Involvement of Homeobox Genes in Early Body Plan of Monocot

Momoyo Ito; Yutaka Sato; Makoto Matsuoka    BioScience Center, Nagoya University, Chikusa, Nagoya 464-8601, Japan

I Introduction

The homeobox, which is characterized by conserved DNA sequence of 180 bp, encodes a 60-amino acid protein motif known as the homeodomain (HD). This structure consists of a helix-turn-helix DNA-binding motif, and thus the homeobox genes are thought to function as transcription factors (Gehring et al., 1994a,b; Gehring, 1987; Qian et al., 1989). Homeobox genes were originally identified in Drosophila homeotic mutants, Antennapedia and bithorax, as the genes that control patterning in Drosophila development (McGinnis et al., 1984; Scott and Weiner, 1984). Since then, homeobox genes have been identified in many evolutionarily distant organisms, including animals, plants, and fungi. In higher plants, many homeobox genes have been found to play important roles in various developmental events, as is the case in animals (Chan et al., 1998).

Angiosperms are subdivided into two classes: dicotyledonous (dicot) and monocotyledonous (monocot) plants. Arabidopsis is commonly used as a model plant of dicots, and maize and rice are used as model plants of monocots. Many molecular and genetic studies using these model plants have revealed the existence of mutually orthologous genes in the dicot and monocot genomes, and they are thought to share essentially common mechanisms for each phenomenon, including various developmental events. On the other hand, there are also many morphological differences between monocots and dicots, and these should be reflected in some differences in the developmental mechanisms. In this review, we first describe the general characteristics of plant homeobox genes and the involvement of homeobox (mainly knox) genes in early development of monocots. Particular attention is then given to the morphological differences in embryogenesis between monocots and dicots.

II Plant Homeobox Genes

A Characteristics of the Plant Homeobox Gene Family

The first homeobox gene to be identified in plants was KNOTTED1 (KN1) from the maize Knotted1 (Kn1) mutant (Vollbrecht et al., 1991). Leaf blades of the Kn1 mutant exhibit abnormal arrangements of the lateral veins, sporadic outgrowths called knots, and ligule displacements. Kn1 is a dominant mutant, caused by ectopic expression of KN1 in leaves, that results in the disorganization of the developmental program of leaf blades (Table I) (Smith and Hake, 1994). Subsequent to the cloning of the KN1 gene from maize, many plant homeobox genes have been isolated from various plant species using library screening with previously identified gene or degenerate oligonucleotides deduced from HDs as probes (Ruberti et al., 1991; Mattsson et al., 1992; Schena and Davis, 1992, 1994; Carabelli et al., 1993; Gonzalez and Chan, 1993; Matsuoka et al., 1993; Boivin et al., 1994; Chan and Gonzalez, 1994; Feng and Kung, 1994; Kerstetter et al., 1994; Lincoln et al., 1994; Ma et al., 1994; Soderman et al., 1994; Baima et al., 1995; Dockx et al., 1995; Kawahara et al., 1995; Meissner and Theres, 1995; Tamaoki et al., 1995, 1997; Di Cristina et al., 1996; Granger et al., 1996; Hareven et al., 1996; Lu et al., 1996; Serikawa et al., 1996; Gonzalez et al., 1997; Meijer et al., 1997; Valle et al., 1997; Watillon et al., 1997; Janssen et al., 1998; Sato et al., 1998; Sentoku et al., 1998, 1999; Nishimura et al., 1999; Ingram et al., 2000), differential screening (Nadeau et al., 1996; Tornero et al., 1996; Ingram et al., 1999; Dong et al., 2000), mutant based cloning (Table I) (Rerie et al., 1994; Müller et al., 1995; Reiser et al., 1995; Schneeberger et al., 1995; Long et al., 1996; Chen et al., 1997; Muehlbauer et al., 1997;Parnis et al., 1997; Mayer et al., 1998; Kubo et al., 1999), and other methods (Bellmann and Werr, 1992; Schindler et al., 1993; Korfhage et al., 1994; Klinge et al., 1996). On the basis of sequence similarities in their HDs and the presence of additional distinctive domains outside of the HD, plant homeobox genes are subdivided into several families: knox, HD-ZIP, glabra2, PHD-finger, BELL1, and WUSCHEL-type (Chan et al., 1998). The characteristic structure and functions of each of these plant homeobox gene families are described below (Fig. 1).