The phenolic compounds in leaves, and glandular trichomes located on the leaf surfaces of silver birch have various effects on leaf-feeding insects. Certain flavonoids and glandular trichomes on the upper leaf surface seem to reduce the consumption and growth of autumnal moth larvae, which suggests a deterrent bioactivity for them. The larvae feeding on leaves with low condensed tannin content grew more efficiently than larvae on unmodified control plants, which suggests that the condensed tannins normally present in silver birch leaves reduce diet quality for plant-feeding insect larvae.
Enhanced ultraviolet B (UVB) radiation increased the accumulation of leaf phenolics and changed the structure of condensed tannins. Silver birch is naturally quite resilient to moderately increased UVB radiation, and the altered phenolic profile did not seem to decrease the resilience of modified silver birches (based on photosynthesis or growth) to enhanced UVB treatment. The accumulation of foliar phenolics and the increased density of glandular trichomes on upper leaf surfaces and on stems supports their suggested defensive roles against harmful levels of solar UVB radiation. By contrast, responses of glandular trichomes to increased temperature, drought or excessive watering did not support their defensive role against these stresses at least on mild intensities.
Genetic modification enables investigating the production and ecological roles of phenolic compounds in plants
Silver birch produces large amounts of phenolic compounds such as flavonoids and condensed tannins. For many of these compounds, the exact ecological roles are unknown. In silver birch, flavonoids and condensed tannins accumulate especially in leaves, but also stems and roots may have high contents of condensed tannins. Investigating the ecological roles of condensed tannins in natural silver birches is difficult, because plants with different flavonoid and tannin contents may also differ in multiple other traits.
RNA interference is a molecular method that can be used to reduce the expression of specific genes.. In this research, RNA interference was used to reduce the levels of enzymes that are involved in the biosynthesis of the precursors of condensed tannins. This resulted in increased accumulation of flavonoids and decreased levels of condensed tannin as well as altered condensed tannin structure in modified plants. Stopping the biosynthesis of condensed tannins greatly reduced the growth of modified plants compared to unmodified controls.
The research provided new information on the ecological roles of phenolic compounds and glandular trichomes in the defence of plants against insects and environmental stress factors. The results may be applied, for example, in the development of pesticides with less unwanted side effects, or in the selection of better silver birch lines for breeding purposes. Additionally, the findings add to the knowledge about biosynthesis of condensed tannins in deciduous trees, and they may help answer open questions about how exactly plants combine flavonoid subunits into condensed tannins.
The doctoral dissertation of MSc Paula Thitz, entitled The ecological roles of condensed tannins, flavonoids and glandular trichomes in silver birch (Betula pendula) will be examined at the Faculty of Science and Forestry on the 11th of December online. The opponent in the public examination will be Docent Maarit Karonen, University of Turku, and the custos will be Docent Tommi Nyman, University of Eastern Finland. The public examination will be held in Finnish.
Link to the event (in Finnish)