in GSH, caused an accumulation of gamma-EC, as if the activity of GSH synthetase was reduced in vivo. [S-35]Cys injected into endosperms of seedlings was partly metabolized to [S-35]sulfate. The scutella absorbed both [S-35]sulfate and [S-35]Cys and transformed 68 to 87% of the radioactivity into [S-35]GSH. [S-35]GSH was translocated to roots and shoots in proportion to the tissue fresh weight. Taken together, the data supported the hypothesis that Cys from the endosperm is absorbed by the scutellum and used to synthesize GSH for transfer through the phloem to the root and shoot. The estimated flux of GSH to the roots was 35 to 60 nanomoles per gram per hour, which totally accounted for the small gain in GSH in roots between days 6 and 7. For Cd-treated roots the GSH influx was similar, yet the GSH pool did not recover to control levels within 24 hours. The estimated flux of GSH to the entire shoot was like that to the roots; however, it was low (11-13 nanomoles per gram per hour) to the first leaf and high (76-135 nanomoles per gram per hour) to the second and younger leaves.