In the two leaves of the plant

In Arabidopsis thaliana, the shoot apical meristem (SAM) plays an essential role for its development. The SAM is formed between the two cotyledon primordia; and it maintains pluripotent stem cells at its tip (Takada et al., 2001). So far, some genes which are required for the SAM formation and maintenance have been studied. These include CUP SHAPED COTYLEDON (CUC) genes, which belong to the NAC (for NAM/ATAF1,2/CUC2) transcription factor family (Aida et al., 1997). The NAC genes form very large family, containing more than 100 members in Arabidopsis. The NAC genes share a highly conserved NAM (NO APICAL MERISTEM) domain, a N-terminal DNA binding domain (Ooka et al., 2003). In Arabidopsis, the CUC genes are first identified among the NAC family. The CUC family has three members, CUC1, CUC2, and CUC3. These three genes were identified because double mutants show cup-shaped cotyledon phenotype (Aida et al., 1997; Hibara et al., 2006; Takada et al., 2001).The three genes are located on different chromosomes of Arabidopsis. CUC1 is on chromosome 3, CUC2 is on chromosome 5, and CUC3 is on chromosome 1. All of the genes have three exons and two introns, and they have similar length, which are around 1800 base pairs. Moreover, the three genes encode proteins with similar size, which are around 30 kDa. CUC genes are highly expressed in SAMs, embryos, and floral buds (Winter et al., 2007). Besides, the three CUC proteins have the capacity to localize to the nucleus (Taoka et al., 2004).RESULTSAdventitious shoot formation in 35S::CUC1- 2 -To study the role of CUC1 in Arabidopsis, Takada et al. 2001 generated an overexpression transgenic line of CUC1 (35S::CUC1). The 35S::CUC1 plants showed several phenotypes compared to wild-type (WT) plants (Figure 1). First, the cotyledons were smaller than those of WT. Second, the two leaves of the plant fused to each other along one lateral margin of the petioles and leaf blades. Third, it was interesting to notice that the transgenic plants form adventitious shoots on the upper surface of the lobed cotyledons. Compare to WT, under the scan electron microscope, the upper surface epidermal cells in cotyledons of the 35S::CUC1 were small and round. The adventitious shoots formed rosette leaves initially, and some of them formed inflorescences and set seeds later. It was found that SHOOT MERISTEMLESS (STM) was essential for the adventitious SAM formation of the 35S::CUC1 plants (Hibara et al., 2003). The 35S::CUC1 was crossed with stm mutant. However, there were no adventitious SAMs arose, and the epidermal cells on the adaxial surface of the cotyledons were not small and round (Figure 1). Taken together, this indicated that the STM plays a central role in the formation of the adventitious SAMs in 35S::CUC1 plants.Identification of strong cuc3 alleles as cuc2 phenotypic enhancerscuc2 mutant shows no obvious phenotypes except for a small fraction with fused cotyledons along one side during Arabidopsis seedling stage (Aida et al., 1997). To identify genes that have redundant function with CUC2, a large population of the ethyl methanesulfonate (EMS)-mutagenized cuc2 plants was screened for strong fused cotyledon phenotypes at the seedling stage. Many independent lines were isolated and showed sever phenotypes compared to cuc2 mutant (Figure 2A). The mutated lines were mapped between ATPASE and nga692 on chromosome 1. This region contains the CUC3. These lines carried mutations in the CUC3 locus. The double mutants of cuc2 cuc3-101 and cuc2 cuc3-105 exhibited the cup-shaped cotyledon


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