Investigation On The Effects Of Extracellular ATP On Photosynthesis Of Bean And Its Physiological Role Under Environmental Changes

Although adenosine 5′-triphosphate(ATP) is usually considered to be localized in intracellular spaces, plant and animal cells can secrete ATP from the cytosol into the extracellular matrix. This extracellular ATP(eATP) is an important signal molecule for many physiological responses in plants. Using bean(Phaseolus vulgaris L) leaves as experiment material, the effects of eATP on photosynthesis of the plant and its physiological role under environmental changes were studied. the main findings are described as below:(1) The effects of eATP on photosynthesis were relate to its concentrations and times. The net photosynthesis rate(Pn), transpiration rate(E) and stomatal conductance(GH2O) were increased obviously after the treatment of leaves with exogenous ATP. And the effects of exogenous ATP on chlorophyll fluorescence parameters become more obvious with the increase of its concentrations. Under different light intersities, treatment of leaves with exogenous ATP obviously increased the values of the potential maximal photochemical efficiency of PSⅡ(Fv/Fm), the maximum quantum efficiency of PSⅡ photochemistry at illumination(Fv′/Fm′), the PS Ⅱ operating efficiency(Y( Ⅱ)), the photochemical quenching coefficient(qP), the rate of photosynthetic electron transport through photosystemⅡ(ETR), and photosynthetic O2 evolution rate, but treatment with exogenous ATP did not affect the values of qN and Y(NPQ). In contrast, the treatment of leaves with β,γ-methyleneadenosine triphosphate(AMP-PCP, an inhibitor of eATP receptors) obviously reduced the values of Fv/Fm, Fv′/Fm′, Y(Ⅱ), ETR, and photosynthetic O2 evolution rate but increased the values of the non-photochemical quenching coefficient(qN) and the quantum yield of regulated energy dissipation of PSⅡ(Y(NPQ)). These results show that eATP has the ability to enhance photosynthesis of the plant, but such function of eATP is dependent on the concentrations of eATP, and the influences of e ATP on the photochemical reaction is also related to the light intensities.(2) In order to explore the possible signaling pathway of eATP in regulating photosynthesis of plant, Ca2+, LaCl3(an inhibitor of Ca2+ channels), H2O2(hydrogen peroxide), and DMTU(dimethylthiourea, a scavenger of H2O2) were used. Similar to the effects of exogenous ATP, treatment with exogenous Ca2+ or H2O2 also led to a significant increase in ΦPSII, qP and ETR and a significant decrease in qN and Y(NPQ). And, LaCl3 and DMTU abolished the effects of exogenous ATP on the photosynthesis of leaves. These results suggest that the role of eATP in enhancing the PSⅡ photochemistry could be related to a Ca2+ or H2O2 singalling pathway.(3) Infection with X.c.p caused a significant decrease in the PS Ⅱ photochemistry and increase in the thermal dissipation of absorbed light energy. and also caused a decrease in eATP level. However, the application of this exogenous ATP alleviated the decreases of the PSⅡ photochemistry and e ATP in the infected leaves. These results suggest that eATP could play a role in mediating the performance of PSII photochemistry under the condition of pathogen infection. Similarity, treatment of leaves with exogenous ATP can also alleviated the decreases of PS Ⅱ photochemistry induced by low temperature. Thus, eATP can alleviate the decline of photochemical efficiency of plants under biological and non-biological stress. SA has been shown to be an important signaling molecules under the stress of pathogen infection or low temperature, and SA is also the main factor for the decrease of photochemical efficiency under these stress. According to our research, eATP can alleviated the decrease of PSⅡ photochemistry caused by SA, suggesting that the effects of e ATP on photosynthesis of leaves under pathogen infection or low temperature, may be relate to the ability of eATP to change the negative effects of SA on the photochemical efficiency of plants.In conclusion, as an important extracellular signaling molecule, e ATP can promote the photosynthesis of plants, and the mechanism for such role of e ATP could be related to a Ca2+ or H2O2 signaling pathway. Furthermore, eATP can alleviate the decrease of photosynthetic efficiency under the condition of low temperature or pathogen infection. Thus, it has important physiological functions in the maintenance of photosynthesis under the environmental changes.