Hypersensitive to Red and Blue 1 and Its Modification by Protein Phosphatase 7 Are Implicated in the Control of Arabidopsis Stomatal Aperture

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.

SAUR Inhibition of PP2C-D Phosphatases Activates Plasma Membrane H+-ATPases to Promote Cell Expansion inArabidopsis

“…In seedlings, hypocotyl length was significantly diminished in both dark-and light-grown seedlings ( Figures 6B and 6C). Unlike PP2C-D1 overexpression, plants overexpressing the A-clade PP2C HAB1 (Figures 6B and 6C; Saez et al, 2004) or the PPP-phosphatase PP7 (Supplemental Figures 6A to 6C online; Liu et al, 2007;Sun et al, 2012) did not exhibit these reduced growth phenotypes. Reciprocally, whereas 35S:HAB1 seedlings exhibited a strong ABA resistance phenotype, 35S:PP2C-D1 seedlings displayed ABA sensitivity comparable to wild-type seedlings (Supplemental Figure 6D).…”

A WRKY Transcription Factor Recruits the SYG1-Like Protein SHB1 to Activate Gene Expression and Seed Cavity Enlargement

“…In vivo affinity-precipitation in a transient Nicotiana expression system was performed as described [39]. MINI3 truncated derivatives, N302 and C189, were PCR-amplified and cloned into a pCR8/GW/TOPO vector and recombined into the modified pCAMBIA1390 vector as described above.…”

Protein phosphorylation in stomatal movement

“…ABA also activates OST1, which could either inhibit K addition, protein phosphatase 7 (PP7) regulates the phosphorylation status of hypersensitive to red and blue 1 (HRB1), thus inhibiting the ability to form complex with other proteins to modulate stomatal movement. 76 Thus, reversible phosphorylation and dephosphorylation by specific kinases and phosphatases is extensive in guard cell signaling. Further studies aimed at finding the molecular links between the light and kinase/phosphatase activation, as well as components downstream of (de)phosphorylation will provide a better understanding.…”