CO2 response screen in grass Brachypodium reveals the key role of a MAP kinase in CO2-triggered stomatal closure

Authors: LOPEZ, BRYN - University Of California, San Diego; CECILIATO, PAULO - University Of California, San Diego; TAKAHASHI, YOHEI - University Of California, San Diego; RANGEL, FELIPE - University Of California, San Diego; SALEM, EVANA - University Of California, San Diego; KERNIG, KLARA - University Of California, San Diego; CHOW, KELLY - University Of California, San Diego; ZHANG, LI - University Of California, San Diego; SIDHOM, MORGANA - University Of California, San Diego; SEITZ, CHRISTIAN - University Of California, San Diego; ZHENG, TINGWEN - University Of California, San Diego; SIBOUT, RICHARD - Inrae; L Chingcuanco, Debbie; WOODS, DANIEL - University Of California, San Diego; MCCAMMON, ANDREW - University Of California, San Diego; VOGEL, JOHN - Joint Genome Institute; SCHROEDER, JULIAN - University Of California, San Diego

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2023
Publication Date: 5/6/2024
Citation: Lopez, B., Ceciliato, P., Takahashi, Y., Rangel, F., Salem, E., Kernig, K., Chow, K., Zhang, L., Sidhom, M., Seitz, C., Zheng, T., Sibout, R., Chingcuanco, D.L., Woods, D., McCammon, A., Vogel, J., Schroeder, J. 2024. CO2 response screen in grass Brachypodium reveals the key role of a MAP kinase in CO2-triggered stomatal closure. Plant Physiology. Article kiae262. https://doi.org/10.1093/plphys/kiae262.
DOI: https://doi.org/10.1093/plphys/kiae262

Interpretive Summary: Global warming will have a major negative impact on agricultural food production. The predicted reduction in water availability and increased drought frequency is expected to reduce the production of food crops including wheat, maize, and rice. Development of crop varieties with improved yield with less water input is, therefore, a research priority. Water used by plants for growth is mainly lost to the environment by transpiration through the leaf stomatal pores. Stomatal pores adjust in response to environmental conditions to regulate CO2 uptake and water loss. CO2 regulated stomatal closure can increase water use efficiency (WUE) in grasses by reducing water loss through transpiration. In this paper, the authors report the use of forward genetic screen for stomatal responses to increased CO2 in grasses. A forward genetics CO2 response screen in Brachypodium distachyon led to the isolation of stomatal CO2 response mutant, chill1, with a strongly impaired stomatal CO2 responses but robust abscisic acid-induced stomatal closure. The chill1 mutant was mapped to the BdMPK5 gene. Generation, and phenotyping of BdMPK5 CRISPR-cas9 alleles and in-vitro reconstitution of the CO2 sensing core with BdMPK5 suggest that chill1 encodes a component of the CO2 sensor in grasses. The characterization of BdMPK5 role in stomatal physiology and function will provide a powerful system for further dissection of stomatal CO2 sensing and signaling mechanisms in grasses.

Technical Abstract: Plants respond to increased CO2 concentrations through rapid stomatal closure which can contribute to increased water use efficiency. Grasses display faster stomatal responses than eudicots due to dumbbell-shaped guard cells flanked by subsidiary cells working in opposition. However, forward genetic screening for stomatal CO2 signal transduction mutants in grasses has not been reported. The grass model Brachypodium distachyon is closely related to agronomically important cereal crops, sharing largely collinear genomes. To gain insights into CO2 control mechanisms of stomatal movements in grasses, we developed a forward genetics screen with an EMS-mutagenized Brachypodium distachyon M5 generation population using infrared imaging to identify plants with altered canopy leaf temperature at elevated CO2. Among isolated mutants, a “chill1” mutant exhibited consistently cooler leaf temperatures than wildtype Bd21-3 parent control plants after exposure to increased [CO2]. chill1 plants showed strongly impaired high CO2-induced stomatal closure, despite retaining a robust abscisic acid-induced stomatal closing response. Through bulked segregant whole-genome-sequencing analyses followed by analyses of further backcrossed F4 generation plants and generation and characterization of CRISPR-cas9 mutants, chill1 was mapped to a protein kinase, BdMPK5. The chill1 mutation impaired BdMPK5 protein-mediated CO2/HCO3- sensing in vitro. Furthermore, AlphaFold2-directed structural modeling suggests that the identified BdMPK5-D90N chill1 mutant residue is located at the interface with the HT1 Raf-like kinase. BdMPK5 is a key signaling component involved in CO2-induced stomatal movements, potentially functioning as a component of the CO2 sensor in grasses.