PARENT SESSION

Symposium S5D Emerging techniques and systems
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 510A
Chair: Conrad Mullineaux
Co-Chair: Barry Osmond

Far-from-equilibrium photosynthesis explored by frequency analysis of forced oscillations. Martin Trtilek^{1, 3}, Michal Sicner¹, Vitezslav Brezina^{2, 3}, Ladislav Nedbal^{*,2, 3}, ¹ Photon Systems Instruments, spol. s r.o., Brno, Czech Republic³ Institute of Physical Biology, Nove Hrady, Czech Republic² Institute of Landscape Ecology, Nove Hrady, Czech Republic

ABSTRACT- Measurements of chlorophyll fluorescence transients occurring in response to a step-wise increase of irradiance are among the most frequently used techniques to study photosynthesis. Here, we demonstrate that the conventional protocols based on the step-wise light modulation do not exploit full potential of this powerful technique. Plants live in a dynamic environment where light typically oscillates around a mean level. We propose that the physiologically relevant regulation responsible for plant homeostasis in the continuously fluctuating light differs from the regulatory motifs investigated by the conventional protocols. To identify the regulation dominating in natural conditions, we use the concept of frequency analysis where the mean level of irradiance is perturbed by a harmonic, sinusoidal modulation. The plant dynamic response is studied as a function of the modulation frequency and amplitude (Nedbal et al. Biochim.Biophys.Acta 1607: 5-17, 2003). Using the cyanobacterium Synechocystis sp. PCC6803 and the higher plant Nicotiana tabacum, we show that the harmonically modulated irradiance induces perpetual, far-from-equilibrium forced oscillations that are strongly non-linear and exhibit a significant hysteresis. The far-from-equilibrium forced oscillations represent a significant correction to the static or linear estimates of the photosynthetic activity in nature. Adding to the relevance of the newly discovered phenomenon, frequency analysis of the forced oscillations is used to resolve photosynthetic regulatory networks and identify new mutants deficient in some of the regulatory functions. Reverse engineering analysis of the input-output relation leads to a model of the photosynthetic regulatory network that can account for the observed dynamic features. The key regulatory motif consists of one positive and one negative feedback loop. We propose that photosynthesis is an exquisite model for systems biology of more complex and less accessible processes.

KEY WORDS: regulation, fluctuating light, fluorescence, model