Researchers at the University of Massachusetts Amherst have discovered that the feature helping weedy rice varieties spread is still present in nearly all cultivated rice, despite 10,000 years of human farming.
The study, published in the Journal of Experimental Botany, 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.
Weedy rice is a type of rice that acts like a pest in cultivated fields.
This persistent problem suggests that the way humans farm rice inadvertently promotes weedy behavior. This is evident as cultivated rice often reverts to its wild, weedy form.
“For years, we thought we understood rice domestication,” said Ana Caicedo, a professor at UMass Amherst and the study’s senior author. “But our detailed study of different rice varieties revealed that the story is more complex.”
Before humans started 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 millennia, humans have selected rice varieties that hold 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.
“The abscission zone is crucial for understanding shattering in rice, but it hasn’t been thoroughly studied,” said Xiang Li, the paper’s co-lead author. “We needed to examine more rice varieties to understand this tissue better.”
Li, Caicedo, and their team collected microscope images of 86 rice samples, including major cultivated groups and weedy rice from various locations, such as 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.
“We could see differences in the abscission zones visually, but we needed to quantify these observations,” explained co-lead author Daniel Lowey. The team developed three new measures to numerically quantify the abscission zones.
They discovered that the length of the abscission zone, not just its presence, best predicted which varieties shatter easily. While cultivated groups varied in abscission zone length, 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.”
Despite 10,000 years of human efforts, weedy rice continues to thrive due to this feature. 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 just look at a rice grain to tell if it was cultivated or if it shattered,” Caicedo said. “Most rice grains have some degree of abscission-zone formation.”
Li added, “Our research sets the stage for examining the genetics behind abscission zones in rice. Understanding these genes will reveal when and how these changes occurred, shaping the history of rice domestication and de-domestication.”
