Description:
<jats:title>Summary</jats:title><jats:p>The red and far‐red light‐absorbing phytochromes interact with the circadian clock, a central oscillator that sustains a 24‐h period, to measure accurately seasonal changes in day‐length and regulate the expression of several key flowering genes. The interactions and subsequent signalling steps upstream of the flowering genes such as <jats:italic>CONSTANS</jats:italic> (<jats:italic>CO</jats:italic>) and <jats:italic>FLOWERING LOCUS T</jats:italic> (<jats:italic>FT</jats:italic>) remain largely unknown. We report here that a photomorphogenic mutant, <jats:italic>red and far‐red insensitive 2‐1</jats:italic> ( <jats:italic>rfi2‐1</jats:italic>), flowered early particularly under long days. The <jats:italic>rfi2‐1</jats:italic> mutation also enhanced the expression of <jats:italic>CO</jats:italic> and <jats:italic>FT</jats:italic> under day/night cycles or constant light. Both <jats:italic>co‐2</jats:italic> and <jats:italic>gigantea‐2</jats:italic> (<jats:italic>gi‐2</jats:italic>) were epistatic to <jats:italic>rfi2‐1</jats:italic> in their flowering responses. The <jats:italic>gi‐2</jats:italic> mutation was also epistatic to the <jats:italic>rfi2‐1</jats:italic> mutation in the expression of <jats:italic>CO</jats:italic> and hypocotyl elongation. However, the <jats:italic>rfi2‐1</jats:italic> mutation did not affect the expression of <jats:italic>GI</jats:italic>, a gene that mediates between the circadian clock and the expression of <jats:italic>CO</jats:italic>. Like many other flowering genes, the expression of <jats:italic>RFI2</jats:italic> oscillated under day/night cycles and was rhythmic under constant light. The amplitude of the rhythmic expression of <jats:italic>RFI2</jats:italic> was significantly reduced in <jats:italic>phyB‐9</jats:italic> or <jats:italic>lhy‐20</jats:italic> plants, and was also affected by the <jats:italic>gi‐2</jats:italic> mutation. As previously reported, the <jats:italic>gi‐2</jats:italic> mutation affects the period length and amplitude of <jats:italic>CCA1</jats:italic> and <jats:italic>LHY</jats:italic> expression, and GI may act through a feedback loop to maintain a proper circadian function. We propose a regulatory step in which RFI2 represses the expression of <jats:italic>CO</jats:italic>, whereas GI may maintain the proper expression of <jats:italic>RFI2</jats:italic> through its positive action on the circadian clock. The regulatory step serves to tune the circadian outputs that control the expression of <jats:italic>CO</jats:italic> and photoperiodic flowering.</jats:p>