The Arabidopsis transcription factor NAI1 is required for enhancing the active histone mark but not for removing the repressive mark on PYK10, a seedling-specific gene upon embryonic-to-postgerminative developmental phase transition.

We have recently shown that the expression onset of a seedling-specific gene, PYK10, occurs in a cell-by-cell manner upon the transition from the embryonic to the postgerminative phase and during embryogenesis in seed maturation regulator mutants such as lec1, and implicated epigenetic mechanisms in the process. Here, the role of the NAI1 transcription factor required for PYK10 expression in the developmental switching of PYK10 was investigated. The cell-by-cell onset of PYK10-EGFP in lec1 embryo was still observed in the nai1 background, but at greatly reduced levels. Decreases in the level of the repressive histone mark, H3K27 trimethylation observed upon the transition to the postgeminative phase normally occurred in nai1. However, concomitant increases in the level of the active mark, H3K4 trimethylation observed in wild type was significantly compromised in nai1. These results indicate that the switching of PYK10 upon developmental phase transition involves 2 separable steps of chromatin state change.

Cell-by-cell developmental transition from embryo to post-germination phase revealed by heterochronic gene expression and ER-body formation in Arabidopsis leafy cotyledon mutants.

LEC1, LEC2, FUS3 and ABI3 (collectively abbreviated LEC/ABI3 here) are required for embryo maturation and have apparent roles in repressing post-germinative development. lec mutant embryos exhibit some heterochronic characteristics, as exemplified by the development of true leaf-like cotyledons during embryogenesis. Although the roles of LEC/ABI3 as positive regulators of embryo maturation have been extensively studied, their roles in the negative regulation of post-germinative development have not been explored in detail. Based on microarray analyses, we chose PYK10, which encodes an endoplasmic reticulum (ER)-body-localized protein, as a molecular marker of post-germinative development. lec/abi3 embryos exhibited PYK10 misexpression and the formation of 'constitutive' ER-bodies, which develop specifically during the seedling stage, confirming the heterochronic nature of these mutants at both the gene expression and cellular levels. The PYK10 reporter expression in lec1 embryos started as early as the globular-heart transition stage. The onset of PYK10 promoter-enhanced green fluorescent protein (EGFP) reporter expression occurred in a stochastic, cell-by-cell manner in both developing lec/abi3 embryos and germinating wild-type seedlings. Additionally, clustered EGFP-positive cells were frequently found along cell files, probably representing the transmission of the expression state via cell division. These observations, together with the results of the experiments using PYK10-EGFP/PYK10-CFP double reporter transgenic lines and the analyses of H3K27me3 levels in the PYK10 chromatin, suggested the involvement of epigenetic mechanisms in repressing post-germinative genes during embryogenesis and derepressing these genes upon the transition to post-germinative development.