Plant Physiol, April 2001, Vol. 125, pp. 2007-2015

Active Oxygen Produced during Selective Excitation of Photosystem I Is Damaging Not Only to Photosystem I, But Also to Photosystem II¹

Plant Biochemistry Laboratory, Department of Plant Biology, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark (S.E.T., H.V.S., B.L.M.); Department of Biochemistry, Stockholm University, SE-10609 Stockholm, Sweden (S.E.T., B.A.); and Division of Cell Biology, Linköping University, SE-58185 Linköping, Sweden (B.A.)

With the aim to specifically study the molecular mechanisms behind photoinhibition of photosystem I, stacked spinach (Spinacia oleracea) thylakoids were irradiated at 4°C with far-red light (>715 nm) exciting photosystem I, but not photosystem II. Selective excitation of photosystem I by far-red light for 130 min resulted in a 40% inactivation of photosystem I. It is surprising that this treatment also caused up to 90% damage to photosystem II. This suggests that active oxygen produced at the reducing side of photosystem I is highly damaging to photosystem II. Only a small pool of the D1-protein was degraded. However, most of the D1-protein was modified to a slightly higher molecular mass, indicative of a damage-induced conformational change. The far-red illumination was also performed using destacked and randomized thylakoids in which the distance between the photosystems is shorter. Upon 130 min of illumination, photosystem I showed an approximate 40% inactivation as in stacked thylakoids. In contrast, photosystem II only showed 40% inactivation in destacked and randomized thylakoids, less than one-half of the inactivation observed using stacked thylakoids. In accordance with this, photosystem II, but not photosystem I is more protected from photoinhibition in destacked thylakoids. Addition of active oxygen scavengers during the far-red photosystem I illumination demonstrated superoxide to be a major cause of damage to photosystem I, whereas photosystem II was damaged mainly by superoxide and hydrogen peroxide.