Most sexual organisms inherit organelles from one parent, commonly by excluding organelles from the smaller gametes. However, post-mating elimination of organelles derived from one gamete ensures uniparental inheritance, where the underlying mechanisms to distinguish organelles by their origin remain obscure. Mating in Chlamydomonas reinhardtii combines isomorphic plus and minus gametes, but chloroplast DNA from minus gametes is selectively degraded in zygotes. Here, we identify OTU2p (otubain protein 2), encoded in the plus mating-type locus MT+, as the protector of plus chloroplast. Otu2p is an otubain-like deubiquitinase, which prevents proteasome-mediated degradation of the preprotein translocase of the outer chloroplast membrane (TOC) during gametogenesis. Using OTU2p-knockouts and proteasome inhibitor treatment, we successfully redirect selective DNA degradation in chloroplasts with reduced TOC levels regardless of mating type, demonstrating that plus-specific Otu2p establishes uniparental chloroplast DNA inheritance. Our work documents that a sex-linked organelle quality control mechanism drives the uniparental organelle inheritance without dimorphic gametes.

Eukaryotic photosynthetic organisms have evolved an array of "antenna" complexes where both light harvesting and photoprotective mechanisms occur. Thermal dissipation of the excitation energy harvested in excess, named non-photochemical quenching (NPQ), is one of the main photoprotective mechanisms evolved in eukaryotic organisms to prevent photooxidative stress. Here, the role of the Photosystem II monomeric antenna CP26 was investigated in Chlamydomonas reinhardtii, model organism for green algae, using a genome editing approach to obtain cp26 knock-out mutant strains (named k6). The absence of CP26 caused a reduced growth at low or medium light but not at high irradiances. Photosystem II were partially affected by the absence of CP26 having reduced photochemical efficiency, light harvesting capacity and excitation energy transfer. However, the main phenotype observed in k6 was a strong reduction of NPQ, being reduced by more than 70% compared to wild type. The NPQ phenotype observed was rescued by genetic complementation of k6 mutant demonstrating that ~50% of CP26 content compared to wild type was able to restore the NPQ capacity. The comparison of k6 mutant with mutants deprived of the other Photosystem II monomeric antenna, CP29 or missing both CP26 and CP29, demonstrated that these monomeric antenna proteins have different specific functions in Chlamydomonas reinhardtii: CP26 plays a pivotal role in NPQ induction while the binding of CP29 to Photosystem II is crucial for its photochemical activity. The genetic engineering of these two proteins could be a promising strategy to regulate photosynthetic efficiency of microalgae under different light regimes.

Thermal dissipation of excess excitation energy, called non-photochemical quenching (NPQ), is one of the main photoprotective mechanisms in oxygenic photosynthetic organisms. Here, we investigated the function of the monomeric photosystem II antenna protein CP26 in photoprotection and light harvesting in Chlamydomonas reinhardtii, a model organism for green algae. We used CRISPR/Cas9 genome editing and complementation to generate cp26 knock-out mutants (named k6#) that did not negatively affect CP29 accumulation, which differed from previous cp26 mutants, allowing us to compare mutants specifically deprived of CP26, CP29 or both. The absence of CP26 partially affected photosystem II activity causing reduced growth at low or medium light but not at high irradiances. However, the main phenotype observed in k6# mutants was a more than 70% reduction of NPQ compared to the wild type. This phenotype was fully rescued by genetic complementation and complemented strains accumulating different levels of CP26, demonstrating that ∼50% of CP26 content, compared to the wild type, was sufficient to restore the NPQ capacity. Our findings demonstrate a pivotal role for CP26 in NPQ induction, while CP29 is crucial for photosystem II activity. The genetic engineering of these two proteins could be a promising strategy to regulate the photosynthetic efficiency of microalgae under different light regimes.