Molecular basis of induction and execution of programmed cell death in plants

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1.Microarray analysis of comprehensive gene expression induced by brassinosteroids (BRs) :To understand tracheary element (TE) cell death mechanism induced by BRs, I analyzed gene expression induced by BRs comprehensively using microarray. As a result, I revealed that most genes involved in TE cell death are induced by BRs rapidly, suggesting that BRs are an initiator of transcriptional activation of cell death-related genes. I also identified transcription factors showing transient and rapid changes by BRs, which are candidates of regulators of cell death related genes.2.Regulation mechanism of the expression of cell death-related genesUsing a cell-death related gene, ZCP4 as a marker, I identified a cis-element responsible for TE-specific expression, which is designated Tracheary Element Regulating cis Element (TERE). This element is conserved in promoters not only of cell death-related genes but also of cell wall-related genes. Using yeast two hybrid system, I isolated two factors that bind the TERE sequence, both of which were bZip-type transcription factors.3.Analysis of nucleasesI performed promoter analysis of all the S1 nuclease gene members in Arabidopsis (BFN1-BFN5). BFN1 was only a S1 nuclease gene that was expressed in developing TEs. In various types of cells going to cell death, different S1 nuclease genes were expressed, suggesting that each S1 nuclease gene is involved in distinctive cell death process. I also identified two candidate genes of calcium-dependent nucleases whose activity increases transiently before TE cell death.4.Function analysis of a nucleus-locating lipaseI have indicated a Zinnia lipase gene whose mRNA increases in a TE-specific manner. Here I isolated its Arabidopsis ortholog and analyzed the expression and function of the Arabidopsis lipase gene. This gene is expressed preferentially in developing TEs and overproduction of the gene caused abnormal vascular formation, suggesting its involvement in cell death process of TEs.