Early development of rice endosperm

Abstract

Plant endosperm stores nutrients for the embryo development at seed maturation and germination. It also provides an important food resource for human populations; the endosperm of rice, for example, represents a large proportion of the world's food supply. Although many aspects of the biology of endosperm have been intensively investigated, such as the cell biology, genetic controls and evolution, the early stages of endosperm development are still unclear. In many species, the endosperm develops in two phases: first, a syncytial phase and then a cellular phase. At the transition point between these phases, the syncytium is divided into individual cells by cellularization. Although cytological approaches have provided a morphological characterization of this aspect of endosperm development, the molecular mechanisms governing these events remain unclear. The endosperm can show the effects of a reproductive barrier following interspecific crosses. Various hypotheses have been proposed to explain the reproductive barriers in the hybrid endosperm of interspecific and interploidy crosses. Among these is genomic imprinting, which is defined as the parent-of-origin dependent expression of selected genes and is associated with specific epigenetic mechanisms such as DNA methylation and histone modification. In Arabidopsis, recent studies have identified several imprinted genes and also the genes required for genomic imprinting. In Arabidopsis, the Polycomb group (PcG) genes, which belong to the plant Fertilization Independent Seed (FIS) class, were identified in mutants that showed spontaneous initiation of endosperm development and seed abortion. Polycomb Repressive Complex 2 (PRC2) controls early aspects of endosperm development through gene silencing by the histone H3 trimethylated at Lysine 27 (H3K27me3). Epigenetically mediated signaling pathways, controlled by DNA methylation and H3K27me3, are essential for the control of endosperm development in Arabidopsis. Here, we summarize the genomic imprinting machinery in Arabidopsis endosperm, and focus on recent advances in our understanding of the role of PcG genes on endosperm development in rice and maize.