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Devising measures for effective use of phosphorus resources through rhizosphere science
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Associate Professor Jun Wasaki is pursuing research into the rhizosphere―the region of soil in the vicinity of plant roots. He points out that plant roots do not only absorb nutrients and water but also secrete organic acids and enzymes, whereby enhancing the nutrient uptake efficiency and influencing microorganisms living around the roots.
“In the rhizosphere, interactions between plants and microorganisms have been established. These plant-microbial interactions are greatly related to plants’ nutrient absorption,” says Associate Professor Wasaki. He added that since many things still remain unknown, his laboratory has carried out various studies both on plants and microorganisms.
Above all, he pays the greatest attention on research into “phosphorus.” Phosphorus is a fertilizer essential for plant growth, and is an important resource relevant to food production. However, Japan produces no phosphate rocks, which are the raw materials of phosphorus. In fact, Japan is totally dependent on imports for phosphate rocks, approximately 80% of which are used for the production of chemical fertilizers for agricultural use. We regret to hear that phosphorus reserves have been continuously declining, and are on the verge of depletion. |
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“Initially I began to study phosphorus nutrition at graduate school, when I realized the fact we were facing a crisis of phosphorus depletion. Since then, I have been pursuing phosphorus research, motivated by the aspiration to find a solution to this crisis, and to develop measures for effective use of phosphorus.
According to a predication by an economist, phosphorus resources will decrease after hitting a peak around 2030, so that phosphorus resources may be available only for the next few decades. On the other hand, most phosphorus applied as fertilizer remains accumulated in soil. Also in Japan, it is considered that there is a large quantity of phosphorus in such a form.
Associate Professor Wasaki says “If we can wisely harness the phosphorus that has been accumulated in soil, we may become able to deal with the phosphorus depletion crisis. The purpose of our research activities is to find such a solution.” |
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Exploring an effective solution to harness phosphorus that remains unused through the utilization of the rhizosphere
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The interviewer asked Associate Professor Wasaki to first explain in what forms phosphorus remains unused in soil.
He answered that “The element has accumulated in soil in the form of either ‘insoluble phosphates’ or ‘organic phosphates.’ Plant roots secrete organic acids that decompose insoluble phosphates, and enzymes that decompose organic phosphates. These root exudates will turn either form of phosphate into an easily-absorbed form―inorganic phosphate. Moreover, in the rhizosphere, microorganisms living there also play a role in promoting the use of inorganic phosphorus.”
In other words, the important function of the rhizosphere is to mobilize the unused phosphorus so as to provide plants with inorganic phosphates, which is the only available form for plants. In this respect, the laboratory of Associate Professor Wasaki conducts research activities regarding the functions of the rhizosphere, from various perspectives.
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| One of them is the research into the formation and functions of cluster roots that feature a high phosphorus uptake efficiency. This research uses a leguminous plant, white lupin (Lupinus albus L.), which has a particularly high ability to make use of phosphorus. |
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“By cultivating white lupin in special flat containers, we can observe the plant’s roots and study the changes in the soil around the roots. Characteristically, white lupin plants form many brush-like roots, called ‘cluster roots.’ These roots show a quite high ability to absorb phosphorus,” he says.
Using white lupin plants, his laboratory also carries out another research to seek effective use of rhizosphere functions aimed at agriculture with low environmental impact.
Associate professor Wasaki says that “We conduct experiments in which white lupin plants with a high phosphorus uptake capacity are intercropped with maize or other crops, so that these crops share the rhizosphere with white lupin. Through this experiment, we check if the phosphorus uptake of maize or other crops could be improved or not.”
A wide variety of other research activities are currently under way. These include: research into dauciform roots (clusters of carrot-shaped rootlets that are smaller than cluster roots); transgenic research on enzyme functions; research aimed to create better rice cultivars by crossing cultivars having high and low tolerance to phosphate deficiency; and research that attempts to clarify the mechanism of high tolerance to phosphate deficiency through comprehensive analysis using large equipment. |
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Some research results included new discoveries, providing us with the satisfaction of discovering something that no one had ever found before.
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These research activities have been carried out mainly through cultivation experiments, and have produced several positive results. Worthy of special attention are the following two discoveries.
“Past studies had already revealed that the transporters involved in the secretion of organic acids are proteins that reside in cell membranes. However, our laboratory became the first to discover the transporters that work under a state of phosphorus deficiency. In addition, when we conducted a field survey on Miyajima Island, one of my undergraduate students newly discovered that Helicia cochinchinensis, a tree belonging to the family Proteaceae, formed cluster roots.”
He adds that it is natural that discovering something new should provide him and his fellow students with a unique pleasure.
“My research activities originally started with a rather childish motivation―I just strongly wanted to resolve the issue of phosphorous depletion. Nevertheless, I was truly happy that my laboratory has been able to continue working on research activities constantly so far,” he says with a smile. Associate Professor Wasaki expresses, with great enthusiasm, his aspiration to further develop their research activities, toward clarification of the phosphorus uptake mechanisms.
“Eventually, I want to overcome the issue of phosphorous depletion by finding methods to enable crops to absorb phosphorous more efficiently, with the use of fertilizer reduced to the minimum amount possible.”
He admits, however, that it is difficult to completely resolve the challenge. Therefore for now he aims to find a solution to make more effective use of the phosphorous accumulated in soil by harnessing the inherent functions of plants, and put it into application for farmers. His laboratory has been attracting many highly-motivated students, and Associate Professor Wasaki is likely to be even more enthusiastic about instructing them. |
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| Jun Wasaki |
Professor
Laboratory of Assessment of Microbial Environment
April 1, 2003 - January 31, 2004: Postdoctoral Fellow (PD), Japan Society for the Promotion of Science
February 1, 2004 - September 30, 2007: Specially Appointed Associate Professor, Creative Research Initiative “Sousei,” Hokkaido University
October 1, 2007 – present: Associate Professor, School of Applied Biological Science, Hiroshima University
April 1, 2016 – present: Professor, School of Applied Biological Science, Hiroshima University
Posted on Mar 26, 2015
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