A Review: Polyamines and Photosynthesis

Polyamines (PAs) are low molecular weight ubiquitous nitrogenous compounds found in all living organisms (Kaur-Sawhney et al., 2003). In higher plants, the most common polyamines are spermidine (Spd), spermine (Spm) and their diamine obligate precursor putrescine (Put). They are formed by aliphatic hydrocarbons substituted with two or more amino groups (Figure.1). Because of the polycationic nature at physiological pH, PAs are present in the free form or as conjugates bound to phenolic acids and other low molecular weight compounds or to proteins and nucleic acids (Childs et al., 2003). Like hormones, PAs displaying high biological activity are involved in a wide array of fundamental processes in plants, such as replication and gene expression, growth and development, senescence, membrane stabilization, enzyme activity modulation and adaptation to abiotic and biotic stresses (Galston et al., 1997; Bais and Ravishankar, 2002; Zapata et al., 2008). Although, according to these reports, PAs seem to be important growth regulators, their precise physiological function and mechanism of action still remain unclear. It has been shown that chloroplasts and photosynthetic subcomplexes including thylakoids, LHCII complex and PSII membranes are enriched with three major polyamines, while PSII core and the reaction center of PSII are exclusively rich in Spm (Kotzabasis et al., 1993; Navakoudis et al., 2003). The potential role of polyamines in maintaining the photochemical efficiency of plants has become a research focus. These studies mainly focused on the effect PAs exert a positive role in the photosynthesis of plants in response to various environmental stresses. In green alga, it was shown that the bound Put content of the thylakoid membrane was increased in environments with high CO2 concentrations, which caused an increase in reaction center density and led to an increased photosynthetic rate (Logothetis et al., 2004). An increase in conjugated Put content can stabilize the thylakoid membrane, thus enhancing resistance of tobacco plants to ozone pollution (Navakoudis et al., 2003) and UV-B radiation (Lütz et al., 2005). Low temperature stress reduced the content of Put as well as the Put/Spm ratio in thylakoids and the light-harvesting complexes LHCII in Phaseolus vulgaris L., leading to a decrease in photosynthetic electron transport rate and inactivation of the PSII reaction center (Sfakianaki et al., 2006). Put is also involved in the induction of a photosynthetic apparatus owning high concentration of reaction center with a small functional antenna that leads to enhance photochemical quenching of the absorbed light energy (Kotzabasis et al., 1999). PAs biosynthesis is controlled by light and the Spm/Put ratio is correlated to the structure and function of the photosynthetic apparatus