A PIP1 Aquaporin Contributes to Hydrostatic Pressure-Induced Water Transport in Both the Root and Rosette of Arabidopsis

Abstract

Aquaporins are channel proteins that facilitate the transport of water across plant cell membranes. In this work, we used a combination of pharmacological and reverse genetic approaches to investigate the overall significance of aquaporins for tissue water conductivity in Arabidopsis (Arabidopsis thaliana). We addressed the function in roots and leaves of AtPIP1;2, one of the most abundantly expressed isoforms of the plasma membrane intrinsic protein family. At variance with the water transport phenotype previously described in AtPIP2;2 knockout mutants, disruption of AtPIP1;2 reduced by 20% to 30% the root hydrostatic hydraulic conductivity but did not modify osmotic root water transport. These results document qualitatively distinct functions of different PIP isoforms in root water uptake. The hydraulic conductivity of excised rosettes (K ros ) was measured by a novel pressure chamber technique. Exposure of Arabidopsis plants to darkness increased K ros by up to 90%. Mercury and azide, two aquaporin inhibitors with distinct modes of action, were able to induce similar inhibition of K ros by approximately 13% and approximately 25% in rosettes from plants grown in the light or under prolonged (11-18 h) darkness, respectively. Prolonged darkness enhanced the transcript abundance of several PIP genes, including AtPIP1;2. Mutant analysis showed that, under prolonged darkness conditions, AtPIP1;2 can contribute to up to approximately 20% of K ros and to the osmotic water permeability of isolated mesophyll protoplasts. Therefore, AtPIP1;2 can account for a significant portion of aquaporin-mediated leaf water transport. The overall work shows that AtPIP1;2 represents a key component of whole-plant hydraulics.

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

“…Plants were transferred to an extended-night (13-21 h of darkness) environment prior to hydraulic measurements, a condition previously shown to enhance K ros and its AQP-mediated component (Postaire et al, 2010;. The 35S:mir1-8 line showed significantly lower K ros than the control, whereas the K ros of 35S:mir1-3 was similar to that of the control (Fig.…”

“…Several studies have reported the involvement of the PIP subfamily (both PIP1 and PIP2) in the regulation of hydraulic conductivity (Kaldenhoff et al, 1998;Martre et al, 2002;Siefritz et al, 2002;Postaire et al, 2010;. Therefore, we chose to silence the PIP1 subfamily using amiRNA.…”

“…We examined the whole-rosette K ros of the 35S:mir1 plants using a previously described pressure-chamber technique (Postaire et al, 2010;. Plants were transferred to an extended-night (13-21 h of darkness) environment prior to hydraulic measurements, a condition previously shown to enhance K ros and its AQP-mediated component (Postaire et al, 2010;.…”

“…For instance, transfer DNA mutant insertion lines lacking the PIP1;2, PIP2;1, or PIP2;6 isoform, which are abundant in leaf tissue (Alexandersson et al, 2005), exhibited a 20% decrease in whole-rosette K ros (Postaire et al, 2010;, and antisense lines targeting PIP AQPs in Arabidopsis and tobacco (Nicotiana tabacum) exhibited decreased whole-plant hydraulic conductivity (Martre et al, 2002;Siefritz et al, 2002). However, these tools are of limited use when it comes to studying the role of gene-tissue interactions or redundancy caused by a high number of targeted isoforms (Ossowski et al, 2008).…”

Signaling Events in Pollen Acceptance or Rejection in the Arabidopsis Species

“…The cargo of these secretory vesicles is also unknown, but they presumably contain stigmatic resources for pollen hydration and pollen tube penetration that are released upon fusion with the papillar plasma membrane and with the subsequent discharge of the contents into the apoplastic space. One such candidate cargo could be plasma membrane aquaporins , which could facilitate water transfer (Verdoucq et al 2008 ;Postaire et al 2010 ) from the stigmatic papilla to the pollen grain for hydration. As well, the vesicles may deliver cell wall-modifying enzymes for stigmatic papillar cell wall loosening and pollen tube penetration (Elleman and Dickinson 1996 ;Samuel et al 2009 ).…”