PGA37 Overexpression Promotes Chloroplast Development in Arabidopsis Roots Through Direct Transcriptional Activation of GLK2, ARR13, and ARR21

Chloroplast biogenesis and development are essential processes in plants, profoundly influencing their growth, survival, and productivity. However, the transcription factors controlling chloroplast development, especially in roots, are poorly characterized. Here, we demonstrate that the ectopic expression of the seed-specific transcription factor Plant Growth Regulator 37 (PGA37) promotes chloroplast development in roots, causing root-greening. Using a steroid-inducible gene expression system and RNA-Seq, we identified 97 potential PGA37 target genes. Notably, PGA37 directly activates the transcription factor GOLDEN2-LIKE (GLK2), which governs chloroplast biogenesis. An overexpression of <i>GLK2</i> replicated the root-greening phenotype observed in PGA37-overexpressing plants, while GLK2 mutation significantly reduced chlorophyll content and suppressed root-greening in PGA37-overexpressing seedlings. Furthermore, PGA37 directly binds to the promoters of type-B response regulators ARR13 and ARR21, thereby activating the cytokinin signaling pathway. Mutations in these regulators partially diminished chlorophyll accumulation in PGA37-overexpressing seedlings, suggesting that PGA37-regulated chloroplast development is partially mediated by the cytokinin signaling through ARR13 and ARR21. Taken together, we propose that PGA37 orchestrates chloroplast development by coordinately regulating transcription factors from various families, including GLK2, ARR13, and ARR21, positioning it as a key regulator of chloroplast development.