RESUMEN
Accumulation of certain phenolics is a well-known response of plants to enhanced UVB radiation (280-315 nm), but few experiments have compared the relative importance of different phenolic groups for UVB resilience. To study how an altered phenolic profile affects the responses and resilience of silver birch (Betula pendula) to enhanced UVB, we used RNA interference (RNAi) targeting dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS), or anthocyanidin reductase (ANR) to change the accumulation of phenolics. The unmodified control line and RNAi-modified plants were grown for 51 days under ambient or +32% enhanced UVB dose in a greenhouse. RNAi greatly affected phenolic profile and plant growth. There were no interactive effects of RNAi and UVB on growth or photosynthesis, which indicates that the RNAi and unmodified control plants were equally resilient. UVB enhancement led to an accumulation of foliar flavonoids and condensed tannins, and an increase in the density of stem glands and glandular trichomes on upper leaf surfaces in both the control and RNAi-modified plants. Our results do not indicate a photoprotective role for condensed tannins. However, decreased growth of high-flavonoid low-tannin DFRi and ANRi plants implies that the balance of flavonoids and condensed tannins might be important for normal plant growth.
RESUMEN
Despite active research, antiherbivore activity of specific plant phenolics remains largely unresolved. We constructed silver birch (Betula pendula) lines with modified phenolic metabolism to study the effects of foliar flavonoids and condensed tannins on consumption and growth of larvae of a generalist herbivore, the autumnal moth (Epirrita autumnata). We conducted a feeding experiment using birch lines in which expression of dihydroflavonol reductase (DFR), anthocyanidin synthase (ANS) or anthocyanidin reductase (ANR) had been decreased by RNA interference. Modification-specific effects on plant phenolics, nutrients and phenotype, and on larval consumption and growth were analyzed using uni- and multivariate methods. Inhibiting DFR expression increased the concentration of flavonoids at the expense of condensed tannins, and silencing DFR and ANR decreased leaf and plant size. E. autumnata larvae consumed on average 82% less of DFRi plants than of unmodified controls, suggesting that flavonoids or glandular trichomes deter larval feeding. However, larval growth efficiency was highest on low-tannin DFRi plants, indicating that condensed tannins (or their monomers) are physiologically more harmful than non-tannin flavonoids for E. autumnata larvae. Our results show that genetic manipulation of the flavonoid pathway in plants can effectively be used to produce altered phenolic profiles required for elucidating the roles of low-molecular weight phenolics and condensed tannins in plant-herbivore relationships, and suggest that phenolic secondary metabolites participate in regulation of plant growth.
