Two Arabidopsis cyclin A3s possess G1 cyclin-like features.

A-type cyclins (CYCAs) are a type of mitotic cyclin and are closely related to cyclin B. Plant CYCAs are classified into three subtypes (CYCA1-CYCA3), among which CYCA3 has been suggested to show a biased expression during the G1-to-S phase. We characterised Arabidopsis CYCA3s (CYCA3;1-CYCA3;4) in terms of expression pattern and protein function. CYCA3;1 and CYCA3;2 transcripts were highly accumulated at the G1/S phase, whereas CYCA3;4 was constantly expressed during the cell cycle. Expressions of CYCA3;1 and CYCA3;2 were observed in actively dividing tissues, such as root and shoot apical meristems and lateral root primordia. Overexpression of CYCA3;1 or CYCA3;2 distorted apical dominance in Arabidopsis, indicating that they have critical functions in shoot meristems. In insect cells, CYCA3;1 formed an active kinase complex with CDKA;1, an orthologue of the yeast Cdc2/Cdc28p, and phosphorylated retinoblastoma-related protein, a key regulator in the transition from the G1 to the S phase. Our results suggest that Arabidopsis CYCA3;1 and CYCA3;2 are distinct members of the G1 cyclin family that play an important role in meristematic tissues.