The C2H2‐type zinc finger transcription factor OSIC1 positively regulates stomatal closure under osmotic stress in poplar

Summary Salt and drought impair plant osmotic homeostasis and greatly limit plant growth and development. Plants decrease stomatal aperture to reduce water loss and maintain osmotic homeostasis, leading to improved stress tolerance. Herein, we identified the C 2 H 2 transcription factor gene OSMOTIC STRESS INDUCED C 2 H 2 1 ( OSIC1 ) from Populus alba var. pyramidalis to be induced by salt, drought, polyethylene glycol 6000 (PEG6000) and abscisic acid (ABA). Overexpression of OSIC1 conferred transgenic poplar more tolerance to high salinity, drought and PEG6000 treatment by reducing stomatal aperture, while its mutant generated by the CRISPR/Cas9 system showed the opposite phenotype. Furthermore, OSIC1 directly up‐regulates PalCuAOζ in vitro and in vivo , encoding a copper‐containing polyamine oxidase, to enhance H 2 O 2 accumulation in guard cells and thus modulates stomatal closure when stresses occur. Additionally, ABA‐, drought‐ and salt‐induced PalMPK3 phosphorylates OSIC1 to increase its transcriptional activity to PalCuAOζ . This regulation of OSIC1 at the transcriptional and protein levels guarantees rapid stomatal closure when poplar responds to osmotic stress. Our results revealed a novel transcriptional regulatory mechanism of H 2 O 2 production in guard cells mediated by the OSIC1‐PalCuAOζ module. These findings deepen our understanding of how perennial woody plants, like poplar, respond to osmotic stress caused by salt and drought and provide potential targets for breeding.