The golden2 mutation in maize was first described in 1927 (Henkins, M.T. 1927). The responsible Golden 2 gene was cloned and characterized in 1998 and found to encode a novel type of transcriptional activator (Hall et al., 1998, PMID: 9634581).  Related G2-like (Glk) genes were subsequently discovered in maize and rice and permitted the definition of a novel class of transcriptional regulators in plants (Rossini et al., 2001, PMID: 11340194).  The Golden2 (G2) gene regulates plastid biogenesis in all photosynthetic cells during the C3 stages of development. However, G2 function is specifically committed to the differentiation of bundle sheath cell chloroplasts in C4 leaf blades. G2 and ZmGLK1 both can transactivate reporter gene transcription and dimerize in yeast, which supports the idea that these proteins act as transcriptional regulators of cell-type differentiation processes (Rossini et al., 2001, PMID: 11340194).

GLK TFs belong to the GARP transcription factor (TF) superfamily which is commonly found in plants. The GARP superfamily is named for the Golden 2 (G2) protein in maize, the type B authentic response regulator (ARR-B) protein in Arabidopsis thaliana, and the phosphate starvation response 1 (PSR1) protein in Chlamydomonas.  The ARR-B TFs harbor a multi-functional domain B motif, which is related to nuclear localization and DNA binding. G2-like TFs have an Myb-DNA binding domain (Myb-DBD), which is highly similar to the B motif (Chen et al 2016., PMID: 27757121, Li et al., 2021, PMID: 34445391).

The NIGT1/HRS1/HHO transcription factor (TF) family has been described as a subfamily of the G2-like TF family in the GARP superfamily and contains two conserved domains: a Myb-DNA binding domain and a hydrophobic and globular domain. Some studies showed that NIGT1/HRS1/HHO TFs are involved in coordinating the absorption and utilization of nitrogen and phosphorus. NIGT1/HRS1/HHO TFs also play an important role in plant growth and development and in the responses to abiotic stresses (Li et al., 2021, PMID: 34445391).

Recently it has been demonstrated in Arabidopsis that Salicylic acid (SA)-induced SIGMA FACTOR-BINDING PROTEIN 1 (SIB1), a transcription coregulator and positive regulator of cell death, interacts with GLK1 and GLK2 to reinforce the expression of photosynthesis-associated nuclear genes (PhANGs) during photosynthesis biogenesis.  It was also found that LESION-SIMULATING DISEASE 1 (LSD1), a transcription coregulator and negative regulator of SA- primed cell death, interacts with GLK1/2 to repress their activities (Li et al 2022., PMID: 34951648).  It was found that overexpression of maize GOLDEN2 gene in rice and maize calli improved regeneration efficiency likely by activating chloroplast development (Luo et al 2023., PMID: 35982378.

There are 59 GLK genes in the B73 maize genome and the vast majority of maize GLK gene duplications resulted from whole genome duplication events rather than tandem duplications. Most maize GLK genes are expressed in green tissues may relate to maize photosynthesis. qRT-PCR showed that the expression of these genes was sensitive to low temperature and drought (Liu et al., 2016, PMID: 27560803). There are approximately 66 putative GLK genes in Tomato (Lycopersicon esculentum)which appear to expressed predominantly under stress and hormone induced signals (Wang et al., 2022. PMID: 35196981). 146 GhGLK genes in cotton (Gossypium hirsutum) of which nine key GLKs appear to be involved in salt, cold, and drought stress (Zhao et al., 2021, PMID: 34820202).

Last updated June 2023 by Ankita Abnave

References:

Jenkins, M.T. (1927). A second gene producing golden plant color in maize. Am. Nat. 60, 484–488.

Hall LN, Rossini L, Cribb L, Langdale JA. GOLDEN 2: a novel transcriptional regulator of cellular differentiation in the maize leaf. Plant Cell. 1998 Jun;10(6):925-36. doi: 10.1105/tpc.10.6.925. PMID: 9634581; PMCID: PMC144032.

Rossini L, Cribb L, Martin DJ, Langdale JA. The maize golden2 gene defines a novel class of transcriptional regulators in plants. Plant Cell. 2001 May;13(5):1231-44. doi: 10.1105/tpc.13.5.1231. PMID: 11340194; PMCID: PMC135554.

Chen M, Ji M, Wen B, Liu L, Li S, Chen X, Gao D, Li L. GOLDEN 2-LIKE Transcription Factors of Plants. Front Plant Sci. 2016 Oct 4;7:1509. doi: 10.3389/fpls.2016.01509. PMID: 27757121; PMCID: PMC5048441.

Li M, Lee KP, Liu T, Dogra V, Duan J, Li M, Xing W, Kim C. Antagonistic modules regulate photosynthesis-associated nuclear genes via GOLDEN2-LIKE transcription factors. Plant Physiol. 2022 Mar 28;188(4):2308-2324. doi: 10.1093/plphys/kiab600. PMID: 34951648; PMCID: PMC8968271.

Li Q, Zhou L, Li Y, Zhang D, Gao Y. Plant NIGT1/HRS1/HHO Transcription Factors: Key Regulators with Multiple Roles in Plant Growth, Development, and Stress Responses. Int J Mol Sci. 2021 Aug 12;22(16):8685. doi: 10.3390/ijms22168685. PMID: 34445391;

Wang ZY, Zhao S, Liu JF, Zhao HY, Sun XY, Wu TR, Pei T, Wang Y, Liu QF, Yang HH, Zhang H, Jiang JB, Li JF, Zhao TT, Xu XY. Genome-wide identification of Tomato Golden 2-Like transcription factors and abiotic stress related members screening. BMC Plant Biol. 2022 Feb 23;22(1):82. doi: 10.1186/s12870-022-03460-9. PMID: 35196981; PMCID: PMC8864820.

Zhao Z, Shuang J, Li Z, Xiao H, Liu Y, Wang T, Wei Y, Hu S, Wan S, Peng R. Identification of the Golden-2-like transcription factors gene family in Gossypium hirsutum. PeerJ. 2021 Nov 16;9:e12484. doi: 10.7717/peerj.12484. PMID: 34820202; PMCID: PMC8603818.

Liu F, Xu Y, Han G, Zhou L, Ali A, Zhu S, Li X. Molecular Evolution and Genetic Variation of G2-Like Transcription Factor Genes in Maize. PLoS One. 2016 Aug 25;11(8):e0161763. doi: 10.1371/journal.pone.0161763. PMID: 27560803; PMCID: PMC4999087.

Luo W, Tan J, Li T, Feng Z, Ding Z, Xie X, Chen Y, Chen L, Liu YG, Zhu Q, Guo J. Overexpression of maize GOLDEN2 in rice and maize calli improves regeneration by activating chloroplast development. Sci China Life Sci. 2023 Feb;66(2):340-349. doi: 10.1007/s11427-022-2149-2. Epub 2022 Aug 12. PMID: 35982378.