Biologists have revealed how plants suppress the formation of stomata, the microscopic pores on their surface, to limit water loss during drought conditions.

Stomata, which are present on the surface of leaves, are important for the gas exchange between plants and the atmosphere. However, stomata are also the major sites of water loss, where water escapes from within the leaves in the form of water vapour. When there is a water shortage, plants execute two stomatal responses to conserve water: they close the pore of existing stomata and limit the formation of new ones. These responses, which are mediated by the phytohormone abscisic acid (ABA), are critical for plant adaptation to drought and are particularly relevant for sustaining agriculture in the changing climate. How ABA inhibits stomatal production, however, was not known.

In the latest issue of Science Advances, the research team led by Assistant Professor LAU On Sun from the Department of Biological Sciences, National University of Singapore reported the mechanism behind this drought-mediated suppression of stomatal development. They found that the core kinases of the ABA signaling, which are activated under drought, directly phosphorylate the master stomatal regulator SPEECHLESS (SPCH). Phosphorylation refers to the addition of a phosphate group to a molecule. This ABA-induced phosphorylation on the SPCH protein occurs at two distinct sites and it triggers the degradation of SPCH. Since SPCH promotes stomatal development, the phosphorylation of SPCH led to reduced SPCH levels (absence of the yellow signals in the S240D sample in Figure A) and stomatal number, and eliminating these sites (S240/271A sample) resulted in a reversed response. Importantly, the researchers also demonstrated that plants with the altered SPCH phosphorylation sites exhibited differential tolerance to drought (see Figure B).

Image: (A) Plants expressing different versions of SPEECHLESS (SPCH) with standard and altered “phosphocodes”, leading to varying SPCH levels and stomatal number. (B) Altering the SPCH phosphocode resulted in plants with improved or reduced tolerance to drought stress. Credit: Modified from figures in Yang et al., Science Advances 2022

Prof Lau said, “Our findings show that a specific “code” underlies a key water conservation response in plants. This is exciting because it suggests that by manipulating the code, we could fine tune the drought tolerance of crop plants, and help optimse them for diverse growing conditions, from traditional to urban farming.”

Read the paper: Science Advances

Article source: National University of Singapore

Image: Alena Kravchenko/Wikimedia