Abstract
Light absorbed by chlorophylls of Photosystems II and I drives oxygenic photosynthesis. Light-harvesting complexes increase the absorption cross-section of these photosystems. Furthermore, these complexes play a central role in photoprotection by dissipating the excess of absorbed light energy in an inducible and regulated fashion. In higher plants, the main light-harvesting complex is trimeric LHCII. In this work, we used CRISPR/Cas9 to knockout the five genes encoding LHCB1, which is the major component of LHCII. In absence of LHCB1, the accumulation of the other LHCII isoforms was only slightly increased, thereby resulting in chlorophyll loss, leading to a pale green phenotype and growth delay. The Photosystem II absorption cross-section was smaller, while the Photosystem I absorption cross-section was unaffected. This altered the chlorophyll repartition between the two photosystems, favoring Photosystem I excitation. The equilibrium of the photosynthetic electron transport was partially maintained by lower Photosystem I over Photosystem II reaction center ratio and by the dephosphorylation of LHCII and Photosystem II. Loss of LHCB1 altered the thylakoid structure, with less membrane layers per grana stack and reduced grana width. Stable LHCB1 knockout lines allow characterizing the role of this protein in light harvesting and acclimation and pave the way for future in vivo mutational analyses of LHCII.
Highlights
Light is the source of energy for photosynthetic organisms and largely fuels the synthesis of organic molecules in the biosphere
We obtained two to four resistant progenies (T1 generation), which were screened for non-photochemical quenching (NPQ), as a loss of Lhcb1 should lower the photoprotective capacity (Pietrzykowska et al, 2014; Supplementary Figure 1)
Even in the presence of non-mutated Lhcb1 genes on Chromosome 2, in both C2-derived lines, there was no detectable LHCB1 protein nor a detectable band by total protein staining at the LHCII level (Figure 1B)
Summary
Light is the source of energy for photosynthetic organisms and largely fuels the synthesis of organic molecules in the biosphere. Light absorbed by PSI drives the electron transport through ferredoxin to reduce NADP+ + H+ to NADPH (reviewed in Merchant and Sawaya, 2005). This process is coupled with the generation of a proton gradient between the lumen and the stroma side of the thylakoid membrane, a gradient used by the ATP synthase for the generation of ATP (Boyer, 1993). Light is an inconstant source of energy, and photosynthetic organisms experience shifts in light spectrum and intensity To cope with these changes and optimize the photosynthetic efficiency, the organization of the pigment-protein complexes must be dynamic (for a recent review, see Rantala et al, 2020). The stacked and unstacked portions of thylakoids are dynamic and contribute to the acclimation to changes in environmental conditions (for a recent review, see Johnson and Wientjes, 2020)
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