Human hormones don't arouse plants' desires. But plant hormones can be stimulating to humans—or at least to their immune cells—based on new work from Santina Bruzzone, Elena Zocchi (University of Genova, Genoa, Italy), and colleagues. The authors identify the plant hormone abscisic acid (ABA) as a human cytokine.
In plants, ABA triggers stress responses such as seed dormancy and stomatal closing. Zocchi previously found that very simple animals such as sponges also use ABA-driven pathways to respond to light and heat. She now finds that ABA's reach extends to mammals.
For humans, the first cells to be exposed to environmental stresses are often immune cells. The group's results show that phagocytosing immune cells called granulocytes synthesize ABA in response to high temperature, like that of a fever.
The ABA calls in more granulocytes, and possibly other immune cell types, by activating chemokinesis. It also stimulates phagocytosis and the production of reactive oxygen species (which help kill pathogens) and nitric oxide (another cytokine).
As in plants, the biochemical pathway that activates granulocyte ABA responses induced intracellular calcium increases via cyclic ADP–ribose. “The capacity to respond to environmental stimuli through biochemical events is really at the heart of life,” says Zocchi. It's no surprise then that it has been so highly conserved.