maternally expressed gene1Is a Novel Maize Endosperm Transfer Cell–Specific Gene with a Maternal Parent-of-Origin Pattern of Expression[W]

Abstract

Growth of the maize (Zea mays) endosperm is tightly regulated by maternal zygotic and sporophytic genes, some of which are subject to a parent-of-origin effect. We report here a novel gene, maternally expressed gene1 (meg1), which shows a maternal parent-of-origin expression pattern during early stages of endosperm development but biallelic expression at later stages. Interestingly, a stable reporter fusion containing the meg1 promoter exhibits a similar pattern of expression. meg1 is exclusively expressed in the basal transfer region of the endosperm. Further, we show that the putatively processed MEG1 protein is glycosylated and subsequently localized to the labyrinthine ingrowths of the transfer cell walls. Hence, the discovery of a parent-of-origin gene expressed solely in the basal transfer region opens the door to epigenetic mechanisms operating in the endosperm to regulate certain aspects of nutrient trafficking from the maternal tissue into the developing seed.

Extensive, clustered parental imprinting of protein-coding and noncoding RNAs in developing maize endosperm

“…S5). These results showed that the imprinted expression of certain endosperm genes depends on developmental stages, with some of them being imprinted only in early stages, consistent with previous reports for Fie1, Fie2, and Meg1 genes (20,28).…”

“…The identification of imprinted genes has long been of interest for maize genetics research (19,20,32). We report here that at least 699 genes are potentially imprinted in maize endosperm.…”

“…Differential methylation between the paternal and maternal alleles in the endosperm is shown to correlate with allele-specific gene expression of several imprinted genes in both Arabidopsis (HDG3 and HDG9) and maize (Fie1 and Mez1) (19)(20)(21)(22)(23)(24), although DNA methylation does not always correlate with the allelic expression of imprinted genes (20,21). Screening of differentially methylated regions (DMRs) between embryo and endosperm resulted in the identification of five imprinted genes in Arabidopsis (22).…”

Balance between maternal and paternal alleles sets the timing of resource accumulation in the maize endosperm

“…The imprinted transcription factor NRP1, which is normally expressed 15-25 DAP (Guo et al 2003), responds similarly to OPAQUE2 and g-ZEIN, suggesting that it is a component of the gene network regulating maturation of the kernel. The expression of MRP1, a transcription factor regulating genes in the BETL (Gómez et al 2002), and MEG1, a putative signalling protein synthesized in the BETL and regulated by MRP1 (Gutiérrez-Marcos et al 2004), normally peaks at about 5 and 10 DAP, respectively-consistent with MRP1 activating MEG1. Paternal excess results in continued accumulation of transcript throughout development, while maternal excess causes expression of both genes to cease at 10 DAP, only to recommence at a lower level by 15 DAP.…”

“…This hypothesis requires that this epigenetic 'marking' (imprinting) should target genes regulating the provision of resources-but to date few have been identified. The MEG1 gene of maize is an exception (Gutiérrez-Marcos et al 2004), for it encodes a putative signal protein located at the basal transfer layer of the endosperm, which is required for grain filling as well as for correct development of the cell layer itself. This absence of imprinted genes involved in resource acquisition may be explained either by imprinting controlling major upstream regulators rather than physiologically functional genes or by the presence of other epigenetic systems that also modulate seed gene expression.…”