Isolation of a novel lateral-rootless mutant in rice (Oryza sativa L.) with reduced sensitivity to auxin Hua Wang a , Shin Taketa a, * , Akio Miyao b , Hirohiko Hirochika b , Masahiko Ichii a a Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795, Japan b Molecular Genetics Department, National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8602, Japan Received 31 March 2005; received in revised form 28 July 2005; accepted 1 August 2005 Available online 6 September 2005 Abstract Lateral roots play an important role in the acquisition of nutrients and anchorage of the whole plant. To better understand the mechanisms underlying lateral root development, we isolated a new lateral-rootless mutant lrt2 in screening for 2,4-dichlorophenoxyacetic acid (2,4-D) resistance in M 2 lines of rice (Oryza sativa L. cv. Nipponbare) generated by tissue culture. lrt2 failed to form lateral roots and exhibited altered root response to gravity. Analysis for auxin resistance showed that lrt2 was less sensitive to various auxins including 2,4-D, indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) and 1-naphthaleneacetic acid (NAA) compared with wild type, but was similarly sensitive to auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). This suggests that the reduced sensitivity to auxin in lrt2 might be caused by a disruption in auxin response rather than in auxin transport. Genetic analysis indicated that the lateral-rootless phenotype of lrt2 is due to a recessive mutation. To map the lrt2 gene, we tested molecular markers by bulk segregant analysis. The lrt2 gene was localized to a 10.8 cM interval on the short arm of chromosome 2, flanked by two sequence-tagged site (STS) markers Lrt2P1 and Lrt2P2. # 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Auxin resistance; Gravitropism; Lateral root formation; lrt2; Rice; STS markers 1. Introduction Lateral root formation is the primary way that plants increase their root mass to absorb water and nutrients as well as anchor the whole plant. By analyzing two rice mutants RM109 and RH2, which lack lateral roots and root hairs respectively, Ma et al. [1] demonstrated that silicon is actively taken up through the lateral roots but not through the root hairs. Likewise, Bailey et al. [2] concluded that lateral roots rather than root hairs play a pivotal role in the anchorage of Arabidopsis plants. Thus, investigating the factors that determine lateral root development is of agronomic importance. To date, however, the precise mechanisms involved in the lateral root formation in crops have not been elucidated. Auxins are known to regulate diverse developmental processes including stem elongation, apical dominance, gravitropism and lateral root formation [3]. Roots are a particularly attractive system to study auxin action because of their morphological simplicity and well-characterized auxin responses [4]. In Arabidopsis,a number of auxin-resistant mutants have been isolated, such as aux1 [5], axr1 [6], axr2 [7], axr3 [8], axr4 [4], axr5 [9], axr6 [10], slr-1 [11], tir1 and tir3 [12,13]. Molecular evidence revealed that these genes are classified into auxin transporter-related proteins, Aux/IAA family, ARF family or ubiquitin-related proteins. Furthermore, almost all of these mutants are defective in lateral root formation and root gravitropism, confirming the importance of auxin in these processes [4]. Previously, Hao and Ichii [14] isolated a dominant lateral- rootless mutant Lrt1 (lateral-rootless) of rice (Oryza sativa L. ssp. japonica cv. Oochikara) in screening for 2,4-dichlorophenox- yacetic acid (2,4-D) resistance. Lrt1 also exhibited resistance to other synthetic and natural auxins including 1-naphthaleneacetic acid (NAA), indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) [15]. In addition to auxin resistance, Lrt1 is characterized by its defects in root gravitropic response and reduced root hairs, suggesting that auxin is required for normal root growth. This paper describes the isolation and characterization of a new recessive mutant defective in lateral root formation in rice, which we have designated lrt2 (lateral-rootless 2). The lrt2 mutant also shows high resistance to auxins and altered root www.elsevier.com/locate/plantsci Plant Science 170 (2006) 70–77 * Corresponding author. Tel.: +81 87 891 3136; fax: +81 87 891 3136. E-mail address: staketa@ag.kagawa-u.ac.jp (S. Taketa). 0168-9452/$ – see front matter # 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2005.08.005