Analysis of the Spatial Expression Pattern of Seven Kip Related Proteins (KRPs) in the Shoot Apex of Arabidopsis thaliana

Cycling prohibited or cycling permitted?

The cell division cycle is controlled at several levels. Within cycling cells there is a complex network of regulatory mechanisms that link the cell cycle phases together. At higher levels there are mechanisms that determine the location and timing of cell division in plant development, for example in the generation of new organs through the activity of meristems. At both levels there are positive and negative regulators, sometimes acting in direct opposition to each other. To add to this complexity there are situations where multiple proteins appear to have the same function. Examples of these are the �Kip-related proteins� (KRPs) studied in Arabidopsis thaliana by Ormenese et al. (Li�ge and Gent, pp. 575-580). These are generally regarded as negative regulators of the cell cycle because they inhibit the cyclin-dependent kinases that act as positive regulators. The recent discovery of these proteins in plants is not surprising, but what is surprising is the number of KRPs. To obtain information on the possible roles of these KRPs, the authors carried out an exquisitely detailed in situ hybridization analysis in the shoot apex, showing clearly that the seven KRPs have different expression patterns. KRPs 1 and 2 are expressed in cells undergoing DNA endoreduplication, KRPs 4 and 5 are expressed in mitotic cells and KRPs 3, 6 and 7 are expressed in a subset of mitotic cells and in cells undergoing endoreduplication. These data clearly suggest that the different KRPs have different roles, but those roles are not entirely clear. Although expression in cells undergoing endoreduplication is consistent with an inhibition of the mitotic cyclin-dependent kinases, it is harder to discern a possible role for those KRPs expressed in cells active in mitotic division. However, it is likely that the possession of several similar proteins with different expression patterns is related to the spatial regulation of cell division in post-embryonic development.