pho2, a phosphate overaccumulator, is caused by a nonsense mutation in a microRNA399 target gene

Institute of BioAgricultural Sciences , Academia Sinica, Taipei 115, Taiwan, Republic of China.
Plant physiology (Impact Factor: 6.84). 08/2006; 141(3):1000-11. DOI: 10.1104/pp.106.078063
Source: PubMed


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.

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Available from: Kyaw Aung, Aug 31, 2014
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    • "The regulation of PHO2, an ubiquitin-conjugating E2 enzyme (UBC24) that tags for degradation of some proteins involved in Pi transport (Liu et al., 2012; Huang et al., 2013; Park et al., 2014), is by now the best described systemic signal linking leaves with roots that regulate Pi absorption (Fujii et al., 2005; Aung et al., 2006; Bari et al., 2006; Chiou et al., 2006). This regulation occurs through the action of the microRNA miR399 that targets and cleaves PHO2 mRNA in the roots. "
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    ABSTRACT: Plants display numerous strategies to cope with phosphate (Pi)-deficiency. Despite multiple genetic studies, the molecular mechanisms of low-Pi-signalling remain unknown. To validate the interest of chemical genetics to investigate this pathway we discovered and analysed the effects of PHOSTIN (PSN), a drug mimicking Pi-starvation in Arabidopsis. We assessed the effects of PSN and structural analogues on the induction of Pi-deficiency responses in mutants and wild-type and followed their accumulation in plants organs by high pressure liquid chromotography (HPLC) or mass-spectrophotometry. We show that PSN is cleaved in the growth medium, releasing its active motif (PSN11), which accumulates in plants roots. Despite the overaccumulation of Pi in the roots of treated plants, PSN11 elicits both local and systemic Pi-starvation effects. Nevertheless, albeit that the transcriptional activation of low-Pi genes by PSN11 is lost in the phr1;phl1 double mutant, neither PHO1 nor PHO2 are required for PSN11 effects. The range of local and systemic responses to Pi-starvation elicited, and their dependence on the PHR1/PHL1 function suggests that PSN11 affects an important and early step of Pi-starvation signalling. Its independence from PHO1 and PHO2 suggest the existence of unknown pathway(s), showing the usefulness of PSN and chemical genetics to bring new elements to this field. © 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.
    New Phytologist 08/2015; DOI:10.1111/nph.13591 · 7.67 Impact Factor
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    • "Pi deficiency deeply affects plant growth, development, and metabolism (Ticconi and Abel, 2004). pho2 is a Pi-overaccumulator mutant with enhanced uptake and root-to-root translocation of Pi. pho2 is defective in UBC24, which encodes a putative E2 enzyme (Aung et al., 2006; Bari et al., 2006). In Pi deficiency conditions, the transcription factor PHR1 upregulates miR399, which targets the 5 UTR of PHO2 for cleavage. "
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    ABSTRACT: Ubiquitination is a major modifier of signaling in all eukaryotes that results in the conjugation of ubiquitin to the lysine residues of acceptor proteins. The targeted protein is then subjected to degradation by the 26S proteasome, the major protein degradation system in eukaryotes. The ubiquitin–proteasome system (UPS) greatly influences plant growth and development by modulating the activity, localization, and stability of proteins. Plants are frequently exposed to various abiotic stresses during their life cycles; they rely on proteomic plasticity achieved by the UPS to adapt to unfavorable environmental conditions. In stress signal pathways, a large number of components are modified by specific ubiquitination machinery. In this review, we highlight recent advances in understanding the roles of ubiquitination in plant responses to abiotic stresses, including salt and drought, temperature, ultraviolet (UV), and nutrient availability. The review focuses primarily on the roles of the UPS. In salt and/or drought stress signaling, a number of E3 ligases mediate the stress response in both abscisic acid (ABA)-dependent and ABA-independant pathways. The UPS-mediated regulation of several key ABA-regulated transcriptional factors, e.g. ABI3 and ABI5, has been well documented. In cold signaling, the transcription factor ICE1 is targeted by E3 ligase HOSI for proteosomal degradation. Under UV stress, CUL4-DDB1A-DDB2 E3 ligase participates in DNA excision repair, and COP1 interacts with the UVR8 mediated UV response. The UPS is also involved in the uptake, transport, and homeostasis of nutrients such as iron, phosphorus, and nitrogen. SIZ1-mediated sumoylation, a ubiquitin-like modification, is necessary for a number of processes involved in plant responses to abiotic stresses. A challenge moving forward for researchers is to define more UPS components and to characterize their functions in plant responses to stress conditions; there is particular interest in identifying the ubiquitination targets that function in specific stress signaling pathways.
    Environmental and Experimental Botany 06/2015; 114. DOI:10.1016/j.envexpbot.2014.07.005 · 3.36 Impact Factor
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    • "They were smaller, accumulated higher levels of Pi in shoots, and had much higher steady-state levels of PHT1;1/2/3, suggesting that they function independently in regulation of phosphate transporter levels (Lin et al., 2013). When Pi is limiting , AtNLA is downregulated by miRNA827 (Hsieh et al., 2009), relieving this inhibition, while PHO2 is a target of miRNA399 (Aung et al., 2006). One interesting observation is that PHO2 is found predominantly in the vasculature based on studies with promoter reporter fusions; however, PHT1s are expressed predominantly in the epidermal, cortex, and root hair cells. "
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    ABSTRACT: The 'phosphorus problem' has recently received strong interest with two distinct strands of importance. The first is that too much phosphorus (P) is entering into waste water, creating a significant economic and ecological problem. Secondly, while agricultural demand for phosphate fertilizer is increasing to maintain crop yields, rock phosphate reserves are rapidly declining. Unravelling the mechanisms by which plants sense, respond to, and acquire phosphate can address both problems, allowing the development of crop plants that are more efficient at acquiring and using limited amounts of phosphate while at the same time improving the potential of plants and other photosynthetic organisms for nutrient recapture and recycling from waste water. In this review, we attempt to synthesize these important but often disparate parts of the debate in a holistic fashion, since solutions to such a complex problem require integrated and multidisciplinary approaches that address both P supply and demand. Rapid progress has been made recently in our understanding of local and systemic signalling mechanisms for phosphate, and of expression and regulation of membrane proteins that take phosphate up from the environment and transport it within the plant. We discuss the current state of understanding of such mechanisms involved in sensing and responding to phosphate stress. We also discuss approaches to improve the P-use efficiency of crop plants and future direction for sustainable use of P, including use of photosynthetic organisms for recapture of P from waste waters. © The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email:
    Journal of Experimental Botany 05/2015; 66(12). DOI:10.1093/jxb/erv210 · 5.53 Impact Factor
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