SEB Bulletin March 2007
You are what you eat
Many people of the Western World and those in developing countries are at risk of poor nutrition; the former because they choose to eat foods of low nutritional value, the latter because many of their staple crops such as cereals are nutrient-poor. Plants are the major source and, in some cases, the sole source of nutrients required for human health (ref 1).
Like humans, plants require many nutrients (which can be divided into two categories; metabolites and minerals) for healthy growth and development, (ref 2,3,4,5). Researchers are working towards increasing levels of important nutrients and reducing toxic chemicals and anti-nutrients (molecules that reduce/prevent nutrient metabolism or absorption in thegut) in the edible parts of plants in order to address this problem (ref 1). In addition, an understanding of plant nutrition could potentially result in increased crop yields, a reduction in fertiliser use and the ability to grow crops in currently infertile soils.
In recent years advances in genomics, such as the sequencing of whole genomes, repositories of expressed sequences and microarrays (ref 6), has seen the emergence of a new discipline of plant nutrition, called plant nutritional genomics (ref 7,8). Defined as "the interaction between a plant's genome and its nutritional characteristics" by Dr. Martin Broadley of Nottingham University (ref 9) and Professor Philip White of Scottish Crop Research Institute (ref 10), plant nutritional genomics is facilitating a much wider understanding of plant nutrition and its underlying genetic basis.
Metabolites
Metabolites are the intermediates and products of metabolism and include vitamins, which are not only important for plant growth, development and defense responses, they are also vital for human nutrition (ref 11). Vitamin supplementation, food fortification and dietary diversification are often not feasible solutions to nutrient malnutrition so an alternative has been suggested; biofortification, the process of increasing the concentrations of important nutrients in the edible portions of crops (ref 12). A successful and perhaps a little controversial example of biofortification is Golden rice, a variety of rice genetically engineered to synthesise increased amounts of pro-vitamin A precursors, designed to be used where vitamin A deficiency, and its accompanying complications such as blindness, is common (ref 13). However, before biofortification can begin to be used to its fullest potential a more detailed knowledge of metabolic pathways is required.
A prevalent nutrient deficiency is a lack of folate, a deficiency that is common even in developed countries. It has severe consequences for health, including anaemia and cardiovascular disease and can cause neural tube defects (e.g. spina bifida and anecephaly) during foetus development. Folates are a group of compounds that belong to the B vitamin family and play a significant role the synthesis of nucleic and amino acids. They are especially important when cells are rapidly dividing such as during infancy and pregnancy (ref 14,15,16).
Unlike plants, humans are unable to synthesise folate and therefore have to obtain sufficient amounts through their diet. For this reason some countries, including the USA, fortify wheat flour with folic acid, a synthetic folate. “Although folic acid is readily used by humans, there is considerable pressure to replace this with naturally occurring folates” comments Professor Gregory Tucker of Nottingham University (ref 17), which has “led to research into methods to enhance levels in plant foods”.
Professor Andrew Hanson and his group at the University of Florida (ref 18) genetically engineered tomato plants to over-express genes involved in the synthesis and transport of folate molecules and their precursors, which successfully increased folate levels in the fruit. However, as folates are readily degraded, Prof. Hanson's group are currently trying to characterise the genes involved in folate degradation and recycling in the hope of further enhancing folate levels by preventing their breakdown (ref 19). “Although there has been some success through the application of genetic engineering, this could meet with consumer resistance” cautions Prof. Tucker who, along with Prof. Malcolm Bennett (ref 20) and other colleagues, is investigating non-transgenic approaches for folate biofortification. “There is also the possibility (although this is controversial) that the form of the folate may impact on either folate stability post harvest and/or bioavailability from the diet”, Dr. David Barrett, also of Nottingham University and his colleagues have therefore developed a system capable of quantifying up to 80 different forms of folate. This group at Nottingham have also demonstrated a four-fold natural variation in the levels of folate in the model plant Arabidopsis and presume similar levels of variation will be found within crops, which they hope to be able to exploit and using quantitative trait loci (QTL) analysis discover the genetic basis for this variation (ref 21). This knowledge could then be used to develop biofortified crops through breeding programmes.
Minerals
Understanding plant mineral nutrition is important as mineral deficiencies are extremely common in humans with up to 80% of the world deficient in iron (ref 22,23). Minerals can only enter the food chain via plants, therefore understanding how plants mobilise minerals from the soil, transport and store them it may be possible to develop food crops with increased mineral content. In addition, plants with the ability to store elevated levels of minerals could also be used to clean soils contaminated with toxic levels of minerals (bioremediation). Finally, not all soils are sufficiently mineral-rich and require fertilisation for crop growth, which can be costly and can have detrimental environmental impacts. Perversely, even infertile soils contain high enough mineral concentrations to support crop growth, but are in forms unavailable to plants. Thus there is considerable interest in deciphering how plants obtain minerals, to develop varieties capable of growing on currently infertile soils.
Professor David Salt of Purdue University (ref 24), a pioneer of plant nutritional genomics, along with colleagues developed a method to simultaneously measure the concentration of multiple minerals in and plants and coined the term “ionomics”, the study of all of the metals, metalloids and non-metallic ions in an organism. By systematically screening thousands of Arabidopsis mutants and wild-type lines they identified 51 mutants and many natural variants that differ in mineral content and uptake. They estimate that 5% of the Arabidopsis genome is involved in regulating mineral content and the observation that only 11% of their mutants have altered levels in just one mineral suggests that these genes are likely to work in complex networks (ref 25). The work to discover the causative genes of the mutant phenotypes is ongoing. Recently they successfully employed DNA microarrays, which allowed the simultaneous genotyping of thousands of loci, to identify the gene (AtHKT1) that is responsible for the natural variation in sodium accumulation (ref 26).
Global studies of mineral nutrition are undoubtedly vital for the rapid discovery of candidate genes, however, more focused studies are important as they are able to address more specific issues, for example selenium (Se) deficiency (ref 27).
“Over the last 20 years the UK population has seen a steady decrease in selenium (Se) intake that is largely attributable to the consumption of increasing amounts of flour from wheat grown in the UK (on Se-poor soils) instead of wheat grown in the Se-rich soils of North America” points out Dr. Martin Broadley, who is currently investigating ways to increase the selenium content of UK wheat grain. Reducing Se deficiency is important as it is associated with a wide variety of health disorders including oxidative stress-related conditions, reduced fertility, impaired immune functions and an increase in cancer. By studying fertilisation regimes and the effect of genotype on Se accumulation, it is hoped an efficient method of Se biofortification can be achieved.
Toxic chemicals in food
Not all plant nutritional genomic research is concerned with increasing the bioavailability of beneficial compounds, indeed, in some cases quite the opposite is true. In 2002 Professor Margareta Törnqvist of Stockholm University (ref 28) and her colleagues announced that they had discovered the presence of acrylamide in food and more specifically foodstuffs cooked at high temperatures (baked, roasted and fried) (ref 29,30). Acrylamide is believed to pose a serious cancer risk and exposure in the workplace is known to cause damage to the nervous system, however its effects through the diet are largely unknown.
This discovery set in motion a wave of research to understand how acrylamide is formed, its affect on humans through the diet and what can be done to reduce it in food. It was soon found that acrylamide is formed as a result of the amino acid asparagine taking part in the Maillard reaction (ref 31); a heat induced chemical reaction between an amino acid and a reducing sugar (e.g. glucose, lactose, maltose but not sucrose) and that the highest levels of acrylamide are found in carbohydrate rich foods e.g. those derived from cereals and potatoes. Professor Nigel Halford of Rothamsted Research (ref 32), who is conducting research to reduce the levels of acrylamide in food, believes its discovery “can be viewed as a positive”. “This is not a new risk, but with this new knowledge science and technology will be able to make food safer”. Dr. Nira Muttucumaru, of Prof. Halford's laboratory and colleagues, demonstrated that when heated to above 1600C the flour from wheat plants grown under sulphate-deprivation contain higher levels of acrylamide than plants grown under normal condition. Thus, even growth conditions can significantly affect the levels of acrylamide formed in the final cooked product. They demonstrated that sulphate-deprivation resulted in dramatic increases in the grain concentration of free asparagine, the precursor for acrylamide formation. Dr. Muttucumaru is furthering this research by over-expressing the gene encoding asparaginase, an enzyme which degrades asparagine to aspartate and water “by overexpressing asparaginase we hope to limit the amount of acrylamide formed”.
It is important to note that to benefit human health it is not simply a case of increasing beneficial nutrients and reducing toxic chemicals and their pre-cursors. The Maillard reaction for example is also responsible for generating many desirable colour and flavour molecules. Also in excess minerals can be toxic as Dr. Ruan Elliott of the Institute of Food Research is keen to point out, “more is not necessarily better, there is likely to be a window of what is good and this will vary with individuals”. Dr. Elliott is currently investigating how the human body responds to various nutrients, as part of the European project NuGo (ref 33) and believes the key to success in reducing worldwide malnutrition is bringing plant and human nutritionists together.
Despite being a relatively new discipline, plant nutritional genomics has already added a great deal to the understanding of plant nutrition and seems likely to continue to do so and will undoubtedly yield results that will be beneficial to both human health and the environment.
Rebecca Poole
University of Bristol
References
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