Effects of co-expressing the plant CDK inhibitor ICK1 and D-type cyclin genes on plant growth, cell size and ploidy in Arabidopsis thaliana.

The cyclin-dependent kinase (CDK) plays a crucial role in regulating the cell cycle of eukaryotic organisms including plants. From previous studies, it is known that ICK1, the first plant CDK inhibitor identified in Arabidopsis plants, interacts with Arath;CycD3;1 (CycD3) and Arath;CDKA;1 (Cdc2a). Overexpression of ICK1 has major effects on cell division, plant growth, and morphology. In this study, approaches were taken to determine the effects on transgenic 35S::ICK1 Arabidopsis plants of introducing another gene that could potentially modulate the activity of ICK1. F1 plants were obtained by crossing 35S::ICK1 plants with wild type (Wt) and transgenic plants expressing 35::GUS, 35S::CycD3, 35S::CycD2, or 35S::antiICK1 ( antiICK1 refers to antisense- ICK1). The major effects on plant growth and morphology observed in the 35S::ICK1 plants were partially reversed in the F1 plants from the crosses [35S::ICK1 x 35S::CycD2] and [35S::ICK1 x 35S::CycD3], and completely restored in the F1 plants from the cross [35S::ICK1 x 35S::antiICK1]. This observation was further supported by the results of ploidy analysis and structural characterization. Overexpression of CycD2 and CycD3 had the opposite effect on leaf cell size to the overexpression of ICK1. In addition, in ICK1-overexpressing plants, the CycD2 and CycD3 transcript levels increased, indicating a possible feedback regulation. The present results demonstrate that the interactions between ICK1 and D-type cyclins previously observed by the yeast two-hybrid and in vitro techniques are biologically relevant. These results illustrate the possibility of modifying plant growth and architecture dynamically by adjusting the levels of positive and negative cell-cycle regulators.