Photoprotective qH in Arabidopsis occurs in the light-harvesting complex II trimer

Excess light can induce photodamage to the photosynthetic machinery, therefore plants have evolved photoprotective mechanisms such as non-photochemical quenching (NPQ). Different NPQ components have been identified and classified based on their relaxation kinetics and molecular players. The NPQ component qE is induced and relaxed rapidly (seconds to minutes), whereas the NPQ component qH is induced and relaxed slowly (hours or longer). Molecular players regulating qH have recently been uncovered, but the photophysical mechanism of qH and its location in the photosynthetic membrane have not been determined. Using time-correlated single-photon counting analysis of the Arabidopsis thaliana suppressor of quenching 1 mutant (soq1), which displays higher qH than the wild type, we observed shorter average lifetime of chlorophyll fluorescence in leaves and thylakoids relative to wild type. Comparison of isolated photosynthetic complexes from plants in which qH was turned ON or OFF revealed a chlorophyll fluorescence decrease specifically in the trimeric light-harvesting complex II (LHCII) fraction when qH was ON. LHCII trimers are composed of Lhcb1, 2 and 3 proteins, so CRISPR-Cas9 edited and T-DNA insertion lhcb1, lhcb2 and lhcb3 mutants were crossed with soq1. In soq1 lhcb1, soq1 lhcb2, and soq1 lhcb3, qH was not abolished, indicating that no single major Lhcb isoform is necessary for qH. Using transient absorption spectroscopy of isolated thylakoids, no spectral signatures for chlorophyll-carotenoid excitation energy quenching or charge transfer quenching were observed, suggesting that qH may occur through chlorophyll-chlorophyll excitonic interaction.