Responses to environmental stresses in metallicolous and non metallicolous Silene paradoxa L. populations

Due to the high heavy metal concentrations, metalliferous soils, both of natural and anthropogenic origin, provide a very restrictive and selective environment for plant life. Some plants, named “metallophytes”, have evolved tolerance mechanisms that allow them to cope with toxic heavy metal concentrations. A good model for comparative studies of metal tolerance is the specie Silene paradoxa L. (Caryophyllaceae), an excluder facultative metallophyte that presents populations living in metalliferous and non-metalliferous soils. This project aims to investigate responses to different environmental stresses, both biotic and abiotic, in metallicolous and non-metallicolous populations of this specie. At the root level, we investigated how Cu affects the morphology and composition of such organ in a non-metallicolous and a Cu-tolerant Silene paradoxa population. We found out that in the Cu-tolerant population some of the possible Cu exclusion strategies could be the mucilage and lignine production and the reduction of subapical zone of the root. Passing to the shoot level, we investigated the different effects of Cu excess on photosynthetic parameters in the same two populations. The Cu-tolerant population showed a more efficient photosynthetic activity in respect to the non-metallicolous population and a different nature of photosynthetic limitations, being mostly stomatal, compared to non-metallicolous mostly diffusional and biochemical limitations. Metalliferous soils are characterized by low macronutrient concentrations and availability, if compared to non-metalliferous ones, so that metallophytes have to adapt even to nutrient scarcity. We compared the ability to use nutrients in a non-metallicolous, a Cu-tolerant and a serpentine Ni-tolerant population exposed to Cu and Ni excess. We found out that metallicolous populations have evolved mechanisms to adapt to Ca and Mg scarcity in a metal-dependent way, optimizing nutrient utilization. Responses to biotic stresses in excluder metallophytes are poorly studied. These plants, unlike hyperaccumulators, cannot rely on the accumulation of the metal to defend themselves from pathogens. Therefore, we quantified the responses to pathogen attack in a metallicolous and Cu-tolerant Silene paradoxa population exposed to Cu. As an elicitor of defense responses, we used a purified fungal protein with PAMP activity, called cerato-platanin (CP). An overproduction of phytoalexins was recorded for the Cu-tolerant population exposed to Cu, suggesting that adaptation to metalliferous soils can affect plant response to biotic stress. Remaining in the same outline and with the same experimental setting, we further investigated whether the pathway leading to induction of defense responses is dependent on ROS (reactive oxygen species) production or not. Our results showed incompatibility between the ordinary ROS-mediated response to fungal attack and the acquired mechanisms of preventing oxidative stress in the Cu-tolerant population. Therefore, the same incompatibility of hyperaccumulators in ROS-mediated biotic stress signals seemed to be exhibited by this excluder metallophyte, but without the advantage of being able to rely on the elemental defense for plant protection from natural enemies.