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A Rice Homeobox Gene, OSHJ, Is Expressed Before Organ Proc. Natl. Acad. Sci. USA Vol. 93, pp. 8117-8122, July 1996 Plant Biology A rice homeobox gene, OSHJ, is expressed before organ differentiation in a specific region during early embryogenesis (embryo/in situ hybridization/organless mutant) YUTAKA SATO*, SOON-KWAN HONGt, AKEMI TAGIRIt, HIDEMI KITANO§, NAOKI YAMAMOTOt, YASUO NAGATOt, AND MAKOTO MATSUOKA*¶ *Nagoya University, BioScience Center, Chikusa, Nagoya 464-01, Japan; tFaculty of Agriculture, University of Tokyo, Tokyo 113, Japan; tNational Institute of Agrobiological Resources, Tsukuba, Ibaraki 305, Japan; and §Department of Biology, Aichi University of Education, Kariya 448, Japan Communicated by Takayoshi Higuchi, Nihon University, Tokyo, Japan, March 25, 1996 (received for review October 20, 1995) ABSTRACT Homeobox genes encode a large family of One of the powerful approaches for understanding the homeodomain proteins that play a key role in the pattern molecular mechanisms involved in plant embryogenesis is to formation of animal embryos. By analogy, homeobox genes in identify molecular rnarkers that can be used both to monitor plants are thought to mediate important processes in their cell specification events during early embryogenesis and to embryogenesis, but there is very little evidence to support this gain an entry into regulatory networks that are activated in notion. Here we described the temporal and spatial expression different embryonic regions after fertilization (10). With this patterns of a rice homeobox gene, OSHI, during rice embry- approach, it has been demonstrated that some genes are ogenesis. In situ hybridization analysis revealed that in the expressed in specific cell types, regions, and organs of embryo wild-type embryo, OSHI was first expressed at the globular (11, 12). The information gained from these studies helps the stage, much earlier than organogenesis started, in a ventral way to elucidate the functional roles of these genes. region where shoot apical meristem and epiblast would later In Drosophila, the principles of the genetic control during develop. This localized expression of OSHI indicates that the embryogenesis have been unraveled through the combined cellular differentiation has already occurred at this stage. At genetic and molecular approaches (for review, see ref. 13). later stages after organogenesis had initiated, OSHI expres- These approaches demonstrate that the homeobox genes, sion was observed in shoot apical meristem [except in the Li which encode evolutionarily conserved 61 amino acid domains (tunica) layer], epiblast, radicle, and their intervening tissues called homeodomains, play important roles in cellular or in descending strength of expression level with embryonic regional differentiation of Drosophila embryo. In plants, ho- maturation. We also performed in situ hybridization analysis meobox genes have been isolated from several species such as with a rice organless embryo mutant, orll, that develops no maize (14, 15), rice (16, 17),Arabidopsis (18-21), and soybean embryonic organs. In the orl) embryo, the expression pattern (22). Plant homeobox genes, by analogy of the functional roles of OSHI was the same as that in the wild-type embryo in spite of animal homeobox genes, have been expected to encode of the lack of embryonic organs. This shows that OSHI is not transcriptional regulators that mediate important develop- directly associated with organ differentiation, but may be mental processes during embryogenesis (19). It has not been related to a regulatory process before or independent of the demonstrated, however, that the plant homeobox genes are organ determination. The results described here strongly directly involved in embryogenesis, although ectopic expres- suggest that, like animal homeobox genes, OSHI plays an sions of the homeobox genes have caused the abnormal leaf important role in regionalization of cell identity during early development in the vegetative phase of the transgenic plants embryogenesis. (16, 21, 23-25). Only very recently, the expression of maize homeobox genes was observed at the localized region of The fundamental body plan in higher plants is established developing maize embryo (26, 27). during the embryogenesis, although most morphogenetic To elucidate the possibility that plant homeobox genes are events occur in the postembryonic phase of the life cycle. In the involved in embryo development, we examined the temporal early stage of embryogenesis, several body axes that cause and spatial expressions of a rice homeobox gene, OSH1, during apical-basal, dorsal-ventral, and radial patterns are estab- rice embryogenesis in wild-type and in organless embryo lished, and two apical meristems (shoot and root) are formed. mutant, orll, that develops no embryonic organs (2). The The apical meristems successively produce various tissues and results described here suggest that OSH1 is not directly asso- organs throughout the plant life cycle while maintaining ciated with shoot development, but may function to specify cell themselves as stem-like cells. In Poaceae species, which have identity and provide regional information for the formation of highly developed embryos, mature embryos contain almost all shoot and its adjacent tissues. organs seen in the vegetative phase such as shoot apex, leaves, vascular systems, and root (1, 2). To elucidate the genetic control of the pattern formation and/or the organ differenti- MATERIALS AND METHODS ation during plant embryogenesis, many embryonic mutants Plant Materials. To observe developmental course of wild- have been isolated in Arabidopsis (3-5), maize (6, 7), and rice type embryos-and expression pattern of OSH1, embryos from (2, 8). The study of available embryo mutants of rice indicate the rice (Oryza sativa L.) cultivar Taichung 65 were used. We the existence of several major developmental processes taking also used embryos from orll mutant, a monogenic, recessive place during early embryogenesis before morphogenetic mutant of rice derived from chemical mutagenesis with N- events start (9). Despite the intensive efforts by researchers, methyl-N-nitroso-urea (2). little is known about the molecular mechanisms regulating Preparation of Rice Embryo Sections. Developmental these developmental processes. course of wild-type rice embryos was examined by standard paraffin method. For paraffin sectioning, seeds at various The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in Abbreviation: DAP, days after pollination. accordance with 18 U.S.C. §1734 solely to indicate this fact. 1To whom reprint requests should be addressed. 8117 Downloaded by guest on September 29, 2021 8118 Plant Biology: Sato et al. Proc. Natl. Acad. Sci. USA 93 (1996) developmental stages were fixed in FAA (formalin/glacial formed successively at 5, 6, and 8 DAP, respectively, in an acetic acid/70% ethanol, 5:5:90), dehydrated in graded etha- alternate phyllotaxis (Fig. 1 E-G). nol series, and embedded in paraffin. Samples were sectioned Based on the developmental course of rice embryo de- at 10 ,tm and stained with hematoxylin. scribed above, we performed in situ hybridization analysis on In Situ Hybridization. In situ hybridization with digoxigenin- embryos at 2-7 DAP to determine the expression pattern of labeled RNA produced from the OSH1 coding region without OSH1 during rice embryogenesis. When the sense-strand poly(A) region was conducted as described (28). Tissues were probe was used, no specific hybridization signal was observed fixed with 4% (wt/vol) paraformaldehyde and 0.25% glutar- in any experiments (data not shown). Therefore, signals with aldehyde in 0.1 M sodium phosphate buffer and embedded in the antisense probe were caused by the specific hybridization Paraplast Plus (Oxford Labware, St. Louis). Microtome sec- between the OSH1 transcripts and the probe. tions (7-10 gm thick) were applied to slide glass treated with We first examined the OSH1 expression in globular em- Vectabond (Vector Laboratories). Hybridization and immu- bryos. The hybridization signals were first detected in globular nological detection of the hybridized probe were performed by embryos comprising around 100 cells and were about 50-55 method of and Hata (28). gm long at late 2 DAP or early 3 DAP (Fig. 2A). At this stage, the Kouchi embryos show no sign of organ differentiation. It is clear that the expression is restricted to a small region just below the RESULTS center of the ventral region of the embryo, where the shoot apex arises later (Fig. 2A). In the late globular through Expression of a Rice Homeobox Gene, OSHJ, During Em- coleoptilar stages of embryos, OSH1 was expressed in an bryogenesis in Wild-Type Plants. Rice embryos develop much enlarged ventral region of embryo (Fig. 2B and C). The signals more quickly than those of other cereal plants such as maize were detected in the outermost several cell layers just below and barley. They complete most of the morphogenetic events the coleoptilar protrusion extending to the basal (Fig. 2B) and within 1 week under normal conditions. Globular stage lasts inner part of the embryo corresponding to the expected nearly 3 days after pollination (DAP) (Fig. 1 A and B). epiblast and radicle, respectively (Fig. 2C). Although organogenetic events are not observed before 3-4 At the shoot apex differentiation stage when embryos are DAP, a dorsiventral polarity is evident from the gradient of 250-300 ,um long (4 DAP), OSHI was expressed in the shoot
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