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Unravelling the Mystery of Vacuolar Phosphate Transporter
Inorganic phosphate (Pi) is an essential nutrient required for plant growth, and thus is one of the main ingredients of fertilizer. Scientists know that more than 70% of total Pi in plants is stored in the vacuole, but for about half a century, no one has been able to discover how it actually gets there. However, an interdisciplinary research team coordinated by Tzu-Yin Liu, assistant professor in the Institute of Bioinformatics and Structural Biology, has recently used magnetic resonance imaging (MRI) to resolve this mystery. Their discovery will make it possible to reduce the amount of phosphate fertilizer used in agriculture, thereby alleviating land pollution. A report on their groundbreaking study was recently published in the renowned journal Nature Communications.
 
This research project was directed by by Tzyy-Jen Chiou of Academia Sinica's Agricultural Biotechnology Research Center, and in addition to Liu included Associate Professor Fu-Nien Wang and Ph.D. student Sheng-Min Huang, both of NTHU’s Department of Biomedical Engineering and Environmental Sciences; and Shang-Yueh Tsai, associate professor at National Chengchi University’s Graduate Institute of Applied Physics. By using the NMR spectrum generated by the 7T-MRI equipment of the Translational Molecular Imaging Center at the Linkou Chang Gung Memorial Hospital, they were able to “visualize” the Pi content in the cytoplasm and in the vacuoles of Arabidopsis thaliana seedlings. They also confirmed that the SPX-MFS gene family is the long-sought-after vacuolar Pi transporters – the plant Pi transporter Type 5.
 
According to Liu, vacuoles can store a variety of nutrients through different membrane proteins on the vacuolar membrane. Yet for a long time scientists had been unable to find the membrane protein by which Pi was transported into the vacuole. By using MRI, they were able to observe the Pi content in the vacuoles of Arabidopsis seedlings and found the "door" by which Pi enters the vacuole, that is, the plant Pi transporter Type 5. Arabidopsis is a cruciferous plant widely distributed in Europe, Asia, and northwest Africa. It has a short life cycle and grows well in the small space available in the basic biology laboratory, and is relatively amenable to mutagenesis, transgene transformation, and genetic analysis. Moreover, it was the first plant to have its genome completely sequenced. Thus it is frequently used as a model organism in plant research.
 
Liu points out that the Arabidopsis mutants lacking the plant Pi transporter Type 5 accumulated more Pi in the cytoplasm. Conversely, overexpression of the Pi transporter Type 5 increases the Pi content in the vacuole and decreases the Pi content in the cytoplasm, leading to the impaired cytoplasmic Pi homeostasis. In other words, a large amount of the Pi transporter Type 5 protein in Arabidopsis facilitates entry of Pi into the vacuole and the storage therein. However, when the external environment provides an adequate supply of Pi, the Arabidopsis plants overexpressing the plant Pi transporter Type 5 accumulate Pi in the vacuole, where it cannot be effectively utilized, so that the plant cell senses the shortage of cytoplasmic Pi, thereby activating the Pi deficiency-responsive genes that retarded plant growth. Taken together, they showed that Pi enters into the vacuole through the plant Pi transporter Type 5.
 
MRI is very commonly used in medical research, but this time it was exploited to open a new avenue for plant study. Liu says that in the past some plant research was conducted using nuclear magnetic resonance (NMR)—the predecessor technology of MRI—but its use with living plants was often subject to limitations of space allowed for placing samples. Wang’s participation in the project was initiated by Professor Rong-Long Pan of NTHU’s College of Life Science.
 
According to Prof. Wang, the main work of his laboratory is to develop imaging technology to measure neural activity and the flow of blood in the brain. Although he has previously allowed his students to use fruits and vegetables to practice scanning, he never expected that this molecular imaging technology could have a practical application in plant science and lead to an important breakthrough.
 
As for the practical future benefits of their discovery, Liu says that it can be used to improve agricultural practices by enhancing the efficient usage of the internal Pi required for plant growth, thereby making it possible to reduce the use of Pi fertilizers in agriculture—one of the major sources of soil pollution. The research findings also create a new territory in the study of vacuolar Pi transporters.
 
Vacuole phosphate transporter protein, in fluorescent green, fusing with protein in (a) Arabidopsis mesophyll cells (without the cell wall); (b) tobacco epidermal cells; and (c) the root of transgenic Arabidopsis (i.e., with a large number of vacuoles containing phosphate transporter).

Vacuole phosphate transporter protein, in fluorescent green, fusing with protein in (a) Arabidopsis mesophyll cells (without the cell wall); (b) tobacco epidermal cells; and (c) the root of transgenic Arabidopsis (i.e., with a large number of vacuoles containing phosphate transporter).

Liu Tzu-yin, assistant professor in the Institute of Bioinformatics and Structural Biology

Liu Tzu-yin, assistant professor in the Institute of Bioinformatics and Structural Biology

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