Modification of the expression of the aquaporin ZmPIP2;5 affects water relations and plant growth

The maize plasma membrane PIP2;5 aquaporin plays a role in controlling root radial water movement, leaf hydraulic conductivity, and plant growth. The plasma membrane intrinsic protein PIP2;5 is the most highly expressed aquaporin in maize (Zea mays) roots. Here, we investigated how deregulation of PIP2;5 expression affects water relations and growth using maize overexpression (OE; B104 inbred) or knockout (KO; W22 inbred) lines. The hydraulic conductivity of the cortex cells of roots grown hydroponically was higher in PIP2;5 OE and lower in pip2;5 KO lines compared with the corresponding wild-type plants. While whole-root conductivity decreased in the KO lines compared to the wild type, no difference was observed in OE plants. This paradox was interpreted using the MECHA hydraulic model, which computes the radial flow of water within root sections. The model hints that the plasma membrane permeability of the cells is not radially uniform but that PIP2;5 may be saturated in cell layers with apoplastic barriers, i.e. the endodermis and exodermis, suggesting the presence of posttranslational mechanisms controlling the abundance of PIP in the plasma membrane in these cells. At the leaf level, where the PIP2;5 gene is weakly expressed in wild-type plants, the hydraulic conductance was higher in the PIP2;5 OE lines compared with the wild-type plants, whereas no difference was observed in the pip2;5 KO lines. The temporal trend of leaf elongation rate, used as a proxy for that of xylem water potential, was faster in PIP2;5 OE plants upon mild stress, but not in well-watered conditions, demonstrating that PIP2;5 may play a beneficial role in plant growth under specific conditions

Non-invasive hydrodynamic imaging in plant roots at cellular resolution

A key impediment to studying water-related mechanisms in plants is the inability to non-invasively image water fluxes in cells at high temporal and spatial resolution. Here, we report that Raman microspectroscopy, complemented by hydrodynamic modelling, can achieve this goal - monitoring hydrodynamics within living root tissues at cell- and sub-second-scale resolutions. Raman imaging of water-transporting xylem vessels in Arabidopsis thaliana mutant roots reveals faster xylem water transport in endodermal diffusion barrier mutants. Furthermore, transverse line scans across the root suggest water transported via the root xylem does not re-enter outer root tissues nor the surrounding soil when en-route to shoot tissues if endodermal diffusion barriers are intact, thereby separating ‘two water worlds’

Nära till naturen : en diskussion om riktlinjer för grundtillgång på friluftsmarker nära tätorter /

This study tested a method to quantify and locate hydraulic lift (HL, defined as the passive upward water flow from wetter to dryer soil zones through the plant root system) by combining an experiment using the stable water isotope 1H218O as a tracer with a soil–plant water flow model. Our methodology consisted in (i) establishing the initial conditions for HL in a large rhizobox planted with Italian ryegrass (Lolium multiflorum Lam.), (ii) labeling water in the deepest soil layer with an 18O-enriched solution, (iii) monitoring the water O isotopic composition in soil layers to find out changes in the upper layers that would reflect redistribution of 18O-enriched water from the bottom layers by the roots, and (iv) comparing the observed soil water O isotopic composition to simulation results of a three-dimensional model of water flow and isotope transport in the soil–root system. Our main findings were that (i) the depth and strength of the observed changes in soil water O isotopic composition could be well reproduced with a modeling approach (RMSE = 0.2‰, i.e., equivalent to the precision of the isotopic measurements), (ii) the corresponding water volume involved in HL was estimated to account for 19% of the plant transpiration of the following day, i.e., 0.45 mm of water, and was in agreement with the observed soil water content changes, and (iii) the magnitude of the simulated HL was sensitive to both plant and soil hydraulic properties

Hydraulic flux–responsive hormone redistribution determines root branching

Plant roots exhibit plasticity in their branching patterns to forage efficiently for heterogeneously distributed resources, such as soil water. The xerobranching response represses lateral root formation when roots lose contact with water. Here, we show that xerobranching is regulated by radial movement of the phloem-derived hormone abscisic acid, which disrupts intercellular communication between inner and outer cell layers through plasmodesmata. Closure of these intercellular pores disrupts the inward movement of the hormone signal auxin, blocking lateral root branching. Once root tips regain contact with moisture, the abscisic acid response rapidly attenuates. Our study reveals how roots adapt their branching pattern to heterogeneous soil water conditions by linking changes in hydraulic flux with dynamic hormone redistribution

Emergent properties of plant hydraulic architecture : a modelling study

In a context of increasing needs for food production and limited availability of freshwater for irrigation, understanding the process of root water uptake (RWU) at the plant scale has become a key issue. The complexity of root system hydraulics as well as the difficulty to measure RWU has made of modelling a valuable tool to investigate this process. However major limitations exist regarding (i) the cost of characterising root segments hydraulic properties, and (ii) the computing time of RWU from that scale. This study demonstrates that simple laws, governing RWU at the plant scale, emerge from water flow equations at the root segment scale. In conditions of uniform soil water potential (SWP), RWU is shown to be distributed proportionally to standard fractions (SUF) along the root system. Under spatially heterogeneous SWP, a compensatory RWU term proportional to a root system conductance parameter (Kcomp) is added, which increases water uptake at locations where SWP is higher. Eventually, another root system conductance parameter (Krs) defines leaf water potential from both plant transpiration rate and sensed SWP, which, itself, is the SUF-weighted-mean SWP. The emergent hydraulic parameters (SUF, Kcomp, and Krs) have a physical meaning and may be estimated or measured directly at the plant scale. They are also shown to be intimately related to the water flow available to plant leaves for transpiration, and may be useful complementary indices to characterise crop strategies against water stress. In addition, the identified emergent properties allow an extreme reduction of RWU computing time, and may even be used accurately in one-dimensional spatial discretisation for densely seeded crops such as wheat.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201

Calibration et amélioration d'un modèle 1-D de prise d'eau racinaire pour l'échelle du champ

Malgré son manque de bases physiques, le modèle 1-D de prise d'eau racinaire a l'avantage d'être très peu demandeur en temps de calcul et est donc la seule approche actuellement viable pour simuler la dynamique de l'eau à l'échelle du champ. Deux problématiques sont ici abordées : Premièrement, comment calibrer ce modèle ? Des tentatives précédentes de calibration par modélisation inverse sur base de mesures de l'évolution du profil de teneur en eau ont montré que l'effet "redistributeur d'eau" du sol masquait une grande partie de l'information sur la prise d'eau racinaire. Une nouvelle approche est proposée : Calibrer le modèle 1-D de prise d'eau racinaire par modélisation inverse sur base de l'évolution du profil de prise d'eau racinaire lui-même. Profil simulé à l'aide d'un modèle 3-D de prise d'eau racinaire (R-SWMS) car non mesurable en réalité. Deuxièmement, quelles améliorations apporter au modèle 1-D de manière à pouvoir obtenir des calibrations valables dans des gammes de conditions hydriques plus larges ? Différents aspects de la fonction de prise d'eau racinaire de FEDDES et al. (1976) sont discutés : dans ce cadre, la pertinence de la Root Length Density est remise en question et confrontée au nouveau concept de Root Conductance Density. D'autre part, des alternatives aux fonctions de stress et de compensation sont étudiée

Plant Water Uptake in Drying Soils

Over the last decade, investigations on root water uptake have evolved toward a deeper integration of the soil and roots compartment properties, with the goal of improving our understanding of water acquisition from drying soils. This evolution parallels the increasing attention of agronomists to suboptimal crop production environments. Recent results have led to the description of root system architectures that might contribute to deep-water extraction or to water-saving strategies. In addition, the manipulation of root hydraulic properties would provide further opportunities to improve water uptake. However, modeling studies highlight the role of soil hydraulics in the control of water uptake in drying soil and call for integrative soil-plant system approaches

A new paradigme for osmotic potential contribution to rrot water uptake : how Nature created a hydraulic pump.

Guttation is the exudation of xylem sap from vascular plant leaves. This process is particularly interesting because in its configuration root water uptake occurs against the hydrostatic pressure driving force. Hence, it emphasizes the contribution of another driving force that lifts water in plants: the osmotic potential gradient. The current paradigm of root water uptake explains that, due to the endodermal apoplastic barrier, water flows radially across roots using the same principles as through selective membranes: driven by the total water potential gradient. This theory relies on the idea that during guttation, osmolites loaded in xylem vessels decrease xylem total water potential, making it more negative than the total soil water potential, and generating water inflow by osmosis as in an osmometer. The theory failed when experiments showed that guttation occurs without sufficient solute loading in root xylem of maize (Enns et al., 1998, 2000) and arrowleaf saltbush (Bai et al., 2007) among others. These studies concluded that experimental observations “could not be explained with the current theories in plant physiology”. Such flow rates towards combined increasing pressure potentials and increasing osmotic potentials between separate apoplastic compartments would require a negative effective root radial conductivity; a mind bender. What piece of hydraulic network would make it possible for water to flow against the total water potential driving force? We implemented Steudle’s composite water transport model in an explicit root cross-section anatomical hydraulic network. Apoplastic, transmembrane and symplastic pathways are interconnected in the network. The results show that while root radial conductivity is particularly sensitive to cell membrane permeability, the combination of conductive plasmodesmata and increasing cell osmotic potentials inwards is a key to explain root water flow towards increasing total potentials. A three-cell theory is proposed as new paradigm of root radial flow

Creation of high resolution root system hydraulic atlas from root cross-section images and modelling tools

Root hydraulic properties play a central role in the global water cycle, agricultural systems productivity, and ecosystem survival as they impact the global canopy water supply. However, the available experimental methods to quantify root hydraulic conductivities, such as the root pressure probing, are particularly challenging and their applicability on thin roots and small root segments is limited. There is a gap in methods enabling easy estimations of root hydraulic conductivities across a diversity of root types and at high resolution along root axes. In this case study, we analysed Zea mays (maize) plants of the var. B73 that were grown in pots for 14 days. Root cross-section data were used to extract anatomical measurements. We used the Generator of Root Anatomy in R (GRANAR) model to generate root anatomical networks from anatomical features. Then we used the Model of Explicit Cross-section Hydraulic Anatomy (MECHA) to compute an estimation of the root axial and radial hydraulic conductivities (kx and kr, respectively), based on the generated anatomical networks and cell hydraulic properties from the literature. The root hydraulic conductivity maps obtained from the root cross-sections suggest significant functional variations along and between different root types. Predicted variations of kr along the root axis were strongly dependent on the maturation stage of hydrophobic barriers. The same was also true for the maturation rates of the metaxylem. The different anatomical features, as well as their evolution along the root type add significant variation to the kr estimation in between root type and along the root axe. Under the prism of root types, anatomy, and hydrophobic barriers, our results highlight the diversity of root radial and axial hydraulic conductivities, which may be veiled under low-resolution measurements of the root system hydraulic conductivity. While predictions of our root hydraulic maps match the range and trend of measurements reported in the literature, future studies could focus on the quantitative validation of hydraulic maps. From now on, a novel method, which turns root cross-section images into hydraulic maps will offer an inexpensive and easily applicable investigation tool for root hydraulics, in parallel to root pressure probing experiments