Genetically Improved Crops - The Oxford Topic of Food, Politics, and Society - page 80

Some novel indirect approaches have also been taken to improve protein content.
Uauy et al. (2006) “rescued” an ancestral wheat allele that encodes a transcription factor
(NAM-B1) to accelerate senescence and increase nutrient remobilization from leaves to
developing grains (modern wheat varieties carry a nonfunctional allele). Reduction in
RNA levels of the multiple NAM homologs by RNA interference delayed senescence by
more than three weeks and reduced wheat grain protein, zinc, and iron content by more
than 30%. Young et al. (2004) used yet another approach to indirectly increase protein
and oil content. With a bacterial cytokinin-synthesizing isopentenyl transferase (IPT)
enzyme, under the control of a self-limiting senescence-inducible promoter, they were
able to block the loss of the lower floret resulting in the production of just one kernel
composed of a fused endosperm with two viable embryos. The presence of two embryos
in a normal-sized kernel leads to displacement of endosperm growth, resulting in ker-
nels with an increased ratio of embryo to endosperm content. The end result is maize
with more protein and oil and less carbohydrate (ILSI 2008; Young et al. 2004).
Macronutrients: Fiber and Carbohydrates
Fiber is a group of substances chemically similar to carbohydrates that nonruminant
animals including humans poorly metabolize for energy or other nutritional uses. Fiber
provides bulk in the diet such that foods rich in fiber offer satiety without contribut-
ing significant calories. Current controversies aside, there is ample scientific evidence
to show that prolonged intake of dietary fiber has various positive health benefits, espe-
cially the potential for reduced risk of colon and other types of cancer.
Recent microbiome twin studies by Jeff Gordon addressing the interrelationships
between diet and gut microbial community structure/function indicated that differ-
ences in our gut microbial ecology affect our predisposition to obesity or malnutrition
and that diet rather than applied probiotics was the single most important characteriza-
tion of gut health (Turnbaugh 2009). These studies involved characterization of the gut
microbiota and microbiome of twins who were concordant or discordant for malnutri-
tion, were living in several developing countries, and were sampled just prior to, during,
and after treatment with various dietry inputs.
When such colonic bacteria (especially Bifidobacteria) ) ferment dietary fiber or other
unabsorbed carbohydrates, the products are short-chain saturated fatty acids. They may
enhance absorption of minerals such as iron, calcium, and zinc; induce apoptosis, thus
preventing colon cancer; and inhibit 3-hydroxy-3-methylglutaryl coenzyme-A reduc-
tase (HMG-CoAR), thus lowering low-density lipoprotein (LDL) production (German
2005). Plants are effective at making both polymeric carbohydrates (e.g., starches and
fructans) and individual sugars (e.g, sucrose and fructose). The biosynthesis of these
compounds is sufficiently understood to allow the bioengineering of their properties
and to engineer crops to produce polysaccharides not normally present. Polymeric car-
bohydrates such as fructans have been produced in sugar beet and inulins and amy-
lase (resistant starch) in potato (Hellwege et al. 2000) without adverse affects on growth