The resistance of plants to natural enemies is determined as a sum of several characters of the plant and the consumer. Among the chemical characters of plants, secondary metabolites (e.g., phenolics), are often implicated in plant defense against pathogens and herbivores. The levels of these metabolites vary according to plant inherent factors (genotype, ontogeny, phenology) and they are also readily affected by changes in environment conditions. My reseach is focusing on the role of carbon-based secondary metabolites in the interactions between forest plants and pathogens. I am especially interested in environmentally-induced changes in plant phenolic levels and in the possible consequences of these changes for plant-parasite interactions. The plant species I am working with are Vaccinium myrtillus, V. vitis-idaea, Deschampsia flexuosa, Filipendula ulmaria, Pinus sylvestris and Populus sp.
Recent results: The vegetative tissues of boreal V. myrtillus contain several phenolic acids and flavonoids with potential antifungal activities. The levels of phenolics in V. myrtillus leaves fluctuate considerably during a growth season, and the fluctuation patterns of individual phenolic compounds differ. This compound-specific within-seasonal variation leads to temporally varying quality of plant tissues to consumers and may thus influence the ecological interactions between bilberry and its natural enemies. In V. myrtillus, also the direction and magnitude of induced effects of biotic and abiotic factors on phenolics seem to vary considerably. All this emphasizes the great importance of multiple sampling occasions in studies where the role of phenolics in plant resistance is studied.
In the field-layer of boreal forests, nitrogen fertilization has been shown to increase the frequency of a fungal parasite (Valdensia heterodoxa Peyronel, Sclerotiniaceae) on V. myrtillus plants (Fig. 1, see also the homepages of my colleagues, Annika Nordin and Joachim Strengbom for more information). In a field-study, we tested a hypothesis that nitrogen amendment causes changes in the content and concentration of phenolics in bilberry leaves, which may contribute to increased susceptibility against V. heterodoxa. Our results indicate that the phenolic metabolism of V. myrtillus leaves is well buffered against environmental disturbance caused by a moderate nitrogen addition (12.5 kg N ha-1 yr-1). However, a higher nitrogen dose (50 kg N ha-1 yr-1) resulted in more pronounced effects on phenolics, and appeared to alter the natural responses of these compounds to infection. The metabolism of different phenolics seemed to be activated at different phased of the fungal infection cycle, and the responses of individual phenolics to nitrogen varied in healthy and diseased leaves. This suggests that in V. myrtillus leaves the possible phenolic-based defence against parasitic fungi occurs in a highly complicated manner and is affected by environmental conditions (such as nitrogen availability).
The role of phenolics in plant resistance may also be realized indirectly by their interactions with other biologically active phytochemicals. Phenolics may, for instance, conjugate with polyamines, and it has been suggested that these conjugates may mediate interactions between plants and micro-organisms. We have analysed the leves of free and conjugated polyamines (putrescine, diaminopropane, spermine and spermidine) in healthy and parasite-infected V. myrtillus plants receiving different amounts of nitrogen fertilizer (collaboration with Dr. Tytti Sarjala, Finnish Forest Research Institute, Parkano). The results indicate that polyamine metabolism is activated in response to parasite infection in V. myrtillus leaves, and that induction of some polyamines may be compromised in fertilized plants.
Future research activities will concentrate on in vitro testing of the possible (individual or synergistic) effects of phenolic metabolites on parasitic fungi of forest plants (collaboration with Dr. Anna Shevtsova, Department of Vegetation Ecology, SLU-Umeå). Moreover, the spatial distribution of phenolic compounds of plant tissues in relation to invading pathogen will be studied using chromatography and microscopic approaches. Due to their biological activity (e.g., antioxidant potential) some plant phenolics, mainly flavonoids and anthocyanins, present in berries of Vaccinium plants are also of interest for human diet (“functional food”). At the present we are analyzing the phenolic content in V. myrtillus berries, and studying how the growth environment affects their levels.
Witzell J, Gref R, Näsholm T. . Plant-part specific and temporal variation in phenolic compounds of boreal bilberry (Vaccinium myrtillus) plants. Biochemical Systematics and Ecology 31(2), 115- 127.
Strengbom J, Olofsson J, Witzell J, Dahlgren J : Effect of repeated damage and fertilization on plant palatability to grey sided voles, Clethrionomys rufocanus (OIKOS, in press)
Witzell J, Shevtsova A. Nitrogen-induced changes in secondary phenolics of blueberry (Vaccinium myrtillus)– implications for parasite resistance (submitted)
Witzell J, Sarjala T, Kuusela T. The effects of parasite infection and nitrogen fertilization on the interaction between polyamines and phenolics in bilberry (manuscript)
Selected earlier publications:
Hakulinen J. and Julkunen-Tiitto, R. : Variation in leaf phenolics of field-cultivated willow (Salix sp.) Clones in relation to occurrence of Melampsora rust. Forest Pathology 30:29-41
Hakulinen J., Sorjonen, S. and Julkunen-Tiitto, R. : Leaf phenolics of willow clones differing in resistance to Melampsora rust. Physiologia Plantarum 105: 662-669.
Hakulinen J. : Nitrogen-induced reduction in leaf phenolic level is not accompanied by increased rust frequency in a compatible willow (Salix myrsinifolia)-Melampsora rust interaction. Physiologia Plantarum 102:101-110.
Hakulinen J., Julkunen-Tiitto R. and Tahvanainen J. : Does nitrogen fertilization have an impact on the trade-off between willow growth and defensive secondary metabolism? Trees - Structure and Function 9:235-240.
