22. Identification of rice genes encoding a LEC1-type HAP3 subunit protein
  T. THIRUMURUGAN1,2, K. MIYOSHI1, Y. ITO1 and N. KURATA1,2

1) Plant Genetics Laboratory, National Institute of Genetics, Mishima, 411-8540 Japan
2) Department of Genetics, School of Life Sciences, The Graduate University for Advanced Studies, Kanagawa, 240-0193 Japan

During seed development, several discrete phenomena, such as establishment of shoot and root apical meristem, accumulation of storage reserves and acquisition of desiccation tolerance and dormancy, progress in a coordinated manner. Molecular genetic studies in Arabidopsis identified genes affecting multiple aspects of the seed development. Of these, LEC1 and L1L have been intensively studied (West et al. 1994; Lotan et al. 1998; Kwong et al. 2003). LEC1 and L1L are members of an Arabidopsis HAP3 (AHAP3) gene family which consists of ten genes and encode a component of a heterotrimeric CCAAT-box binding transcription factor. Based on the amino acid identity of an evolutionally conserved region called B domain, the AHAP3 proteins were divided into two classes, LEC1-type and non-LEC1-type. Transgenic studies demonstrated that only the LEC1-type genes (i.e. LEC1 and L1L) could activate the expression of a set of genes required for embryo formation in a precisely coordinated manner (Lee et al. 2003). The LEC1-type HAP3 genes were found not only in Arabidopsis but also in many other plant species. Comparative study of HAP3 gene(s) among plant species would provide a tool for understanding of a general mechanism of embryogenesis in higher plants.

By taking two approaches, cDNA cloning and a database search, we identified eleven members of rice HAP3 genes. A Phylogenic tree generated on the basis of amino acid sequences of conserved B domain of the HAP3 subunits exhibited distinct clusters, and OsHAP3D and OsHAP3E were located in the same cluster as LEC1 and L1L (Fig. 1). Fig. 2 shows the

predicted amino acid sequences of the B domain of eleven members of the rice HAP3 and two LEC1-type Arabidopsis proteins, LEC1 and L1L. Among the rice members, OsHAP3E exhibited highest similarity to LEC1 (83% amino acid identity) and L1L (91%) followed by OsHAP3D (74% and 75% respectively). In addition, most of amino acid residues characteristic to the LEC1-type HAP3s were specifically conserved in OsHAP3D and OsHAP3E, but not in the other rice members. These results indicate that OsHAP3D and OsHAP3E encode the LEC1-type HAP3 protein. RT-PCR analysis revealed the expression of OsHAP3D and OsHAP3E in developing rice seeds (data not shown), suggesting the involvement of LEC1-type HAP3s in monocot embryogenesis.

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