CSB plant scientists have identified the finely tuned way plants adjust to high humidity conditions. These results provide important insights, as high humidity conditions will be experienced more often by Canadian crops due to increased rainfall and flooding. Professor Eiji Nambara led this research as part of his studies on the ways plants integrate signals of stress in their environment, like drought, pathogens or cold.
Nambara is originally from East Asia, where excess water environments occur frequently in a climate heavily influenced by the ocean. Rice is anatomically shaped to handle excess water such as floods, but wheat crops or the model plant Arabidopsis are adapted to a more temperate climate.
Research by Saad Hussain in Nambara’s lab showed that exposing Arabidopsis to high humidity resulted in catabolization of the plant hormone ABA by the ABA-8’hydroxylase enzyme CYP707A3.
He traced the pathway from early high humidity signaling to the decrease of ABA, which facilitates transpiration. Their bioinformatic analysis found that proteins associated with calcium ion signaling were initially activated.
Focusing on calcium, they made an unexpected discovery. When they looked at their plants to see how quickly they responded to high humidity, they saw a glowing calcium signal within a few minutes!
This rapid signal appeared around the periphery of the leaf, near regions called hydathodes. The hydathode is a pore that opens as a relief valve to drain condensed water from the leaf, which helps the plant control water levels in high humidity.
A potential drawback of this response is that bacteria thrive in a humid environment and can infect plants by invading the leaf through the open pore.
Plants have evolved to manage these tradeoffs. Hussain’s results show that high humidity triggering calcium influx into the cytosol goes on to activate ABA catabolism, which in turn induces plant immunity genes. So even though stomatal pores open to increase evaporation under high humidity, the plant is protected from infection.
The plant’s highly tuned regulatory pathways mean that cytosolic calcium concentration can induce opposite responses using same regulatory modules, depending on the input signals. ABA plays a role in cold response, but calcium channels responsible for high humidity responses were activated selectively while cold-responsive calcium channels were unaffected.
This insightful research addressing potential responses to the effects of climate change has been published by the journal PNAS as “Calcium signaling triggers early high humidity responses in Arabidopsis thaliana”.