Early response of wheat seminal roots growing under copper excess.

Growth reduction caused by copper excess during plant photoautotrophic metabolism has been widely investigated, but information regarding early responses of root apical meristem (RAM) to toxic concentrations of this metal at the initial heterotrophic stage is certainly scarce. We analysed some determinants of seminal root growth in developing wheat seedlings germinated in the presence of 1, 5 and 10 µM CuCl2, focussing on oxidative damage to cell membrane and to proteins, and investigated the expression patterns of some genes relevant to cell cycle progression and cell expansion. The proliferation zone of the RAM was shorter under 5 and 10 µM CuCl2. Cyclin D and CDKA levels remained unchanged in the root apexes of wheat seedlings grown under these Cu(2+) concentrations, but more carbonylated levels of both proteins and less ubiquitinated-cyclin D was detected under 10 µM CuCl2. Increased levels of ROS were revealed by fluorescent probes at this Cu(2+) dose, and severe cell membrane damage took place at 5 and 10 µM CuCl2. Several genes related to retinoblastome phosphorylation and therefore involved in the transition from G1 to S cell cycle stage were found to be downregulated at 10 µM CuCl2, while most expansin genes here analysed were upregulated, even at a non-toxic concentration of 1 µM. These results together with previous findings suggest that a "common" signal which involves oxidative posttranslational modifications of specific cell cycle proteins may be necessary to induce root growth arrest under Cd(2+) and Cu(2+) stress.