Transcriptional activator that functions with GLK2 to promote chloroplast development. Acts as an activator of nuclear photosynthetic genes involved in chlorophyll biosynthesis, light harvesting, and electron transport. Acts in a cell-autonomous manner to coordinate and maintain the photosynthetic apparatus within individual cells. May function in photosynthetic capacity optimization by integrating responses to variable environmental and endogenous cues (PubMed:11828027, PubMed:12220263, PubMed:17533111, PubMed:18643989, PubMed:19376934, PubMed:19383092, PubMed:19726569). Prevents premature senescence (PubMed:23459204). Interacts with NAC92. A number of isoforms are produced. According to EST sequences. Expressed in rosette and cauline leaves. Expressed at low levels in cotyledons and shoots. By light. Repressed by BZR2. No visible phenotype (PubMed:12220263). Pale-green seedlings in double mutants glk1/glk2 (PubMed:23459204). Plants overexpressing GLK1 have a delay in flowering under long days, show high constitutive expression of genes encoding disease defense related proteins and are resistant to the pathogen F.graminearum.