The root-hair pattern of Arabidopsis is determined through a regulatory circuit composed of transcription factor genes. The homeobox gene GLABRA2 (GL2) has been considered a key component, acting farthest downstream in this regulation. GL2 modified to include a transactivating function caused epidermal cells to develop ectopic root hairs or root hair-like structures. With this system, the phospholipase Dzeta1 gene (AtPLDzeta1) was identified as a direct target of GL2. Inducible expression of AtPLDzeta1 promoted ectopic root-hair initiation. We conclude that GL2 exerts its regulatory effect on root-hair development through modulation of phospholipid signaling.

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The temporal and spatlal expression of one member of the Arabidopsis 1-aminocyclopropane-1-carboxylate (ACC) synthase gene family (ACS7) was analyzed using a promoter-P-glucuronidase fusion. The expression of ACSl is under developmental control both in shoot and root. High expression was observed in young tissues and was switched off in mature tissues. ACS7 pmmoter activity was strongly correlated with lateral root formation. Dark-grown seedlings exhibited a different expression pattern from light-grown ones. The ACC content and the in vivo activity of ACC oxidase were determined. ACC content correlated with ACS7 gene activity. ACC oxidase activity was demonstrated in young Arabidopsis seedlings. Thus, the ACC formed can be converted into ethylene. In addition, ethylene production of immature leaves was fourfold higher compared to that of mature leaves. The possible involvement of ACSl in influencing plant growth and development is discussed. INTRODUCTIONEthylene is involved in several aspects of plant development from germination and seedling growth to flowering, fruit ripening, leaf abscission, and organ senescence. It also plays a key role in the response to environmental factors (Abeles, 1973; Yang and Hoffman, 1984; Moore, 1989; Van Der Straeten and Van Montagu, 1991).With the recent cloning of genes encoding the two most important enzymes in the biosynthesis of ethylene, l-aminocyclopropane-1-carboxylate (ACC) synthase (Nakajima et al., 1990; Van Der Straeten et al., 1990; Dong et al., 1991;Huang et al., 1991; Olson et al., 1991; and ACC oxidase (ethylene-forming enzyme or EFE) (Hamilton et al., 1990; Spanu et al., 1991;Wang and Woodson, 1991), more information about the molecular regulation of the synthesis of this hormone has been made available. The main focus has been on the regulation of ACC synthase gene expression because of its key regulatory role in the pathway. This implies that diverse inducers of ethylene production are also inducers of de novo synthesis of ACC synthase (Yang and Hoffmann, 1984; Van Der Straeten and Van Montagu, 1991).ACC synthases have been cloned from tomato (Van Der Straeten et al., 1990; Olson et al., 1991; To whom correspondence should be addressed.Yip et al., 1992), winter squash (Nakajima et al., 1990), zucchini (Huang et al., 1991), Arabidopsis (Liang et al., 1992; Van Der Straeten et al., 1992), mung bean (Botellaet al., 1992; Kim et al., 1992), carnation (Park et al., 1992), orchid (ONeill et al., 1993), tobacco (Bailey et al., 1992), and apple (Dong et ai., 1991; Kim et al., 1992). In several cases, it has been shown that ACC synthase genes belong to a multigene family and that they are differentially responsive to various ethyleneinducing factors and conditions such as wounding, fruit ripening, and auxins, although in certain cases some leve1 of coordination was demonstrated (Nakajima et al., 1990; Van Der Straeten et al., 1990; Dong et al., 1991; Olson et al., 1991; ; Yip et ai., 1992). These studies have provided information about general aspects of AC...

A genomic clone of one member of the Arabidopsis thaliana (L.) Heynh. 1-aminocyclopropane-1-carboxylate (ACC) synthase (S-adenosyl-L-methionine methylthioadenosine-lyase, EC 4.4.1.14) gene family (AT-ACC1) was isolated and sequenced. A region of homology was found in the 5'-untranslated region with the promoter of a zucchini and a tomato ACC synthase gene. Comparison of its primary structure with other ACC synthases revealed conservation of seven peptide regions as well as similarity with 11 amino acids of the catalytic site of aminotransferases. Genomic DNA gel blotting suggested the existence of an ACC synthase multigene family in Arabidopsis, possibly with three other members, none of which is very closely related to AT-ACC1. The existence of at least one other gene was confirmed by the isolation of a cDNA (AT-ACC2) from a flower-specific cDNA library. The AT-ACC1 gene was mapped on the Arabidopsis restriction fragment length polymorphism map and is located on the top of chromosome 1. This position does not correspond to any known mutation on the genetic map. Expression of the AT-ACC1 gene was studied by reverse transcription-PCR on total RNA. Messenger accumulation was strong in young leaves and flowers. The gene was not induced by wounding of young leaves or in seedlings in the presence of auxin. Ethylene exposure of mature plants led to an induction of AT-ACC1 gene expression. It is suggested that AT-ACC1 protein has a role in developmental control of ethylene synthesis.