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First published online May 5, 2006; 10.1104/pp.106.078063 Plant Physiology 141:1000-1011 (2006) © 2006 American Society of Plant Biologists pho2, a Phosphate Overaccumulator, Is Caused by a Nonsense Mutation in a MicroRNA399 Target Gene1,[W]Institute of BioAgricultural Sciences (K.A., S.-I.L., C.-C.W., Y.-T.H., C.-l.S., T.-J.C.), and Molecular and Biological Agricultural Sciences Program, Taiwan International Graduate Program (K.A., T.-J.C.), Academia Sinica, Taipei 115, Taiwan, Republic of China; Department of Life Science, National Chung-Hsing University, Taichung 402, Taiwan, Republic of China (K.A., T.-J.C.); and Graduate Institute of Life Sciences, National Defense Medical Center, Taipei 114, Taiwan, Republic of China (S.-I.L., T.-J.C.)
We recently demonstrated that microRNA399 (miR399) controls inorganic phosphate (Pi) homeostasis by regulating the expression of UBC24 encoding a ubiquitin-conjugating E2 enzyme in Arabidopsis (Arabidopsis thaliana). Transgenic plants overexpressing miR399 accumulated excessive Pi in the shoots and displayed Pi toxic symptoms. In this study, we revealed that a previously identified Pi overaccumulator, pho2, is caused by a single nucleotide mutation resulting in early termination within the UBC24 gene. The level of full-length UBC24 mRNA was reduced and no UBC24 protein was detected in the pho2 mutant, whereas up-regulation of miR399 by Pi deficiency was not affected. Several characteristics of Pi toxicity in the pho2 mutant were similar to those in the miR399-overexpressing and UBC24 T-DNA knockout plants: both Pi uptake and translocation of Pi from roots to shoots increased and Pi remobilization within leaves was impaired. These phenotypes of the pho2 mutation could be rescued by introduction of a wild-type copy of UBC24. Kinetic analyses revealed that greater Pi uptake in the pho2 and miR399-overexpressing plants is due to increased Vmax. The transcript level of most PHT1 Pi transporter genes was not significantly altered, except PHT1;8 whose expression was enhanced in Pi-sufficient roots of pho2 and miR399-overexpressing compared with wild-type plants. In addition, changes in the expression of several organelle-specific Pi transporters were noticed, which may be associated with the redistribution of intracellular Pi under excess Pi. Furthermore, miR399 and UBC24 were colocalized in the vascular cylinder. This observation not only provides important insight into the interaction between miR399 and UBC24 mRNA, but also supports their systemic function in Pi translocation and remobilization.
1 This work was supported by Academia Sinica (grant no. AS91IBAS2PP) and the National Science Council of the Republic of China (grant no. NSC94–2311–B–001–057) to T.-J.C. 2 These authors contributed equally to the paper. 3 Present address: Vita Genomics, Inc., Taipei 248, Taiwan, Republic of China. 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: Tzyy-Jen Chiou (tjchiou{at}gate.sinica.edu.tw). [W] The online version of this article contains Web-only data. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.078063. * Corresponding author; e-mail tjchiou{at}gate.sinica.edu.tw; fax 886–2–26515600. Received February 2, 2006; returned for revision April 27, 2006; accepted May 1, 2006. Related articles in Plant Physiol.:
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