Cell | MYB-Related Transcription Factors Regulate Plant Chloroplast Development

Photosynthesis is the basis of life, and this process occurs in chloroplasts. Chloroplasts first originated from endosymbiosis among plants and have been around for more than a billion years [1]. Most of the genes that regulate or contribute to chloroplast biogenesis in plants are encoded in the nucleus and subsequently translated and transported into chloroplasts. Chloroplast biogenesis is dependent on the GLK (GOLDEN2-LIKE) family of transcription factors, yet glk mutants still contain residual chlorophyll, suggesting that other proteins are involved in chloroplast biogenesis [2,3].

Recently, Julian M. Hibberd's group at the University of Cambridge, UK, and Eftychios Frangedakis (first author) published a paper in Cell titled MYB-related transcription factors control chloroplast biogenesis , and found that MYB related transcription factors in Marchantia polymorpha and Arabidopsis thaliana, and unravelled the mechanism by which MYB-related transcription factors coordinate with GLK transcription factors in plant chloroplast development.

To search for factors controlling chloroplast biogenesis, the authors first analysed published gene expression data during the transition from non-photosynthetic to photosynthetic growth in Groundnut and Arabidopsis thaliana [4,5], and identified 108 as well as 144 factors whose expression was up-regulated after exposure to light, respectively (Figure 1). By overlapping the two databases, the authors identified 14 candidate genes in ground money, two of which were MpGLK as well as MpGATA4, which are homologues in Arabidopsis thaliana and are known regulators of photobiosynthesis. The authors performed CRISPR-Cas9-mediated gene editing of these candidate genes using ground money and found that the Mp5g11830 gene annotated as MpRR-MYB5, when mutated, causes the ground money to turn pale and lose chloroplast biosynthesis. However, this phenomenon does not occur in mutants of the MpRR-MYB2 gene.

Further, by mutating both MpRR-MYB5 as well as MpRR-MYB2, the authors found that it leads to a 95% reduction in chloroplast content compared to the control, which is realised as an extremely pale plant. Since the Mprr-myb5,2 double mutant exhibited residual chloroplast development, the authors suggested that limited photoautotrophic growth might be associated with the activity of GLK, a major regulator of chloroplast biosynthesis. To this end, the authors attempted to construct double mutants of Mpglk with either Mprr-myb5 or Mprr-myb2. The authors found that double mutants of Mpglk with the Mpmyb gene would be paler than single mutants (Figure 2). Thus, a simultaneous lack of MpRR-MYB5 as well as MpGLK causes the assembly of the photosynthetic synthesis apparatus to almost no longer occur.