The formation of chloroplasts is regulated by light and by endogenous inputs such as hormones. Most genes required for chloroplast biogenesis are encoded in the nucleus, and their encoded proteins are imported to the chloroplast. The GOLDEN2-LIKE (GLK) family of transcription factors are known master regulators of chloroplast biogenesis. Frangedakis et al. now report that MYB-related transcription factors function in chloroplast biogenesis alongside GLK in the distantly related species Marchantia polymorpha and Arabidopsis thaliana.

Taking advantage of the compact M. polymorpha genome, in which many transcription factors are represented by only one or a few copies, and of the liverwort being haploid during the dominant phase of its life cycle, the authors performed CRISPR–Cas9-mediated editing of all candidate genes and screened for pale mutants. In addition to the expected glk mutants, mutant alleles of RR-MYB5, encoding a MYB-related transcription factor, produced pale plants. RR-MYB5 has a paralogue, RR-MYB2, that did not produce a pale phenotype when mutated, but simultaneously mutating RR-MYB5 and RR-MYB2 produced very pale plants with a 95% reduction in chlorophyll content. The double mutants had poorly developed chloroplasts but had residual photosynthetic function, which indiates that although assembly of the photosynthetic apparatus was limited, it was still operational. The authors propose that M. polymorpha RR-MYB5 and RR-MYB2 are necessary for chloroplast biogenesis and act redundantly, but unlike GLK they are not sufficient to initiate the process. Mutating all three genes appeared to be lethal.