Fertilization-independent seed development in Arabidopsis thaliana

Abstract

We report mutants in Arabidopsis thaliana (fertilization-independent seed: fis) in which certain processes of seed development are uncoupled from the double fertilization event that occurs after pollination. These mutants were isolated as ethyl methanesulfonate-induced pseudo-revertants of the pistillata phenotype. Although the pistillata (pi) mutant has short siliques devoid of seed, the fis mutants in the pi background have long siliques containing developing seeds, even though the f lowers remain free of pollen. The three fis mutations map to loci on three different chromosomes. In fis1 and fis2 seeds, the autonomous endosperm nuclei are diploid and the endosperm develops to the point of cellularization; the partially developed seeds then atrophy. In these two mutants, proembryos are formed in a low proportion of seeds and do not develop beyond the globular stage. When FIS͞fis plants are pollinated by pollen from FIS͞FIS plants, Ϸ50% of the resulting seeds contain fully developed embryos; these seeds germinate and form viable seedlings (FIS͞FIS). The other 50% of seeds shrivel and do not germinate; they contain embryos arrested at the torpedo stage (FIS͞fis). In normal sexual reproduction, the products of the FIS genes are likely to play important regulatory roles in the development of seed after fertilization.Arabidopsis seed, like the seed of other angiosperms, is a product of double fertilization, in which one of the two sperm cells fertilizes the haploid egg cell, giving rise to a diploid embryo, and the other sperm cell fertilizes the polar nuclei in the central cell, giving rise to the triploid endosperm (1). As the embryo and the endosperm develop, the ovule enlarges into a seed; the maternal tissues of the inner and outer integuments surrounding the embryo sac form the seed coat.We have proposed a strategy for identifying genes that uncouple components of seed development from the fertilization process (2). Inactivation of genes that normally repress seed development may lead to precocious seed development without fertilization. In many apomictic plants, seed development does occur without fertilization or with only partial fertilization (3). In autonomous apomixis, seed development occurs without pollination and thus without fertilization of either the egg cell or the polar cell. In pseudogamous apomixis, pollination is required; in some cases, the pollination event results in fertilization of the polar cell but not of the egg cell. Apomixis has been described in a close relative of Arabidopsis, Arabis holboellii (3), and some genetic data support a one or two gene control of apomixis (3), so we reasoned that a mutational approach in Arabidopsis might detect mutants displaying some components of apomixis.For the isolation of these mutants, we used stamenless pistillata (pi) (4). If pi plants are not pollinated, the siliques remain short; they only elongate when seed is formed. We identified mutants in which the siliques elongated without pollination (5), and recently Ohad et al (6) described ...

Endosperm: an integrator of seed growth and development

“…The Pc-G complex FERTILIZATION INDEPENDENT SEED (FIS) contains the SET domain protein MEDEA (MEA) [11], the VEFS domain protein FIS2 [12], and the WD40 domain protein FERTILIZA-TION INDEPENDENT ENDOSPERM (FIE) [13]. Loss-of-function mutations of FIS genes cause autonomous onset of cell division in the central cell in the absence of fertilization [14,15]. The FIS complex also includes the WD40 protein MULTICOPY SUPPRESSOR OF IRA 1 (MSI1) [16,17].…”

“…The paternal allele of MEA and FIS2 is silenced. As a consequence, the maternal inheritance of a null allele, mea or fis2, cannot be rescued by a paternal wildtype MEA or FIS2 allele and is sufficient to cause maternal effects on endosperm development [11,14,15,17].…”

Mutations in the FIE and MEA Genes That Encode Interacting Polycomb Proteins Cause Parent-of-Origin Effects on Seed Development by Distinct Mechanisms

“…Loss-of-function mutations in the FERTILIZATION-INDE-PENDENT ENDOSPERM ( FIE ) gene allow diploid endosperm development in the absence of fertilization (Ohad et al, 1996Chaudhury et al, 1997). This phenotype suggests that the wild-type FIE gene functions to suppress endosperm development until fertilization occurs.…”

“…Alternatively, a gene dosage-dependent mechanism might underlie the parent-of-origin effect of fie on seed viability; that is, a single copy of the paternal wild-type FIE gene may not be able to compensate for the lack of gene activity associated with two maternal mutant fie alleles within the triploid endosperm. Finally, FIE could be an imprinted gene in which the maternal allele is expressed and the paternal allele is silenced.Plants with mutations in either the MEDEA ( MEA ) or FER-TILIZATION-INDEPENDENT SEED2 ( FIS2 ) gene display phenotypes similar to those of fie mutant plants (Chaudhury et al, 1997;Grossniklaus et al, 1998;Kiyosue et al, 1999). The MEA gene encodes a SET (Jenuwein et al, 1998) domain polycomb protein (Grossniklaus et al, 1998;Kiyosue et al, 1999;Luo et al, 1999), whereas FIS2 encodes a zinc finger protein (Luo et al, 1999).…”

“…FIE is related to polycomb proteins from Drosophila ( EXTRA SEX COMBS [ ESC ;Gutjahr et al, 1995;Sathe and Harte, 1995]), mammals ( EM-BRYONIC ECTODERM DEVELOPMENT [ EED ;Schumacher et al, 1996;Sewalt et al, 1998]), and Caenorhabditis ( MA-TERNAL EFFECT STERILE6 [ MES6 ; Korf et al, 1998]). Drosophila ESC and mammalian EED promote interactions with other polycomb proteins that repress gene transcription at specific sites within the genome (Wall et al, 1995;Sondek et al, 1996;Ng et al, 1997).After fertilization, mutations in the FIE gene cause parentof-origin effects on seed development (Ohad et al, 1996;Chaudhury et al, 1997). For example, when a heterozygous fie/FIE plant is pollinated with wild-type pollen, half of the seed inherit a maternal mutant fie allele and subsequently abort their development, even in the presence of the wildtype paternal allele.…”