A YAK1-type protein kinase, triacylglycerol accumulation regulator 1, in the green alga Chlamydomonas reinhardtii is a potential regulator of cell division and differentiation into gametes during photoautotrophic nitrogen deficiency

Yet another kinase (YAK) 1 is a conserved eukary-otic protein kinase coordinating growth and devel-opment. We previously isolated a mutant of Chlamydomonas reinhardtii defective in the YAK1 ortholog triacylglycerol (TAG) accumulation regu-lator 1 (TAR1). The mutant tar1-1 displayed higher levels of chlorophyll, starch, TAG, and biomass than the parental strain C9 (renamed as C9-3) in photo-autotrophic nitrogen (N)-deficient conditions. How-ever, we found that the parental C9-3 showed faster chlorosis upon N-deficiency than the original C9 (C9-1) freshly recovered from cryopreservation, suggesting that C9-3 had acquired particular char-acteristics during long-term subculturing. To exclude phenotypes dependent on a particular parental strain, we newly created tar1 mutants from two wild-types, C9-1 and CC-125. Like tar1-1, the new tar1 mutants showed higher levels of chloro-phyll and TAG/starch than the parental strain. Upon removal of N, Chlamydomonas cells divide once before ceasing further division. Previously, the sin-gle division after N-removal was arrested in tar1-1 in photomixotrophic conditions, but this phenotype was not observed in photoautotrophic conditions because of the particular characteristics of the parental C9-3. However, using C9-1 and CC-125 as parental strains, we showed that cell division after N-removal was impaired in new tar1 mutants in photoautotrophic conditions. Consistent with the view that the division under N-deficiency is neces-sary for gametic differentiation, new tar1 mutants showed lower mating efficiency than the parental strains. Taken together, TAR1 was suggested to pro-mote differentiation into gametes through the reg-ulation of cell division in response to N-deficiency.