The Origin of the Beefsteak Tomato
Have you ever wondered how plants grow? You might have known that in humans a special tissue called a growth plate exists at the end of each long bone in children and adolescents. This growth plate contains special cells called osteogenic cells that are responsible for dividing and adding more bone cells to existing bones thereby making them longer. Plants grow is a very similar fashion. However, in plants, the tissue where growth originates is called the meristem. Meristems contains un-specialized cells called stem cells. Similar to osteogenic cells, these stem cells continuously divide allowing plants to grow and form new organs. Plants contain different meristematic regions that control the growth in the region they are found. One particular region called the shoot apical meristem gives rise to specialized plant organs such as leaves and flowers. However, when these specialized organs are formed, the shoot apical meristem must produce more stem cells in order to replace the ones it lost as these organs developed. In tomatoes, stem cell production in the shoot apical meristem is controlled by a negative feedback loop involving two important genes called WUSCHEL (WUS) and CLAVATA (CLV). WUS promotes stem cell formation and is therefore responsible for replacing the cells that have been lost due to organ formation while CLV is responsible for repressing WUS activity thereby inhibiting stem cell production. This negative feedback loop therefore regulated the production of stem cells in the shoot apical meristem and an overproduction of stem cells in this region.
The discovery of the genes responsible for maintaining meristem size and therefore fruit size in tomatoes is significant because it can help agricultural breeders adjust the size of fruit in fruit-bearing plants. For example, wild tomato plants originally bore tiny, “berry-sized” fruits. However, a mutation in the CLV3 gene lead to an increase in tomato size. Plant cultivators artificially selected for this mutate making this mutant more common which is why we harvest larger tomatoes today. Therefore, understanding the pathway that leads to larger fruit size can allow breeders to customize fruit size in tomatoes and other fruit bearing plants.
Figure 1: Genes controlling stem cell production in tomatoes are responsible for the increase in tomato size.
Posted by: Katie Kossack