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Mutants of a new gene, TRANSPARENT TESTA GLABRA2 (TTG2), show disruptions to trichome development and to tannin and mucilage production in the seed coat. The gene was tagged by the endogenous transposon Tag1 and shown to encode a WRKY transcription factor. It is the first member of this large, plant-specific family known to control morphogenesis. The functions of all other WRKY genes revealed to date involve responses to pathogen attack, mechanical stress, and senescence. TTG2 is strongly expressed in trichomes throughout their development, in the endothelium of developing seeds (in which tannin is later generated) and subsequently in other layers of the seed coat, and in the atrichoblasts of developing roots. TTG2 acts downstream of the trichome initiation genes TTG1 and GLABROUS1, although trichome expression of TTG2 continues to occur if they are inactivated. Later, TTG2 shares functions with GLABRA2 in controlling trichome outgrowth. In the seed coat, TTG2 expression requires TTG1 function in the production of tannin. Finally, TTG2 also may be involved in specifying atrichoblasts in roots redundantly with other genes) but independently of TTG1 and GLABRA2.
INTRODUCTION
In many species, plant hairs (trichomes) are believed to provide physical protection from attack by predators. In Arabidopsis, trichomes occur as large single cells that develop on the upper surface of rosette leaves, the lower surface of cauline leaves, the edges of leaves, stems, and sometimes sepals (Hulskamp et al., 1994). Trichome development on the leaf surface is first seen as the enlargement of a single epidermal cell. Such cells are spaced relatively evenly, approximately three or four cells apart at this stage. As the trichome increases in size and grows outward from the leaf surface, its nucleus endoreduplicates several times, and three branches arise in a stereotypic manner. The surface of the trichome is glassy until it reaches the final stages of differentiation, when it develops surface papillae. At maturity, the trichome is surrounded by a ring of supporting epidermal cells.
Genes involved in controlling the initiation and early stages of morphogenesis of trichomes are now being discovered in Arabidopsis through molecular genetic studies (Hulskamp et al., 1999; Szymanski et al., 2000) (Table 1). The first to be studied in detail was GLABROUS I (GL1), mutants of which lack hairs on the surface...