"We cannot sit back and watch plant species disappear -- plants are the basis of all life on Earth, providing clean air, water, food and fuel. All animal and bird life depends on them, and so do we," Hopper added.
The study comes ahead of a meeting in Nagoya, Japan, from October 18 to 29, where members of the UN's Biodiversity Convention will set new targets to save endangered wildlife.
Craig Hilton-Taylor of the IUCN said he hoped the Nagoya meeting would set the goal of preventing the extinction of any known threatened species by 2020.
"We want to make sure that plants will not be forgotten," he said.
In their study, researchers assessed almost 4,000 species, of which 22 percent were classed as threatened, especially in tropical rain forest.
Plants were more threatened than birds, as threatened as mammals and less threatened than amphibians or corals, it said. Gymnosperms, the plant group including fir trees, were the most threatened.
The greatest peril came from man-induced habitat loss, mostly the conversion of natural habitats for crops or livestock. Human activity accounted for 81 percent of threats, said Kew researcher Neil Brummitt.