The development of what food resource began to revolutionize Native American societies around 1200BC?

Maize, or corn, is a staple part of the diets of modern Native Americans, as well as in Mexican and other central and South American societies. The systematic cultivation of this plant was one of the most important achievements of very early Native American societies, as it led to their being able to sustain themselves in various climates throughout the year. Maize was originally a sort of grass, but by careful selection, early Native Americans were able to encourage maize plants to grow 'cobs', or clusters of kernels. Over a period of several millennia, maize grew to the size we now know it to be.
One of the most interesting Native American applications of corn was a development of New England maize specifically. Around a thousand years ago, Native Americans living in New England were able to transform maize into a plant that could grow ears of corn from multiple joints of the stalk, and could also mature during the shorter new England season. This corn was therefore suitable for planting in individuals' homes and gardens. People were able to harvest large amounts of surplus corn to see them through the winter. They later passed on their knowledge of this plant to European colonists.
http://www.nativetech.org/cornhusk/cornhusk.html