State transitions and light adaptation require chloroplast thylakoid protein kinase STN7

Abstract

Photosynthetic organisms are able to adjust to changing light conditions through state transitions, a process that involves the redistribution of light excitation energy between photosystem II (PSII) and photosystem I (PSI). Balancing of the light absorption capacity of these two photosystems is achieved through the reversible association of the major antenna complex (LHCII) between PSII and PSI (ref. 3). Excess stimulation of PSII relative to PSI leads to the reduction of the plastoquinone pool and the activation of a kinase; the phosphorylation of LHCII; and the displacement of LHCII from PSII to PSI (state 2). Oxidation of the plastoquinone pool by excess stimulation of PSI reverses this process (state 1). The Chlamydomonas thylakoid-associated Ser-Thr kinase Stt7, which is required for state transitions, has an orthologue named STN7 in Arabidopsis. Here we show that loss of STN7 blocks state transitions and LHCII phosphorylation. In stn7 mutant plants the plastoquinone pool is more reduced and growth is impaired under changing light conditions, indicating that STN7, and probably state transitions, have an important role in response to environmental changes.

Protein kinases and phosphatases involved in the acclimation of the photosynthetic apparatus to a changing light environment

“…Together, they form a duo of kinases that is conserved in land plants where the orthologues of Stt7 and Stl1 are called STN7 and STN8, respectively [29]. Although these two kinases appear to have distinct substrates and functions with Stt7/STN7 required for LHCII phosphorylation and state transitions [27,29] and Stl1/STN8 for PSII core protein phosphorylation [30,31], there is some overlap between these two kinases as seen by the comparison of the protein phosphorylation patterns of stn7 and stn8 with that of the stn7-stn8 double mutant. While several proteins are still phosphorylated both in the stn7 or stn8 mutant, most of these phosphorylations are lost in the double mutant.…”

“…In this way, two allelic mutants were identified which were unable to dephosphorylate LHCII upon transfer from state 2 to state 1. The gene inactivated in these mutants encodes a PP2C phosphatase [27,[29][30][31]. STN7 is also involved in retrograde signalling (LTR) [30].…”

Photosynthesis Control: An underrated short-term regulatory mechanism essential for plant viability

“…Surprisingly, plants devoid of NPQ (npq4), 43 State Transitions (stn7 and tap38/pph1) [21][22][23][24] and CET [(pgr5, pgrl1) 31,32 and different subunits of the NDH complex 11 ] grew similarly as WT plants or even better, as was the case of tap38/pph1, 23 under constant light conditions (see Fig. 1).…”

“…After a decrease in the level of PQH 2 , LHCII is dephosphorylated by the TAP38/PPH1 phosphatase and returns to PSII, thus balancing the redox state of the PQ pool. 5,[19][20][21][22][23][24] Another example of the importance of Cyt b 6 f in regulating thylakoid electron transport is provided by the Photosynthesis Control mechanism. 15,16,18 In particular, the observation that PSI remains oxidized after exposure to high light intensities 25,26 indicates that electrons do not flow freely from PSII to PSI and that a limiting step to the electron transport must exist within the ETC.…”

The Transiently Generated Nonphotochemical Quenching of Excitation Energy in Arabidopsis Leaves Is Modulated by Zeaxanthin

“…State transitions are regulated by the redox state of the chloroplast (Hamel et al, 2000;Rintamäki et al, 2000). The thylakoid protein kinase that is responsible for light-harvesting complex II phosphorylation has recently been identified in Chlamydomonas (Chlamydomonas reinhardtii; Bellafiore et al, 2005) and Arabidopsis (Arabidopsis thaliana; Bonardi et al, 2005).…”