maize: eco-conscious genetic modification

Yun young Hwang

Bio 213

Research Paper – Maize

15 June 2011

Maize (Poaceae, Zea Z. mays): eco-conscious genetic modification.  

Genetic modification (GM) is a significant scientific advancement. It has great impacts in global food systems and ecosystems. Affects of GM produce in our diets and ecosystem need to be analyzed and directed consciously. Maize is a common GM produce. Affects of GM maize diets and fate of GM maize protein in ecosystems will be examined.

Maize; Zea Z. mays, belongs to the order Poales and family Poaceae. The family poaceae is in the class Liliopsida of flowering plants are generally known as the grasses. Approximately 700 genera and 10,000 species belong to the grass family. The USDA lists 338 generas and 1935 accepted taxa overall for the Poaceae family. Most well known are the cereals, wheat Triticum, corn Zea, rice Oryza, and Sorghum Sorghum bicolor. Poaceae are perennial herbs, but many are also annuals (USDA 2011). Nearly three-quarters of grasses are distributed in Africa, Australia, Eurasia north of Himalayas, Southeast Asia, North America, temperate South America, and tropical America. A broader distribution pattern is found throughout temperate or tropical conditions (Britannica 2011).   

Corn varieties are directly and indirectly used mostly for food, livestock feed, ethanol production, and ingredients. Main ingredients produced from are starch, corn oil, corn syrup, corn flour, and beverage alcohol (Iowa Corn 2011). Corn is a commonly fed grain in United States beef cattle. Relative to other feed grains, corn is lower in protein and slightly higher in energy. Due to its lower protein content, corn based diets may require additional source of protein (NDSU 2002).

Livestock producers use corn feed in attempts of reducing cost of production. Corn can be “grazed” any time of the year regardless of seasons. In terms of cattle feeding; terms grass-fed and corn-fed are misleading because corn is grass. However the distinctions are clear when considering the corn fed cattle are in concentrated animal feeding operations (CAFOs). CAFO cattle are typically fed high starch contents from corn or soy diets (NDSU 2002). When cattle are completely dependent on feed diet; nutrient variety is limited.

Ethanol is a high octane fuel. At its most basic level, ethanol is grain alcohol and is mostly produced from corn. It is promoted as the domestically produced biomass-fuel which will help America’s independence for energy source. Ethanol is blended into 46% of American gasoline. It also comprises 3.5% of total United States gas consumption which is 140 billion gallons each year (Iowa Corn 2011). Sustainability of corn-based bio-ethanol is highly debated. Large amounts of arable land, fertilizer, water, and pesticides are needed for efficient mass-production of corn. Environmental impacts of such chemicals and land fertility are of concern. Contrastingly, ethanol readily biodegrades without environmental harm and is a replacement fuel for additives such as methyl tert-butyl ether (MTBE) (USDE 2011).

Poaceae family shares several distinctive morphological characteristics. Poaceaes have parallel leaf venation and absent sepals and petals. Root systems consist of fine and fibrous taproots. Stems are hollow and round. Leaf sheaths enclose the stems (Ohio State University 2005). Niklas’s study showed such clasping of leaf sheaths were largely responsible for the stem of the grass species Arundinaria tecta’s stiffness. On average, leaf sheaths contributed 33% of the bending stiffness and 43% of overall torsional stiffness of the stem segments by acting as an “external cylindrical brace.” Materials of sheaths are stiffer than that of the internodes which they envelope. Such construction of the stem allows support of the potentially-weak hollow stem. It also provides flexibility of twisting through winds (Niklas 1998).

Grass flowers are relatively simple and small. They are hidden with lemma and palea and can be seen briefly during some parts of flowering. Opening of floret allows exertion of pollen sacs (anthers) on their filaments and stigmas for cross-pollination (Britannica 2011). Grasses are wind-pollinated (Carter 2004).

Grass plants have equivalent shoots. The shoot system consists of the shoot apex and embryonic leaves (NDSU 2011). The shoot apical meristem (SAM) is a congregation of stem cells which generate shoot-derived organs. The SAM initiates leaves and preserves the stem cell pool (American Society of Plant Biologists 2011). The embryonic leaves are covered by coleoptile. The coleoptile is the protective leaf sheath covering the emerging shoot of monocotyledons and consists of cells which are specialized to fast stretch growth. Without dividing, coleoptile increases in size as they accumulate water. It protects the first true leaf as it grows towards the surface from its seed (NDSU 2011). Internodes are the stem regions between the nodes which are round in cross section and are either hollow or filled with “spongy pith” (Britannica 2011). Internodes allow flexibility to the grass’s tall height without stiffly breaking.

The intercalary meristem is an interesting feature of grass which is the evolutionary product to the selective pressure of grazing. The grass has accommodated to grazing rather than deterring it. Grasses carry an intercalary meristem. Meristems are classified by their location in the plant as apical, lateral, and intercalary. The intercalary meristem occur at bases of internodes and leaf blades.Unlike most flowering plants whose new growth occurs at shoot tips only, having the meristems at bases of each stem between leaves allows re-growth even after removal of tips by grazers, fire, or lawnmowers (Britannica 2011). Such adaptation is responsible for grasses’ diversification even under the pressure of grazing.

Grass fruits are also known as grains. A fruit wall of grains completely covers the single seed. The pericarp (ovary wall) and seed coat are fused into one layer. Such dry and simple fruit types are called caryopsis. Caryopsis in itself consists of endosperm and embryo. Endosperm is the starchy storage tissue. Embryo is between the endosperm and fruit wall (Britannica 2011).

The symbiotic relationship between AM and roots contribute significantly to the plant’s nutrient and growth. AM colonization attributed to enhance uptake of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), and importantly nitrogen (N). Also water consumption differences have been measured between AM and non-AM plants. AM plants dry soils aft faster rates than non-Am plants. Such affect results in greater evaporative surface area or more extensive root systems in AM plants. AM fungus also has significant influence on drought tolerance levels of maize. Subramanian et al. conducted a study of tropical maize. During drought stress period; leaf water potential, stomatal resistance and transpiration rate differed between the control group and AM fungus Glomus intraradices inoculation plants. Mycorrhizal plants of cultivators had higher leaf water potential and transpiration rate a lower stomatal resistance. The green leaf area of mycorrhizal plants was 27.5% higher than that of non-mycorrhizal plants under drought conditions (Subramanian et al. 2005).

Maize has been subjected to many biotechnology variations and is the second most genetically modified organism (Barbour 2006). A common biotechnology application is the application of Bacillus thuringiensis (Bt) crystalline (Cry) protein gene to crops. The application protects plants from specific insect infection. The cry gene application increased crop yield and reduced chemical insecticide usage. The Bt protected corn also showed decreased levels of fungal toxins such as fumonisin B1 (He et al. 2008).

Commercial Bt-hybrids variously express cry1Ab, cry9C, cry1Ac and cry1F genes (Wisniewski et al. 2002). The most frequent GM maize cultivars contain different sequences of synthetic crylA(b) gene. The gene is in maize Bt 176, MON 810 and Bt 11. The crylA(b) gene along with insecticidal usage; enhances and strengthens expression in its corn (Dinon et al. 2008). The transgenic hybrid MON 810 was developed specifically to protect maize against herbivorous Lepidoptera larvae. Kernels of such hybrid crops store average of 20 times less Cry1Ab toxin than leaves (Hubert et al. 2008).

Hubert et al. analyzed transgenic hybrid MON 810 diet on four species of moths: Ephestia kuehniella, Ephestia elutella, Cadra cautella and Plodia interpunctella. The diets were designed with kernels obtained at two different experiment fields containing the same concentration of Cry1Ab (0.3570.056 mg g_1). Results showed 100% mortality in E. ellutella, C. cautella and P. interpunctella, and 65% mortality rate in E. kuehniella (Hubert et al. 2008).

Similar study was conducted on animals by Trabalza-Marinucci et al. A longitudinal study of insect–resistant Bt176 maize diet was done on ewes. The study showed no adverse effects on the ewes’ health or performance nor horizontal gene transfer to ruminal microorganisms or animal tissues. Behaviors, reproductive traits, antioxidant defense, phagocytosis, and intracellular killing of macrophages, and ruminal microbial population characteristics between control and GM maize fed animals showed no difference. Immune response to Salmonella abrortus ovis vaccination was more efficient in GM maize-fed ewe (Trabalza-Marinucci et al. 2008).

Ruminal epithelium showed evidence of proliferative activation of basal cells in all GM maize-fed ewes. Proliferative activation of basal cells is the deepest layer of the epidermis. Because Cry1 protein of GM maize is able to bind to intestinal mucosal surface; it may have influenced epithelial cell functions. Liver and pancreas analysis revealed smaller cell nuclei and increased amounts of heterochromatin and perichromatin granules in GM maize-fed ewes. Heterochromatin are tightly packed deoxyribonucleic acid (DNA) located on the surface of the nucleus’ inner membrane. Perichromatin granules are ribonucleo-protein which transport and/or store readily spliced pre mRNA. No transgenic DNA was found in the animals’ tissues (Trabalza-Marinucci et al. 2008).

Wiedemann et al. performed an experiment to tract the dispersal of modified DNA after being digested. Wild boars (Sus scrofa) and two diet plans of endogenous rapeseed and GM maize expressing Cry1Ab protein (Bt 176) were used. The study found signals of Cry1Ab protein was observed from samples of stomach, colon and rectum of the boars. Scarce amounts of both GM and non-GM seeds were intact to the feces. However, neither seeds were able to germinate. The feed-ingested DNA was partially resistant to mechanical, chemical, and enzymatic activities of the gastrointestinal tract of wild boar and is not completely degraded after digestion. Thus it is possible for transgenic DNA forms are able to pass onto its environment via excretion from the consuming animal (Wiedemann et al. 2008).

Cry1Ab protein from Bt maize was carried through root exudates from the crop and post-harvest crop residues. Highest amounts of Cry1Ab were found from living root fragments. Adsorption experiments showed Cry1Ab toxins adsorbing onto clay minerals. Adsorbed toxins retained their structure and insecticide activity while biodegradability decreased. Cry1Ab toxin was detectible for up to 234 days. Emmerling et al. studied performance of earthworm species Lumbricus terrestirris L. on fragmentation of such Bt maize litter Cry1Ab protein. Results concluded soil with earthworm showed significantly higher levels of Cry1Ab protein fragmentation than the controlled sample without earthworms (Emmerling et al. 2011). Impact of GM protein on earthworm is unknown.

(a) Cry1Ab concentration of Bt maize for both samples at start; (b) Maize control sample Cry1Ab Bt maize concentration at end; (c) Maize earthworm treatment sample Cry1Ab Bt maize concentration at end (Emmerling et al. 2011).

Impacts and dispersal of the transgene raises questions of gene flow and possible growth of gene-plant combination (Barbour 2006). Possibilities may stretch to genetic diversification. Wild relatives of maize may increase in fitness to the transgenes. Herbicide tolerance and insecticide resistance are likely. Introgression of such pressure may potentially lead to weed problems and decrease of herbicide efficiency.

Concerns of insecticide resistance pressure promoted by the Bt-genes are not far fetched because maize is cultivated widely in large quantities all over the globe. Biotechnological advancement of agriculture now requires social attention. Public health and ecosystems are at risk. Because gene flow is constant; affects of the modified maize protein requires monitoring.

Regarding genetic modification, Wisniewski et al. said, “… agricultural scientists and leaders have a moral obligation to warn the political, educational, and religious leaders about the magnitude and seriousness of the arable land, food, and population problems that lie ahead, even with breakthroughs in biotechnology.” Wisniewski also mentions advancements in biotechnology must be “deployed for their benefit by a strong public-sector agricultural research effort” (Wisniewski et al. 2002).

Genetic modification is a significant pressure on evolution and ecosystems. As Wisniewski included, the idea must be taken politically. Conscious and directional policies and personal choices must be made for ecological benefits rather than monetary profit.
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