Wild coyote tobacco, Nicotiana attenuata. Left: A healthy, fertile plant with intact reproductive organs. Right: A genetically modified plant that lacks the enzyme NaMaT1. These plants have a shorter style and are sterile. Image credit: Jiancai Li (CC BY 4.0)
Plants produce tens of thousands of molecules called secondary metabolites that are thought to help them cope with threats from their environment, such as attack by insects or ultraviolet radiation from the sun. Wild coyote tobacco plants produce large amounts of a particular class of secondary metabolite known as DTGs. Insects feeding on tobacco plants containing DTGs cause less damage and produce fewer offspring
Plants modify many secondary metabolites by attaching tags known as malonyl groups to them. Enzymes called malonyl transferases take a malonyl group from another substrate and attach it to the secondary metabolite. This process can be repeated so that an individual secondary metabolite molecule may have many malonyl groups attached to it. Previous studies have shown that insects feeding on tobacco plants trigger more malonyl groups to be attached to DTGs, but it is not clear what effect this has on the plants.
To simulate attack by an insect, Li et al. punctured holes in the leaves of tobacco plants and applied saliva from tobacco hornworms. The experiments show that more DTGs modified with malonyl groups accumulated in these plants compared to untreated plants. However, there was no change in the average number of malonyl groups added to individual DTGs during the modification process. Further experiments show that a malonyl transferase enzyme called NaMaT1 adds malonyl groups to DTGs in coyote tobacco plants. The flowers of plants that produce less of this protein have shorter styles (a tube structure that guides pollen to the egg cells at the base of the flower) and are less fertile than flowers in normal plants.
These experiments demonstrate that, along with helping plants to defend themselves from herbivores, DTGs regulate how flowers grow and develop. It was generally thought that secondary metabolites do not play important roles in how plants grow when they are not under stress. Indeed, plant breeders frequently select crops that produce lower levels of secondary metabolites in order to increase their nutritional value. Therefore, the findings of Li et al. may help improve the outcomes of crop breeding programs.
