An ARF2-GRF5 module, operating early and upstream of GLK factors, reveals genetic hierarchy during chloroplast biogenesis

During photosynthetic development, GOLDEN2-LIKE (GLK) factors drive the expression of photosynthesis-associated nuclear (PhAN) genes. Simultaneous loss of GLK1 and GLK2 in Arabidopsis leads to pale-green plants with small, incompletely developed chloroplasts, revealing involvement of additional components. Following mutagenesis of a glk1-1 knock-down, glk2-1 knock-out (KO) mutant we identified suppressor of glks (sgl) 2, sgl2. The sgl2 glk1 glk2 triple mutant showed increases in chlorophyll, PhAN gene transcripts and thylakoid stacking in mesophyll cell chloroplasts, and enhanced cellular content of chloroplasts in mesophyll and bundle-sheath cells. The suppressor mutation delayed flowering but greening phenotypes preceded flowering. sgl2 was caused by loss of function of AUXIN RESPONSE FACTOR2, ARF2. arf2sgl2 boosted chloroplast content. It dramatically suppressed the developmental and chloroplast delay of the phytochrome-deficient long hypocotyl 1 (hy1) mutant. We observed in glk1-1 glk2-1 low but detectable expression of GLK1, which was elevated in arf2sgl2 glk1-1 glk2-1, and saw reduced suppression by arf2sgl2of a glk1-2 glk2-2 mutant carrying KO alleles, revealing partial epistasis. Overexpression of Growth Regulating Factor 5 (GRF5), upregulated in arf2sgl2, rescued chlorophyll levels in glk1-1 glk2-1, consistently with ARF2 and GRF5 as a transcription module. In developing cereal leaves presumed orthologs of ARF2 and GRF5 are expressed at early stages of plastid proliferation and assembly, preceding GLK expression during greening stage. ARF2 is therefore a negative switch modulating GRF5 early in chloroplast development in photosynthetic cells, upstream of the GLKs. Our results reveal a hierarchy of transcription factor action and feed-forward regulation in chloroplast biogenesis during leaf development.