ARR-B 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 Arabidopsis 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, Liu et al., 2016., PMID: 27560803 ).  While the A-type ARR proteins likely function via protein-protein interaction, B-type ARRs constitute a subfamily of the GARP-domain transcription factor family. The B-motif (GARP-like motif) comprises about 60-80 amino acids and has been shown to be responsible for binding to its DNA-binding consensus GAT (Hosoda et al., 2002).

ARR-B TFs are the second component of two-component regulatory systems that were originally discovered and functionally characterized in prokaryotes. They have emerged as important sensing/response mechanisms in higher plants. Generally, hybrid histidine kinases function as receptors for the incoming signals and the so-called 'response regulators' transmit this information via canonical His-to-Asp phosphorelay. Histidine containing phosphotransfer domain proteins exist in plants as a kind of shuttle mechanism, which can receive the phosphate residue from the upstream histidine kinases and transfer it to the response regulators. So far, there are 10 A-type ARRs (Arabidopsis Response Regulators) and 11 B-type ARR proteins known from Arabidopsis thaliana. Approximately 8 ARR-B members are found in maize. Besides these classical ARRs the APRR (Arabidopsis Pseudo response Regulator) family consists of several members of pseudo response regulators, where the phospho-accepting Asp (D) residue has been replaced by a Glu (E) in the receiver domain of these proteins (and might not necessarily be involved in a canonical His-to-Asp phosphorelay dependent pathway.

Functionally the two component systems in plants play a major role in cytokinin perception and signaling and contribute to ethylene signal transduction and osmosensing. Furthermore, developmental processes like megagametogenesis in Arabidopsis thaliana and flowering promotion in rice (Oryza sativa) involve elements of two-component systems. Two-component-like elements also function as components of the Arabidopsis circadian clock. Because of the molecular mode of signaling, plant two-component systems also appear to serve as intensive cross talk and signal integration machinery.

Last updated June 2023 by John Gray

References:

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; PMCID: PMC8395448.

Hosoda K, Imamura A, Katoh E, Hatta T, Tachiki M, Yamada H, Mizuno T, Yamazaki T. Molecular structure of the GARP family of plant Myb-related DNA binding motifs of the Arabidopsis response regulators. Plant Cell. 2002 Sep;14(9):2015-29. doi: 10.1105/tpc.002733. PMID: 12215502; PMCID: PMC150752.

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.