Root Caps and Rhizosphere Martha C. Hawes, 1, * Glyn Bengough, 2 Gladys Cassab, 3 and Georgina Ponce 3 1 Department of Plant Pathology, University of Arizona, Tucson, Arizona 85721, USA; 2 Scottish Crop Research Institute, Dundee, Scotland DD2 5DA; 3 UNAM/iBT, Cuernavaca, Mexico ABSTRACT In this paper we discuss recent work on the physi- ological, molecular, and mechanical mechanisms that underlie the capacity of root caps to modulate the properties of the rhizosphere and thereby foster plant growth and development. The root cap ini- tially defines the rhizosphere by its direction of growth, which in turn occurs in response to gradi- ents in soil conditions and gravity. The ability of the root cap to modulate its environment is largely a result of the release of exudates and border cells, and so provides a potential method to engineer the rhizosphere. Factors affecting the release of border cells from the outer surface of the root cap, and function of these cells and their exudates in the rhizosphere, are considered in detail. Release of border cells into the rhizosphere depends on soil matric potential and mechanical impedance, in ad- dition to a host of other environmental conditions. There is good evidence of unidentified feedback signals between border cells and the root cap meristem, and some potential mechanisms are dis- cussed. Root border cells play a significant me- chanical role in decreasing frictional resistance to root penetration, and a conceptual model for this function is discussed. Root and border cell exudates influence specific interactions between plant hosts and soil organisms, including pathogenic fungi. The area of exudates and border cell function in soil is an exciting and developing one that awaits the production of appropriate mutant and transgenic lines for further study in the soil environment. Key words: Root caps; Rhizosphere; Plant growth INTRODUCTION Haberlandt (1914) defined the cap as a mucilage- covered, bullet-shaped barrier functioning primarily to protect the meristem physically and to lubricate its passage through the soil. This much-cited clas- sical description of the cap as a slimy battering ram may have led inadvertently to an impression of this organ as an important yet relatively passive part of the root system, like cuticle or bark. Recent studies outlined below suggest a more complex, dynamic, and specialized system that does provide mechanical protection of the root, but also serves to ÔengineerÕ the properties of the invaded environment (re- viewed in Hawes and others 1998; Hawes 2000). Thus, the root cap responds to a symphony of sig- nals from the soil environment to (1) control di- rection of movement; (2) facilitate penetration into soil; and (3) define microbial ecology by the regu- lated delivery of biologically active root exudates into the rhizosphere. An often-cited distinction between plants and animals is that of mobility: whereas animals can propel themselves away from danger and toward nutrients and safety, plants are less obviously mo- bile. Survival of plants nevertheless depends on the Received: 3 October 2002; accepted: 10 December 2002; Online publica- tion: 28 March 2003 *Corresponding author; e-mail: mhawes@u.arizona.edu J Plant Growth Regul (2003) 21:352–367 DOI: 10.1007/s00344-002-0035-y 352