NIP aquaporins: new tools to reduce rice arsenic accumulation

Rice is the staple food for about half of the world's population. Among major food crops, rice is especially efficient at the accumulation of arsenic which is toxic and carcinogenic. This accumulation presents a potentially serious health risk, because consumption of rice contributes a large proportion of inorganic As intake for people living on a rice-diet anywhere in the world. The problem is exacerbated in many rice-producing regions by the past use of arsenic-based herbicides and insecticides, mining, and irrigation with arsenic-contaminated groundwater. There is an urgent need to develop strategies to reduce this widespread contamination of the food chain. This requires a better understanding of the mechanisms responsible for uptake, transport and distribution of arsenic into rice grain. Unlike aerobic soils where arsenate is the predominant chemical species of arsenic, the arsenite form dominates in the reducing environment of flooded paddy soils. We have recently discovered that arsenite is taken up by rice roots through the silicon uptake pathway. Rice accumulates a large amount of silicon, which protects the plant against biotic and abiotic stresses. An aquaporin channel protein called NIP2;1 transports silicon, and also inadvertently arsenite, into the root cells. There is a family of 10 NIP proteins in rice, some of which are expressed mainly in leaf and grain tissues. We hypothesise that some of these NIP channel proteins are involved in arsenic transport to the rice grain. We will evaluate the role of NIP proteins in arsenic distribution to the leaf and rice grain using a range of molecular and physiological methods. We will investigate the pattern of expression of different NIP genes in leaf and grain tissues during grain development and the transport function of NIP proteins for arsenic compounds. We will determine the specificity of different NIP proteins for arsenic compounds and manipulate the amino acid composition in a key region of the proteins to alter their transport selectivity for arsenic. Results obtained from this project will provide insight into the mechanisms of arsenic transport in plants and help the development of counter measures to reduce arsenic accumulation in rice grain through molecular breeding or transgenic approaches.

Paddy rice accumulates more arsenic than other cereal crops because arsenite is mobilised in flooded soil and then taken up by the highly efficient silicon uptake pathway. We have recently discovered that the aquaporin NIP2;1 (Nodulin 26-like Intrinsic Protein), a silicon transporter, also mediates the influx of arsenite into root cells of rice. Preliminary evidence suggests that arsenite permeability is more widespread among NIP proteins than silicon permeability. Based on preliminary evidence that we have obtained, we hypothesise that NIP proteins are involved in the distribution of arsenite and methylated arsenic species to leaf and grain tissues and that the relative specificity for arsenic and silicon is determined mainly by the amino acid composition of the aromatic/arginine filter of the NIP proteins. We will investigate the functions and substrate specificity of different rice NIP proteins using heterologous systems of Xenopus oocytes and yeast, and establish the structure-function relationship using site-directed mutagenesis to manipulate the amino acid composition of the aromatic/arginine filter. We will evaluate the in planta functions of rice NIP genes in terms of arsenic distribution in leaf and grain tissues using loss of function mutants and transgenic plants with altered expression of specific genes. This proposal will provide important insight into the mechanism of arsenic transport in plants, particularly toward rice grain. The results can be used to develop counter measures to reduce arsenic accumulation in rice grain through molecular breeding or transgenic approaches.

Who will benefit from this research? Rice is a staple food for about half of the world population. Excessive accumulation of arsenic in rice poses a significant problem in terms of food chain contamination which has adverse impacts on human health. Recent studies have shown that intake of arsenic from rice can significantly elevate cancer risk. Methods to reduce arsenic accumulation will benefit people in the general public anywhere in the world who consume rice. This includes babies and children who consume rice-based products; in fact they are the most vulnerable group to arsenic contamination. Reduced arsenic accumulation by rice is likely to enhance the crop tolerance to arsenic and increase yields under arsenic stress; this can bring benefit to rice farmers in parts of Asia and Africa where soil and irrigation water are contaminated with arsenic, leading to a more sustainable use of natural resources. How will they benefit from this research? The proposed research will address the fundamental question of the mechanism of transport and distribution of arsenic to rice grain, which is currently poorly understood. This knowledge is essential for devising strategies to develop low arsenic rice. It is envisaged that key NIP genes involved in arsenic distribution to rice grain will be identified, and the structure-function relationship governing the selectivity of arsenic by NIP proteins established in this project. This information will greatly aid the development of low arsenic rice through molecular breeding or transgenic approaches. The impact of low arsenic rice on human health will be substantial and long-term. Use of GM technology to produce rice plants that avoid the accumulation of arsenic in the grain will be very important for improving public opinion on the beneficial use of this technology in food production and provision of a healthier diet. There will be a clear and simple advantage to health resulting from the plants produced in this research, potentially benefitting some of the poorer parts of the world. This research has the potential to grab the public's imagination and can be simply presented by the media. What will be done to ensure that they benefit from this research? Potential intellectual property will be identified and properly protected; Rothamsted and the University of York have offices to advise on this process. Non-patentable findings will be disseminated to the scientific community through conference presentations and peer-reviewed publications; to agronomists and crop breeders through a workshop, and our existing links with the south and southeast Asian countries and the International Rice Research Institute; and the wider public through the media, national and local events. Material Transfer Agreements will be arranged between the partners in this proposal. MTAs represent the first step in a process to safeguard the ownership and rights of the research organisations involved, liabilities and to cover any IP arising. This also covers other important aspects of working together such as acknowledgement of the source of the materials in any publication, provision of raw data, reports or publications or inventions relating to the materials and arising from the specific research programme. The agreement also requires subsequent discussion of any intellectual property arising, and negotiation on how it should be exploited, with what division of work related to its exploitation and the revenues between the partners. The supplier of materials will retain the right to use an Invention for non-commercial research purposes. The PI and Co/Is have relevant experience and track record in public dissemination of research findings. Any publicity activities will be coordinated through the Science Communication offices in both host organisations. Examples include recent highlights and interviews on our arsenic research in Nature, Science, New York Times, other websites, TV and radio.