Modification of fruit colour and sweetness

It is becoming widely accepted that plant biotechnology is entering a second phase of development that looks for differentiated crop development (Kishore and Shewmaker, 1999). This phase is characterised for being more focused on output traits than input traits. Some of these output traits have been described in the previous section, but still there are others that are related to fruit quality for human consumption. In general, market trends predict growth in the consumption of fruit if quality can be improved. These traits remain a challenge for plant biotechnology developers. However, there are two aspects that are present in the efforts of various research laboratories. They focus on fruit colour and sweetness, and they may be on the market in the near future.

Carotenoids are a group of natural pigments that determine the colour of many flowers, fruits and vegetables. In fact, the golden-yellow colour of high β-carotene transgenic rice has previously been indicated to be a side effect of rice plant transformation. In some cases, the goal has been specifically to increase the colour of the transgenic product. This trait is particularly important in fruits that are going to be processed to produce jams, marmalades, pastes, and even wine. Recently, the carotenoid biosynthesis pathway has been modified in tobacco plants using the CrtO gene encoding the β-carotene ketolase, from the alga Haematococcus pluvialis (Mann et al., 2000). The transgenic plants accumulated several ketocarotenoids that changed the colour of the nectary from yellow to red. The researchers claim that plant transformation with this gene may be used in the future to change the colour of fruit.

Flavonoids are a major class of secondary plant products well known for the colouration they provide in blue, red and purple flowers, fruits and leaves. Flavonoids are derived from simple metabolites as phenylalanine and acetyl- CoA in a highly branched pathway that leads to flavonols, flavanones, isoflavonoids and anthoycianins. Each type may undergo further modifications that result in a great diversity of colours. It is known that this complex pathway is regulated at the level of transcription of structural genes thus giving a main role to transcriptional factors, i.e. proteins that control the expression of some other genes at transcriptional level. This has been applied to the model plant Arabidopsis and tobacco, which were transformed with the maize R and C1 genes. These are regulators that produced an activation of genes encoding for enzymes of the anthocyanine production pathway in the transgenic plants. In general, changes at the level of intermediate enzymes of the flavonoids pathway may change the final balance of these coloured compounds and eventually the colour of a given plant organ, for example, the production of yellow flowers in acyanic lines of Petunia after its transformation with a chalcone reductase gene from Medicago sativa (Davies et al., 1998). Only redirection of the flavonoid biosynthesis pathway could explain the reported effect since chalcone reductase activity is not naturally present in Petunia as it is not the product of the reaction, which is further transformed in a coloured compound. This illustrates the complex equilibrium of the complete pathway and the difficulty of predictable effects after plant transformation with heterologous genes.

A critical component of fruit palatability is sweetness. There are two goals associated with this food property to be achieved. The first is that many fruits are not sweet enough to make them appetising to consumers. The second is that the most common natural sweetener is sucrose, whose caloric content has caused a change in consumer preference for other sweeteners. Both problems have been addressed by plant biotechnologists.

It was known that a protein from the plant Dioscorephyllum cumminsii, named monellin, was about 10⁵ sweeter than sucrose on a molar basis. This property is common among some plant proteins of the thaumatin-like class. Tomato plants overexpressing the monellin gene under the control of a fruitspecific promoter (E8) have been produced (Peñarrubia et al., 1992). Although no further reports on fruit properties have been reported, the experience indicates a possible way to increase fruit sweetness, at the time that protein content is enhanced.

Other researchers have also looked for substitutes for the natural sweetener sucrose (Sévenier et al., 1998). They identified the low molecular weight fructans, polymers of fructose, as an adequate replacement. These compounds resemble sucrose in their organoleptic properties, but are indigestible by humans. In addition, they cannot be used as a carbon source by cariogenic bacteria. To obtain high fructan plants, the gene encoding 1-sucrose:sucrose fructosyl transferase from Helianthus tuberosus was introduced into sugar beet. Most of the stored sucrose in the root was converted to low molecular weight fructans. The experience is ready to be extended to other commercial crops whose sugar content make them unattractive to consumers.