Corn endosperm significance

​All flowering plants utilize the endosperm as storage of starch as an energy source for seed germination.  Grains used for human food have been chosen for having larger endosperms than simply needed for germination energy.  The variety of teosinte (Zea mayssubspeciesparviglumis) known as Balsas teosinte, believed to be the source of human domestication of maize (Zea mays subspecies mays).  Mutations in the Balsa teosinte resulted in absence of the hard encasement around the kernel and absence of abscission layers leading to early release of the kernels from the central rachis (cob).  Occurrence and acknowledgment of these mutations occurred to humans about 9000 years ago in the Tehuacan Valley, south of the current Mexico City.  Fortunately, this occurrence coincided with the migration of people from northern Asia across the Bering Strait into North America, spreading south through Mexico to South America.
 
The attraction of these primitive Zea maysseeds as a food source and ease of transportation must have allowed for distribution from that original source to Peru by 6500 years ago and to the eastern base of the Amazon River by 4030 years ago.  Recently published research indicates that much development into current corn occurred near the Andes (Kistler et al., Science 362, 1309–1313 (2018).

Human’s interested centered on the endosperm of the corn seed.  The fact that this species has at least one generation per year, and that it was cultivated by multiple growers who essentially were corn breeders, selecting for endosperm size and characters.  These people did not need to study genetics to realize that if they chose kernels with the most desirable characters in the endosperm to plant, they could increase kernels with those characters.  Essentially, there were thousands of corn breeders in a huge number of environments selecting for corn genetics favoring their desired corn endosperm characters.  Some preferred certain starch components that could be ground into flour for culture desirable foods.  Others chose strong carotene (yellow) colors, still desired because it results in dark yellow egg yolks when fed to chickens.  Others preferred the blue or red anthocyanin colors in the aleurone layer of the endosperm.

Selection for desirable endosperm in multiple environments of South and North Americas also allowed for diversity of genetics for other plant characteristics including root growth, time to flowering, leaf size and structure.  Multiple internal characteristics affecting photosynthesis, disease resistance and transportation of sugars to the endosperm were also affected by the selection for desirable endosperm by these many corn breeders, across many environments for thousands of years.
An interesting read on the internet on the teosinte and maize relationship can be found at
https://bsapubs.onlinelibrary.wiley.com/doi/full/10.3732/ajb.0900059