STATE TRANSITIONS, LIGHT-HARVESTING ANTENNA PHOSPHORYLATION AND LIGHT-HARVESTING ANTENNA MIGRATION IN-VIVO IN THE HIGHER-PLANT SPIRODELA-OLIGORRHIZA

Plants can balance the relative levels of excitation energy reaching t he two photosystems of photosynthesis via state transitions. This proc ess was investigated in vivo using the aquatic higher plant Spirodela oligorrhiza. State transitions were followed by 77 K chlorophyll a (Ch l a) fluorescence and phosphorylation of the Chl a/b light harvesting complex (LHCII). A response spectrum for the state transition indicate d that light absorbed predominantly by Chi b led to state 2 and light absorbed predominantly by Chl a resulted in state 1. The kinetics of L HCII phosphorylation (t(1/2) = 4 min) during a state 1 to state 2 tran sition were similar to the rise in fluorescence at 77 K from Photosyst em I (PS I) relative to Photosystem II (PS II) (t(1/2) = 3 min). As we ll, for the transition from state 2 to state 1, the kinetics of LHCII dephosphorylation (t(1/2) = 13 min) and the rate of loss of fluorescen ce from PS I relative to PS II (t(1/2) = 10 min) were comparable. The phosphatase inhibitor, NaF, suppressed both LHCII dephosphorylation an d the decrease in the PS I/PS II fluorescence emission ratio, thus sho wing prevention of a transition to state 1 under PS I illumination. 3- (3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU), which indirectly inactiv ates the LHCII kinase, triggered a transition to state 1 in PS II ligh t. Upon exposure to PS II light, phosphorylated LHCII was observed to appear in granal thylakoid fractions prior to its appearance in stroma l thylakoid fractions, indicating vectoral movement between the two me mbrane compartments. Determinations of the absolute yield of PS II and PS I fluorescence showed a complementary decrease and increase in PS II and PS I emission, respectively, in state 2 compared to state 1. Ou r results strongly support the hypothesis that LHCII phosphorylation a nd antenna migration between the photosystems are key components of th e state transition mechanism in vivo.