The Role of Plasma Membrane Aquaporins in Regulating the Bundle Sheath-Mesophyll Continuum and Leaf Hydraulics

Nir Sade,Gilor Kelly,Stephen Lerner,Yann Boursiac,Christophe Maurel,Arava Shatil-Cohen,David Granot,Menachem Moshelion,Ziv Attia,Adi Yaaran

doi:10.1104/pp.114.248633

Abstract

Our understanding of the cellular role of aquaporins (AQPs) in the regulation of whole-plant hydraulics, in general, and extravascular, radial hydraulic conductance in leaves (K(leaf)), in particular, is still fairly limited. We hypothesized that the AQPs of the vascular bundle sheath (BS) cells regulate K(leaf). To examine this hypothesis, AQP genes were silenced using artificial microRNAs that were expressed constitutively or specifically targeted to the BS. MicroRNA sequences were designed to target all five AQP genes from the PLASMA MEMBRANE-INTRINSIC PROTEIN1 (PIP1) subfamily. Our results show that the constitutively silenced PIP1 (35S promoter) plants had decreased PIP1 transcript and protein levels and decreased mesophyll and BS osmotic water permeability (P(f)), mesophyll conductance of CO2, photosynthesis, K(leaf), transpiration, and shoot biomass. Plants in which the PIP1 subfamily was silenced only in the BS (SCARECROW:microRNA plants) exhibited decreased mesophyll and BS Pf and decreased K(leaf) but no decreases in the rest of the parameters listed above, with the net result of increased shoot biomass. We excluded the possibility of SCARECROW promoter activity in the mesophyll. Hence, the fact that SCARECROW:microRNA mesophyll exhibited reduced P(f), but not reduced mesophyll conductance of CO2, suggests that the BS-mesophyll hydraulic continuum acts as a feed-forward control signal. The role of AQPs in the hierarchy of the hydraulic signal pathway controlling leaf water status under normal and limited-water conditions is discussed.

Highlights

Our understanding of the cellular role of aquaporins (AQPs) in the regulation of whole-plant hydraulics, in general, and extravascular, radial hydraulic conductance in leaves (Kleaf), in particular, is still fairly limited
Using artificial microRNA (amiRNA) for the Down-Regulation of Several studies have reported the involvement of the PLASMA MEMBRANEINTRINSIC PROTEIN (PIP) subfamily in the regulation of hydraulic conductivity (Kaldenhoff et al, 1998; Martre et al, 2002; Siefritz et al, 2002; Postaire et al, 2010; Prado et al, 2013)
We designed a single sequence of synthetic miRNA containing a consensus sequence of 21 nucleotides targeted at the entire Arabidopsis PLASMA MEMBRANE-INTRINSIC PROTEIN1 (PIP1) subfamily (AtPIP1;1–AtPIP1;5; Fig. 1A)

Summary

Introduction

Our understanding of the cellular role of aquaporins (AQPs) in the regulation of whole-plant hydraulics, in general, and extravascular, radial hydraulic conductance in leaves (Kleaf), in particular, is still fairly limited. Have suggested that, in Arabidopsis (Arabidopsis thaliana), the leaf radial in-flow rate is controlled by the vascular BS, which is a bottleneck for leaf hydraulic conductance (Ache et al, 2010) These cells may be involved in regulating the radial transport activity of the xylem system (Kinsman and Pyke, 1998; Leegood, 2008), partially due to the extremely low apoplastic flow through the BS (Shatil-Cohen and Moshelion, 2012). New evidence has emerged to support the hypothesized role of the BS, as well as xylem parenchymal cells, in the regulation of radial Kleaf, possibly via the regulation of water channels (i.e. aquaporins [AQPs]; for review, see Moshelion et al, 2014). The AtPIP1 group includes five proteins (AtPIP1;1–AtPIP1;5), and the AtPIP2 group includes eight proteins (AtPIP2;1–AtPIP2;8)

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Discussion

Conclusion