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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Iron Deficiency-Induced Secretion of Phenolics Facilitates the Reutilization of Root Apoplastic Iron in Red Clover¹

State Key Laboratory of Plant Physiology and Biochemistry, College of Life Science, Zhejiang University, Hangzhou 310058, China (W.C.J., G.Y.Y., P.W., S.J.Z.); Ministry of Education Key Laboratory for Environmental Remediation and Ecosystem Health, College of Environmental and Resource Science, Zhejiang University, Hangzhou 310029, China (W.C.J., Y.F.H.); and Department of Agricultural Sciences, La Trobe University, Bundoora (Melbourne) Victoria 3086, Australia (C.T.)

Phenolic compounds are frequently reported to be the main components of root exudates in response to iron (Fe) deficiency in Strategy I plants, but relatively little is known about their function. Here, we show that removal of secreted phenolics from the root-bathing solution almost completely inhibited the reutilization of apoplastic Fe in roots of red clover (Trifolium pratense). This resulted in much lower levels of shoot Fe and significantly higher root Fe compared with control and also resulted in leaf chlorosis, suggesting this approach stimulated Fe deficiency. This was supported by the observation that phenolic removal significantly enhanced root ferric chelate reductase activity, which is normally induced by plant Fe deficiency. Furthermore, root proton extrusion, which also is normally increased during Fe deficiency, was found to be higher in plants exposed to the phenolic removal treatment too. These results indicate that Fe deficiency-induced phenolics secretion plays an important role in the reutilization of root apoplastic Fe, and this reutilization is not mediated by proton extrusion or the root ferric chelate reductase. In vitro studies with extracted root cell walls further demonstrate that excreted phenolics efficiently desorbed a significant amount of Fe from cell walls, indicating a direct involvement of phenolics in Fe remobilization. All of these results constitute the first direct experimental evidence, to our knowledge, that Fe deficiency-induced secretion of phenolics by the roots of a dicot species improves plant Fe nutrition by enhancing reutilization of apoplastic Fe, thereby improving Fe nutrition in the shoot.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Shao Jian Zheng (sjzheng{at}zju.edu.cn).

www.plantphysiol.org/cgi/doi/10.1104/pp.107.095794

^* Corresponding author; e-mail sjzheng{at}zju.edu.cn; fax 86–571–8820–6438.

Received January 10, 2007; accepted March 13, 2007; published March 16, 2007.

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