Maize tissue culture and transformation: The first 20 years

Maize is one of the world's three most widely cultivated crops (along with wheat and rice) and is arguably the most economically important cereal crop on a worldwide basis. That status is only likely to become more apparent in the next decade. Demand for maize is projected to increase by 50% to over 800 million tons per year by the year 2020 and will surpass both rice and wheat in global demand (Pingali and Pandey 2001). Biotechnology is expected to play an increasingly important role in maize genetic improvement to meet this expanding demand. The genetic marvel that is modern maize belies its humble origins. While still a topic of some debate, the preponderance of evidence clearly suggests that modern maize (Zea mays L.) was domesticated from teosinte (Zea mays ssp. parviglumis) approximately 9000 years ago (Doebley 1990; Doebley and Stec 1991; Matsuoka et al. 2002). Teosinte is an innocuous annual plant with a two-rowed spikelet that produces 6–12 hard cupule-encased kernels, and a growth habit consisting of many, long, lateral branches that are topped by a male inflorescence. Maize, on the other hand, has one central stalk which terminates in a male inflorescence, the tassel, and several modified short axillary shoots that terminate in the female inflorescence, or ear. Unlike the simple teosinte inflorescence, modern maize is capable of producing a large multi-rowed ear that typically produces hundreds of kernels. So different is teosinte from maize that the evolutionary relationship between the two is not readily apparent and is still disputed. This remarkable and rapid transformation of teosinte into domesticated maize was certainly facilitated by Native Americans. Their skillful selection of the useful traits from the existing variation within teosinte created the basis for maize to become the centerpiece of a major agricultural revolution, a revolution that continues today.