Light-dependent regulation of state transitions in gymnosperms compared with angiosperms

Background/Question/Methods

The mobile light-harvesting antenna (LHCII) moves laterally between the two major photosystems (PSI and PSII) of plants to regulate light through a process called state transitions. This process is controlled by the phosphorylation of LHCII and is dependent on the intensity of light. The goal of this study was to determine whether state transitions in gymnosperms have a light-dependent response that is unique compared to angiosperms and that may be regulated by the redox status of the chloroplast stroma. Two species of conifer were examined, eastern white pine (Pinus strobus L.) and white spruce (Picea glauca (Moench) Voss), as well as the angiosperm pumpkin (Cucurbita pepo L.). Diurnal time courses were conducted of each species growing outdoors under full sun conditions. Each species was also subjected to manipulated light conditions in growth chambers. Light and chlorophyll fluorescence were measured, and needle/leaf tissue samples were collected. Thylakoids were isolated from all samples and phosphorylation patterns of thylakoid proteins were monitored by Western blotting using anti-phosphothreonine antibody. The redox status of the chloroplast stroma was measured by examining the activity of the enzyme NADP-malate dehydrogenase.

Results/Conclusions

In both sets of experiments, the pumpkin maintained a higher photosynthetic efficiency and a lower photosystem II reduction state (both estimated using chlorophyll fluorescence) at a given light intensity relative to the conifers, indicating that the pumpkin is capable of using a larger fraction of absorbed light for photosynthesis compared with the conifers.  In all species, LHCII was maximally phosphorylated at low light intensities and the phosphorylation decreased at higher light intensities as has been previously reported.  However, both conifers showed relatively higher phosphorylation at high light compared with the angiosperm.  Additionally, the conifers maintained higher LHCII phosphorylation under a given excitation pressure compared with pumpkin. We also found the photosystem II core proteins (CP43, D1 and D2) increased in phosphorylation as light increased in pumpkin as reported previously, while in the conifers, CP43 and D2 remained fairly highly phosphorylated at all light intensities, and we were unable to detect D1 phosphorylation with our antibody.  The results suggest that the two species of conifers have a different light-dependent response of LHC phosphorylation, and other core proteins, compared to pumpkin.