Introduction

tobeeTom Beeckman leads the Root Development Group. The root system of Arabidopsis thaliana is an excellent model to study the relationship between cell cycle regulation and growth and development. Understanding this offers a great potential for altering root architecture and water uptake, allowing to design plants to survive under dryer conditions. The research of this group focuses on the formation of lateral root primordia in the pericycle to investigate how cell cycle regulation is involved in the initiation of new organs. Plant roots serve a multitude of functions. They anchor plants and supply them with water and nutrients and exchange various growth substances with the shoots. At the root-soil interface, numerous interactions between plants and their environment take place. The diversity of functions and broad range of interactions with the environment render the biology of roots complicated. During the last ten years, Arabidopsis thaliana has been proven to be an efficient model plant to study root development and time has come to extrapolate the obtained insights to crop species such as maize, a species that is currently also under investigation.
 
Besides the various advantages this plant offers in genetic and molecular studies, the Arabidopsis root boasts quite predictable ontogeny, very simple anatomy and a high level of transparency, making it very suitable for morphogenetic and cell biology studies. Understanding how root systems develop is crucial for maximizing crop production in a world in which the population is increasing and the amount of arable land is decreasing. Lateral root formation or root branching is one of the major regulators of root system architecture. During root branching, developmental and environmental controls of cell cycle regulation are crucial. Mitotic activity must be renewed in differentiated pericycle cells that have left the cell cycle. In Arabidopsis, the lateral roots are initiated by local activation of the pericycle cells at the xylem poles (Dhooge et al., 1999). Earlier, a model describing the cell cycle progression preceding the first formative divisions for lateral root initiation was proposed by members of our group (Beeckman et al., 2001). Unique cell cycle control, dissimilar from that in other pericycle cells, was shown to take place in the xylem pericycle. Good knowledge of this tissue-specific cell cycle regulation is therefore needed for a better understanding of lateral root initiation.
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To efficiently study the molecular and cytological events during the early stages of pericycle activation, a lateral root-inducible system was developed. Successive treatments with an auxin transport inhibitor and exogenous auxin were used to prevent the first formative divisions and to activate the whole pericycle, respectively. The morphological and molecular results show that, in this inducible system, pericycle activation corresponds to that of spontaneous lateral root initiation, but is enhanced and covers the whole length of the xylem pericycle. In this system, cell cycle progression during the early course of lateral root induction was monitored using histochemical and molecular techniques. The results demonstrated that CDK inhibitory proteins (KRPs) play a previously unknown role in specifically preventing lateral root initiation at the G1-to-S transition (Himanen et al., in preparation). In addition, the analysis of auxin distribution patterns in the system confirmed that auxin is important for the positioning and frequency of the first formative divisions for lateral root initiation (Casimiro et al., 2001).
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Publications:
  • Beeckman T., Burssens S. and Inzé D. (2001). The peri?cell?cycle in Arabidopsis.J. Exp. Bot. 52, 403?411. [free full text]
  • Casimiro I., Marchant A., Bhalerao R.P., Beeckman T., Dhooge S., Swarup R., Graham N., Inzé D., Sandberg G., Casero P.J. and Bennett M. (2001). Auxin transport promotes Arabidopsis lateral root initiation. Plant Cell 13, 843?852. [free full text]
  • Dhooge S., Beeckman T., Redant C., Viane R. and Inzé D. (1999). The initiation of lateral roots in Arabidopsis thaliana (L.) Heyhn. Biol. Jb. Dodonaea 66, 157?169.