With the world population tipping the scale at 7 billion people, the question of food sustainability has been brought to the forefront. Feeding the growing human populace with a finite amount of natural resources is becoming an increasingly difficult task. Genetically engineered foods serve to enhance the health benefits of foods and production rates while retaining consumer expectations of flavor. In apple, the anthocyanin pathway has been shown to be controlled by the MYB transcription factor, MYB10(Espley et al. 2007). Espley et al. (2012) raised the polyphenolic content of apple by genetically engineering the anthocyanin pathway using the apple transcription factor MYB10. The authors did a series of tests that compared genetically modified (GM) apples to their wild type (WT). These tests examined anthocyanin metabolite and transcription levels, flavonoids and proanthocyanin expression, and consumer reaction to MYB10 over-expressed apples. This study aims to enhance novelty and food crop nutritional status, and in a larger sense, produce food that can more adequately sustain our growing populace. Their results indicate that MYB10 over-expressed apples are likely to retain all the current consumer expectations of flavor, with the added appeal of elevated colour and potential health enhancement.—Paloma Medina
Espley, R. V., Bovy, A., Bava, C., Jaeger, S. R., Tomes, S., Norling, C., Crawford, J., Rowan, D., McGhie, T. K., Brendolise, C., Putterill, J., Schouten, H. J., Hellens, R. P. and Allan, A. C. (2012), Analysis of genetically modified red-fleshed apples reveals effects on growth and consumer attributes. Plant Biotechnology Journal. doi: 10.1111/pbi.12017
In the majority of plant species, pigmentation is controlled by the relative amount of anthocyanins, chlorophyll, and carotenoids. These compounds are essential for plant health and function, but are also considered as markers for dietary health. For this reason, plant-breeding programs have targeted their efforts to produce red-fleshed apples, similar to those from Central Asia, with white commercial varieties. Previous examples of GM apples have resulted in obtaining beneficial characteristics such as disease resistance, dwarfing, and extended fruit storage. In apple, the anthocyanin pathway has been shown to be controlled by the MYB transcription factor, MYB10. When MYB10 is fused to a 35S promoter, it is able to drive the accumulation of anthocyanin pigments. The researchers show that a large increase in anthocyanin produces red-fleshed apples. The trees that had an over expression of MYB10 had characteristics of deep pigmentation in the leaves and trunk. A microscopic analysis of the leaves revealed anthocyanin accumulation predominantly in the spongy mesophyll and lower epidermis and in tissue close to the vascular rays.
Though pigmentation increased, anthocyanin accumulation did not appear to compromise photosynthetic electron transport activity (P < 0.05). The electron transport rate (ETR) versus photosynthetically active radiation (PAR) was plotted, and the curves were used to estimate maximum photosynthetic capacity (PSmax) and photosynthetic efficiency (initial slope of the ETR versus PAR). Interestingly, researchers found that photosynthetic capacity was significantly higher in the red MYB10 leaves. Furthermore, firmness of the fruit was measure by a penetrometer, and both MYB10 apples and Royal Gala apples were found to be similar in firmness.
In addition to photosynthetic data analysis of pigmented fruit, anthocyanin concentration of mature fruit was analyzed by high-performace liquid chromatography (HPLC). MYB10 fruit showed a significant increase of five cyanidin glycosides, of which cyanidin-3-galactoside was the most abundant. This glycoside is the leading anthocyanin found in many apple varieties. Futhermore, a qPCR analysis of transcript profiles of genes in the anthocyanin pathway showed significant increases in expression of all the anthocyanin biosynthesis-related genes tested. Not only this but the red-fleshed fruit showed elevated concentrations of flavonols and proanthocyanidins (Pas). Specifically, in MYB10 plants, PA concentrations increased approximately three-fold in all the tissues tested. Flavonols are polyphenolic compounds present in fruits, vegetables, tea, and berries, and have effects similar to antioxidants. Thus, this is positive evidence to support the breeding of MYB10 apples on the grounds of nutritional status.
But there are disadvantages to red-fleshed apples. After storage at 0.5 °C for 12 weeks, researchers observed internal browning in some MYB10 fruit which isn’t normally observed in Royal Gala. In addition, average fruit size for the MYB10 fruit was similar to WT, but average WT fruit were heavier. A variety of apples were tested in a consumer panel of 15 individuals. In total, there was an overall preference for WT fruit due to its overall ‘crisp’ texture and quality. In addition, metabolite analysis revealed that the cortex of MYB10 fruit contained 1.5-fold and 3.6-fold higher concentrations of fructose and glucose and 0.44-fold lower concentrations of sucrose than WT fruit, although these differences were not significantly different (P > 0.05). The diminished sucrose concentration is reiterated in a consumer panel in which individuals preferred the sweetness of a WT apple to a MYB10 apple. However, when presented with the additional health benefits that red-fleshed apples may have, the consumer panel response towards red-fleshed apples remained positive.
All in all, the changed metabolite profile of the MYB10 fruit did not appear to alter the flavor or aroma qualities of the fruit significantly. If further research is able to find mechanisms to increase sucrose levels, red-fleshed fruit may have more value on the market. Espley et al. presents a surprising multitude of health benefits in altering the transcription factor MYB10. The authors are hopeful that these findings will help plant-breeding programs enhance novelty and food crop nutritional status.
Espley, Richard V. (2007). “Red colouration in apple fruit is due to the activity of the MYB transcription factor, MdMYB10 A MYB transcription factor controlling apple fruit colour”. The Plant journal: for cell and molecular biology (0960-7412), 49 (3), p. 414.