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Developing tissues such as meristems and reproductive organs require high zinc, but the molecular mechanisms of how zinc taken up by the roots is preferentially delivered to these tissues with low transpiration are unknown. Here, we report that rice (Oryza sativa) heavy metal ATPase2 (OsHMA2), a member of P-type ATPases, is involved in preferential delivery of zinc to the developing tissues in rice. OsHMA2 was mainly expressed in the mature zone of the roots at the vegetative stage, but higher expression was also found in the nodes at the reproductive stage. The expression was unaffected by either zinc deficiency or zinc excess. OsHMA2 was localized at the pericycle of the roots and at the phloem of enlarged and diffuse vascular bundles in the nodes. Heterologous expression of OsHMA2 in yeast (Saccharomyces cerevisiae) showed influx transport activity for zinc as well as cadmium. Two independent Tos17 insertion lines showed decreased zinc concentration in the crown root tips, decreased concentration of zinc and cadmium in the upper nodes and reproductive organs compared with wild-type rice. Furthermore, a short-term labeling experiment with (67)Zn showed that the distribution of zinc to the panicle and uppermost node I was decreased, but that, to the lower nodes, was increased in the two mutants. Taken together, OsHMA2 in the nodes plays an important role in preferential distribution of zinc as well as cadmium through the phloem to the developing tissues.
Yamaji et al. (Wed,) studied this question.