Abstract
Maize (Zea mays) is a major cereal crop that originated at low latitudes, and thus photoperiod sensitivity is an important barrier to the use of tropical/subtropical germplasm in temperate regions. However, studies of the mechanisms underlying circadian regulation in maize are at an early stage. In this study we cloned ZmCCA1a on chromosome 10 of maize by map-based cloning. The gene is homologous to the Myb transcription factor genes AtCCA1/AtLHY in Arabidopsis thaliana; the deduced Myb domain of ZmCCA1a showed high similarity with that of AtCCA1/AtLHY and ZmCCA1b. Transiently or constitutively expressed ZmCCA1a-YFPs were localized to nuclei of Arabidopsis mesophyll protoplasts, agroinfiltrated tobacco leaves, and leaf and root cells of transgenic seedlings of Arabidopsis thaliana. Unlike AtCCA1/AtLHY, ZmCCA1a did not form homodimers nor interact with ZmCCA1b. Transcripts of ZmCCA1a showed circadian rhythm with peak expression around sunrise in maize inbred lines CML288 (photoperiod sensitive) and Huangzao 4 (HZ4; photoperiod insensitive). Under short days, transcription of ZmCCA1a in CML288 and HZ4 was repressed compared with that under long days, whereas the effect of photoperiod on ZmCCA1a expression was moderate in HZ4. In ZmCCA1a-overexpressing A. thaliana (ZmCCA1a-ox) lines, the circadian rhythm was disrupted under constant light and flowering was delayed under long days, but the hypocotyl length was not affected. In addition, expression of endogenous AtCCA1/AtLHY and the downstream genes AtGI, AtCO, and AtFt was repressed in ZmCCA1a-ox seedlings. The present results suggest that the function of ZmCCA1a is similar, at least in part, to that of AtCCA1/AtLHY and ZmCCA1b, implying that ZmCCA1a is likely to be an important component of the circadian clock pathway in maize.
Highlights
The photoperiod and its annual changes with seasons provide exogenous signals that organisms use to predict upcoming environmental changes
Major QTLs that affect flowering time have been mapped to bin10.04 on Chr10 in maize (Ducrocq et al, 2009; Wang et al, 2010)
As core components of negative feedback loops of the oscillator, the genes regulate the expression of numerous downstream genes and participate in maintenance of the circadian clock rhythm (Schaffer et al, 1998; Green and Tobin, 1999; Alabadıet al., 2002; Esther et al, 2009; Lu et al, 2009; Seo et al, 2012; Nagel et al, 2015)
Summary
The photoperiod and its annual changes with seasons provide exogenous signals that organisms use to predict upcoming environmental changes. The circadian clock regulatory system comprises an input pathway, an oscillator, and an output pathway (Esther et al, 2009; Wang et al, 2011). Among these elements, the oscillator, which functions through multiple negative feedback loops, is the core component that maintains the circadian clock period of approximately 24 h. The rhythmicity and period of the oscillator can be entrained and reset by environmental signals received and transmitted by input pathway components. The oscillator transmits these messages to downstream components of the output pathway
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