Functional importance of conserved domains in the flowering‐time gene CONSTANS demonstrated by analysis of mutant alleles and transgenic plants

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<jats:title>Summary</jats:title><jats:p> <jats:italic>CONSTANS</jats:italic> promotes flowering of <jats:italic>Arabidopsis</jats:italic> in response to long‐day conditions. We show that <jats:italic>CONSTANS</jats:italic> is a member of an <jats:italic>Arabidopsis</jats:italic> gene family that comprises 16 other members. The CO‐Like proteins encoded by these genes contain two segments of homology: a zinc finger containing region near their amino terminus and a CCT (CO, CO‐Like, TOC1) domain near their carboxy terminus. Analysis of seven classical <jats:italic>co</jats:italic> mutant alleles demonstrated that the mutations all occur within either the zinc finger region or the CCT domain, confirming that the two regions of homology are important for CO function. The zinc fingers are most similar to those of B‐boxes, which act as protein–protein interaction domains in several transcription factors described in animals. Segments of CO protein containing the CCT domain localize GFP to the nucleus, but one mutation that affects the CCT domain delays flowering without affecting the nuclear localization function, suggesting that this domain has additional functions. All eight <jats:italic>co</jats:italic> alleles, including one recovered by pollen irradiation in which DNA encoding both B‐boxes is deleted, are shown to be semidominant. This dominance appears to be largely due to a reduction in <jats:italic>CO</jats:italic> dosage in the heterozygous plants. However, some alleles may also actively delay flowering, because overexpression from the CaMV 35S promoter of the <jats:italic>co‐3</jats:italic> allele, that has a mutation in the second B‐box, delayed flowering of wild‐type plants. The significance of these observations for the role of <jats:italic>CO</jats:italic> in the control of flowering time is discussed.</jats:p>

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