Maternal Gametophytic baseless1 Is Required for Development of the Central Cell and Early Endosperm Patterning in Maize (Zea mays)

Abstract

In angiosperms, double fertilization of an egg cell and a central cell with two sperm cells results in the formation of a seed containing a diploid embryo and a triploid endosperm. The extent to which the embryo sac controls postfertilization events in the seed is unknown. The novel gametophytic maternal-effect maize mutation, baseless1 (bsl1) affects central cell development within the embryo sac, frequently by altering the position of the two polar nuclei. Despite this irregularity, fertilization is as efficient as in wild type. The spatial expression of basal endosperm-specific transcripts is altered in free-nuclear and cellular mutant endosperms. At later stages of seed development, bsl1 predominantly affects development of the basal endosperm transfer layer (BETL). When bsl1/1 diploid plants were pollinated by wild-type tetraploid plants, the BETL abnormalities observed in bsl1/bsl1/1/1 tetraploid endosperms were diverse and of variable severity. Moreover, the frequency of kernels with severely perturbed BETL development correlated with the percentage of severely affected bsl1 central cells. Therefore, BSL1 is likely required in the central cell before fertilization for correct BETL patterning to occur. These findings provide new genetic evidence that a maternal gametophytic component is necessary for correct endosperm patterning.

The Development of Endosperm in Grasses

“…Transfer cell fate specification apparently occurs during a narrow temporal window of syncytial development, as shown by the phenotype of the maize globby-1 mutant, which has an abnormal basal layer of transfer cells . Patterning events in the central cell of the maize embryo sac are also important for patterning of the transfer cell layer (Gutierrez-Marcos et al, 2006a). Three groups of maize genes are preferentially expressed in transfer cell layers, BETL, BAP, and EBE (Magnard et al, 2003).…”

Plant Somatic Embryogenesis: Some Useful Considerations

“…The single-celled zygote soon acquires the potential to develop into an embryo undergoing a series of complex cellular and morphological processes that finally produce the sporophyte or plant (Rao, 1996). Further information in these topics can be found in Russell (1992); Rotman et al (2003); Gutiérrez-Marcos et al (2006); He et al (2007) and Capron et al (2008), among many others.…”

Analysis of stunter1, a Maize Mutant with Reduced Gametophyte Size and Maternal Effects on Seed Development

“…Since some maternal effect mutants cause kernels to develop patches lacking aleurone (Gavazzi et al 1997) or BETL (Gutierrez-Marcos et al 2006), and some zygotic mutants ectopically express aleurone tissue (Costa et al 2003), we examined the expression of markers for these two tissues. For aleurone, we crossed stt1/1 females by males carrying Pro Vp1TGUS and stained for GUS activity in mutant and wild-type endosperms at 20 and 34 DAP (Figure 7) (Cao et al 2007).…”

“…Studies of these mutants and others have revealed several causes for maternal effects. Possible mechanisms include: changes in functional gene dosage in the endosperm (e.g., floury3; Ma and Nelson 1975), abnormal embryo sac morphology (e.g., baseless1; Gutierrez- Marcos et al 2006), loss of embryo sac proteins normally stored cytoplasmically to act after fertilization (perdurance) (e.g., PROLIFERA; Springer et al 2000), or imprinting (e.g., Polycomb group and other genes in maize and Arabidopsis (recently reviewed in Huh et al 2008;Johnson and Bender 2009;Jullien and Berger 2009). Known maternal gametophyte effect mutants are involved in basic cellular process, such as DNA demethylation, DNA repair, and cell cycle regulation, as well as seed specific processes such as the suppression of autonomous endosperm development (Golden et al 2002;Holding and Springer 2002;Ngo et al 2007;Andreuzza et al 2009).…”

“…Endosperms and embryos from developing kernels were analyzed from stt1/1 heterozygotes crossed as females by M14 or TB-5S-2L males, and ears were fixed at multiple DAP. Samples were processed according to Gutierrez-Marcos et al (2006) and visualized on a Leica SP5 (Wetzlar, Germany) laser scanning confocal microscope. Excitation was performed at 405 nm, 488 nm, and 561 nm and emission was collected at 410-480 nm, 495-555 nm, and 565-730 nm for the merged images.…”