2101 Introduction In higher plants, the shoot apical meristem (SAM) is the source of all aerial organs (Brand et al., 2001; Clark, 2001), whereas the root apical meristem (RAM) produces the below-ground structures (Dolan et al., 1993; Scheres et al., 1994). Both meristems are formed during embryogenesis, and start to produce cells and organs after germination (Jürgens, 1995; Scheres et al., 1994). In these post-embryonic meristems, cell division and cell differentiation are highly regulated, and the cellular organization is maintained in the meristem as it grows. The Arabidopsis RAM has a typical cell arrangement that contains four central cells, known as the quiescent center (QC), surrounded by initial cells (Dolan et al., 1993). The QC is mitotically inactive, whereas the initial cells have stem cell-like activity, with regularly repeated cell divisions. During post- embryonic development, each initial cell divides in a plane parallel to that in which it touches the QC (Dolan et al., 1993). The daughter cell that is adjacent to the QC is kept as the initial, and the other daughter differentiates into a specific cell type according to its position (van den Berg et al., 1995). When the QC is laser ablated, the adjacent initial cells lose their stem cell-like activity and differentiate in the same way as their daughter cells (van den Berg et al., 1997). These observations revealed that the QC plays essential roles in the maintenance of the post-embryonic RAM. Several genes involved in the specification of QC identity have been isolated by analyses with Arabidopsis mutants. For example, in the mutant of a putative transcription factor SCARECROW, several QC- specific markers lost their expression, and the root growth was ceased prematurely (Di Laurenzio et al., 1996; Sabatini et al., 2003). In addition, the mutation of a putative auxin efflux carrier PIN4 disrupted both the expression pattern of QC- specific markers and the cellular organization of the RAM (Friml et al., 2002). Similar to the role of QC in the RAM, the organizing center (OC) in the SAM is involved in the maintenance of meristematic activity (Laux et al., 1996; Mayer et al., 1998). The Arabidopsis SAM consists of a dome of cells, which is organized into a central zone (CZ) that harbors the OC and stem cells, and a peripheral zone wherein organ primordium are developed. The OC and the stem cells are located in the lower and the upper region of the CZ, respectively, and the OC is known to confer the stem cell state on its upper cells, which in turn restrict the size of the OC (Mayer et al., 1998; Laux, In higher plants, post-embryonic development is dependent on the activity of the root and shoot apical meristem (RAM and SAM). The quiescent center (QC) in the RAM and the organizing center (OC) in the SAM are known to be essential for the maintenance of meristematic activity. To understand the mechanism that maintains post-embryonic meristems, we isolated an Arabidopsis mutant, halted root (hlr). In this mutant, the cellular organization was disrupted in post-embryonic meristems both in the root and in the shoot, and their meristematic activity was reduced or became abnormal. We showed that the mutant RAM lost its QC identity after germination, which was specified during embryogenesis, whereas the identity of differentiated tissues was maintained. In the post- embryonic SAM, the expression pattern of a typical OC marker gene, WUSCHEL, was disturbed in the mutant. These observations indicate that the HLR gene is essential to maintain the cellular organization and normal nature of the RAM and SAM. The HLR gene encodes RPT2a, which is a subunit of the 26S proteasome that degrades key proteins in diverse cellular processes. We showed that the HLR gene was expressed both in the RAM and in the SAM, including in the QC and the OC, respectively, and that the activity of proteasomes were reduced in the mutant. We propose that proteasome-dependent programmed proteolysis is required to maintain the meristem integrity both in the shoot and in the root. Key words: HALTED ROOT (HLR), Arabidopsis, Proteasome, Quiescent center, Organizing center, Meristem Summary The HALTED ROOT gene encoding the 26S proteasome subunit RPT2a is essential for the maintenance of Arabidopsis meristems Minako Ueda 1 , Keisuke Matsui 1, *, Sumie Ishiguro 1,† , Ryosuke Sano 2 , Takuji Wada 2 , Ivan Paponov 3 , Klaus Palme 3 and Kiyotaka Okada 1,2,4,‡ 1 Department of Botany, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan 2 Plant Science Center, RIKEN, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan 3 Institut für Biologie II, Zellbiologie, Universität Freiburg, Schänzlestrasse 1, D-79104 Freiburg, Germany 4 Core Research of Science and Technology (CREST) Research Project *Present address: Institute for Advanced Technology, Suntory Limited, 1-1-1 Wakayamadai, Shimamoto, Mishima, Osaka 618-8503, Japan † Present address: Graduate School of Bioagricultural Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan ‡ Author for correspondence (e-mail: kiyo@ok-lab.bot.kyoto-u.ac.jp) Accepted 26 January 2004 Development 131, 2101-2111 Published by The Company of Biologists 2004 doi:10.1242/dev.01096 Research article