WEG1, which encodes a cell wall hydroxyproline-rich glycoprotein, is essential for parental root elongation controlling lateral root formation in rice Nonawin Lucob-Agustin a , Tsubasa Kawai a , Misuzu Takahashi-Nosaka b , Mana Kano-Nakata c , Cornelius M. Wainaina d , Tomomi Hasegawa a , Mayuko Inari-Ikeda c , Moeko Sato e , Hiroyuki Tsuji e , Akira Yamauchi a and Yoshiaki Inukai c,f, * a Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi, 464-8601, Japan b Department of Genomics and Evolutionary Biology, National Institute of Genetics, Mishima, Shizuoka, 411-8540, Japan c International Center for Research and Education in Agriculture, Nagoya University, Nagoya, Aichi, 464-8601, Japan d Department of Horticulture, Jomo Kenyatta University of Agriculture and Technology, Nairobi, Kenya e Kihara Institute for Biological Research, Yokohama City University, Yokohama, Kanagawa, 244-0813, Japan f PREST, JST, Kawaguchi, Saitama, 332-0012, Japan Correspondence *Corresponding author, e-mail: inukaiy@agr.nagoya-u.ac.jp Received 18 September 2019; revised 18 December 2019 doi:10.1111/ppl.13063 Lateral roots (LRs) determine the overall root system architecture, thus enabling plants to efficiently explore their underground environment for water and nutrients. However, the mechanisms regulating LR development are poorly understood in monocotyledonous plants. We characterized a rice mutant, wavy root elongation growth 1 (weg1), that produced higher number of long and thick LRs (L-type LRs) formed from the curvatures of its wavy paren- tal roots caused by asymmetric cell growth in the elongation zone. Consistent with this phenotype, was the expression of the WEG1 gene, which encodes a putative member of the hydroxyproline-rich glycoprotein family that regu- lates cell wall extensibility, in the root elongation zone. The asymmetric elon- gation growth in roots is well known to be regulated by auxin, but we found that the distribution of auxin at the apical region of the mutant and the wild- type roots was symmetric suggesting that the wavy root phenotype in rice is independent of auxin. However, the accumulation of auxin at the convex side of the curvatures, the site of L-type LR formation, suggested that auxin likely induced the formation of L-type LRs. This was supported by the need of a high amount of exogenous auxin to induce the formation of L-type LRs. These results suggest that the MNU-induced weg1 mutated gene regulates the auxin-independent parental root elongation that controls the number of likely auxin-induced L-type LRs, thus reflecting its importance in improving rice root architecture. Introduction The fibrous root system of rice, similar to other cereal crops, is comprised of a seminal root and numerous crown roots and their lateral roots (LRs) whose overall growth and angle determine the root system architecture (Rebouillat et al. 2009, Uga et al. 2015). Furthermore, LRs have been categorized into distinct types substantially varying in length, diameter and histological structures (Kono et al. 1972). The S-type LRs are short with no branches and have a thin diameter and simple vasculature, but are normally Abbreviations – FAA, formaldehyde acetic acid; GUS, β-glucuronidase; HRGP, hydroxyproline-rich glycoprotein; IAA, indole-3 acetic acid; LRs, lateral roots; weg1, wavy root elongation growth 1. Physiol. Plant. 169, 2020 214 Physiologia Plantarum 169: 214–227. 2020 © 2020 Scandinavian Plant Physiology Society, ISSN 0031-9317