Structural Biology of Salt and Drought Tolerance in Plants

Abstract

Climate disruption is an increasing pressure on food supply worldwide. Consequently, it is required to develop strategies to improve the yield of crops under abiotic stress as plants have to endure novel adverse environmental conditions. Among them, drought and salinity constrain agricultural productivity most dramatically. Many of the plant adaptive responses occur at the cell membrane. There, the communication between those processes that are disrupted as a consequence of the adverse environmental stimuli and those involved in the plant adaptive response is established. The available data show that this communication is achieved by a regulated localisation of different signalling molecules to the vicinity of ion channels and transporters. The structural characterisation of those complexes constitutes a major challenge to understand the mechanism to confer plant resistance to stress and to implement novel biotechnological approaches to ensure food security.

Key Concepts

  • The understanding at the molecular level of the mechanisms for plant adaptation to drought and salinity helps to implement novel biotechnological approaches to ensure food security.
  • The cytosolic levels of the phytohormone abscisic acid (ABA) and the Ca2+ ion (Ca2+) act as a molecular switch to plant adaptive response to stress.
  • Plant cell response to abiotic stress is mediated by supramolecular complexes that are formed in the vicinity or at the cell membrane.
  • Structural biology has provided the bases for ABA and Ca2+ roles as they affect the molecular architecture of the signalling complexes regulating plant response.
  • The structural information can be used to drive a genetic approach and for the identification of small molecules that can be used as new agrochemicals for plant improvement.

Keywords: protein structure; plant biology; abiotic stress; signal transduction; abscisic acid; agriculture

Figure 1. Plant response to drought and salt stresses. Root cells incorporate Na+ from the soil to the cytosol through low‐affinity channels. Cell response includes extruding Na+ ions from the cell to the vascular system; there they are transported by diffusion to the leaves where it is compartmentalised into the vacuoles. If the water availability is scarce, plant prevents water transpiration by the closure of the stomata. These processes involve the Ca2+‐ and ABA‐mediated regulation of ion channels and transporters to prevent misadjustments of intracellular ion homeostasis.
Figure 2. Structural biology of the ABA‐mediated responses to drought. (a) Schematic representation of the proteins involved in the pathway. (b) Structural basis of ABA signalling. Cartoon representation of the structures of the SlPYL1 ABA receptor (PYR/PYL), the ABA‐mediated complex between HAB1 and CsPYL1 (PP2C–ABA–PYR/PYL), the SnRK2.6/OST1 (active SnRK2) and the HAB1–SnRK2.6/OST1 complex (PP2C–SnRK2). The inset represents details of the ABA‐binding pocket showing the ABA molecule totally buried by the latch and gate loops of PYR/PYL and the interaction with PP2C. (c) A superimposition of the structures of the apo form of CsPYL1 (Apo), the complex of CsPYL1 with ABA (ABA) and the ternary complex CsPYL1–ABA–HAB1 (ABA–PP2C). (d) A cartoon representation showing that the gate loop of the PYR/PYL ABA receptor and the activation loop of the SnRK2 kinase compete for the active site of PP2C, these interactions being mutually exclusive (see references in the text; the PDB codes are 5MOA, 5MN0, 3ZUT, 3UJG, 5MMQ and 5MMX).
Figure 3. The structure‐based chemical‐biology approach for plant performance under drought stress. (a) Chemical structures of ABA and quinabactin (left); superimposition of the ABA‐binding pocket of the PYL2 in complex with ABA and QN (right). (b) Details comparing the hydrogen bond and hydrophobic interaction of ABA and QN with the PYL2 ABA‐binding site (PDB codes: 3KB3, 4LA7). (c) Close view of the ligand binding site of the AtPYL2 in complex with pyrabactin (PDB code 3MNH) (left), the modelled complex between SlPYL1–ABA and PBI686; the structure of the ternary complex CsPYL1–ABA–PP2C is overlaid in semitransparent mode to highlight the predicted clashes between the PBI686 and F70 and L96 from the CsPYL1 in closed conformation (centre); and AtPYR1 in complex with AS6 (PDB code 3WG8) (right). The gate and the latch loops are highlighted in blue and cyan colours, respectively. Ligands are represented in a stick mode. The insets on each panel represent the chemical structures of the antagonist molecules.
Figure 4. Structural biology of the Ca2+‐ and ABA‐mediated response to salt stress. (a) Scheme showing the molecular mechanism of action of the plant cellular machinery dedicated to respond to salt stress. (b) Structural model of the activation of SOS1 Na+ extrusion by the CBL–CIPK complex SOS3–SOS2 and PP2C‐type phosphatase. (c) Domain organisation of CBL and CIPK proteins. (d) On the left, molecular surface representation of the crystallographic structure of the Ca2+ sensor SOS3 bound to the regulatory domain of the kinase SOS2 (ribbons), showing how SOS3 sequesters the self‐inhibitory domain of SOS2 leaving at the opposite side the PPI protein phosphatase interaction domain. On the right, the structure of the CIPK23 kinase domain that activates AKT1‐mediated K+ transport is depicted. The cavity where the self‐inhibitory NAF motif inserts to block kinase activity is shown as a light‐green volume. The activation domain (magenta) displays a close conformation and reaches the NAF docking region, thus stabilising an inactive conformation of the kinase. A black dashed line has been depicted to understand how the kinase and regulatory domain of CIPKs are segregated in the CBL–CIPK complex. The same colour code has been used throughout the figure for a better comprehension of the mechanism of action of these proteins.
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Further Reading

Food and Agriculture Organization of the United Nations (2016) FAOs' Work on Climate Change Conference, 2016. Food and Agriculture Organization of the United Nations, www.fao.org/3/a‐i6273e.pdf

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Sánchez‐Barrena, María J, Martinez‐Ripoll, Martín, and Albert, Armando(Mar 2018) Structural Biology of Salt and Drought Tolerance in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0027628]