Scientific objectives
The long term goal of the research is a mechanistic understanding of how photosynthesis acclimates to overcome environmental stresses limiting plant performance. Particular attention is paid to the function of the two photosystems, energy distribution between photosystems and the intersystem electron transfer in relation to molecular, organizational and metabolic changes as plants are exposed to high light intensities, low temperatures, nutritional and water stress. The cyanobacterium
Synechococcus sp. PCC is used as a model organism but also algae, lichens and higher plants such as Pinus sylvestris, winter cereals and
Arabidopsis thaliana are used when the scientific question so motivates.
A flexible photosynthetic system
The well known Z-scheme with photosystem II and I linked in series is well accepted and described in text books in plant physiology. However, the partitioning of photosystems II and I between grana and stroma lamellae, respectively, and the variable ratio between the two photosystems and between grana and stroma lamellae as dependent of the prevailing light environment demonstrate a flexible interaction between the two photosystems (Anderson et al. , Aust. J. Plant Physiol. 15: 11-26; Chow et al. , PNAS 87: ). Furthermore, subpopulations of the the two photosystems have been described (Albertsson , Photosynth. Res. 46: 141-149), and are likely to be involved in the dynamic degradation and resynthesis of the whole machinery as exemplified by the so called photosystem II repair cycle (Anderson and Aro , Photosynth. Res. 41: 315-326). Photosystem II appears to be a particularly dynamic system with respect of degradation triggered by light (photoinhibition) and resyn-thesis adjusting the amount of photosystem II to fit the capacity of temperature dependent pro-cesses downstream of photochemistry (Anderson et al. , Physiol. Plant. 100: 214-223). The nonphotochemical quenching mechanism as regulated pH is furthermore an integrated function of photosystem II and its associated antennae chlorophyll protein complexes (Horton et al. , Annu. Rev. Plant Physiol. Plant Mol. Biol. 47: 655-684). In cyanobacteria, the interplay between photosynthesis and respiration adds an additional component of great significance as demonstrated by respiration being of great importance in keeping the primary stabile electron acceptor of photosystem II, QA, oxidized (Campbell et al. , Physiol.Plant. 105:746-755).
Our own studies have shown that depending on the environmental conditions the standard, linearly arranged Z-scheme for electron transport is much more flexible than previously thought. In many cyanobacteria, a psbA gene family codes for two alternative forms of the reaction center D1 protein of photosystem II. In Synechococcus, the transiently light stress induced D1 form (D1:2) forms reaction centra with an increased capacity to transfer electrons beyond pheophytine or QA as compared with cells containing the light acclimated D1 form D1:1. In addition, Synechococcus responds to iron deficiency by a functional separation of the two photosystems with photosystem I conducting cyclic electron transport and photosystem II probably reducing NADP on its own. Such a scheme was originally proposed by Arnon (see Arnon , Photosynth. Res. 46: 47-71) although it has seriously been questioned. In Scots pine, winter stress induces an almost complete inhibition of photosystem II while photosystem I still show good activity after thawing. By studies of the P700 oxidation/reduction kinetics it now appears that photosystem I of Scots pine becomes highly active upon occasional thawing during the winter, probably supporting cyclic photophosphorylation. Furthermore, winter stressed pine has a very high stromal pool of electrons donating electrons to P700, the origin of this pool being unknown. Another example of a very dynamic photosynthetic system is given by desiccation tolerant species such as the intertidal red alga Porphyra perforata showing an extreme transfer of excitation energy in favor of photosystem I thus probably protecting photosystem II from photodynamic damages in the dry thallus.
Selected publications:
Hurry, V.M., GardestrŲm, P. and ÷quist, G. () Reduced sensitivity to photoinhibition following frost-hardening of winter rye is due to increased phosphate availability. Planta 190: 484-490.
Clarke, A.K., Soitamo, A., Gustafsson, P. and ÷quist, G . () Rapid interchange between two distinct forms of cyanobacterial photosystem II reaction-center protein D1 in response to photoinhibition. PNAS, USA 90: .
Ottander, C., Campbell, D. and ÷quist, G . () Seasonal changes in photosystem II organization and pigment composition of Pinus sylvestris. Planta 197: 176-183.
Campbell, D., Zhou, G., Gustafsson, P., ÷quist, G . & Clarke, A.K. () Electron transport regulates exchange of two forms of photosystem II D1 protein in the cyanobacterium Synechococcus. EMBO Journal 14:
Campbell, D., Eriksson, M.-J., ÷quist, G ., Gustafsson, P. and Clarke, A. K. () The cyanobacterium Synechococcus resists UV-B by exchanging photosystem II reaction center D1 proteins. PNAS 95: 364-369.
Park, Y.-I., SandstrŲm, S., Gustafsson, P. and ÷quist, G . () Expression of the isiA gene is essential for the survival of the cyanobacterium Synechococcus sp. PCC by protecting photosystem II from excess light under iron limitation. Molecual Biology. 32: 123-129.
