PARENT SESSION
Posters P5C Biosynthesis and assembly: Pigments. Abstracts (643-659)

The mechanisms of light and dark protochlorophyllide reduction in vivo in angiosperms. Victor Raskin^*,1, Amnon Schwartz¹, ¹ The Hebrew University of Jerusalem, Rehovot, Israel

ABSTRACT- The fluorescence lifetime of photoactive protochlorophyllide (P) (protochlorophyllide incorporated in NADPH:protochlorophyllide-oxidoreductase (POR)) at room temperature has been estimated as 300 ps, which approximates the time constant of photoinduced electron transfer (ET) from H-D (donors of electrons and protons) to P. First, the excited P (P*) and H-D diffuse together to form a collision or contact complex (P*---H-D), which is isoenergetic with one or more of a set of vibrationally excited charge-transfer Franck-Condon states [(P*--- H-D) <---> (P ^----H-D⁺)]. This equilibrium state (ES) may relax to the intermediate ET state - (P^----H-D⁺). As suggested in the literature, the accumulation of the ET state - the non-fluorescent paramagnetic intermediate - is coupled with proton transfer or a shift in the hydrogen-bonding excited complex. The singlet ET state yields the triplet ET state. The existence of a triplet ET state is proposed because the time interval (35-250 ns) between recording of the ES and appearance of the charge transfer complex ground state (CTC GS) does not feet the singlet ET state. In the temperature range of normal physiological activity, the lifetime of CTC GS transformation into chlorophyllide is from 7 to 12 microseconds. It is known that genes of light-independent protochlorophyllide-reductase (LIPR) are absent in angiosperms. However, etiolated seedlings of angiosperms, illuminated for a few hours and then returned to darkness, continue to accumulate chlorophyll a (Chl). Here, it is provided evidence that salicylic acid, an inhibitor of ascorbate peroxidase (APX), inhibits dark Chl accumulation in greening barley seedlings at the P reduction step. This means that in absence of LIPR, P reduction in the dark can be achieved via POR by using the energy of electronically excited species generated in the enzymatic APX system.

KEY WORDS: non-fluorescent intermediate, protochlorophyllide dark reduction, charge-transfer complex, protochlorophyllide photoreduction