The CO (CONSTANS) gene of Arabidopsis has an important role in the regulation of flowering by photoperiod. CO is part of a gene family with 17 members that are subdivided into three classes termed Group I to III here. All members of the family have a CCT (CO CO-like TOC1) domain near the carboxy terminus (PFAM06203).  This short motif is found in a number of plant proteins. It is rich in basic amino acids and has been called a CCT motif after Co, Col and Toc1. The CCT motif is about 45 amino acids long and contains a putative nuclear localisation signal within the second half of the CCT motif. Group I genes which include CO have two zinc finger B-boxes (PFAM00643) near the amino terminus. Group II genes have one B-box and Group III genes have one B-box and a second diverged zinc finger.  In maize at least 19 members have been identified (Song et al., 2018).  

Of these ZmCOL1 (CONZ1), is a gene with extensive sequence homology to photoperiod genes CONSTANS (CO) in Arabidopsis and Heading date1 (Hd1) in rice (Oryza sativa L.) (Miller et al., 2008). Sequence homology and comparative mapping showed that HvCO1 (from barely) was the counterpart of OsA (Hd1) a major determinant of photoperiod sensitivity in rice. Major genes determining photoperiod response have been mapped in barley and wheat (Triticum aestivum) but none corresponded to CO-like genes. Thus selection for variation in photoperiod response has affected different genes in rice and temperate cereals. The peptides of HvCO1 HvCO2 (barley) and Hd1 (rice) show significant structural differences from CO particularly amino acid changes that are predicted to abolish B-box2 function suggesting an evolutionary trend toward a one-B-box structure in the most CO-like cereal genes.  

Flowering in maize (Zea mays) is also influenced by photoperiod. The CO, CO-like/COL and TOC1 (CCT) domain protein-encoding genes in maize, ZmCCTs, are particularly important for photoperiod sensitivity. The numbers of ZmCCTs identified were 58 in B73, 59 in W22, 48 in Mo17, and 57 in Huangzao4 for temperate maize inbred lines, and 68 in tropical maize inbred line SK. Some ZmCCTs underwent duplications and presented chromosome collinearity. Expression of 37 ZmCCTs in embryonic leaves during seed germination of maize under DD and DL cycles could be roughly divided into five patterns indicating some of them have a potential to perceive dark and/or dark-light transition. Thirty-three ZmCCTs were co-expressed with 218 other maize genes; and 24 ZmCCTs were associated with known QTLs. The data presented in this study will help inform further functions of ZmCCTs (Dong et al., 2021).

Last updated June 2023 by John Gray

References:

Song N, Xu Z, Wang J, Qin Q, Jiang H, Si W, Li X. Genome-wide analysis of maize CONSTANS-LIKE gene family and expression profiling under light/dark and abscisic acid treatment. Gene. 2018 Oct 5;673:1-11. doi: 10.1016/j.gene.2018.06.032. Epub 2018 Jun 13. PMID: 29908279.

Miller TA, Muslin EH, Dorweiler JE. A maize CONSTANS-like gene, conz1, exhibits distinct diurnal expression patterns in varied photoperiods. Planta. 2008 May;227(6):1377-88. doi: 10.1007/s00425-008-0709-1. Epub 2008 Feb 27. PMID: 18301915.

Dong MY, Lei L, Fan XW, Li YZ. Analyses of open-access multi-omics data sets reveal genetic and expression characteristics of maize ZmCCT family genes. AoB Plants. 2021 Aug 16;13(5):plab048. doi: 10.1093/aobpla/plab048. PMID: 34567492; PMCID: PMC8459886.