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 the two photosystems of photosynthesis via state transitions. This process was investigated in vivo using the aquatic higher plant Spirodela oligorrhiza. State transitions were followed by 77 K chlorophyll a (Chl a) fluorescence and phosphorylation of the Chl a/b light harvesting complex (LHCII). A response spectrum for the state transition indicated 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 LHCII phosphorylation (t(1/2) = 4 min) during a state 1 to state 2 transition were similar to the rise in fluorescence at 77 K from Photosystem I (PS I) relative to Photosystem II (PS II) (t(1/2) = 3 min). As well, 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 fluorescence from PS I relative to PS II (t(1/2) = 10 min) were comparable. The phosphatase inhibitor, NaF, suppressed both LHCII dephosphorylation and the decrease in the PS I/PS II fluorescence emission ratio, thus showing prevention of a transition to state 1 under PS I illumination. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU), which indirectly inactivates the LHCII kinase, triggered a transition to state 1 in PS II light. Upon exposure to PS II light, phosphorylated LHCII was observed to appear in granal thylakoid fractions prior to its appearance in stromal thylakoid fractions, indicating vectoral movement between the two membrane 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. Our results strongly support the hypothesis that LHCII phosphorylation and antenna migration between the photosystems are key components of the state transition mechanism in vivo.