ABSTRACT
The genetic analysis of root formation in maize has long been neglected, although maize has been a favored plant object for genetic analysis (cf. Coe et al., 1988; Sheridan and Clark, 1988). The lack of genetic research of root structure and its formation in monocotyledonous plants was largely influenced by the difficulty of observing the complex underground root systems. Furthermore, the large environmental influence on root formation makes it difficult to identify mutants in large plant populations. Thus, only one maize root mutant had been characterized until the mid-1990s, rt1 (Jenkins, 1930), a mutant that shows a reduced lodging resistance caused by defects in shootborne root formation. The deficiency of available root mutants was recently overcome in the model plant Arabidopsis by applying new methods and concepts, and this led to the isolation of an impressive number of mutants allowing new insights into the general mechanisms of root formation and morphogenesis (cf. Scheres and Wolkenfelt, 1998). However, as the root system of maize is more complex (Feldman, 1994) than that of Arabidopsis, and because a detailed knowledge of root structure and formation of food plants like maize is still needed, a concerted effort was recently undertaken in search for root mutants in maize (Wen and Schnable, 1994; Hetz et al., 1996; Hochholdinger and Feix, 1998a). This effort has concentrated so far on the isolation and characterization of monogenic mutants with an influence on the formation and morphogenesis of various root types and root hairs. Identification and correct phenotypic assessment of such mutants were essential for detailed analysis of the structure and development of the root system (cf. Feldman, 1994; Kisselbach, 1999). As this aspect remains important for further mutant work, a short structural account of the root system of maize is given below.
