Abstract
The stomatal pores are located on the plant leaf epidermis and regulate CO2 uptake for photosynthesis and the loss of water by transpiration. Their stomatal aperture therefore affects photosynthesis, water use efficiency, and agricultural crop yields. Blue light, one of the environmental signals that regulates the plant stomatal aperture, is perceived by the blue/UV-A light-absorbing cryptochromes and phototropins. The signal transduction cascades that link the perception of light to the stomatal opening response are still largely unknown. Here, we report two new players, Hypersensitive to Red and Blue 1 (HRB1) and Protein Phosphatase 7 (PP7), and their genetic and biochemical interactions in the control of stomatal aperture. Mutations in either HRB1 or PP7 lead to the misregulation of the stomatal aperture and reduce water loss under blue light. Both HRB1 and PP7 are expressed in the guard cells in response to a light-to-dark or dark-to-light transition. HRB1 interacts with PP7 through its N-terminal ZZ-type zinc finger motif and requires a functional PP7 for its stomatal opening response. HRB1 is phosphorylated in vivo, and PP7 can dephosphorylate HRB1. HRB1 is mostly dephosphorylated in a protein complex of 193 kDa in the dark, and blue light increases complex size to 285 kDa. In the pp7 mutant, this size shift is impaired, and HRB1 is predominately phosphorylated. We propose that a modification of HRB1 by PP7 under blue light is essential to acquire a proper conformation or to bring in new components for the assembly of a functional HRB1 protein complex. Guard cells control stomatal opening in response to multiple environmental or biotic stimuli. This study may furnish strategies that allow plants to enjoy the advantages of both constitutive and ABA-induced protection under water-limiting conditions.
Highlights
Phytochromes are photo-reversible red and far-red light receptors with five members in Arabidopsis, phyA to phyE [1,2]
Stomatal apertures vary over diurnal cycles, and stomata tend to be open during the day in response to blue light and tend to be closed at night
The blue/UV-A lightabsorbing cryptochromes and phototropins are the receptors for the blue light response
Summary
Phytochromes (phy) are photo-reversible red and far-red light receptors with five members in Arabidopsis, phyA to phyE [1,2]. The major red and far-red light responses include de-etiolation, photoperiodic flowering, and circadian rhythm. Cryptochromes (crys), including cry and cry, are blue light-absorbing flavoproteins that regulate hypocotyl elongation, flowering time, circadian rhythm, and stomatal aperture [3,4]. Phototropins (phots), including phot and phot, have a C-terminal serine/threonine kinase domain and repeated LOV1 (light, oxygen, or voltagesensing domain 1) and LOV2 motifs in their N-terminus [5,6]. Phots regulate blue light-induced plant movements such as phototropism [5], chloroplast movement [6], and stomatal opening [7]. The phototropic and chloroplast movement responses allow plants to capture light energy efficiently or to avoid damage from high light intensity
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