Root caps (RC) are the terminal tissue of roots of most plants (Barlow 2003). The root cap is divided between the apical part (Columella) and the lateral part (lateral root cap) this tissues originate from different intials. The lateral root cap is tought to be involved in the control of the root meristem size (Werner et al 2003). The columella contains statocytes (i.e. gravity perceiving cells). Some basic functions relating to root biology, such as lubrication of root growth and gravitropism, were ascribed to RCs by Haberlandt (1914, reviewed in Barlow 1975). Within the past 25 years, however, the functions of the RC have been shown to be considerably more diverse and to include regulation of many aspects of root development (Scheres et al . 1996; Tsugeki & Federoff 1999). The RC perceives and processes many environmental stimuli, and mediates the direction of root growth accordingly. Gravity (gravitropism), light (phototropism), obstacles (thigmotropism), gradients of temperature (thermotropism), humidity (hydrotropism), ions and other chemicals (chemotropism) are all examples of environmental stimuli that are perceived and processed by the cap (Hasenstein & Evans 1988; Ishikawa & Evans 1990; Okada & Shimura 1990; Fortin & Poff 1991; Takahashi 1997; Eapen et al . 2003).
In roots with a ‘closed’ type of construction, such as maize (Fig. 1a) and Arabidopsis , a distinct cap meristem with a set of RC initials (RCI) is present. The initial layer consists of the most rapidly dividing and least differentiated cells in the RC. As new RC cells are produced by the RCI, these derivatives are displaced through the RC until they are finally released into the soil as border cells (Hawes & Lin 1990; Hawes et al . 2003). During their passage to the outside of the cap, cells change from statocytes (i.e. gravity perceiving cells) into secretory cells which produce mucilage, and then finally differentiate into border cells which detach from the cap (Barlow 1975; Hawes & Lin 1990). If these cells are allowed to accumulate, and are prevented from being sloughed off (e.g. by suspending the root in air), the RC meristem ceases to produce new cells, suggesting communication between border cells and the RCI (Hawes & Lin 1990; Brighman et al . 1998). There is also evidence that the RCI communicate with adjacent cells located basally in the root proper. If the cap is excised, the adjacent root tissues alter their development and regenerate a new cap (Barlow 1974; Feldman 1976). The new cap regenerates from a population of previously mitotically inactive root cells located at the tip of the root proper and designated as a quiescent centre (QC). In experiments in which both the RC and QC are together excised a new RC reforms, but not until after a new QC re-develops from an even more proximal portion of the root, suggesting that the QC functions as an architectural template (Barlow 1976), because it mainly retards cell differentiation, and not cell division (Feldman 1976, 1998). Recent work using laser ablation to destroy one or more QC cells in Arabidopsis roots has extended this view (van den Berg et al . 1997).
Genetic ablation of RC cells in Arabidopsis has revealed that the roots of transgenic plants (carrying a RC-specific promoter that directs the expression of a diphteria toxin Achain gene [DT-A tsM ]) have more lateral roots, and these, in turn, are more highly branched than those of wild-type plants (Tsugeki & Federoff 1999). This may indicate that the RC is a complex and dynamic tissue essential for normal root development. Tsugeki & Fedoroff (1999) propose that there is an auxin sink in the RC or that access to the sink is through the RC auxin transport system. Recently, AtPIN4 , a novel member of the PIN family of putative auxin efflux carriers, has been linked to the establishment of an auxin sink in the RCI that is essential for auxin distribution and patterning (Friml et al . 2002).