University of Massachusetts Amherst Researchers Discover Humans Inadvertently Help Weedy Rice to Thrive
Weedy rice is a type of rice that acts like a pest in cultivated fields. Researchers at the University of Massachusetts Amherst have discovered that despite 10,000 years of human farming, the anatomical adaptation that helps weedy rice varieties to proliferate is not, as previously believed, confined to pest varieties.
Published in the Journal of Experimental Botany, the research study shows that a type of cell tissue allowing rice plants to drop their seeds called the “abscission zone,” remains in most cultivated rice varieties, although to varying degrees.
The continued proliferation of weedy rice suggests that the way humans cultivate rice inadvertently also selects for weedy behavior. This is evident as cultivated rice often reverts to its wild, weedy form.
Ana Caicedo, a professor at UMass Amherst and the study’s senior author stated, “For the past few decades, we biologists have been telling ourselves a story about how rice domestication occurred. But when we started looking very closely at a wide array of different rice varieties, it turns out that old story is far too simplistic and obscures what is actually happening.”
Before humans began cultivating rice, its wild ancestor developed a way to “shatter” or easily drop its seeds which helped it spread quickly. This was due to the formation of the abscission zone, a special tissue at the base of each rice grain. While shattering helps wild plants spread, it is a disadvantage for harvesting, as seeds are lost before they can be collected.
Over many millennia, humans have selected different cultivated rice varieties, each of which has its own domestication history that holds onto their grains better. It was believed that the abscission zone had been mostly bred out of cultivated rice.
However, cultivated rice has evolved into weedy varieties that still shatter and are so successful that they limit cultivated rice production worldwide. These weedy rice varieties have evolved independently from different cultivated varieties.
Xiang Li, the paper’s co-lead author who completed this research as part of his graduate studies at UMass Amherst stated, “The abscission zone has long been recognized as a critical factor influencing shattering in rice, but it has not been thoroughly examined. We needed to look at more rice individuals to uncover the pattern of this specialized tissue in different rice varieties. This anatomic investigation will improve our understanding of rice evolution and lay the foundation for further examination of underlying genetics.”
Li, Caicedo, and their team, as well as co-lead author Daniel Lowey who was an undergraduate at UMass Amherst, collected microscopy images of 86 samples from both the five major cultivated rice groups as well as from de-domesticated weedy rice from multiple locations, including the Iberian Peninsula, South Asia, Northeast Asia and the U.S.
They found that while one group of cultivated rice, temperate japonica, has almost lost its abscission zone, most other cultivated groups still have it to some extent.
However, there were noticeable differences in these zones. “Visually, we could see clear differences between the abscission zones of the rice we examined, but we could not rely on perception alone. To put numbers to our observations, we developed a set of three novel measures to numerically quantify aspects of the abscission zones we were most interested in,” explained Lowey.
The team also discovered that the length of the abscission zone, not just its presence, best predicted which varieties shatter easily. All cultivated groups varied in abscission zone length but all weedy varieties converged on a longer zone.
“This finding shows that weedy rice consistently evolves to have a long abscission zone, aiding its spread,” Caicedo said. “Conversely, domesticated rice varieties have modified their zones differently to reduce shattering.”
Weedy rice continues to thrive due to this feature, despite 10,000 years of human cultivation. This also means that understanding the long history of humans and rice needs updating. This challenges previous beliefs that cultivated rice had lost its abscission zone and that the roughness at the base of a rice grain could indicate whether it had shattered.
“You can’t simply look at the surface of a rice grain, either one from today or an archaeological sample dating back thousands of years, and tell whether or not it was cultivated or if it shattered because almost all of them have some degree of abscission-zone formation,” Caicedo said.
Li added, “An exciting aspect of our research is that it lays the foundation for examination of underlying genetics. We now know we need to figure out the genes that control the length of the abscission zones in different rice groups. Then we’ll be able to understand when and how all of these changes in the abscission zone arose, and how they have shaped the story of rice domestication and de-domestication.”
