Recent advances in technical innovations for DNA sequencing and their expression array analyses have accelerated understanding of mechanisms for gene regulation in diverse organisms from prokaryotes to eukaryotes including plants. During analyses of these data, it has become clear that heritable epigenetic modifications, in which the modifications are attributed to the degree of methylation of a gene and not to the change in its sequence, sometimes occur and can affect the level of gene expression by reducing its transcriptional level. DNA methylation at cytosine residues has been found to affect many important biological processes, including maintenance of genome integrity, developmental regulation, and regulation of gene expression. In many organisms, allelic diversity generates phenotypic variations and contributes to many events, such as development, adaptation to changing environment, and genome evolution. Allelic diversity is generally defined by the difference in nucleotide sequences that code for a gene. However, recent studies on recessive allele-specific DNA methylation of Brassica self-incompatibility alleles demonstrate that DNA methylation patterns in plants can vary temporally and spatially in each generation. In this review, we firstly present overview of self-incompatibility system in Brassica and then describe dominance relationships in Brassica self-incompatibility regulated by allele-specific DNA methylation and their possible underlying mechanisms.

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[in Japanese]

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[in Japanese]

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Seed germination completes when radicle emergence occurs through the surrounding tissues (testa and endosperm). To do this, embryonic growth potential should overcome the mechanical resistance caused by testa and endosperm. Therefore, germination processes require the activation of embryonic growth potential and weakening of surrounding tissues after seed imbibition. Hydrolytic enzymes of cell wall components are activated to weaken endosperm after seed imbibition. Gibberellin (GA) and abscisic acid (ABA) regulate the activation of these enzymes antagonistically. On the other hand, embryonic growth potential is activated in part by enhancing protein synthesis after imbibition. It is predicted that the protein synthesis is regulated by GA and ABA thorough the AtTCP/site II motif transcription complex. In this article, we summarize the functions of embryo and endosperm in terms of hormone actions.

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The majority of genes encoding gibberellin (GA) biosynthesis and deactivation enzymes have been identified to date. Recent studies have highlighted the occurrence of previously unrecognized deactivation mechanisms, including epoxidation and methylation. GA concentrations in plant tissues are carefully optimized by a number of internal and external signals. Studies have recently shown that, in many cases, bioactive GA levels are modulated through coordinated regulation of GA biosynthesis and deactivation. In addition, the identification of transcription factors that directly regulate GA biosynthesis and deactivation genes has started to uncover the molecular mechanisms for fine-tuning the hormone levels. In this article, I summarize our current understanding of the GA biosynthesis and deactivation pathways in plants, and discuss how the concentration of bioactive GAs is regulated in several selected systems.

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[in Japanese]

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Fusicoccins are a class of diterpene glucosides produced by the plant-pathogenic fungus Phomopsis amygdali. As modulators of 14-3-3 proteins, fusicoccin A functions as a potent activator of plasma memblane H^+-ATPase in plants. From the mycelia, a cDNA encoding fusicocca-2,10(14)-diene (a tricyclic hydrocarbon precursor for fusicoccins) synthase (PaFS) was isolated. This enzyme is chimera protein, which possesses not only cyclase activity but also prenyltransferase (geranylgeranyl diphosphate synthase) activity. The functional analysis of truncated mutants and site-directed mutagenesis demonstrated that PaFS consists of two domains, a terpene cyclase domain at the N terminus and a prenyltransferase domain at the C terminus. Our data base survey suggested that such chimeric terpene synthase also occur in other fungal species. According to these results, we can propose the unusual diterpene synthase family, specific to fungi.

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Plant hormones are signal compounds, which regulate various responses at a low concentration in plants. Endogenous levels of them had been quantified mainly by using GC-EI-MS. Recently, microanalysis by using LC-ESI-MS/MS has been carried out frequently. This method does not require complicated purification and derivatization that GC-EI-MS requires. However, because the ionization efficiency in LC-ESI-MS/MS method is susceptible to impurities, this requires purification suitable to this system. In RIKEN Plant Science Center, we aim to establish a system for comprehensive analysis of all bioactive plant hormones and their precursors/metabolites. Here, we report our current method for purification and quantification of bioactive hormones (GA_1, GA_4, ABA, IAA, JA, SA, CKs and BRs) in dry seeds, leaves and flowers of Arabidopsis.

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[in Japanese]

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[in Japanese]

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