Investigation of Photosynthesis Regulation in Different Bryophyte Accessions

Oxygenic photosynthesis is the process that land and aquatic photoautotrophic organisms use to convert light into chemical energy. As with other physiological processes, photosynthesis is affected by variations in natural environmental conditions like light exposure, temperature, humidity, and it must therefore be regulated to face these changesThe ‘light reactions’ of oxygenic photosynthesis are carried out thanks to the linear electron flow (LEF) that takes place in the thylakoidal proteins of the chloroplasts. Water is the initial electron donor and NADP+ is the last acceptor, to fill the energy gap between the two, are required two photochemical reactions carried out by the two photosystems, PSII and PSI, where charge separation occurs. Excess light can excite photosystems II (PSII) for a prolonged period, which in turn, excite other molecules leading to the production of reactive oxygen species (ROS) such as singlet oxygen O2-. During evolution different photoprotective mechanisms were developed by photosynthetic organisms to protect the reaction centers of PSII, one of those is non-photochemical quenching (NPQ). Through NPQ the plant can dissipate into heat the energy from excited singlet state chlorophylls. This mechanism is triggered by a drop in pH in the lumen, a signal transduced by chlorophyll-binding proteins into a quenching reaction. The latter are PSBS (Photosystem II subunit S) and LHCSR (Light-Harvesting Complex Stress-Related), proteins similar to those present in the antenna system of the photosystem. An important role is also played by xanthophylls, in particular zeaxanthin which can act as a ROS scavenger and can also bind to LHCSR to enhance NPQ. Studying the variability present in nature is a promising tool for a better comprehension of this mechanism. In this thesis, we used mosses because of their fundamental role in evolution as first land colonizers, and because of their presence even in extreme environmental conditions. In particular, were used 17 accession from 4 genera endemic of various environments of different parts of the world, along with Physcomitrium patens, a model organism widely used for physiological and genetic studies. Biochemical and physiological techniques were utilized to investigate the efficiency and composition of the photosynthetic apparatus in the gametophore and protonema stages of growth. After an initial screening, the accessions were classified based on their photosynthetic efficiency and NPQ variability. Three accessions were then selected based on their NPQ phenotype to further investigate this photoprotective mechanism in the protonema growth stage: P. patens Gransden (high NPQ), P. patens Californica (high NPQ) and P.readeri Okayama (low NPQ). The results of western blotting led to attributing the low NPQ phenotype of P. readeri Okayama to the lack of the LHCSR1 protein. The overall results indicated that high light acclimation induced higher stress levels in mosses compared to control light, confirmed by lower Fv/Fm values and higher NPQ activation in HL compared to CL. Fluorimetric measurements revealed a higher NPQ activation in the protonema compared to gametophore, a physiological difference to date little described in the scientific literature.