Over the last 10,000 years, we have altered the genetic make-up of plants to produce food, fibres and other materials we need (1). For most of that time we have relied on fairly primitive selective breeding, with farmers selecting plants by the usefulness or desirability of their obvious traits, for example grain size, taste or colour. In the last 300 years, our expanding knowledge of plant science has provided more sophisticated technologies for improving crops. Here are some of the major ones:
The offspring of two very different individuals of the same species tend to be healthier, larger and more fertile. We call this hybrid vigour. When Charles Darwin noticed hybrid vigour while studying toadflax in the 1870s, he became the first person to study it systematically (2). In the 1920s, the first hybrid crop seeds were released to farmers. Although the seeds produced by hybrids can be re-planted, they do not have the same combination of good traits as their parents. Therefore, farmers tend to buy new hybrid seeds every year, if they can afford to.
It is possible to deliberately introduce random DNA mutations into plants by exposing them to a chemical or radiation treatment. Most of the time, these mutations will either be harmful to the plants or have no obvious effects, but sometimes they can generate a desirable trait. Crop breeders can screen large numbers of plants to search for individuals with these good traits, and then breed from them.
In the last 10 years, a new technique called TILLING (targeted induced local lesions in genomes) has been used to generate mutations in crops. It is proving to be especially useful in plants whose genetics have been less-well studied than other crops, for example to improve shelf-life in melons.
Marker-assisted selection (MAS)
Marker-assisted selection (MAS) is a more sophisticated form of traditional selective breeding (1). Now that it is possible to identify the genes that are responsible for producing a desired trait, crop breeders can select for the presence of the particular genes in the plants, instead of the trait itself. This is useful because genetic testing can be carried out when the plants are very young, so it is not necessary to carry out big field trials. Also, it is easier to track down, and eliminate any undesirable traits that might also happen to be in the plants.
MAS is especially useful for introducing genes from wild relatives or primitive varieties of crops where there are many difference between the two parent plants. For example, flood-tolerant rice was developed using MAS by crossing a popular commercial rice variety and another, less useful rice variety that carried a version of the Sub1 gene that made it flood-tolerant.
Genetic engineering techniques allow DNA to be inserted into the genome of plants. It makes it possible to produce crops with traits that would be difficult or very time-consuming to achieve with conventional breeding. For example, purple tomatoes, which contain high levels of the antioxidant anthrocyanin were created by inserting a gene that comes from snapdragon.
Since the first release of GM crop varieties in 1996, the production of GM crops has increased massively, and they are now widely grown in 29 countries. One of the traits introduced into crops is the ability to produce the Bt toxin, a naturally-occuring pesticide. Bt cotton is especially popular, and accounts for 70-90% of cotton grown in the US, China and India (1). Growing Bt cotton enables farmers to control pest populations, whilst using less synthetic pesticides, which tend to kill beneficial insects alongside the pests. An added benefit is the improved health of farmers, many of whom used to become ill from the large quantities of synthetic pesticides they were using.
The human population is predicted to rise by 50 % by 2050. To produce enough food to feed these people we need to increase agricultural production. Given that land and water are both in short supply, this is going to be a challenge.
One new plant science technology that could potentially benefit agriculture is called genome editing. With genome editing it is possible to precisely alter DNA sequences in living cells. It can be used to recreate naturally-occurring mutations (found in other plants) in popular crop varieties without having to introduce new genes. For this reason, scientists and farmers believe that genome editing might be more readily accepted than genetic engineering has been (1).
Whatever technologies we use to help increase agricultural production in the future, it is important that they are combined with sustainable agricultural practices. Unfortunately this does not always happen at the moment. For example, in the US it is a legal requirement that all Bt cotton crops must be surrounded by “refuge” areas containing crops that don’t produce the Bt toxin (1). These areas promote the survival of beneficial insects, and delay the evolution of resistance to the Bt toxin in the pest. In India, refuge areas are not typically used and Bt toxin resistance in the pink bollworm, a major pest, is on the rise. To make the most of future advances we need to learn from the lessons of the past.
1) Ronald (2014) Lab to Farm: Applying Research on Plant Genetics and Genomics to Crop Improvement. PLOS Biology.
2) Darwin (1876) Cross and self-fertilisation of plants (via Darwin Online)