Promoter region characterization of ZmPhyB2 associated with the photoperiod-dependent floral transition in maize (Zea mays L.)

Flowering is one of the most important steps in the life cycle of a plant. Plants utilize light as a source of information to determine the timing of flowering. Recent molecular genetics studies have implicated phytochrome B (PhyB)-mediated responses to light in the control of floral transition. However, the molecular mechanism by which ZmPhyB2 mediates flowering time remains uncertain, although it is known to control the photoperiod-dependent floral transition in maize. In this study, an "rpt_family" (MITE) sequence was detected in the ZmPhyB2 promoter region of early flowering maize. The presence or absence of the MITE was associated with phenotypic variation of the flowering time following association analysis. ZmPhyB2 transcript levels were expressed rhythmically under short-day (SD) conditions, with two relatively sharp peaks. The mRNA expression of ZmPHYB2 was higher in CML288 (without MITE) than in B73 (with MITE). The two peaks appeared similar to 3 h earlier in ZmPhyB2 expression from CML288 and slightly ahead of time from B73, and ZmPhyB2 expression increased dramatically in CML288 and increased slightly in B73 following 30-min night break (NB) treatments under SD conditions. The results showed that the ZmPhyB2 allele without MITE insertion was associated with dramatically enhanced gene expression following an NB treatment under SD conditions, resulting in a delayed flowering time, which suggested that the MITE insertion functioned as a repressor of gene expression. In addition, our findings showed that ZmPhyB2 peaked earlier after the NB treatment, with an increase in the accumulation of ZmCCA1 mRNA, giving rise to a longer circadian clock period. The longer circadian clock period produced two peak expression patterns of Zmconz1, which further inhibited ZmZCN8 expression, which may result in flowering delay.