Editorial |
Plant Nutrition—Roots of Life for Fundamental Biology and Better Crop Production
1Biotechnology Research Center, The University of Tokyo, Japan
2Solution-Oriented Research for Science and Technology (SORST), Japan Science and Technology Agency (JST), Japan
3Graduate School of Agriculture, Kyoto University, Japan
The world's population reached 6.7 billion in 2008 and continues to grow. 2008 was also a year marked by high food prices, and indeed food crises have arisen in developing countries. Given the increase in population and decrease in available arable land, the public expects us, the plant science community, to provide technologies that maintain and increase food production. Plants grow in the soil, take up mineral nutrients and generate food for us. Therefore, the uptake of mineral nutrients by plants from the soil is a critical step, in terms of both food production and global element cycling.
The human body contains about 1.5 kg of nitrogen atoms. Every human eats nearly three times this quantity of nitrogen every year in the form of protein, equivalent to 73 g of protein a day. The current world population consumes some 28 million tonnes of protein-nitrogen every year. Eighty-five percent of the nitrogen in food proteins comes from agriculture, either directly in plant-derived foods or indirectly via animals fed with plant material. Synthetic fertilizers derived from the Haber–Bosch synthesis of ammonia provide 44–51% of all the nitrogen absorbed by crops. Therefore, roughly 40% of nitrogen in foods derives from synthetic ammonia (Smil 2002
). Nitrogen fertilizers were first introduced using the few natural deposits of nitrate salts such as Chilean nitrate, and as ammonium sulfate produced as a by-product from gasworks, but the Haber process was established on an industrial scale in 1913 and has since ensured unlimited supplies from atmospheric nitrogen (Jenkinson 2001).
These figures indicate that nitrogen fertilizers are absolutely essential for human life on earth. Borlaug summed up the role that N fertilizers played in the Green Revolution by using a memorable kinetic analogy: ‘If the high-yielding dwarf wheat and rice varieties are the catalysts that have ignited the Revolution, then chemical fertilizer is the fuel that has powered its forward thrust’ (Borlaug 1970 cited by Smil 2002). Even though the requirement will increase further as the world's population grows, research into nitrogen metabolism in the soil has barely begun.
It is known that rice grain yield is a function of nitrogen uptake by rice plants, and a larger yield is attained only under higher nitrogen uptake (Haefele et al. 2008). On the other hand, it is also appreciated that the absolute efficiency of nitrogen uptake varies according to rice variety, and is thus controlled genetically. The molecular mechanism underlying this has not been fully revealed, even though all the enzymes involved in nitrate reduction and ammonium assimilation have been identified, along with their corresponding genes and several other regulatory genes. A more or less similar situation exists for other fertilizers, including P and K. Low-input, high yield production is desirable given the limited resources of fertilizers and energy consumption in production, transportation and application of fertilizers.
Plant nutrition is a complex process that has developed over the course of plant evolution. Plants support our life by extending their leaves into the air and their roots in the soil. Roots take up nutrients from the soil and transport them to the leaves to support photosynthesis. Most soils are poor in nutrients, and plants have evolved accordingly, regulating their transport systems depending on the nutritional conditions. In many cases, nutrient deprivation induces high affinity uptake systems. The development of roots and leaves is also influenced by nutritional conditions. In particular, it is well known that the root/shoot ratio and lateral root development are regulated by nutrition. For such regulatory systems to function, nutrient conditions need to be sensed, signals need to be transduced, gene expression needs to be transcriptionally and post-transcriptionally regulated, transporters must be properly trafficked through endomembrane systems, and cell cycles and cell elongation need to be coordinated. Metabolism is also under the influence of nutritional conditions. Such a wide range of responses may be a reflection of the very sophisticated systems that have evolved in plants over time. In other words, a proper understanding of plant nutrition requires an understanding of all of these processes.
In the last few decades, the field of plant nutrition has advanced rapidly, incorporating a wide range of plant sciences. At the same time, plant nutrition research has enormous potential to contribute to other plant and biological sciences. For example, the first boron transporters in living systems were identified in plants, and human and yeast boron transporters were identified subsequently (Takano et al., 2002). This transporter also contributed to the understanding of endomembrane trafficking systems (Takano et al. 2005). The recent discovery of silicon transporters and their subcellular localization attracted much attention (Ma et al. 2006, Ma et al. 2007). Another recent breakthrough in plant nutrition was the successful generation of plants that tolerate nutritional stresses (Takahashi et al. 2001, Yanagisawa et al. 2002, Miwa et al. 2006, Ishimaru et al. 2007, Miwa et al. 2007). Transgenic rice lines engineered to be tolerant to iron deficiency were successfully tested in the field (Kobayashi et al. 2008, Suzuki et al. 2008). Generation of nutrient stress-tolerant crops will contribute to increase yields; but, to achieve this goal, a wide range of nutrient studies need to be conducted and coordinated among the research community.
The demand for increased food production has a long history, and plant nutrition has played significant roles in responding to the challenge. Indeed, from soil and plant diagnosis to suggestions for appropriate fertilizer applications, current levels of food production would never have been possible without knowledge of plant nutrition. In developed countries where fertilization is optimized, however, traditional plant nutrition approaches can no longer boost food production any further. By incorporating new knowledge and technologies, the field of plant nutrition has reached a new level where crop production can potentially be improved without significant application of fertilizers. Such research will give us insights into not only plant physiology, but also the ways in which we can manipulate plants for better production.
In this special issue, we asked innovative plant nutritionists to report on their exciting work. All the manuscripts were peer reviewed. First, Mitani et al. describe silicon transporters in maize (pp. 5–12). This is an extension of the group's discovery of silicon transporters in rice. The silicon transporter is essential for normal growth of rice, and its interesting polar localization revealed the importance of membrane trafficking in plant nutrient transport. In the present study, silicon transporters in maize are described, extending the understanding of silicon transport mechanisms in crop plants. Next, Yuan et al. describe a pollen-specific ammonium transporter in Arabidopsis (pp. 13–25). The group has characterized ammonium transporters and the manuscript describes the unique cell-specific expression of an uncharacterized member of the ammonium transporter family. Koshiba et al. describe the involvement of oxidative stresses in cell death following boron deprivation (pp. 26–36). The group has made a significant contribution to boron physiology, especially regarding the roles of boron in cell walls. The present study may lead to a novel strategy to generate plants tolerant to low B conditions. Sawada et al. describe a metabolomic analysis of a number of plant species and characterize their properties (pp. 37–47). It is obvious that metabolism is an important area in plant science, but, due to the technical challenges of analysis, only a limited number of metabolites have been analyzed. Their report is a good example of how a novel research technology can contribute to our understanding of plant metabolism. Tsukamoto et al. describe real-time tracer analysis to visualize Fe movement in barley (pp. 48–57). This technology allows us to understand the movement of nutrients throughout plant development, which has not been possible until recently. Finally, Kato et al. describe the successful development of plants highly tolerant to low B conditions, through manipulation of boron transporters (pp. 58–66). Together, these articles cover a wide range of plant nutritional studies representing the current status of the field.
It is our view that with the recent demand for high quality laboratory research exploiting novel technologies, the importance of field science tends to be ignored. It is certainly important to understand more deeply how plants function, but we believe that there is no future for ‘in vitro’ plant biology on its own. Without accompanying knowledge generated under field conditions, we could not transfer our new understanding to real agriculture. We strongly believe that this final application is essential if we hope to answer the public's expectations. I hope this special issue will be perceived as an invitation for a wide range of plant scientists to focus their attention on plant nutrition, and that it will stimulate a wider exchange of ideas and expertise. We also hope that it will encourage the submission of more excellent studies to Plant and Cell Physiology in the field of nutrition in the future.
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  M. Matsuoka Greetings from the Editor-in-Chief Plant Cell Physiol., January 1, 2009; 50(1): 1 - 1. [Full Text] [PDF]
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