The plant hormones auxin and ethylene have been shown to play important roles during root hair development. However, cross talk between auxin and ethylene makes it difficult to understand the independent role of either hormone. To dissect their respective roles, we examined the effects of two compounds, chromosaponin I (CSI) and 1-naphthoxyacetic acid (1-NOA), on the root hair developmental process in wild-type Arabidopsis, ethylene-insensitive mutant ein2-1, and auxin influx mutants aux1-7, aux1-22, and double mutant aux1-7 ein2. ␤-Glucuronidase (GUS) expression analysis in the BA-GUS transgenic line, consisting of auxin-responsive domains of PS-IAA4/5 promoter and GUS reporter, revealed that 1-NOA and CSI act as auxin uptake inhibitors in Arabidopsis roots. The frequency of root hairs in ein2-1 roots was greatly reduced in the presence of CSI or 1-NOA, suggesting that endogenous auxin plays a critical role for the root hair initiation in the absence of an ethylene response. All of these mutants showed a reduction in root hair length, however, the root hair length could be restored with a variable concentration of 1-naphthaleneacetic acid (NAA). NAA (10 nm) restored the root hair length of aux1 mutants to wild-type level, whereas 100 nm NAA was needed for ein2-1 and aux1-7 ein2 mutants. Our results suggest that insensitivity in ethylene response affects the auxin-driven root hair elongation. CSI exhibited a similar effect to 1-NOA, reducing root hair growth and the number of root hair-bearing cells in wild-type and ein2-1 roots, while stimulating these traits in aux1-7and aux1-7ein2 roots, confirming that CSI is a unique modulator of AUX1.Root hairs are tip-growing, tubular-shaped outgrowths that help to anchor roots, interact with soil microorganisms, and assist in the uptake of water and nutrients (Cutter, 1978). The relatively simple and invariant cellular organization of the primary roots of Arabidopsis and the ease of isolation and characterization of mutants make it a very attractive material for studying the root hair developmental process. The first committed step for root hair development is epidermal cell specification. In many species, including Arabidopsis, the root epidermis consists of two epidermal cell types, root hair-forming trichoblast cells and hairless atrichoblast cells (Cormack, 1947(Cormack, , 1949Bunning, 1951;Cutter, 1978). Within the Arabidopsis root epidermis, cells adopt distinct fates in a position-dependent manner. Epidermal cells that overlay the junction between two cortical cell files adopt a root hair cell fate, whereas the epidermal cells that contact only one cortical cell file become hairless cells (Dolan et al., 1994;Galway et al., 1994; Berger et al., 1998).Once the immature epidermal cell adopts a root hair cell fate, it goes through characteristic changes in its shape and size (Schiefelbein, 2000). Genetic analysis revealed that the root hair initiation mutations axr2 ), axr3 (Leyser et al., 1996, and ctr1 (Kieber et al., 1993) exhibit changes in their response to two i...

p-Chlorophenoxyisobutyric acid (PCIB) is known as a putative antiauxin and is widely used to inhibit auxin action, although the mechanism of PCIB-mediated inhibition of auxin action is not characterized very well at the molecular level. In the present work, we showed that PCIB inhibited BA::␤-glucuronidase (GUS) expression induced by indole-3-acetic acid (IAA), 2,4-dichlorophenoxyacetic acid, and 1-naphthaleneacetic acid. PCIB also inhibited auxin-dependent DR5::GUS expression. RNA hybridization and quantitative reverse transcriptase-polymerase chain reaction analyses suggested that PCIB reduced auxin-induced accumulation of transcripts of Aux/IAA genes. In addition, PCIB relieved the reduction of GUS activity in HS::AXR3NT-GUS transgenic line in which auxin inhibits GUS activity by promoting degradation of the AXR3NT-GUS fusion protein. Physiological analysis revealed that PCIB inhibited lateral root production, gravitropic response of roots, and growth of primary roots. These results suggest that PCIB impairs auxin-signaling pathway by regulating Aux/IAA protein stability and thereby affects the auxin-regulated Arabidopsis root physiology.The plant hormone auxin (indole-3-acetic acid [IAA]) plays an important role in every aspect of plant growth and development (Thimann, 1977; Davies, 1995). Despite its physiological significance, the molecular mechanism of auxin action is not fully understood yet. One of the earliest events in the auxin action involves the changing of expression pattern of some specific genes with a lag period of 5 to 30 min (Abel and Theologis, 1996). Auxin-dependent degradation of Aux/IAA proteins is a key event for early auxin-dependent gene induction (Leyser, 2002). Aux/IAA proteins interact with auxin response factors (ARFs) that bind to auxin-responsive elements (AuxREs) in the auxin-responsive promoters (Guilfoyle, 1998). Aux/IAA proteins are short-lived proteins, and their stability is regulated by auxin through the ubiquitin-mediated protein degradation pathway (Leyser, 2002). In the absence of auxin, large amounts of Aux/IAA proteins are present, bind to ARFs, and prevent ARFs to activate transcription of auxin-induced genes from AuxREs. In the presence of auxin, degradation of Aux/IAA proteins is promoted, and then ARF proteins are released from Aux/IAA proteins and activate transcription of auxin-responsive genes (Tiwari et al., 2003). Although detailed molecular mechanism of the role of ARF and Aux/IAA proteins and their regulation has been revealed in these recent years, the mechanisms of the very early step of auxin signaling, i.e. how auxin is recognized by plant cell and how ubiquitin proteolysis system is activated, still remains unknown (Benfey, 2002).p-Chlorophenoxyisobutyric acid (PCIB), also called ␣-(4-chlorophenoxy) isobutyric acid, 2-(p-chlorophenoxy)-2-methylpropionic acid, or clofibric acid, has been most widely used to inhibit auxin action (e.g. Kim et al., 2000;Xie et al., 2000). Because of the structural similarity of PCIB with a synthetic auxin 4-chlorophenoxyac...

Gamma rays are the most frequently used ionizing radiation in plant mutagenesis; however, few studies are available on the characteristics of mutations at a genome-wide level. Here, we quantitatively and qualitatively characterized the mutations induced by acute/chronic gamma ray irradiation in Arabidopsis. The data were then compared with those previously obtained for carbon ion irradiation. In the acute irradiation of dry seeds at the same effective survival dose, gamma rays and carbon ions differed substantially, with the former inducing a significantly greater number of total mutation events, while the number of gene-affecting mutation events did not differ between the treatments. This may result from the gamma rays predominantly inducing single-base substitutions, while carbon ions frequently induced deletions ≥2 bp. Mutation accumulation lines prepared by chronic gamma irradiation with 100-500 mGy/h in five successive generations showed higher mutation frequencies per dose compared with acute irradiation of dry seeds. Chronic gamma ray irradiation may induce larger genetic changes compared with acute gamma ray irradiation. In addition, the transition/transversion ratio decreased as the dose rate increased, suggesting that plants responded to very low dose rates of gamma rays (∼1 mGy/h), even though the overall mutation frequency did not increase. These data will aid our understanding of the effects of radiation types and be useful in selecting suitable radiation treatments for mutagenesis.