Cadmium localization and its toxic effects on root tips of barley

Toxic effects of different cadmium (Cd) concentrations on root tip cells of barley (Hordeum vulgare L.) for 1, 4, 12, 24, 48 and 72 h were investigated in order to better understand the mechanisms both in metal toxicity and tolerance. The results indicated that the roots were the main sites of cadmium uptakes. Cadmium ions initially entered elongation zone cells and were accumulated in this area. Then, the cadmium extends up to mature zone and down to meristem zone gradually. The degree of the cell damage in the root tips was associated with the amounts of cadmium absorption and accumulation. After cadmium ions entered meristem zone, cadmium disturbed the mechanisms controlling the organization of microtubule (MT) cytoskeleton and tubulin assembly/disassembly processes. The formation of abnormal MT arrays was formed, including discontinuous wavy MTs and short MT fragments at the cell periphery. The toxic effects resulted in a decrease of mitotic index and inducement of abnormal mitosis, inducing aberrant chromosomes, such as C-mitosis, anaphase bridges and chromosome stickiness. Therefore, MT cytoskeleton can be thought to be one of target sites of cadmium toxicity in root tip cells of plants. The direct cellular evidence of cadmium toxic effects on root tips of barley obtained here is valuable in evaluation of mechanisms both in metal toxicity and tolerance.