| Abstract |
Photosynthesis comprises nowadays a wide and most active research field. This incomparable plant process constitutes not only the primary basis for sustaining plant life, but also a primary source for supporting life earth wide, since it comprises the only pathway for introducing the sunlight energy needed to the living organisms for biomass production. The processes of chloroplast photodevelopment, and photoadaptation of the photosynthetic apparatus are considered to be most important for the structural and functional integrity of the photosynthetic performance. The chloroplast photodevelopment is a dynamic process triggering the building of the photosynthetic apparatus and thus the basis for photosynthesis, whereas photoadaptation permits the fine tuning of the stucture and function of the developed photosynthetic apparatus, by both exploiting the environmental conditions the best way, and at the same time being able to protect itself from any putative harmfull environmental conditions. The three main polyamines (Put, Spd, Spm) were found to be localized in the chloroplast, whereas most importantly they were found to be attached to photosynthetic subcomplexes [Kotzabasis et al., 1993; del Duca et al., 1994]. Furthermore, their role in the chloroplast was associated with drastic prosseces, such as the stabilization of chlorophyll protein complexes [Besford et al., 1993] and the inhibition of the light-independent chlorophyll biosynthesis [Beigbeder and Kotzabasis, 1994]. Thus, the present dissertation aimed to the dissection of the role of polyamines in the dynamic processes of the photodevelopment and photoadaptation of the photosynthetic apparatus. For the purpose of the experimentation the wild type strain of the unicellular green alga (Chlorophycae) Scenedesmus obliquus was used and also its mutant strains C-2A- and C-6D, since the three strains comprise different levels of the chloroplast developmental stage. The results showed that polyamines, and especially Put are readily being regulated by onset of light and this process promotes the gradual decline of both their intracellular and intraplastidal concentrations, beginning with an oscillatory pattern. The primary photoreceptor for this regulation was identified as the -active- Protochlorophyllide, which also comprises the primary photoreceptor for chlorophyll biosynthesis and chloroplast photodevelopment. Thus, polyamine regulation and chloroplast photodevelopment share the shame photoreceptor. Other components of the signaling pathway leading to the polyamine regulation by light are heterotrimeric G-proteins, cyclic nucleotides (cNMPs), protein kinases protein phosphatases, calmodulin and Ca2+. The studied pathway reveals similarity to the pathway leading to the photosynthetic gene expression. Thus, polyamines seem to play a central role in the pathway which coordinates the chlorophyll and appoprotein biosynthesis, for the concerted development of the photosynthetic constituents. In parallel, whereas the development of the photosynthetic apparatus is still ongoing, the newly assembled Photosystem II subcomplexes trigger a second regulation of polyamine levels, by means of increasing their bound levels to the major antenna complex of PSII, the LHCII. LHCII has a unique role in photoadaptation processes, by readily adjusting its size, as a response to the changing light intensities. By externally manipulating the polyamine levels, an indisputable relation was found between the LHCII size and polyamine levels: low Put or increased Spm levels promote increase in the LHCII size, simulating low light conditions, whereas, increased Put concentration produces a smaller LHCII antenna size, an effect normally seen in high light conditions. Further dissection on the effect of polyamines in the polymerization/apopolymerization of LHCII, showed that Put and Spm affect the autoproteolytic properties of LHCII, thus modifying its size independently of the environmental light conditions. Other studies performed in the laboratory of Plant Biochemistry and Photobiology (Univ. of Crete, dep. Biology) revealed the participation of the LHCII size regulation in plant photosynthetic responses to many different environmental stressors, such as high ozon concentration [Navakoudis et al., 2003] (study performed with the contribution of the GSF-Research Center (Munich), UVB radiation [Navakoudis et al., in preparation; Sfichi et al., 2004], high CO2 [Logothetis et al., 2004). In all these cases, externally manipulated polyamine concentrations were effective in reversing the sensitivity of the photosynthetic apparatus to the stressor. Thus, polyamines play a major role in regulating the LHCII size status and concomitantly the photosynthetic efficiency, thus increasing plant production under unfavourable environmental conditions.
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Δομικές και λειτουργικές δράσεις των πολυαμινών στη φωτοανάπτυξη και φωτοπροσαρμογή του φωτοσυνθετικού μηχανισμού
