In most flowering plants, fertilization is necessary for development of the central cell into endosperm, but in the fie-1 mutant of Arabidopsis, the central cell can proliferate autonomously. However, autonomous fie-1 endosperms do not develop completely: They have fewer nuclei than sexually produced endosperms, cellularization does not take place, and no clear distinction is seen between the different endosperm compartments. Here, we show that autonomous endosperm develop much further in hypomethylated than normally methylated fie-1 mutants, undergoing cellularization and regional specification to resemble endosperm in sexually produced wild-type seeds. Therefore, the combination of maternal hypomethylation and loss of FIE function enables formation of differentiated endosperm without fertilization. A maternal fie-1 mutation is also lethal to sexual seeds, even if the pollen donor is wild type. We report that sexual mutant fie-1 endosperms fail to cellularize and overproliferate, consistent with the hypothesis that embryo abortion may be due, at least in part, to a defect in endosperm development. Finally, we show that pollen from hypomethylated plants rescues fie-1 mutant seeds provided that it also donates a wild-type paternal FIE allele. These results are discussed in light of models for parent-of-origin effects on seed development.

Genomic imprinting, the parent-of-origin-specific expression of genes, plays an important role in the seed development of flowering plants. As different sets of genes are imprinted and hence silenced in maternal and paternal gametophyte genomes, the contributions of the parental genomes to the offspring are not equal. Imbalance between paternally and maternally imprinted genes, for instance as a result of interploidy crosses, or in seeds in which imprinting has been manipulated, results in aberrant seed development. It is predominantly the endosperm, and not or to a far lesser extent the embryo, that is affected by such imbalance. Deviation from the normal 2m:1p ratio in the endosperm genome has a severe effect on endosperm development, and often leads to seed abortion. Molecular expression data for imprinted genes suggest that genomic imprinting takes place only in the endosperm of the developing seed. Although far from complete, a picture of how imprinting operates in flowering plants has begun to emerge. Imprinted genes on either the maternal or paternal side are marked and silenced in a process involving DNA methylation and chromatin condensation. In addition, on the maternal side, imprinted genes are most probably under control of the polycomb FIS genes.

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The introduction of apomixis to crops would allow desirable genotypes to be propagated while preventing undesirable gene flow, but so far there has been little success in transferring this trait from a natural apomict to another species. One explanation is the sensitivity of endosperm to changes in relative maternal and paternal contribution owing to parental imprinting, an epigenetic system of transcriptional regulation by which some genes are expressed from only the maternally or paternally contributed allele. In sexual species, endosperm typically requires a ratio of two maternal genomes to one paternal genome for normal development, but this ratio is often altered in apomicts, suggesting that the imprinting system is altered as well. We present evidence that modification of DNA methylation is one mechanism by which the imprinting system could be altered to allow endosperm development in apomicts. Another feature of natural apomixis is the modification of the normal fertilization programme. Sexual reproduction uses both sperm from each pollen grain, but pseudogamous apomicts, which require a sexual endosperm to support the asexual embryo, often use just one. We present evidence that multiple fertilization of the central cell is possible in Arabidopsis thaliana , suggesting that pseudogamous apomicts may also need to acquire a mechanism for preventing more than one sperm from contributing to the endosperm. We conclude that strategies to transfer apomixis to crop species should take account of endosperm development and particularly its sensitivity to parental imprinting, as well as the mechanism of fertilization.

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