Systemic analysis of CO gene family has not been reported in such an important species. Although COL proteins are known to have important roles in various aspects of plant growth and development, their evolutionary history, as well as functions, remain poorly characterized in maize (Bendix et al., 2013 Miller et al., 2008). Maize is not only an important cereal crop but also a model plant for extensive research in plant science (Zhao et al., 2011 Li, 2017). The expression of AtCOL4 was strongly induced by abscisic acid (ABA), salt and osmotic stress (Ji-Hee et al., 2015). For example, AtCOL4 is an important regulator for plant tolerance to abiotic stress. Additionally, some COL genes have been characterized to be stress responsive (Holefors et al., 2009). Remarkably, COL genes also have been found to be involved in the photoperiodically regulated developmental processes (Datta et al., 2006), and playing a key role in the photoperiodic response to flowering (Holefors et al., 2009). And these COL genes were characterized to participate in diverse molecular and genetic processes, including light signal transduction (Zobell et al., 2005), regulation of circadian rhythms (Salomé et al., 2006), and control of the photoperiod response and flowering time (Zhang et al., 2011). The last group incorporates COL9 to COL15 with one CO-like B-box, one more diverged zinc finger domain and three introns (Griffiths et al., 2003). The second group includes COL6 to COL8 and COL16 with one B-box domain and one intron. The first group contains CO and COL1 to COL5 with two B-box domains and one intron. Furthermore, the outcome of analysis on CO-like genes in Arabidopsis showed that the family could be divided into three broad groupings, mainly due to the differences of B-box domain (Robson et al., 2001). In Arabidopsis, 16 remaining CO-like genes were identified, all of which encoded proteins harboring the two conserved domain (CCT, B-box). In the past decades, CO gene family has been extensively studied. Further analysis of seven classical co mutant alleles demonstrated that the mutations all occurred on either the zinc finger region or the CCT domain, implying the functionally importance of these two conserved domains for CO genes (Robson et al., 2001). This CCT domain is similar to the DNA-binding region of HAP2, possessing the function of nuclear localization (Wenkel et al., 2006). The other one is CCT domain (CO, CO-like, TOC1), consisted of 43 amino acids near the carboxy terminus (Robson et al., 2001 Strayer et al., 2000). This conserved domain has been reported to be involved in the regulation of protein-protein interactions in several animal TFs and formed a special structure comprised by four cysteine (C-X2-C-X16-C-X2-C) (Borden, 1998 Wenkel et al., 2006). The first one is a zinc finger region near the amino terminus that was named as B-box. ![]() It belongs to a protein family in which all members harbor two conserved domain (Putterill et al., 1995 Robson et al., 2001). ![]() The CONSTANS (CO) protein identified in Arabidopsis, was characterized as an important transcription factor (TF), which could regulate flowering via photoperiod (Gong et al., 2004 Mouradov et al., 2002 Putterill et al., 1995). ![]() Our results laid the foundation for functional characterization of Zm COL proteins and uncovering the biological roles of COL genes in response to stress in future. Therefore, qRT-PCR was performed to explore the expression patterns of ZmCOL genes in group A under light/dark conditions and ABA treatment. Additionally, several light-responsive and one ABA-responsive cis-elements could be detected for ZmCOL genes in group A. Comparing interspecies gene collinearity between three major grasses species, extensive microsynteny was detected among maize, sorghum and rice COL-containing segments. Calculation of Ka/Ks ratio demonstrated that most duplicated ZmCOLs experienced purifying selection, which suggested limited functional divergence after duplication events. Analysis of compound phylogenetic tree (maize, sorghum, rice and Arabidopsis) showed high bootstrap, as well as conserved domain and semblable gene structures among members within the same clade, revealing that COL genes in same clade were from the common ancestral and prior to the divergence of monocots and dicots lineages. In the present study, we performed a genome-wide analysis of maize COL gene family and identified 19 non-redundant ZmCOL genes, which were unequally distributed on ten maize chromosomes. The CONSTANS-LIKE ( COL) gene has an important role both in regulation flowering through photoperiodic control and response to abiotic stress.
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