Ecological significance of flavonoid accumulation in reproductive and vegetative tissues of silene littorea (caryophyllaceae)

Abstract

Anthocyanins are one of the main pigments conferring color to flowers. These compounds may be also accumulated in vegetative tissues, where perform multiple protective roles against different biotic and abiotic stressors. In the same metabolic route of anthocyanins, other non-anthocyanin flavonoids that confer protection against a wide diversity of environmental stresses are also synthesized. Thus, the study of anthocyanins and other flavonoids is key to understand the importance of these metabolites in the adaptation process of plants to their environments. In this thesis, we investigate the adaptive role of flavonoids analyzing the ecological and evolutionary significance of flavonoid accumulation in reproductive and vegetative tissues of plants, using Silene littorea (Caryophyllaceae) as a model organism. This species is endemic of the Iberian Peninsula, growing at the sea level on coastal ecosystems. The flowers of S. littorea are pink due to the accumulation of anthocyanins. The presence of anthocyanins in calyces, leaves and stems confers a coloration that vary from green to red depending on the concentration of pigments accumulated. In addition, non-anthocyanin flavonoids are also produced in all plant organs. The ability of S. littorea to synthesize anthocyanins and non-anthocyanin flavonoids in the whole plant allow us to study how these compounds are accumulated in different plant tissues, the possible relationships among tissues in flavonoid production and the degree of phenotypic plasticity of this response. We also studied the physiological, biochemical and genetic changes that UV light exposure induces in this species. In addition, S. littorea also displays white plants with localized absence of anthocyanins in petals or a complete absence of these pigments in the whole plant. This feature of the species offers an excellent opportunity to investigate the relevance of these secondary metabolites in plant development and survival. Finally, we develop a fast, efficient and non-invasive method for estimating anthocyanin concentrations in plants by using digital images. In S. littorea, each plant organ exhibited considerable variability in the content of anthocyanins and other flavonoids both within and among populations. At the plant level, the flavonoid content in petals, calyxes, and leaves was not correlated in most of the populations. However, at the population level, the mean amount of anthocyanins in all organs was positively correlated, which suggests that the variable environmental conditions of populations may play a role in anthocyanin accumulation. In populations on the west coast of the Iberian Peninsula, a general pattern of increasing accumulation of flavonoids toward southern latitudes was observed in photosynthetic tissues. This pattern corresponds to a gradual increase of UV-B radiation and temperature, and a decrease of rainfall toward the south. The study of plasticity in flavonoid production revealed that the synthesis of both anthocyanins and flavones is mostly plastic in photosynthetic tissues, being mainly influenced by environmental effects. In addition, flavones exhibited approximately half the degree of phenotypic plasticity compared to anthocyanins. On the other hand, petals showed limited plasticity in flavonoid production. These results suggest that anthocyanins are usually constitutive in petals, yet susceptible to environmental conditions in photosynthetic tissues. We found that UV light stimulated the production of both anthocyanins and flavones, especially these latter in photosynthetic tissues. The synthesis of effective antioxidant flavones in photosynthetic tissues suggest that these compounds may have a key role as antioxidants and a minor role as light filters. In addition, UV light also induced a modest activation of specific transcription factors, decreased the photochemical efficiency of photosystem II and had a negative influence on plant fitness. The study of polymorphism in the flavonoid production showed that loss of anthocyanins, either in petals or in the whole plant, does not influence the ability of this species to synthesize flavones. Flavones have important protective functions for plants, similar to those of anthocyanins. We suggested that lack of anthocyanins may involve deleterious pleiotropic effects for plant survival not associated to their protective functions, which would explain the scarcity of non-pigmented plants in natural populations. Finally, we showed that our method based on digital images can be applied for accurately estimate anthocyanin concentration in photosynthetic and non-photosynthetic tissues. The most accurate estimations were obtained from color indices that stated the ratio of the G channel over the R and/or B channels or that reflect variations in the G channel.