Do state transitions control CEF1 in higher plants?

Abstract

Cyclic electron flow around photosystem I (CEF1) in oxygenic photosynthesis is thought to augment the production of ATP/NADPH to balance the chloroplast energy budget. In the green alga Chlamydomonas, CEF1 has been shown to be regulated by the so-called state transitions, which involve reversible phosphorylation of antenna and other complexes in response to changes in plastoquinone redox state (Allen, 1981). Whether this regulatory system operates in C3 plants, which have much less robust state transitions, have been unclear. Moreover, recent works by Iwai et al. (2010) and Livingston et al. (2010) suggest that CEF1 in Chlamydomonas and Arabidopsis may operate through different pathways, with the later involving the NDH complex. In this work, we test for the involvement of state transitions in CEF1 using a newly isolated mutant, hcef2, with constitutively high CEF1. We find that the high CEF1 mutant is predominantly in state II. We also find that the mutants stn7 and tap38, locked in state I and state II (Bellafiore et al., 2004; Pribil et al., 2010), respectively, show elevated CEF1. This indicates STN7 is not necessary for activation of CEF1. We suggest a role of CEF1 as a compensatory mechanism to balance the redox state of the chloroplast when the state transition response is impaired.