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Maize Reproductive Development

Figure 1. Angiosperm life cycle.

The plant life cycle alternates between a diploid sporophyte and a haploid gametophyte generation (Figure 1). Specialized cells of the diploid sporophyte undergo meiosis to produce male or female spores. The sexually dimorphic spores divide to develop into haploid organisms, the gametophytes, which produce the gametes--the sperm and egg cells. In flowering plants, the female gametophyte, called the embryo sac, consists of four cell types: the egg cell, the central cell, the synergids, and the antipodal cells (Figure 2). The male gametophyte (the pollen grain) contains a vegetative cell, which performs the metabolic functions of the pollen grain and delivers the gametes to the embryo sac, and two sperm cells that fertilize the egg and central cell to produce the embryo and endosperm, respectively. The embryo develops into a new plant to continue the life cycle. The endosperm produces the nutritive material used by the embryo and seedling and comprises the bulk of the grain weight in cereals such as wheat, rice, and maize. The embryo and endosperm together make up the seed. The gametophytes of plants have active haploid genomes, in contrast to animals, where the products of meiosis are genetically inactive.

Figure 2. Male and female gametophyte development. a=antipodal cells; cc=central cell; pn=polar nuclei; e=egg cell; sy=synergid; end=endosperm; emb=embryo; vn= vegetative cell nucleus; v= vegetative cell; va=vacuole gc=generative cell; sp=sperm cells; pt=pollen tube. FG1-FG8=female gametophyte stage 1 to 8. FM=free microspore; VM=vacuolated microspore; BC=bicellular pollen; MP=mature pollen; GP=germinating pollen; dm=degenerated microspores.

Maize, like the majority of Angiosperm species, undergoes the Polygonum type of embryo sac development. Three of the spores produced by meiosis abort, and the chalazal most megaspore undergoes three rounds of free nuclear division to produce an eight nucleate syncytium. Cellularization produces a seven-celled embryo sac consisting of: two synergids, which attract the male gametophyte; the egg cell; the homodiploid central cell; and three antipodal cells (Figure 3). In maize the antipodal cells persist and continue to divide.

Figure 3. Maize embryo sac. Confocal image of cellularized embryo sac. The embryo sac is imbedded within the nucellus (n) of the ovule. Antipodal cells (a) have already divided a few times. The egg (e) and one of the synergids (s) are in the plane of focus. The synergid has not yet degenerated. The central cell has a large vacuole and two polar nuclei adjacent to the egg.

The male gametophyte undergoes one round of division to produce the vegetative cell and the generative cell. The generative cell undergoes a second round of division to produce the two sperm cells, which are both contained within the cytoplasm of the vegetative cell. Upon interaction between the pollen grain and the female stigma, the pollen grain germinates and the vegetative cell produces a pollen tube that grows through the style and is guided to a synergid of the embryo sac. Interaction with the synergid causes pollen tube rupture and release of the two sperm cells, which then fuse with the egg and central cell to achieve fertilization.

Fertilization of the embryo sac leads to rapid development of the embryo and endosperm. By four days after pollination (DAP) a small proembryo of approximately 24 cells has formed consisting of the large basal cells of the suspensor and the smaller apical cells of the embryo proper. By eight DAP the embryo has entered the transition stage with a long suspensor and a globular embryo proper. The scutellum (cotyledon) enlarges rapidly after the transition stage, and the coleoptile and shoot apical meristem (SAM) form on the face of the scutellum by approximately 10 DAP.

Maize endosperm development begins rapidly after fertilization in comparison to the embryo (Figure 4). It begins with a period of free nuclear divisions to produce a syncytium of over 250 nuclei located at the periphery of the cell. This period is followed by cellularization at approximately three to four DAP, when the embryo is still in the proembryo phase. The endosperm differentiates into at least four distinct domains: the aleurone, the basal endosperm transfer layer (BETL), the starchy endosperm, and the embryo surrounding region (ESR).

Figure 4. Endosperm development in maize. The growth of the embryo (gray) is shown for comparison. A syncytial phase is followed by cellularization. A phase of mitotic growth is followed by cycles of endoreduplication and eventually programmed cell death. Blue indicates cells that have undergone programmed cell death.

News

May

2019

How corn’s ancient ancestor swipes left on crossbreeding

By Carnegie HQ

Palo Alto, CA— Determining how one species becomes distinct from another has been a subject of fascination dating back to Charles Darwin. New research led by Carnegie’s Matthew Evans and published in Nature Communications elucidates the mechanism ...

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Matt Evans

Research Staff Scientist

Plant Biology

Carnegie Institution for Science

mevans@carnegiescience.edu
650-739-4283
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260 Panama Street
Stanford, CA 94305, US

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Yuanlin Duan (Visitor)
Yongxian Lu (Postdoctoral Fellow)
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Warman, C., Panda, K., Vejlupkova, Z., Hokin, S., Unger-Wallace, E., Cole, R. A., Chettoor A. M., Jiang D., Vollbrecht, E., Evans, M. M. S., Slotkin, R. K., Fowler, J. E. (2020) High expression in maize pollen correlates with genetic contributions to pollen fitness as well as with coordinated transcription from neighboring transposable elements. PLOS Genetics 16 (4):e1008462. http://doi.org/10.1371/journal.pgen.1008462

Phillips, A. R., Evans, M. M. S. (2020) Maternal regulation of seed growth and patterning in flowering plants. In: Marlow F (ed) (Academic Press, Cambridge, MA) Current Topics in Developmental Biology, Maternal Effect Genes in Development, vol 140: 257-282. https://doi.org/10.1016/bs.ctdb.2019.10.008

Lu, Y., Hokin, S. A., Kermicle, J. L. Hartwig, T., and Evans, M. M. S. (2019) A Pistil-Expressed Pectin Methylesterase Confers Cross-Incompatibility Between Strains of Zea mays. Nature Communications 10: 2304, http://doi.org/10.1038/s41467-019-10259-0 .

Han, L., Li, L., Muehlbauer, G. J., Fowler, J. E., and Evans, M. M. S. (2019) RNA isolation and analysis of lncRNAs from gametophytes of maize. In: Chekanova J.A. (ed) (Humana Press, New York, NY) Methods in Molecular Biology, Plant Long Non-Coding RNAs: Methods and Protocols, vol. 1933: 67-86. https://doi.org/10.1007/978-1-4939-9045-0_4

Chettoor, A. M., and Evans, M. M. S. (2017) Live-Cell Imaging of Auxin and Cytokinin Signaling in Maize Female Gametophytes. In: Schmidt A. (ed) (Humana Press, New York, NY) Methods in Molecular Biology, Plant Germline Development. vol. 1669: 95-101. http://DOI.org/10.1007/978-1-4939-7286-9_9 .

Vollbrecht, E. and Evans, M. M. S. (2017) Gametophyte interactions establishing kernel development. In, Maize Kernel Development, B. Larkins, (ed) (CAB International, Boston) pp. 16-27.

Chettoor, A. M., Phillips, A. R., Coker, C. T., Dilkes, B. and Evans, M. M. S. (2016) Maternal Gametophyte Effects on Seed Development in Maize. Genetics 204: 233-248. https://doi.org/10.1534/genetics.116.191833

Bai, F., M. Daliberti, A. Bagadion, M. Xu, Y. Li, Baier, J., Tseung, C. W., Evans, M. M.S., Settles, A. M., (2016) Parent-of-Origin-Effect rough endosperm Mutants in Maize. Genetics 204: 221-231. https://doi.org/10.1534/genetics.116.191775

Chettoor, A. M. and Evans, M. M. S. (2015). Correlation between a loss of auxin signaling and a loss of proliferation in maize antipodal cells. Frontiers in Plant Science 6: 187. https://doi.org/10.3389/fpls.2015.00187

Chettoor, A. M., Givan, S. A., Cole, R. A., Coker, C. T., Unger-Wallace, E., Vejlupkova, Z., Vollbrecht, E., Fowler, J. E. and Evans, M. M. S. (2014). Discovery of novel transcripts and gametophytic functions via RNA-seq analysis of maize gametophytic transcriptomes. Genome Biology 15, 414. https://doi.org/10.1186/s13059-014-0414-2

Lu, Y., Kermicle, J. L. and Evans, M. M. S. (2014) Genetic and cellular analysis of cross-incompatibility in Zea mays. Plant Reproduction 27: 19-29 https://doi.org/10.1007/s00497-013-0236-5 .

Li, L., Eichten, S. R., Shimizu, R., Petsch, K., Yeh, C. T., Wu, W., Chettoor, A. M., Givan, S. A., Cole, R. A., Fowler, J. E., Evans, M. M. S., Scanlon, M. J., Yu, J., Schnable, P. S., Timmermans, M. C., Springer, N. M., Muehlbauer, G. J. (2014). Genome-wide discovery and characterization of maize long non-coding RNAs. Genome Biol 15, R40. https://doi.org/10.1186/gb-2014-15-2-r40

Phillips, A. R., and Evans, M. M. S. (2011). Analysis of stunter1, a Maize Mutant with Reduced Gametophyte Size and Maternal Effects on Seed Development. Genetics 187: 1085-1097. https://doi.org/10.1534/genetics.110.125286

Kermicle, J. L. and Evans, M. M. S. (2010) The Zea mays Sexual Compatibility Gene ga2: Naturally-occurring Alleles, their Distribution and Role in Reproductive Isolation. J. Hered 101: 737-749. https://doi.org/10.1093/jhered/esq090

Evans, M. M. S. and Grossniklaus, U. (2009) The maize megagametophyte. In Handbook of Maize: its Biology, J.L. Bennetzen and S. Hake, eds (New York: Springer), pp. 79-104. https://DOI.org/10.1007/978-0-387-79418-1_5

Wenkel, S., Emery, J., Hou, B.-H., Evans, M. M. S., and Barton, M.K. (2007) A Feedback Regulatory Module Formed by LITTLE ZIPPER and HD-ZIPIII Genes. Plant Cell 19: 3379-3390. https://doi.org/10.1105/tpc.107.055772

Evans, M. M. S. (2007) The indeterminate gametophyte1 gene of maize encodes a LOB domain protein required for embryo sac and leaf development. Plant Cell 19: 46-62. https://doi.org/10.1105/tpc.106.047506

Gutierrez-Marcos, J. F., Costa, L. M., and Evans, M. M. S. (2006) Maternal gametophytic baseless1 is required for development of the central cell and early endosperm patterning in maize (Zea mays). Genetics 174: 317-329. https://doi.org/10.1534/genetics.106.059709

Kermicle, J. L., Taba, S., and Evans, M. M. S. (2006) The gametophyte-1 locus and reproductive isolation among Zea mays subspecies. Maydica, 51: 219-225.

Kermicle, J. L. and Evans, M. M. S. (2005) Pollen-pistil barriers to crossing in maize and teosinte result from incongruity rather than active rejection. Sex. Plant Reprod. 18: 187-194. http://doi.org/10.1007/s00497-005-0012-2

Walbot, V. and Evans, M. M. S. (2003) Unique features of the plant life cycle and their consequences. Nature Rev. Genet. 4: 369-379. https://doi.org/10.1038/nrg1064

Evans, M. M. S. and Kermicle, J. L. (2001) Interaction between maternal effect and zygotic effect mutations during maize seed development. Genetics 159: 303-315. https://www.genetics.org/content/159/1/303

Evans, M. M. S. and Kermicle, J. L. (2001) Teosinte crossing barrier1, a locus governing hybridization of teosinte with maize. Theor. Appl. Genet. 103: 259-265. https://doi.org/10.1007/s001220100549

Evans, M. M. S. and Barton, M. K. (1997) Genetics of angiosperm shoot apical meristem development. Ann. Rev. Plant Physiol. and Plant Mol. Biol. 48: 673-701. https://doi.org/10.1146/annurev.arplant.48.1.673

Evans, M. M. S. and Poethig, R. S. (1997) The viviparous8 mutation delays vegetative phase change and accelerates the rate of seedling growth in maize. Plant Journal 12: 769-779. https://doi.org/10.1046/j.1365-313X.1997.12040769.x

Bongard-Pierce, D. K., Evans, M. M. S., and Poethig, R. S. (1996) Heteroblastic features of leaf anatomy in maize and their genetic regulation. Int. J. Plant Sci. 157: 331-340.

Evans, M. M. S. and Poethig, R. S. (1995) Gibberellins promote vegetative phase change and reproductive maturity in maize. Plant Physiol. 108: 475-487. https://doi.org/10.1104/pp.108.2.475

Evans, M. M. S., Passas, H. J., and Poethig, R. S. (1994) Heterochronic effects of glossy15 mutations on epidermal cell identity in maize. Development 120: 1971-1981. https://dev.biologists.org/content/120/7/1971

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