measuring-osmotic-water-permeability-coefficient-pf-spherical-cells

Measuring the Osmotic Water Permeability Coefficient (P<sub>f</sub>) of Spherical Cells: Isolated Plant Protoplasts as an Example

Studying AQP regulation mechanisms is crucial for the understanding of water relations at both the cellular and the whole plant levels. Presented here is ...

The Hebrew University of Jerusalem

The hydraulic conductivity of the leaf vascular system (K(leaf) ) is dynamic and decreases rapidly under drought stress, possibly in response to the stress phytohormone ABA, which increases sharply in the xylem sap (ABA(xyl) ) during periods of drought. Vascular bundle-sheath cells (BSCs; a layer of parenchymatous cells tightly enwrapping the entire leaf vasculature) have been hypothesized to control K(leaf) via the specific activity of BSC aquaporins (AQPs). We examined this hypothesis and provide evidence for drought-induced ABA(xyl) diminishing BSC osmotic water permeability (P(f) ) via downregulated activity of their AQPs. ABA fed to the leaf via the xylem (petiole) both decreased K(leaf) and led to stomatal closure, replicating the effect of drought. In contrast, smearing ABA on the leaf blade, while also closing stomata, did not decrease K(leaf) within 2-3 h of application, demonstrating that K(leaf) does not depend entirely on stomatal closure. GFP-labeled BSCs showed decreased P(f) in response to 'drought' and ABA treatment, and a reversible decrease with HgCl(2) (an AQP blocker). These P(f) responses, absent in mesophyll cells, suggest stress-regulated AQP activity specific to BSCs, and imply a role for these cells in decreasing K(leaf) via a reduction in P(f) . Our results support the above hypothesis and highlight the BSCs as hitherto overlooked vasculature sensor compartments, extending throughout the leaf and functioning as 'stress-regulated valves' converting vasculature chemical signals (possibly ABA(xyl) ) into leaf hydraulic signals.

Bundle-sheath cell regulation of xylem-mesophyll water transport via aquaporins under drought stress: a target of xylem-borne ABA?

Signs of abiotic toxicity often appear first at the margins of leaves and gradually spread toward the midrib. It has been suggested that the bundle sheath tissue surrounding the shoot vascular system acts as a solute transport-regulating barrier that prevents excessive quantities of toxic ions from entering the leaf and pushes them toward the hydathodes. We examined this hypothesis by examining the distribution of toxic boron (B) in mutant Arabidopsis leaves with flooded mesophyll and comparing it with that observed in control leaves that exuded guttation drops. As opposed to the control plants, which showed classical symptoms of B toxicity (necrosis starting at the leaf margins), in the mutants, necrosis was first observed inside the leaf. We will discuss this result and how it supports the hypothesis that the bundle sheath serves as a selective barrier filtering the xylem-to-leaf radial transport flow and pushing toxic solutes toward the hydathodes.

Smart pipes: the bundle sheath role as xylem-mesophyll barrier.

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. We hypothesized that the aquaporins of the vascular bundle sheath (BS) cells regulate Kleaf. To examine this hypothesis, AQP genes were silenced using artificial microRNAs (amiRNAs) that were expressed constitutively or specifically targeted to the BS. MicroRNA sequences were designed to target all five AQP genes from thePIP1 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 (Pf), mesophyll conductance of CO2 (gm), photosynthesis (AN), Kleaf, transpiration and shoot biomass. Plants in which the PIP1 subfamily was silenced only in the BS (SCR:mir plants) exhibited decreased mesophyll and BS Pf and decreased Kleaf, but no decreases in the rest of the parameters listed above, with the net result of increased shoot biomass. We excluded the possibility of SCR promoter activity in the mesophyll. Hence, the fact that SCR:mir mesophyll exhibited reduced Pf, but not reduced gm 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. Keywords: Plasma membrane intrinsic proteins (PIPs), Aquaporins (AQPs), Bundle sheath, Leaf hydraulic conductivity (Kleaf), Artificial microRNA (amiRNA).

Article	Year
Bundle-sheath cell regulation of xylem-mesophyll water transport via aquaporins under drought stress: a target of xylem-borne ABA? The Plant journal : for cell and molecular biology\| PubMed ID: 21401747	2011
Smart pipes: the bundle sheath role as xylem-mesophyll barrier. Plant signaling & behavior\| PubMed ID: 22899076	2012
The role of plasma membrane aquaporins in regulating the bundle sheath-mesophyll continuum and leaf hydraulics. Plant physiology\| PubMed ID: 25266632	2014

Arava Shatil-Cohen

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