Regulation of hair cell and stomatal size by a hair cell-specific peroxidase in the grass Brachypodium distachyon

Authors: NUNES, TIAGO - Heidelberg University; BERG, LEA - Bern University Of Applied Sciences; SLAWINSKA, MAGDALENA - Heidelberg University; ZHANG, DAN - Heidelberg University; REDT, LEONIE - Heidelberg University; SIBOUT, RICHARD - Inrae; VOGEL, JOHN - Department Of Energy Joint Genome; L Chingcuanco, Debbie; JESENOFSKY, BARBARA - Heidelberg University; LINDNER, HEIKE - Heidelberg University; RAISSIG, MICHEAL - Heidelberg University

Submitted to: Current Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2023
Publication Date: 4/21/2023
Citation: Nunes, T.D., Berg, L.S., Slawinska, M.W., Zhang, D., Redt, L., Sibout, R., Vogel, J.P., Chingcuanco, D.L., Jesenofsky, B., Lindner, H., Raissig, M.T. 2023. Regulation of hair cell and stomatal size by a hair cell-specific peroxidase in the grass Brachypodium distachyon. Current Biology. 33(9):1844-1854. https://doi.org/10.1016/j.cub.2023.03.089.
DOI: https://doi.org/10.1016/j.cub.2023.03.089

Interpretive Summary: The spatial organization and functionality of leaf epidermal cells play fundamental roles in plant productivity and in biotic and abiotic stress tolerance. In the model grass B. distachyon, the leaf blade epidermis is dominated by rectangular pavement cells, stomatal complexes (SC), trichomes consisting of prickle hair cells and few interspersed macrohairs. The stomatal complexes, cellular epidermal pores that drive plant evapotranspiration, enable carbon dioxide uptake for photosynthesis and limit excessive water loss. To identify novel factors associated with grass stomatal morphology and functionality, we performed RNA-sequencing of mature zones of 7-day old leaves in wild-type B. distachyon Bd21-3 and bdmute mutant plants. The bdmute leaf epidermis features abnormal stomata that lack SCs which strongly affects stomatal responsiveness and gas exchange. About 50 candidate genes out of 179 genes downregulated in bdmute were selected for a reverse genetic screening. Candidates were chosen according to their annotated gene function, and lower expression in the developmental zone and the availability of mutants from a collection of sodium azide (NaN3) mutagenized and fully resequenced lines. Two mutants of the class III peroxidase BdPRX76/BdPOX were found which showed lower water-use efficiency and higher ambient-adapted stomatal conductance. The molecular and physiological characterization of these two genes is reported in this paper.

Technical Abstract: The leaf epidermis is the outermost cell layer forming the interface between plants and the atmosphere that must accomplish contrasting functions; it must provide a robust barrier against biotic and abiotic stressors while simultaneously facilitating efficient exchange of carbon dioxide and water. To achieve these opposing requirements, the plant epidermis developed a wide range of specialized cell types such as stomata and hair cells (=trichomes). While factors forming these individual cell types are known, it is poorly understood how their number and size is coordinated. Here, we identified a role for BdPRX76/BdPOX, a class III peroxidase, in regulating hair cell and stomatal size in the model grass Brachypodium distachyon. In bdpox mutants prickle hair cells were smaller and stomata were longer. Because stomatal density remained unchanged, the negative correlation between stomatal size and density was disrupted in bdpox and resulted in higher stomatal conductance and lower intrinsic water-use efficiency. Reporter lines revealed that BdPOX was exclusively expressed in hair cells suggesting that BdPOX cell-autonomously promotes hair cell size and only indirectly restricts stomatal length. Cell wall autofluorescence and lignin stainings indicated a role for BdPOX in lignification or crosslinking of related phenolic compounds at the hair cell base. Ectopic expression of BdPOX in the stomatal lineage increased phenolic autofluorescence in guard cell walls and restricted stomatal elongation in bdpox. Together, we highlight a developmental interplay between hair cells and stomata that optimizes epidermal functionality. We propose that cell-type-specific changes disrupt this interplay and lead to compensatory developmental defects in other epidermal cell types.