Light stress proteins from Arabidopsis thaliana : identification, characterisation and expression

Sammanfattning: Plants are major players in the vital process of photosynthesis that takes place in organelles called chloroplasts. Plant's immobility makes them victims of the tremendous fluctuations in light intensity and quality that often lead to inhibition of their physiological functions. In order to maintain the photosynthetic processes under light stress conditions, plants have developed several protective mechanisms. In this thesis, one such protective strategy operating in the chloroplasts, namely the accumulation of light stress proteins from the Elip (early light-induced proteins) family of Arabidopsis thaliana is presented. Elips in higher plants are nuclear-encoded proteins located in the thylakoid membranes and related to the chlorophyll a/b-binding proteins of photosystem I and II. A photoprotective function either by binding of released free chlorophylls and thus preventing the formation of free radicals or by thermal dissipation of excess energy was proposed for Elip family members. Using genomic resources available for A. thaliana, two members of the classical three-helix Elips, a new class of two-helix stress-enhanced proteins (called Sep) and a Elip-like one-helix protein (called Ohp 2) were identified and characterised at the molecular level. We demonstrated that both light and retrograde signalling are involved in the regulation of these proteins and that this regulation occurrs independently at transcript and protein levels. We showed that while Elips and Seps are located in photosystem II, Ohp 2 was found in photosystem I, thus providing evidence for a novel protective mechanism present in this photosystem. This work further demonstrates the involvement of chlorophyll a in the posttranslational regulation of Elip expression, thus supporting the proposed photoprotective function of these proteins. Finally, the distribution of the Elip family members through oxygenic photosynthetic organisms from the prokaryotic cyanobacteria, all throughout primitive algae and finally to higher plants was analysed and a novel model for the evolution of Elips and chlorophyll a/b-binding proteins in higher plants and green algae is proposed.