![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
U. S. Department of Agriculture, Agricultural Research Service, U. S. Plant, Soil and Nutrition Laboratory, Ithaca, NY 14853
2To whom correspondence should be addressed. E-mail: rmw1{at}cornell.edu
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
KEY WORDS: • micronutrient malnutrition • trace minerals • agricultural productivity • bioavailability • sustainability
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
,2
). These tools should be fully exploited by the nutrition and public health communities to combat micronutrient malnutrition (3
). Biofortifying crops that feed the world’s poor can significantly improve the amount of these nutrients consumed by these target populations (4
). Additionally, increasing the micronutrient metals stored in seeds and grains of staple food crops increases the yield potential of these crops when they are sown to micronutrient-poor soils (5
). Much of the developing world has significant areas of such soils (6
). Enhancing seeds with micronutrient metals will act as an incentive to farmers affected by micronutrient-poor soils to adopt seeds that are micronutrient-enriched for use in their cropping systems (3
).
Genetically modifying plants in ways that will increase the density of Fe and Zn in edible portions of seeds and grains requires that several barriers to metal accumulation within the plant be overcome (7
). Therefore, the physiological processes controlling metal accumulation by plants is discussed below. Furthermore, because plant foods contain substances (i.e., antinutrients and promoters;Tables 1
and 2
) that influence the bioavailability of these nutrients to humans, it is necessary to demonstrate the efficacy of micronutrient enrichment of plant foods toward meliorating the nutritional health of targeted populations. (Bioavailability is defined as the amount of a nutrient that is potentially available for absorption from a meal and once absorbed, utilizable for metabolic processes in the body.) This requires that the bioavailability of Fe, Zn and provitamin A carotenoids in select micronutrient-enriched genotypes of staple plant foods must be determined before advancing genotypes (3
).
|
|
|
Physiological bases for micronutrient accumulation |
|---|
|
TOPABSTRACTINTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
,7
–10
). A brief outline of the processes that determine micronutrient element concentrations in edible plant tissues follows.
There are several barriers to overcome in genetically modifying plants to accumulate more micronutrient metals (e.g., Fe and Zn) in their edible parts (7
). These barriers to micronutrient metal uptake and distribution in plants are the result of tightly controlled homeostatic mechanisms that regulate metal uptake and distribution in plants, allowing adequate but nontoxic levels of these nutrients to accumulate in plant tissues. The first and most important barrier to micronutrient uptake resides at the root-soil interface (i.e., the rhizosphere). To increase micronutrient metal uptake by roots, the available levels of the micronutrient in the root-soil interface must be increased to allow for more absorption by root cells. This could be enhanced by stimulating certain root-cell processes that modify micronutrient solubility and movement to root surfaces, such as by stimulating the rate of root cell efflux of H+, metal-chelating compounds and reductants, and by increasing root absorptive surface area such as the number and extent of fine roots and root hairs. Second, the root-cell plasma membrane absorption mechanisms (e.g., transporters and ion channels) must be sufficient and specific enough to allow for the accumulation of micronutrient metals once they enter the apoplasm (i.e., the intercellular free space between cells) of root cells from the rhizosphere. Third, once taken up by root cells, the micronutrients must be efficiently translocated to edible plant organs. For seeds and grains, phloem sap loading, translocation and unloading rates within reproductive organs are important characteristics that must be considered in increasing micronutrient metal accumulation in edible portions of seeds and grains (11
). Finally, to be effective, the forms that the micronutrient metals are accumulated in edible portions of seeds must be bioavailable to people that eat the seeds (5
,11
).
|
Breeding for micronutrient enrichment: the question of yields |
|---|
|
TOPABSTRACTINTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
) and is reviewed by Gregorio in this volume.
The effects of breeding for micronutrient-dense staple seeds and grains on crop yields have been addressed in a number of recent reviews (8
,12
–14
). Briefly, increasing the micronutrient stores in seeds increases seedling vigor and viability, which enhances the performance of seedlings when seeds are planted in micronutrient-poor soils. This improved seed vigor allows for the production of more and longer roots under micronutrient-deficient conditions, allowing seedlings to scavenge more soil volume for micronutrients and water early in growth, an advantage that can lead to improved yields compared with seeds with low-micronutrient stores when grown under micronutrient deficiency stress. Figure 1
presents an example of such enhancement of crop productivity through micronutrient enrichment of seeds. In the experiment depicted, micronutrient-enriched wheat grain were sown to farmers’ fields having micronutrient-poor soils in three farming regions of Bangladesh. Seven of the nine farms had significant increases in wheat grain yield when produced from micronutrient-enriched grain compared with control or farmers’ grain not so enriched, clearly showing the advantage of using micronutrient-enriched grain on wheat productivity in these regions of Bangladesh.
|
). Thus, improving seed vigor with respect to micronutrient stores should be very beneficial to agricultural production in these countries. Additionally, disease resistance and stress tolerance are improved in micronutrient-dense seeds, which would also aid agricultural production in target countries (5
,11
). Thus, selecting for these traits in staple food crops is a win-win opportunity. It has the potential to enhance crop yields without additional farmer inputs and to improve their nutritional quality at the same time. |
Bioavailability issues |
|---|
|
TOPABSTRACTINTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
). Because of this complexity, the data obtained using various bioavailability model systems are always ambiguous (16
,17
). Only data obtained on reducing the prevalence of micronutrient deficiencies among those afflicted using feeding trials in test populations under free living conditions can delineate the actual efficacy of using micronutrient-enriched varieties of plant foods as an intervention tool. However, it is impractical to test the bioavailability of selected micronutrients in numerous genotypes of staple plant foods that can be generated in plant-breeding programs in this way (3
,8
). Micronutrient inhibitor and promoter substances.
Plant foods (especially seeds and grains) contain various antinutrients (Table 1)
in differing amounts depending on both genetic and environmental factors that can reduce the bioavailability of dietary nonheme Fe, Zn and other nutrients to humans (18
). Dietary substances that promote the bioavailability of Fe and Zn in the presence of antinutrients are also known whose levels are also influenced by both genetic and environmental factors (Table 2)
. Current plant molecular biological and genetic modification approaches now make it possible to reduce or eliminate antinutrients from staple plant foods or to significantly increase the levels of promoter substances in these foods (19
). Given these options (i.e., to decrease antinutrients or to increase promoters in staple plant foods), which is the wisest path to pursue?
Plant breeders could breed for genotypes that contain lower concentrations of antinutrients or molecular biologists could alter plant genes in ways that reduce or even eliminate antinutrients from plant food meals. However, doing so is not without risk and should be done with caution because many antinutrients are major plant metabolites that may play important roles in plant metabolism, in plant abiotic stress resistance and in plant resistance to crop pests or pathogens. Additionally, some of the antinutrients, such as phytate and polyphenols, may play important beneficial roles in human diets by acting as anticarcinogens or by promoting health in other ways, such as decreasing the risk of heart disease or diabetes (20
–22
). Thus, plant breeders and molecular biologists should be aware of the possible negative consequences of changing antinutrients in major plant foods before they attempt to alter food crops in this manner (8
,23
).
Other substances, as shown in Table 2
, promote the bioavailability of micronutrients in plant foods to humans even in the presence of antinutrients in those foods. Many of these compounds are normal plant metabolites and only small changes in their concentration may have significant effects on the bioavailability of micronutrients (24
). Therefore, it is highly recommended that plant breeders and molecular biologists closely scrutinize the strategy of increasing promoter substances in food crops when attempting to improve food crops as sources of micronutrients for people (23
).
Bioavailability enigmas.
Anomalies in some studies of the effects of various dietary factors on micronutrient bioavailability suggest that some aspects of the current dogma concerning the mechanisms of action of some antinutrients in plant foods on Fe and Zn bioavailability are not fully understood. Under some experimental conditions, antinutrients, such as phytic acid, do not have large negative effects on Fe and Zn bioavailability in human subjects. The reasons for these anomalous findings are unknown. For example, Table 3
lists some data reported by Morris and Ellis (25
) from a study with humans fed either low- or high-phytate muffin diets. As expected, the subjects fed the dephytinized muffins remained in positive Fe balance for the entire period that they were fed the dephytinized muffins. Interestingly, subjects fed the high-phytate diet during the first 5 d were in negative Fe balance (as expected), but by study d 10, these same subjects demonstrated positive Fe balances (which was not expected), suggesting that there must have been some adaptation to the high-phytate meals in these test subjects. Others have reported similar results from balance studies to those of Morris and Ellis (26
).
|
presents some recent findings reported by Manary et al. (27
) concerning phytate action on Zn bioavailability in the human gut (i.e., precipitation of Zn as a Zn-phytate complex in the lumen of the small intestine making it unavailable for absorption). They studied the effects of phytate on Zn homeostasis in four groups of children: two groups recovering from tuberculosis (27
) were fed high-phytate and low-phytate diets and two groups that were well but in the hospital for elective surgery and other treatments were fed high-phytate and low-phytate diets. As expected, in the children recovering from tuberculosis fed meals high in phytate, fractional Zn absorption, total Zn absorption and net Zn absorption were significantly reduced, while endogenous fecal Zn decreased compared with those fed low-phytate diets. However, unexpectedly, for the well children fed a high-phytate diet, more phytate had no effect on fractional Zn absorption, while total Zn absorption and net Zn absorption were higher, compared with well children fed the low-phytate diet. A major difference between the recovering children and the well children was the fact that the recovering children had received four potent antibiotics for over 60 d, whereas the well children received none. This suggests that the activity of microorganisms in the gut may have a large influence on the effects of phytate in meals on Zn bioavailability. Possibly, certain microorganisms in the gut may have active phytases that hydrolyze phytate making it inactive toward Zn absorption from the gut. If this is found to be the case, the composition of the diet and how it affects the microorganism population in the gut may be an important factor in determining the effects of phytate on Fe and Zn bioavailability. Further studies are needed to clarify this possibility.
|
|
Conclusion |
|---|
|
TOPABSTRACTINTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
|
FOOTNOTES |
|---|
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONPhysiological bases for...Breeding for micronutrient...Bioavailability issuesConclusionLITERATURE CITED |
|---|
1. Bouis, H. E. (2000) Enrichment of food staples through plant breeding: a new strategy for fighting micronutrient malnutrition. Nutrition 16:701-704.[Medline]
2. Combs, G. F., Jr, Welch, R. M., Duxbury, J. M., Uphoff, N. T. & Nesheim, M. C. (1996) Food-Based Approaches to Preventing Micronutrient Malnutrition: An International Research Agenda 1996:1-68 Cornell International Institute for Food, Agriculture, and Development, Cornell University Ithaca, NY. .
3. Graham, R. D., Welch, R. M. & Bouis, H. E. (2001) Addressing micronutrient malnutrition through enhancing the nutritional quality of staple foods: principles, perspectives and knowledge gaps. Adv. Agron. 70:77-142.
4. Welch, R. M. & Graham, R. D. (1999) A new paradigm for world agriculture: meeting human needs—productive, sustainable, nutritious. Field Crops Res 60:1-10.
5. Welch, R. M. (1999) Importance of seed mineral nutrient reserves in crop growth and development. Rengel, Z. eds. Mineral Nutrition of Crops: Fundamental Mechanisms and Implications 1999:205-226 Food Products Press New York, NY. .
6. White, J. G. & Zasoski, R. J. (1999) Mapping soil micronutrients. Field Crops Res 60:11-26.
7. Welch, R. M. (1995) Micronutrient nutrition of plants. Crit. Rev. Plant Sci. 14:49-82.
8. Graham, R. D. & Welch, R. M. (1996) Breeding for Staple-Food Crops with High Micronutrient Density: Agricultural Strategies for Micronutrients Working Paper 3 1996:1-72 International Food Policy Research Institute Washington, DC. .
9. Marschner, H. (1995) Mineral Nutrition of Higher Plants 1995:1-889 Academic Press London, UK. .
10. Yang, X. & Römheld, V. (1999) Physiological and genetic aspects of micronutrient uptake by higher plants. Gissel-Nielsen, G. Jensen, A. eds. Plant Nutrition—Molecular Biology and Genetics. Proceedings of the Sixth International Symposium on Genetics and Molecular Biology of Plant Nutrition 1999:151-186 Kluwer Academic Publishers Dordrecht, The Netherlands .
11. Welch, R. M. (1986) Effects of nutrient deficiencies on seed production and quality. Adv. Plant Nutr. 2:205-247.
12. Bouis, H. (1996) Enrichment of food staples through plant breeding: a new strategy for fighting micronutrient malnutrition. Nutr. Rev. 54:131-137.[Medline]
13. Graham, R. D., Senadhira, D., Beebe, S. E. & Iglesias, C. (1998) A strategy for breeding staple-food crops with high micronutrient density. Soil Sci. Plant Nutr. 43:1153-1157.
14. Graham, R. D., Senadhira, D., Beebe, S., Iglesias, C. & Monasterio, I. (1999) Breeding for micronutrient density in ed 1999 ible portions of staple food crops conventional approaches. Field Crops Res. 60 57–80. .
15. Fairweather-Tait, S. & Hurrell, R. F. (1996) Bioavailability of minerals and trace elements. Nutr. Res. Rev. 9:295-324.
16. House, W. A. (1999) Trace element bioavailability as exemplified by iron and zinc. Field Crops Res 60:115-141.
17. Van Campen, D. R. & Glahn, R. P. (1999) Micronutrient bioavailability techniques: accuracy, problems and limitations. Field Crops Res 60:93-113.
18. Welch, R. M. & House, W. A. (1984) Factors affecting the bioavailability of mineral nutrients in plant foods. Welch, R. M. Gabelman, W. H. eds. Crops as Sources of Nutrients for Humans 1984:37-54 American Society of Agronomy Madison, WI. .
19. Forssard, E., Bucher, M., Mächler, F., Mozafar, A. & Hurrell, R. (2000) Review: potential for increasing the content and bioavailability of Fe, Zn and Ca in plants for human nutrition. J. Sci. Food Agric. 80:861-879.
20. Saied, H. T. & Shamsuddin, A. M. (1998) Up-regulation of the tumor suppressor gene p53 and WAF1 gene expression by IP6 in HT-29 human colon carcinoma cell line. Anticancer Res. 18:1479-1484.[Medline]
21. Shamsuddin, A. M. (1999) Metabolism and cellular functions of IP6: A review. Anticancer Res 19:3733-3736.[Medline]
22. Zhou, J. R. & Erdman, J. W., Jr (1995) Phytic acid in health and disease. Crit. Rev. Food Sci. Nutr. 35:495-508.[Medline]
23. Welch, R. M. (1996) Viewpoint: the optimal breeding strategy is to increase the density of promoter compounds and micronutrient minerals in seeds; caution should be used in reducing antinutrients in staple food crops. Micronutr. Agric. 1:20-22.
24. Welch, R. M. & House, W. A. (1995) Meat factors in animal products that enhance iron and zinc bioavailability: implications for improving the nutritional quality of seeds and grains. 1995 Cornell Nutrition Conference for Feed Manufacturers 1995:58-66 Department of Animal Science and Division of Nutrition, Cornell University Agricultural Experiment Station Ithaca, NY .
25. Morris, E. R. & Ellis, R. (1982) Phytate, wheat bran, and bioavailability of dietary iron. Kies, C. eds. Nutritional Bioavailability of Iron 1982:121-141 American Chemical Society Washington, DC. .
26. Apte, S. V. & Venkatachalam, P. S. (1962) Iron absorption in human volunteers using high phytate cereal diet. Indian J. Med. Res. 50:516-520.
27.
Manary, M. J., Hotz, C., Krebs, N. F., Gibson, R. S., Westcott, J. E., Arnold, T., Broadhead, R. L. & Hambidge, K. M. (2000) Dietary phytate reduction improves zinc absorption in Malawian children recovering from tuberculosis but not in well children. J. Nutr. 130:2959-2964.
This article has been cited by other articles:
|
|
 |
|
  S. K. Thakkar, B. Maziya-Dixon, A. G. O. Dixon, and M. L. Failla {beta}-Carotene Micellarization during in Vitro Digestion and Uptake by Caco-2 Cells Is Directly Proportional to {beta}-Carotene Content in Different Genotypes of Cassava J. Nutr., October 1, 2007; 137(10): 2229 - 2233. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. M Beiseigel, J. R Hunt, R. P Glahn, R. M Welch, A. Menkir, and B. B Maziya-Dixon Iron bioavailability from maize and beans: a comparison of human measurements with Caco-2 cell and algorithm predictions Am. J. Clinical Nutrition, August 1, 2007; 86(2): 388 - 396. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Yasuda, K. R. Roneker, D. D. Miller, R. M. Welch, and X. G. Lei Supplemental Dietary Inulin Affects the Bioavailability of Iron in Corn and Soybean Meal to Young Pigs J. Nutr., December 1, 2006; 136(12): 3033 - 3038. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. A. Underwood Scientific Research: Essential, but Is it Enough to Combat World Food Insecurities? J. Nutr., May 1, 2003; 133(5): 1434S - 1437. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. Lonnerdal Genetically Modified Plants for Improved Trace Element Nutrition J. Nutr., May 1, 2003; 133(5): 1490S - 1493. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
