Functional characterisation of HvCO1, the barley (Hordeum vulgare) flowering time ortholog of CONSTANS

Variation in photoperiod response is a major factor determining plant development and the agronomic performance of crops. The genetic control of photoperiodic flowering has been elucidated in the model plant Arabidopsis, and many of the identified genes are structurally conserved in the grasses. In this study, HvCO1, the closest barley ortholog of the key photoperiod response gene CONSTANS in Arabidopsis, was over-expressed in the spring barley Golden Promise. Over-expression of HvCO1 accelerated time to flowering in long- and short-day conditions and caused up-regulation of HvFT1 mRNA under long-day conditions. However, the transgenic plants retained a response to photoperiod, suggesting the presence of photoperiod response factors acting downstream of HvCO1 transcription. Analysis of a population segregating for HvCO1 over-expression and natural genetic variation at Ppd-H1 demonstrated that Ppd-H1 acts downstream of HvCO1 transcription on HvFT1 expression and flowering. Furthermore, variation at Ppd-H1 did not affect diurnal expression of HvCO1 or HvCO2. Over-expression of HvCO1 increased transcription of the spring allele of Vrn-H1 in long- and short-day conditions, while genetic variation at Ppd-H1 did not affect Vrn-H1 expression. Over-expression of HvCO1 and natural genetic variation at Ppd-H1 accelerated inflorescence development and stem elongation. Thus, HvCO1 probably induces flowering by activating HvFT1 whilst Ppd-H1 regulates HvFT1 independently of HvCO1 mRNA, and all three genes also appear to have a strong effect in promoting inflorescence development.