Cell Signalling Mechanisms in Plants

Abstract

Plants are exposed to a wide range of environmental and developmental signals to which they must respond if they are to grow and reproduce. To allow this, plants have evolved complex mechanisms by which these different signals are perceived and transduced to bring about an appropriate physiological response. While these signalling pathways are highly diverse, they all possess two key properties: signal amplification and signal specificity. In addition, many of the components of plant cell signalling pathways are common to all eukaryotes. These include membrane receptors that recognise individual stimuli and numerous proteins, including kinase and phosphatase enzymes, and small molecules that transfer the signals from where they are perceived to their site of action within cells. However, the manner in which these components function in plants can often be different from how they function in other organisms.

Key Concepts

  • Plants monitor and respond to a diverse and continually changing stream of environmental and developmental signals.
  • Chains of proteins and other signalling intermediates connect the perception of signals to the appropriate physiological response in cells.
  • Signal transduction pathways share common properties.
  • The plasma membrane is the main site of signal perception.
  • Interaction between signalling pathways to form signalling networks within cells allows the integration of information from different signals.
  • Second messengers and protein phosphorylation cascades allow amplification of signals.
  • Cell signalling pathways can result in changes to cellular metabolism, function and movement, and altered gene expression and development.

Keywords: cell signalling; protein phosphorylation cascade; MAP kinase; second messenger; cytosolic calcium; reactive oxygen species; receptor; ethylene; abscisic acid

Figure 1. A simplified scheme of cell signalling mechanisms in plants. Extracellular stimuli (e.g. temperature, light, nutrient concentration and water availability) are typically perceived by receptors located in the plasma membrane. Signals are subsequently transduced to the point within the cell where they generate a physiological response by intracellular signal transduction pathway, second messengers that comprised that include second messengers and kinase cascades, resulting in amplification of the original signal. Physiological response includes changes to cellular metabolism, function or movement, and alterations in gene expression and development.
Figure 2. The mitogen‐activated protein kinase (MAPK) signalling cascade. A signal perceived by the receptor results in activation of a mitogen‐activated protein kinase kinase kinase (MAPKKK), which phosphorylates and activates a mitogen‐activated protein kinase kinase (MAPKK), which then phosphorylates a MAPK. The active MAPK phosphorylates downstream signalling intermediates, for example transcription factors which result in changes in gene expression. MAPK signalling is switched off by a MAPK phosphatase which dephosphorylates and inactivates the MAPK.
Figure 3. A simplified model of abscisic acid (ABA) signalling mechanisms in guard cells: ABA‐induce stomatal closure. In the presence of ABA, PYR/PYL/RCAR (PYRABACTIN RESISTANCE 1/PYR1‐LIKE/REGULATORY COMPONENT OF ABA RECEPTOR) receptors bind to and inhibit PP2Cs (protein phosphatase 2Cs) which activates the Ca2+‐independent protein kinases SnRK2s (SNF1‐RELATED KINASE 2), possibly by autophosphorylation. This removes PP2C‐dependent inhibition of hyperpolarisation‐dependent Ca2+‐permeable cation (ICa) channels which, together with IP3 (inositol 1,4,5‐trisphosphate) and cADPR (cyclic ADP‐ribose)‐mediated Ca release from endomembrane stores, result in an increase in [Ca2+]cyt. Increased [Ca2+]cyt activates CPKs (Ca2+‐dependent protein kinases) which are also required for activation of ICa channels) that with SnRK2s phosphorylate and activate the S‐type anion channel SLAC1 (SLOW ANION CHANNEL‐ACCOCIATED 1). Phosphorylation and activation of the R‐type anion channel ALMT12/QUAC1 (ALUMINIUM‐ACTIVATED MALATE TRANSPORTER 12/QUICKLY ACTIVATING ANION CHANNEL 1) allows anion efflux. The resultant membrane depolarisation activates K+ efflux through the voltage‐dependent outward K+ (K+out) channel GORK (GUARD CELL OUTWARD RECTIFYING K+ CHANNEL) causes loss guard cell turgor decrease and stomatal closure. NADPH oxidase‐mediated ROS (reactive oxygen species) production activates GHR1 (GUARD CELL HYDROGEN PEROXIDE‐RESISTANT1) contributing to the activation of ICa and S‐type anion channels.Reproduced with permission from Munemasa et al. 2015 © Elsevier.
Figure 4. A simplified model of ethylene signalling in Arabidopsis. Ethylene receptors act as negative regulators of ethylene signalling. (a) In the absence of ethylene, the receptor (e.g. ETR1) activates the CTR1 (CONSTITUTIVE TRIPLE RSPONSE1) kinase which phosphorylates and inactivates EIN2 at its C‐terminal end. The levels of EIN2 (ETHYLENE INSENSITIVE2) are also negatively regulated by ETP1/2 (ETHYLENE INSENSITIVE2‐TARGETING PROTEIN1/2) through the proteasome. Similarly, in the nucleus, the levels of the transcription factors EIN3/EIL1 (ETHYLENE INSENSITIVE3 and EIN3/ETHYLENE INSENSITIVE‐LIKE PROTEIN1 are also negatively regulated by EBF1/2 (ETHYLENE INSENSITIVE3‐BINDING F‐BOX PROTEIN1/2), through the proteasome. In the absence of EIN3/EIL1, transcription of the ethylene response genes is switched off. (b) In the presence of ethylene, the receptors are inactivated by ethylene binding which in turn inactivates CTR1 preventing the phosphorylation of EIN2. The C‐terminal end of EIN2 is cleaved off and migrates to the nucleus where it stabilises EIN3/EIL1 and induces degradation of EBF1/2. EIN2‐mediated activation of EIN5 (ETHYLENE INSENSITIVE5) also contributes to the degradation of EBF1/2. This allows the transcription factors EIN3/EIL1 to activate the expression of ethylene target genes.
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Further Reading

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Wang WF, Bau MY and Wang ZY (2014) The brassinosteroid signaling network – a paradigm of signal integration. Current Opinion in Plant Biology 21: 147–153.

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McAinsh, Martin R, and Taylor, Jane E(Jan 2017) Cell Signalling Mechanisms in Plants. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026507]