STN7 is not essential for developmental acclimation of Arabidopsis to light intensity.

Plants respond to changing light intensity in the short term through regulation of light harvesting, electron transfer and metabolism to mitigate redox stress. In the long-term, a sustained shift in light intensity leads to a long-term acclimation response (LTR) that involves adjustment in the stoichiometry of photosynthetic complexes, through de novo synthesis and degradation of specific proteins associated with the thylakoid membrane. The light-harvesting complex II (LHCII) serine-threonine kinase (STN7) plays a key role in both short-term light harvesting regulation to moderate redox state and the LTR to allow acclimation. In the short-term Arabidopsis plants lacking STN7 (stn7) shifted to low light experience higher PQ redox pressure than the wild-type or plants lacking the cognate phosphatase TAP38 (tap38), while the reverse is true at high light where tap38 plants suffer more. In principle, the LTR should allow optimisation of the stoichiometry of photosynthetic complexes to mitigate these effects in the mutants. We used quantitative label-free proteomics to assess the how the relative abundance of photosynthetic proteins varied with growth light intensity in the wild-type, stn7 and tap38 plants. All plants were able to adjust photosystem I, LHCII, cytochrome b 6 f, and ATP synthase abundance with changing white light intensity, demonstrating neither STN7 nor TAP38 is crucial to the LTR. However, stn7 plants grown for several weeks at low light (LL) or moderate light (ML) still showed high PQ redox pressure and correspondingly lower photosystem II efficiency, CO 2 assimilation and leaf area compared to wild-type and tap38 plants. In contrast, under high light growth conditions the mutants and wild-type behaved similarly. These data are consistent with the vital role of STN7-dependent LHCII phosphorylation in tuning PQ redox state for optimal growth in LL and ML conditions.