Overall shoot architecture in higher plants is highly dependent on the activity of embryonic and axillary shoot meristems, which are produced from the basal adaxial boundaries of cotyledons and leaves, respectively. In Arabidopsis thaliana, redundant functions of the CUP-SHAPED COTYLEDON genes CUC1, CUC2, and CUC3 regulate embryonic shoot meristem formation and cotyledon boundary specification. Their functional importance and relationship in postembryonic development, however, is poorly understood. Here, we performed extensive analyses of the embryonic and postembryonic functions of the three CUC genes using multiple combinations of newly isolated mutant alleles. We found significant roles of CUC2 and CUC3, but not CUC1, in axillary meristem formation and boundary specification of various postembryonic shoot organs, such as leaves, stems, and pedicels. In embryogenesis, all three genes make significant contributions, although CUC3 appears to possess, at least partially, a distinct function from that of CUC1 and CUC2. The function of CUC3 and CUC2 overlaps that of LATERAL SUPPRESSOR, which was previously shown to be required for axillary meristem formation. Our results reveal that redundant but partially distinct functions of CUC1, CUC2, and CUC3 are responsible for shoot organ boundary and meristem formation throughout the life cycle in Arabidopsis.

The shoot apical meristem (SAM) is a group of stem cells that are responsible for plant development. Mutations in rice SHOOTLESS2 (SHL2), SHL4/SHOOT ORGANIZATION2 (SHO2), and SHO1 cause complete deletion or abnormal formation of the SAM. In this study we showed that defects in SAM formation in shl mutants are associated with the loss of expression of the homeodomainleucine zipper (HD-ZIPIII) family genes. Rice SHL2, SHL4/SHO2, and SHO1 encoded orthologues of Arabidopsis RNA-dependent RNA polymerase 6, ARGONAUTE (AGO) 7, and DICER-like 4, respectively, whose mutations affect leaf development through the trans-acting siRNA (ta-siRNA) pathway. This suggested that the ta-siRNA pathway regulates the critical step of SAM formation during rice embryogenesis. The gain-of-function experiment by the ectopic expression of SHL4 resulted in reduced accumulation of an microRNA, miR166, and partial adaxialization of leaves, supporting a role for the ta-siRNA pathway in the maintenance of leaf polarity as previously reported in maize. Analysis of the spatiotemporal expression patterns of HD-ZIPIII and miR166 in wild-type and shl mutant embryos suggested that the loss of HD-ZIPIII expression in the SAM region of the developing embryo is the result of ectopic expression of miR166. Our analysis of shl mutants demonstrated that HD-ZIPIII expression regulated by miR166 is sensitive to the ta-siRNA pathway during SAM formation in rice embryogenesis.embryogenesis ͉ shoot apical meristem ͉ shootless ͉ shoot organization I n multicellular organisms, precise control of the spatial and temporal expression of developmental regulatory genes is essential. Recent studies have revealed the existence of a posttranscriptional mechanism of regulation of these genes through small RNAs (1, 2). Among the various types of small RNA molecules, microRNAs (miRNAs) control the expression of developmental regulatory molecules in both plants and animals, and their target sequences are often conserved between species. Recently, it has been shown that repeat-associated siRNAs and trans-acting siRNAs (ta-siRNAs) are also involved in the regulation of development (1, 3-5). However, the downstream developmental and genetic events that are regulated by these small RNAs are poorly understood.The shoot apical meristem (SAM) is a center of morphogenesis in plants and the developmental origin of most of the aboveground parts of the plant, including the leaves, stems, and axillary buds (6). Embryonic SAM formation is the initial process that establishes stem cells, providing an appropriate local environment for the maintenance of the stem cells, and ensures postembryonic development of the shoot architecture. Thus, understanding the mechanism of embryonic SAM formation is a priority issue in the developmental biology in plants, and doing so provides a unique system to study the mechanism of stem cell establishment. In rice, shootless (shl) mutants were originally identified as embryonic mutants that completely lack a SAM (7). In most shl mutants, the scutellum and ra...

SummaryCUP-SHAPED COTYLEDON (CUC )1 encodes members of the NAC family. These are functionally redundant genes that are involved in shoot apical meristem (SAM) formation and cotyledon separation during embryogenesis in Arabidopsis. We analyzed transgenic plants overexpressing CUC1 (35S::CUC1 ). The cotyledons of these transgenic seedlings regularly had two basal lobes, small and round epidermal cells between the sinuses, and adventitious SAMs on the adaxial surface of this region. This suggests that CUC1 promotes adventitious SAM formation by maintaining epidermal cells in an undifferentiated state. In 35S::CUC1 cotyledons, the class I knotted-like homeobox (KNOX ) genes, including SHOOT MERISTEMLESS (STM ) and BREVIPEDICELLUS (BP ), which are involved in SAM formation and/or maintenance, were ectopically expressed before adventitious SAM formation. In stm mutants, ectopic expression of CUC1 could not induce adventitious SAMs, whereas they continued to be observed in bp mutants. These results suggest that STM, but not BP, is necessary for the formation of adventitious SAMs in 35S::CUC1 cotyledons. Furthermore, we examined the relationship between CUC1 and ASYMMETRIC LEAVES (AS )1 and AS2. The as1 and as2 mutations genetically enhance 35S::CUC1 phenotypes even in the absence of STM function. Interestingly, the as1 mutation can partially rescue the mutant vegetative development phenotypes in the cuc1 cuc2 double mutant. Our results suggest that CUC1 positively regulates SAM formation not only through STM but also through an STM-independent pathway that is negatively regulated by AS1 and AS2.

The CUP-SHAPED COTYLEDON (CUC) genes CUC1, CUC2 and CUC3 act redundantly to control cotyledon separation in Arabidopsis. In order to identify novel regulators of this process, we have performed a phenotypical enhancer screen using a null allele of cuc2, cuc2-1. We identified three nonsense alleles of AtBRM, an Arabidopsis SWI/SNF chromatin remodeling ATPase, that result in strong cotyledon fusion in cuc2-1. atbrm also enhances cotyledon fusion in loss-of-function cuc1 and cuc3 mutants, suggesting a general requirement for this ATPase in cotyledon separation. By contrast, a null allele of SPLAYED (SYD), the closest homolog of AtBRM in Arabidopsis, enhances only the loss-of-function cuc1 mutant. By investigating the activities of the CUC promoters in the cotyledon boundary during embryogenesis in sensitized backgrounds, we demonstrate that AtBRM upregulates the transcription of all three CUC genes, whereas SYD upregulates the expression of CUC2. Our results uncover a specific role for both chromatin remodeling ATPases in the formation and/or maintenance of boundary cells during embryogenesis. Development 133, 3223-3230 (2006) DEVELOPMENT 3224 and the complex composition of the SWI/SNF remodelers is becoming better understood, understanding their role in the organism has been hampered by the fact that mutations in most SWI/SNF ATPases are embryo lethal (Bultman et al., 2000; Bultman et al., 2005;Indra et al., 2005;Reyes et al., 1998;Sawa et al., 2000;Tamkun et al., 1992). We show here that the two SWI/SNF ATPases AtBRM and SYD are specifically and differentially required for cotyledon separation in Arabidopsis via the regulation of expression of a small gene family. KEY WORDS: Arabidopsis, Embryo, Boundary formation, CUP-SHAPED COTYLEDON, Chromatin remodeling ATPase MATERIALS AND METHODS Plant lines and growth conditionscuc2-1, stm-1, stm-2, cuc1-1 and syd-2 mutants have been described (Aida et al., 1997; Barton and Poethig, 1993; Clark et al., 1996;Takada et al., 2001;Wagner and Meyerowitz, 2002). stm-2 syd-2 was described in a previous study (Kwon et al., 2005). cuc3-101, isolated as an enhancer mutant of cuc2-1, carries a point mutation at the junction of the first exon and intron (AG:GT to AG:AT) and represents a strong allele (K.i.-H., M.A. and M.T., unpublished). Double-mutant plants made in this study were genotyped for cuc1 -1, cuc3-101, syd-2, stm-2, atbrm-1, atbrm-2 and atbrm-3 using dCAPS. Homozygous cuc2-1 was genotyped by amplifying a genomic DNA region flanking the transposon insertion site (Aida et al., 1997). Segregating populations of syd-2 and atbrm-1 were individually maintained in Ler, cuc1-1, cuc2-1 and cuc3-101 homozygous backgrounds. stm-2 atbrm-1 was maintained as double heterozygotes. Seeds were sown on fertilized soil mix (Promix BX; Premier Horticulture, Quakertown, PA), stratified in the dark at 4°C for 7 days, and then grown at 22°C in long day (16 hours light) conditions at 120 mol/m 2 sec of cool white light. Plants were photographed using an Olympus SZX12 dissecting microscope e...

SUMMARYAmong angiosperms there is a high degree of variation in embryo/endosperm size in mature seeds. However, little is known about the molecular mechanism underlying size control between these neighboring tissues. Here we report the rice GIANT EMBRYO (GE) gene that is essential for controlling the size balance. The function of GE in each tissue is distinct, controlling cell size in the embryo and cell death in the endosperm. GE, which encodes CYP78A13, is predominantly expressed in the interfacing tissues of the both embryo and endosperm. GE expression is under negative feedback regulation; endogenous GE expression is upregulated in ge mutants. In contrast to the loss-of-function mutant with large embryo and small endosperm, GE overexpression causes a small embryo and enlarged endosperm. A complementation analysis coupled with heterofertilization showed that complementation of ge mutation in either embryo or endosperm failed to restore the wild-type embryo/endosperm ratio. Thus, embryo and endosperm interact in determining embryo/endosperm size balance. Among genes associated with embryo/endosperm size, REDUCED EMBRYO genes, whose loss-of-function causes a phenotype opposite to ge, are revealed to regulate endosperm size upstream of GE. To fully understand the embryo-endosperm size control, the genetic network of the related genes should be elucidated.