High Light-Dependent Phosphorylation of Photosystem II Inner Antenna CP29 in Monocots Is STN7 Independent and Enhances Nonphotochemical Quenching

Abstract

Phosphorylation of the photosystem II antenna protein CP29 has been reported to be induced by excess light and further enhanced by low temperature, increasing resistance to these stressing factors. Moreover, high light-induced CP29 phosphorylation was specifically found in monocots, both C3 and C4, which include the large majority of food crops. Recently, knockout collections have become available in rice (Oryza sativa), a model organism for monocots. In this work, we have used reverse genetics coupled to biochemical and physiological analysis to elucidate the molecular basis of high light-induced phosphorylation of CP29 and the mechanisms by which it exerts a photoprotective effect. We found that kinases and phosphatases involved in CP29 phosphorylation are distinct from those reported to act in State 1-State 2 transitions. In addition, we elucidated the photoprotective role of CP29 phosphorylation in reducing singlet oxygen production and enhancing excess energy dissipation. We thus established, in monocots, a mechanistic connection between phosphorylation of CP29 and nonphotochemical quenching, two processes so far considered independent from one another.In eukaryotic photosynthesis, light-dependent reactions are performed by two supramolecular complexes, PSII and PSI, which catalyze light harvesting and electron transport from water to NADP + . To this aim, water is oxidized by PSII, which, in turn, is oxidized by PSI, which becomes a reductant for ferredoxin-NADP(1) oxidoreductase and NADP + (Nelson and Ben-Shem, 2004). The two photosystems are functionally connected by the plastoquinone (PQ) and cytochrome (cyt) b 6 /f, which catalyze the building of the transthylakoid proton gradient, which is dissipated by ATP synthase (ATPase) activity for ATP synthesis from ADP and inorganic phosphate (P i ). PSII and PSI have clearly distinct absorption spectra, with PSI-lightharvesting complex I (LHCI) complexes being enriched in red-shifted absorption forms (Gobets and van Grondelle, 2001). Within canopies, this leads to differential excitation depending on available light quality. This effect needs to be compensated to avoid imbalance of electron transport rates, yielding into either photoinhibition or decrease of photon use efficiency. Two major regulatory mechanisms counteract these effects. (1) State 1-State 2 transitions are active in limiting light conditions (Rintamäki et al., 2000) and inhibited by reduction of a disulfide bridge in high light (HL; Lemeille et al., 2009). This mechanism is activated by overreduction of PQ to plastoquinol (PQH 2 ) through activation of a thylakoid bound kinase, STN7, acting on LHCII (Depège et al., 2003;Bellafiore et al., 2005). This causes a fraction of PSII antenna system, mainly Lhcb2 (Leoni et al., 2013), to be transferred to PSI in stroma-exposed membranes. The consequent increase in PSI antenna size (Galka et al., 2012) bursts the electron transfer rate and reequilibrates PQ/PQH 2 redox poise, thus causing feedback inactivation of kinase activity. A phosphatase, PPH1-TA...

Thylakoid proteome modulation in pea plants grown at different irradiances: quantitative proteomic profiling in a non‐model organism aided by transcriptomic data integration

“…Considering the similar rate of accumulation of these proteins in ML, we propose their direct and concerted involvement in turnover of PsbA, which is already enhanced at moderate irradiances (Aro et al ., ). It is well known that the functioning and repair of photosynthetic protein complexes, especially PSII, is largely regulated by reversible protein phosphorylation (Allen et al ., ; Betterle et al ., ). A roughly twofold increase in the amount of the luminal kinase NDPK3, which uses ATP to synthesize the GTP required for the degradation of PsbA (Spetea et al ., ; Spetea and Lundin, ), was already observed in ML versus LL plants and further exacerbated in HL versus ML plants (Figure ), pointing out its major role in efficiently sustaining PSII turnover even at moderate intensities.…”

Adjustment of photosynthetic activity to drought and fluctuating light in wheat

“…In the same study, barley and maize plants showed increased CP29 phosphorylation also under high light, cold, and salt stress. The increase of CP29 phosphorylation upon exposure to high light was further observed both in dicots and monocots and associated with PSII damage (Betterle, Ballottari, Baginsky, & Bassi, 2015; Fristedt & Vener, 2011). As a result, no common pattern of PSII‐LHCII phosphorylation in drought stress can be deduced among different species.…”

Thylakoid Protein Phosphorylation Dynamics in a Moss Mutant Lacking SERINE/THREONINE PROTEIN KINASE STN8

“…The amino acid sequence of P. patens STN8 kinase showed a conserved C-terminal catalytic domain with other plant species, while the main differences were found in the N terminus, already shown to be variable among species, possibly related to the different substrate specificity of the kinase in different species (Betterle et al, 2015;Supplemental Fig. S5).…”

“…Although the exact physiological role of phospho-LHCB6 in P. patens has still to be elucidated, the phosphorylation of minor antenna proteins in other plant species has been mainly related to a photoprotective role, e.g. NPQ (Betterle et al, 2015) or energy spillover (Ferroni et al, 2018). This might be the case also in P. patens, where LHCB6 phosphorylation is enhanced particularly upon the HL treatment.…”