Academic journal article Genetics

Maternal Gametophyte Effects on Seed Development in Maize

Academic journal article Genetics

Maternal Gametophyte Effects on Seed Development in Maize

Article excerpt

THE process of double fertilization is unique to fl owering plants and results in the formation of the seed. The two sperm cells of the pollen grain fertilize the egg and central cell of the female gametophyte, or embryo sac, to form the diploid (1 maternal:1 paternal) embryo and typically triploid (2 maternal:1 paternal) endosperm, respectively (Sheridan and Clark 1994; Walbot and Evans 2003). The endosperm is thus a genetic sister of the embryo and is functionally equivalent to the mammalian placenta, acting as a nutritive tissue that supports the growth of the developing embryo and seedling. The maize endosperm consists of several morphologically and transcriptionally distinct domains: the aleurone, the basal endosperm transfer layer (BETL), the starchy endosperm, the conducting zone (CZ), the basal intermediate zone (BIZ), and the embryo-surrounding region (Olsen et al. 1999; Olsen 2004; Leroux et al. 2014; Li et al. 2014).

Haploid gene expression and patterning of the female gametophyte prior to fertilization can significantly affect the development of both the endosperm and embryo (Drews et al. 1998; Walbot and Evans 2003; Marton et al. 2005; Vernoud et al. 2005). The maize embryo sac is produced from a single megaspore by three rounds of free nuclear divisions generating an eight-nucleate syncytium which then cellularizes to produce seven cells of four types (Evans and Grossniklaus 2009): the egg cell, two synergids, the central cell, and three antipodal cells. Division of the antipodal cells, associated with auxin signaling, produces a cluster of 20-100 antipodal cells in maize (Chettoor and Evans 2015). The function of antipodal cells is undetermined but they are hypothesized to act as a transfer tissue based on the presence of cell wall invaginations on the surfaces facing the maternal nucellus (Diboll 1968). Alternatively, they could act as a signaling center by providing positional information for the embryo sac, or even for the endosperm since they persist in the maize seed after fertilization (Weatherwax 1926; Randolph 1936).

Two types of mutants with maternal effects on seed development can be distinguished based on their mode of inheritance: those in genes required in the maternal sporophyte (Li and Berger 2012; Li and Li 2015) and those in genes required in the maternal female gametophyte (Luo et al. 2014). They can be distinguished from each other by the mode of transmission (Grossniklaus and Schneitz 1998; Evans and Kermicle 2001). Recessive maternal sporophyte effect mutants will only have consequences if parent plants are homozygous. Both maternal gametophyte effect mutants and dominant maternal sporophyte effect mutants produce abnormal seeds when heterozygotes are crossed as females; but in the case of gametophyte mutants the abnormal seeds inherit the mutant allele because the embryo sac must carry the mutation to cause an effect, while the allele present in the embryo sac (and hence the seed) is irrelevant in the case of maternal sporophyte effects. Consequently, in the case of dominant maternal sporophyte effects, wild-type and mutant alleles are equally represented in both abnormal and normal seeds.

Gametophytic maternal-effect mutants have been identified in both Arabidopsis and maize (Gavazzi et al. 1997; Evans and Kermicle 2001; Grini et al. 2002; Olsen 2004; Köhler and Grossniklaus 2005; Pagnussat et al. 2005; Gutierrez-Marcos et al. 2006b; Pien and Grossniklaus 2007; Phillips and Evans 2011). Although not affecting postfertilization seed development when transmitted through the pollen, many gametophytic maternal-effect mutations in Arabidopsis (Pagnussat et al. 2005; Boavida et al. 2009) and maize (Evans and Kermicle 2001; Gutierrez-Marcos et al. 2006b; Phillips and Evans 2011) have reduced male transmission, indicating a separate role for the gene in pollen development/function.

Studies of these mutants have revealed several causes for maternal effects, as identified through genetic and cellular analysis. …

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