Over-expression of the photoperiod response regulator ZmCCT10 modifies plant architecture, flowering time and inflorescence morphology in maize

Maize originated as a tropical plant that required short days to transition from vegetative to reproductive development. ZmCCT10 [CO, CONSTANS, CO-LIKE and TIMING OF CAB1 (CCT) transcription factor family] is a photoperiod regulator and was identified as a major QTL controlling photoperiod sensitivity in maize. We modulated expression of ZmCCT10 in transgenic maize using two constitutive promoters which cause differing expression levels. Transgenic plants over expressing ZmCCT10 with either promoter was delayed in their transition from vegetative to reproductive development but were not affected in their change from juvenile-to-adult vegetative growth. Strikingly, transgenic plants containing the stronger expressing construct had a very prolonged period of vegetative growth accompanied with dramatic modifications to plant architecture that impacted both vegetative and reproductive traits. These plants did not produced ears, but tassels were heavily branched, and more than half of the transgenic plants showed conversion of shoot apices into \"bushy tops\", which were composed of vegetative reversion plantlets. Analysis of expression modules controlling the floral transition and meristem identity linked these networks to photoperiod dependent regulation, whereas phase change modules appeared to be photoperiod independent. Results from this study clarified the influence of the photoperiod pathway on vegetative and reproductive development and allowed to fine-tune the flowering time model for maize.