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The gene Os08g0174700 encode a somatic embryogenesis（SERK-family）receptor-like protein kinase 1(OsSERK1).Functionally, OsSERK1 is the closest homolog of AtBAK1 in rice. Therefore, we named it as OsBAK1( BRI1-Associated receptor Kinase 1). The expression of OsBAK1changed important agricultural traits of rice such as plant height, leaf erectness, grain morphologic features, and disease resistance responses. A new rice variety with erect-leaf and normal reproduction can be generated simply by suppressing the expression level of OsBAK1.Reduceing levels of OsBAK1 and decreasing sensitivity to BL will lead to semidwarfism in overall growth and result in abnormal growth patterns(Fig.1).
1.Expression of OsSERK1 and in various organs of riceUsing a set of OsSERK1 primers (K-1; 5’AT(CT)AT(ATC)CATCG(AT)GATGTCAA3’ and K6-2; 5’CCATCTTGGGGCGTTCTGTG3’, where nucleotides in parentheses are a mixture). Actin (RAc-1; 5’AACTGGGATGATATGGAGAA3’, RAc-2; 5’CCTCCAATCCAGACACTGTA3’) was used as an internal control. The result showed that OsSERK1 was expressed in all organs and calli examined with different levels.Expression of OsSERK1 was relatively strong in a 1 DAP flower which contained a very early embryo, a leaf blade and a shoot apex. A callus on a regeneration medium with both cytokinin and auxin did not show significant change of the level of the OsSERK1expression in comparison to a callus on a callus-inducing medium(Fig.3 ).
2.OsSERK genes were expressed in various organs but with some tissue specificityThe OsSERK1 promoter showed reporter gene activities in some specific tissues in a germinating seed, leaf and root, but not in a developing embryo. This promoter activity suggests that OsSERK1 may have roles in non-embryonic tissues rather than in the embryo.OsSERK1 might also be involved in signaling of or response to phytohormones in these tissues(Fig.4 ).
3.Rice SERKs are involved in BR signalling Overexpress the OsSERK1 rice gene in a weak BRI1 mutant,bri1-5.The transgenic Arabidopsis lines ectopically expressing OsSERK1 partially rescued the defective phenotypes of bri1-5(Fig.9).Compared to bri1-5, transgenic plants expressing OsSERK1 showed larger statures, longer petioles and earlier ﬂowering phenotypes.The expression of CPD has been widely used as a molecular marker to detect the effectiveness of the BR signalling pathway . BRI1 mutants usually show higher CPD expression levels than that of wild-type plants. Compared to bri1-5, the transgenic lines expressing OsSERKs showed signiﬁcantly decreased expression of CPD (Fig.10).These results indicated that OsSERKs, like AtBAK1, can partially rescue bri1-5 mutant phenotypes.Functionally, OsSERK1 is the closest homolog of AtBAK1 in rice. Therefore, we named it as OsBAK1. 4.The subcellular localization of OsBAK1 protein The OsBAK1 protein in onion epidermis cells is located at plasma membrane (Fig.5) 5.OsBAK1 interacts with OsBRI1 in vivo Full length OsBAK1,the extracellular domain (OsBAK1-ECD), the intracellular domain (OsBAK1-ICD), and a truncated intracellular domain (OsBAK1-ICDD)(Fig.11)
Phylogenetic analysis and suppression of a weak Arabidopsis mutant bri1-5 indicated that OsBAK1 (Os08g0174700) is the closest relative of Arabidopsis BAK1(AtBAK1). Genetic, physiological, and biochemical analyses all suggest that the function of OsBAK1 is conserved with AtBAK1(Fig.6). Phylogenetic analysis of SERK proteins in Arabidopsis and rice. In order to search for AtBAK1 orthologs among the rice genes that were reported to be as OsSERKs or OsSERLs in the public database using the amino acid sequence of AtBAK1 as a query. It was difficult to determine which gene was an exact AtBAK1 ortholog, because many similar genes were identified. OsSERKs and OsBISERK1 cluster together, closer to each other than to any other AtSERKs based on amino acid sequence homology (Fig.7).
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- Dan Li; Lei Wang; Min Wang; Yun-Yuan Xu; Wei Luo; Ya-Ju Liu; Zhi-Hong Xu; Jia Li; Kang Chong Engineering OsBAK1 gene as a molecular tool to improve rice architecture for high yield Plant Biotechnology Journal, 2009, 7(8): 791-806.
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