In a review article, “Genetic and epigenetic processes in seed development”, published in the February, 2002 issue of Curr. Trends in Plant Biol. (5:19-25), AR Lohe and Abid Chaudhury at CSIRO, Canberra, Australia, discuss various molecular mechanisms such as interaction between genetic and epigenetic factors that operate and control the seed development processes both in sexually- reproducing and apomictic plants.
The article accompanies a description of the development of the embryo sac of Arabidopsis thaliana. The eight nuclei that result from three divisions of the megaspore are at first arranged into two groups of 4 at the two ends of the embryo sac. Three out of four nuclei at both the micropylar and chalazal end develop into an egg apparatus and antipodals, respectively. One nucleus from each pole moves to the center; they fuse and give rise to what is called a central cell. One of the two male gametes fuses with the egg to give rise to the diploid zygote. The latter after repeated divisions in an organized fashion turns into an embryo; the second male gamete fuses with the nucleus of the central cell to give rise to the triploid endosperm nucleus. After fertilization the endosperm nucleus divides to produce endosperm and provides nutrition to the developing embryo.
The authors point out that in light of recent findings the paternal genome is of less importance during the early stages of seed development and that the genes contributed by the parent-of-origin have their impact on the development of characters in the progeny. The recent finding also shows that seed devlopment is orchestrated by DNA methylation and chromatin remodelling.
The finding that genes in Arabidopsis encode proteins that are homologous to the PcG class of trancriptional repressors in Drosophila suggests that global control of gene repression evolved before the separation of plant and animal kingdoms.
Endosperm polarity. A close examination of endosperm nuclear divisions has revealed that cells divide in three different domains independent of each other. Unlike the rest of nuclei in the endosperm, those in the chalazal domain divide by a process called endoduplication. (DNA replication without cell wall formation). In fis mutants polarity is lost.
Interestingly, some genes belonging to the MADS-box family have been reported to be expressed in the endosperm and in the developing male and female gametophytes. For instance, RbAp (retinoblastoma-associated protein) proteins were found in abundance in maize endosperm, shoot apical meristems and leaf primordia of the embryo.
Epigenetic effects in endosperm development: Gene expression in both the embryo and and the endosperm is largely maternal in origin until 3-4 days after fertilization. The activity of 20 paternal genes was first detected only after that time. Evidence is emerging that not all genes from the paternal genome are inactive in early embryo and endosperm development. For instance, the prolifera gene is expressed early in development in both the embryo and the endosperm
The genes that repress the embryo and the endosperm development until fertilization are of interest in understanding how apomictic systems function at the molecular level. Intensive studies on the embryo and endosperm development of Arabidopsis thaliana, which is strictly a sexually reproducing species, need to be undertaken in order to identify all genes that contribute to seed development. Studies on their mutant counterparts will shed light how a sexually reproducing population of a species such as A. thaliana may turn into an asexual entity, i.e., producing seeds without fertilization. To date apomictic Arabidopsis mutants are not known but several instances in which proliferation of endosperm tissue without fertilization have been reported. For instance, mutation in each of the three FIS (fertilization independent seed) leads to proliferation of endosperm tissue, independent of fertilization. In spite of the endosperm proliferation, the egg cell remains undeveloped, regardless whether the male gamete fuses with it or not.
In this context, the authors mention of two other genes, namely, CYP78A9 and FWF (Fruit without fertilization) which are essential for pericarp development. Fruits of individuals with the mutant genotype cyp78A9 are seedless. In the other mutant with fwf genotype, cells undergo rapid divisions affecting mesocarp- and lateral vascular layer development. Functional studies of these two genes will shed light on the molecular aspect of apomictic development.
Citing the example of Hieracium, an apomictic species belonging to the family Asteraceae, the authors describe three closely linked components that essentially contribute to the development of apomixis in this taxon. These are: (a) apospory (development of pollen grains without the intervention of meiosis), (b) development of both embryo and (c) endosperm without fertilization. All the three genes invloved in the process have been found to segregate in a simple Mendelian fashion. However, it is yet to be determined whether the three associated genes are co-adapted and transmitted en block to the progeny without the involvement of crossover. Analysis of the progeny of a cross between a sexual and an apomictic Taraxacum (Asteraceae) taxon indicates that the linkage between the two genes controlling diplospory and parthenogenesis may be delinked. Working on another apomictic species, Erigeron annus of the same family, the authors discovered that the two phenomena, diplospory and parthenogenesis, are independently inherited and that the locus controlling parthenogenesis produces a lethal mutation.