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.