Minisymposium 15: Photosynthesis
Abs #
28005: Photosynthesis in seeds: localisation, feature and function
Many contradicting evidence about the ability of seeds to photosynthesise in vivo has been published. In order to analyse photosynthetic ability of crop plant seeds and to define the parameters and input of photosynthesis on seed development we applied a broad spectrum of topographical and analytical techniques, such as analysis of distribution of chlorophyll by CLS-microscopy, photosynthetic electron transport analysed by PAM-fluorescence and inhibitors, photosynthetic O2 evolution by microsensors and ATP distribution using quantitative bioluminescence imaging (Borisjuk et al., Plant J. 36: 318-329, 2003). The influence of photosynthesis on metabolic fluxes was monitored by metabolite profiling (LC-MS/MS) and 14C-tracers.
Developing legume seeds contain very low internal O2 levels that can fall <2.5 µM (<0.1 % of atmospheric saturation!, Rolletschek et al., Plant Physiol. 132: 1196-1206, 2003). Internal hypoxia can restrict energy metabolism and storage. During development the main storage organ (embryo) becomes green and photosynthetically active. The photosynthetic electron transport was saturated at low light conditions. However, photosynthetic O2 release was sufficient for significant increase in internal O2 levels, thereby preventing hypoxia. Gradients in chlorophyll, photosynthetic electron transport and O2 release were observed within seeds. Photosynthetic capacity temporally and spatially corresponded to the ATP distribution. The highest biosynthetic fluxes were observed when the embryos turned green and contained high ATP. We suggest that seed photosynthesis has an impact on storage by providing ATP/reducing equivalents and O2, which is instantly used for respiration.
In cereal grains, photosynthetic activity is restricted to distinct cell layers within the pericarp, enveloping the developing embryo and endosperm (main storage organ). Endosperm is lacking chlorophyll. Seed photosynthesis leads to superambient O2 levels (up to 250 %) within pericarp, and supplies the interiorly located endosperm with O2. This allows the endosperm to overcome internal hypoxia and to generate high ATP levels required for storage (Rolletschek et al., J. Exp. Bot., in press, 2004). By supplying O2 to hypoxic endosperm, photosynthesis is of fundamental importance for storage processes in cereal grains, despite of their low photosynthetic CO2-fixation potential. We propose the model for the role of seed photosynthesis to improve the energy state of seeds.
