were made resistant to incoming pathogenic viruses (Fitch et al. 1992). Varieties of
transgenic papaya were first introduced commercially in Hawaii in 1998, and so far, the
transgenic lines have remained virus-resistant over the years. Just as in other transgenic
crops, after the initial transgenic transformation in a single variety, the gene of interest
was transferred to other desirable commercial varieties using standard breeding techniques.
A similar approach has been used successfully to control cucumber mosaic virus (CMV)
in transgenic squash production. A particularly exciting application of RNA-mediated virus
resistance might be viable in the control of the feathery mottle virus in sweet potato in
Africa. Sweet potato serves as a staple crop in some countries, such as Kenya, and viral
diseases can be especially severe there and in developing countries. Transgenic varieties
resistant to this virus have been developed and might be an effective tool in managing
production and increasing yields.
8.4 Traits for Improved Products and Food Quality
In the early years of commercialization of plant biotechnology, efforts and products focused
on traits that aid in the growing of crop plants, such as resistance to herbicides or insects—
these are calledinput traits. It is likely that many future applications of plant biotechnology
will also targetoutput traits, centered on improved plant-based products that will find their
way to consumers.
8.4.1 Nutritional Improvements
Humans depend on plants as food for survival. In addition to the calories that they provide,
plants produce nutrients, vitamins, and essential amino acids that we require. Much more so
than animals, plants have an incredible capacity for producing a variety of complex
chemical compounds. Through methods in biotechnology, efforts are being made to take
advantage of this capacity for chemical synthesis to improve or alter the nutritional
values of plants.
One of the best known examples of nutritional improvement of a food crop has been the
development ofGolden Rice, a transgenic plant that produces high levels ofb-carotene or
provitamin A in the grain (Ye et al. 2000). Over one-third of the world’s population depends
on rice as a major component of their diet. Although rice can be a good source of calories, it
is not high in protein or vitamins. Although dietary vitamin deficiencies are uncommon
today in industrialized countries, they can still be a serious problem in developing countries
in parts of southern Asia and sub-Saharan Africa, where rice is a staple and there is a lack of
a diverse diet including meat, fruits, and vegetables. Vitamin A deficiency is especially
serious, and the World Health Organization estimates that as many as 4 million children
suffer from a severe deficiency. Humans depend on dietary sources of vitamin A, and
deficiency of this vitamin is the leading preventable cause of blindness in children and
significantly increases the mortality rate due to illnesses such as measles and malaria.
Providing vitamin A supplements as capsules to children and new mothers is one approach
to solving this problem, but to be effective, supplements need to be administered several
times per year, which can present logistical challenges in many areas. An alternative strat-
egy is to provide provitamin A in the form ofb-carotene in rice.
Carotenoids are a subset of compounds within a large and variable class of plant
metabolites calledterpenoidsorisoprenoids. This class of compounds is all based on a
8.4. TRAITS FOR IMPROVED PRODUCTS AND FOOD QUALITY 205