Academic journal article Genetics

Genetic and Genomic Toolbox of Zea Mays

Academic journal article Genetics

Genetic and Genomic Toolbox of Zea Mays

Article excerpt

Working with Maize

THE term maize is often used synonymously with corn, particularly in the United States and in reference to its agricultural use. While both terms are correct, maize is a name that refers uniquely to this plant. Maize is a large grain plant that evolved from its wild-grass ancestors by the direct intervention of human agriculture. Many varieties or "races" differ in physical properties (Goodman and Brown 1988), but generally maize is a single-stalk plant that grows to approximately 8 feet tall with about 20 long, narrow leaves growing individually from nodes along the stalk (Figure 1A) (Kiesselbach 1999). Several characteristics make it an attractive genetic system (Strable and Scanlon 2009). It is easy to culture on any scale, from a few plants in pots to many acres (Figure 1B). It can be grown successfully year round in greenhouses and growth chambers with proper lighting; it is also quite hardy and can be grown outdoors under a range of conditions, from tropical to temperate climates (Shaw 1988). Maize is a naturally outcrossing species, which makes its genetic architecture (diversity, linkage, recombination, etc.) more similar to other outcrossing organisms such as humans rather than self-pollinating plants (Rafalski and Morgante 2004; Wallace et al. 2013). While its genetics are similar to humans, maize retains the major strength of plant genetics: the ability to self-cross and quickly produce homozygotes or F2 populations.

The male and female reproductive organs are accessible and separable, making controlled crosses easy to perform. The male germ cells are produced in the tassel found at the top of the plant (Figure 1C). Tassels contain anthers that open upon maturation, releasing up to 107 wind-dispersed pollen grains (Coe et al. 1988). The female germ cells are located in one or more ears, which grow from the base of leaves in the midsection of the plant (Figure 1, A and D). An ear generally contains several hundred egg cells that will develop into kernels after fertilization (Neuffer et al. 1997). Each young kernel contains a silk, an elongated stigma, which emerges out of the husk leaves of the ear (Figure 1D). Pollen grains land on the silk and produce a pollen tube that grows down through the length of the silk, ultimately delivering two sperm to the female gametophyte (Figure 2A). Double fertilization occurs as one sperm fertilizes the egg to create the embryo (2n), and the other sperm fertilizes the central cell with two polar nuclei that gives rise to the aleurone and the starchy endosperm (3n). The aleurone and endosperm nourish the young embryo during germination (Figure 2, D and E, and Figure 4A) (Dresselhaus et al. 2011).

Controlled crosses are made by placing a bag over the tassel and shaking gently to collect the pollen, which is then sprinkled onto silks (Neuffer 1994a; Neuffer 1994b). The Copyright ears are covered before silks emerge to prevent contamination (Figure 1E). Crosses can be made over a large time window, as a normal tassel will produce new pollen for up to 7 days and silks can be receptive to fertilization for 10 days. Ear development can also be hastened by removing tassels, and pollen shedding can be delayed by cool temperatures or accelerated by warm temperatures (Coe et al. 1988). Although pollen is short lived, it can be kept viable by refrigeration for 24 hr and by liquid nitrogen freezing for up to 1 year (Barnabas and Rajki 1976, 1981). The physiology of maize makes it an excellent system for large-scale progeny screens. The vast amounts of pollen produced by a single plant allows many crosses to be performed; one tassel can produce enough pollen to fertilize 50+ ears in a single day. Experienced maize researchers can perform 300-500 crosses in a day, and with each cross yielding several hundred seeds, maize can quickly generate large numbers of offspring for genetic analysis (Neuffer 1994a). Progeny output can also be amplified many fold by growing plants in isolated plots and removing tassels from female parents with alternating rows of the male parent. …

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