Expression of maize D-type cyclins: Comparison, regulation by phytohormones during seed germination and description of a new D cyclin

While cloning maize D-type cyclins previously reported in databases (described as of D1, D2 and D4 types), a fourth D cyclin was cloned that showed high homology (75%) with the D1 cyclin. Because this D1 cyclin has been recently described as a D5-type cyclin (D5;1), the new cyclin was named D5;2. All maize cyclins have been compared among themselves and among D cyclins from other plant species. All maize D cyclins possess the retinoblastoma protein-binding motif and cyclin boxes but no PEST sequences or destruction box sequences are required for protein degradation. D5 and D2 cyclins also have canonical cyclin-dependent kinase (Cdk)-phosphorylation sites. Every cyclin showed a different expression pattern during seed germination, standing out cyclin D5;2, which seems to be expressed only during the early stages (equivalent to postmitotic interphase), and cyclin D4;1, which progressively accumulates from an almost undetectable level in dry seed embryo axes. Phytohormones like cytokinins and auxins, which accelerate the germination process, change the expression pattern of all cyclins, with cytokinins promoting an increase in expression during the early hours of germination (by 6 h), whereas auxins promote a constant increase in the levels of three out of the four D cyclins (except D5;1). Cyclin D5;1 is the least expressed of all cyclins in all tissues measured (embryo axes, seedlings and plantlets), and all cyclins are expressed in both meristematic and non-meristematic tissues. We discuss their relevance for the germination process and plantlet establishment.