DNA methylation is an epigenetic modification involved in transposon silencing and regulation of gene expression. In plants, developmentally induced re-programming of DNA methylation profiles are critical for gene imprinting and embryogenesis. Yet, little is known about the integration of DNA methylation control and vegetative development. We identified several spontaneous stably-inherited epialleles of the Arabidopsis thaliana Qua-Quine Starch (QQS, At3g30720) gene and their expression levels were found to be inversely correlated with the DNA methylation levels in the promoter and 5´-UTR. Since the expression of QQS varies between different organs, we thought to use it as a probe to explore the interplay between developmental programs and DNA methylation. QQS expression was found to be quite contrasted between epialleles throughout development and correlated inversely with their DNA methylation levels. Relative expression analysis revealed that methylated epialleles presented more pronounced expression differences during development than demethylated epialleles. A correlation between reduction of DNA methylation levels and increase of expression was found in rosette leaves of the methylated QQS epiallele in comparison to inflorescence tissues (stem, flowers and fruits). Since inflorescence tissues are formed after rosette leaves, we hypothesized that QQS could be undergoing a demethylation process from the vegetative meristem towards the formation of specific organs. Accordingly, GUS activity in Arabidopsis transgenic lines containing a chimerical QQS:GUS gene was seen in tissues where QQS expression was previously detected. However, a comparative analysis of QQS expression and methylation levels between organs (roots, cotyledons, meristem and leaves) revealed that increase of expression is not necessarily accompanied by a decrease in methylation. This result suggests that in addition to methylation, other factors also contribute to establishment of QQS expression pattern. Alternatively, the technique used is not so sensitive to detect subtle DNA methylation changes which could be restricted to few cells linages. Further detailed analysis should confirm the possibility of a programed QQS demethylation process during development and the possible underlying mechanisms will be discussed.
Financial support: FAPESP