Helping solve World Hunger by modifying photosynthesis
Some plants use light from the sun more efficiently than others
James Routledge | 3 April 2017

The world's population is growing exponentially and is expected to reach 9 billion by 2050. Even today, around 1 billion people suffer from serious hunger, with 14,000 children dying of hunger-related causes every day. This is only going to get worse as the world's population increases.

It is estimated that, to be able to feed the future's population, the yields of plants such as rice, a staple food crop in areas of the world such as Asia, are going to have to increase by 60%. This is going to be made even more complicated by the shift caused by climate change leading to lower water availability and changing temperatures.

While it may be possible to increase crop yields to a certain extent by breeding plants to improve growth in drought or saline conditions, it is unlikely that this will be enough to increase crop yields by the 60% needed to feed the world’s population. We therefore have to start looking at more dramatic measures to increase crop yields.

All plants need light to photosynthesise and produce energy for respiration in order to grow, so improving this area could prove highly beneficial for crop yields. However, some plants use light from the sun more efficiently than others when under warmer and drier conditions. These are called C4 photosynthetic plants, while the others plants are called C3 photosynthetic plants. They have different methods of photosynthesis because, at the time and place they developed, they required different mechanisms to be the most efficient.

Where C3  and C4 plants differ is in how they complete the Calvin cycle. This is the second stage of photosynthesis where carbon is fixed into sugars. A process called photorespiration can occur during the Calvin cycle. This happens when an enzyme called Rubisco which fixes carbon into the cycle uses oxygen instead of carbon dioxide. This results in wasted energy and carbon, causing the plant’s growth to be reduced. This can happen when plants close their stomata to reduce water loss when there is a high temperature and low humidity.

C3  plants complete the Calvin cycle in the same cell which allows for photorespiration to occur. In C4 plants, the first stage of photosynthesis and the Calvin cycle are physically separated. It is a two stage process in which the first stage takes place in the mesophyll cells in the middle of the leaf and the Calvin cycle takes place in the bundle-sheath cells around the leaf veins.

Energy in the form of a compound called ATP is used to pump carbon dioxide from the outer mesophyll cell to the inner bundle sheath cell. This causes there to be a higher concentration of carbon dioxide in the bundle sheath cells relative to oxygen near the Rubisco enzyme so photorespiration is reduced.

In areas of the world that are hot and dry, C4 photosynthesis may be beneficial as it reduces photorespiration and allows the plant to focus more of its energy on growing. If a staple food crop such as rice is able to be modified from a C3  -using plant to a C4 -using plant, rice yields may be able to increase enough to support a growing global population.

Currently a group of researchers called the C4 rice consortium is looking into ways of adapting a rice plant to use a C4 photosynthetic pathway. This involves trying to identify existing genes controlling C4 -like characteristics in rice plants that could be encouraged to develop fully. Transgenic plants are also being used to study the effects of adding genes related to C4 photosynthesis into rice plants to see the changes.

If the C4 rice project results in a rice crop, or another similar crop, capable of helping feed the world's growing population, it would be a great achievement. As is normally the case, issues about genetic modification may be brought up. Only time will tell what the true outcome of these experiments will be, but it is likely that genetic modification of crops will become an ever increasing part of our lives, more so than it is already, whether we like it or not.

Original image by James Routledge

James Routledge 2016