Signal Perception and Transduction

Abstract

While investigating the effect of unilateral light on the bending response of canary grass (Phalaris canariensis) seedlings, Charles Darwin (1881) observed that although light signal is perceived at the shoot tip, the bending of coleoptile due to differential growth occurs in the subapical region. This classic case of environmental signal perception and transduction resulting in growth response leads to the concept of signal transduction. It is now known that plant growth and development are modulated by a variety of environmental (external) and physiological (internal) signals. Some of the major signals (stimuli) to which plant cells are sensitive include light, mineral nutrients, organic metabolites, gravity, water status, soil quality, turgor, mechanical tensions, heat, cold, wind, freezing, growth hormones, pH, gases (CO₂, O₂, NO, C₂H₄), volatile compounds (e.g., jasmonates), electrical fluxes, wounding, and disease (Fig. 23.1). These signals can vary in quality and quantity over a period. Some signals penetrate across the plasma membrane, while others are carried over long transcellular distances through vessel elements (xylem) and sieve tubes (phloem). Plasmodesmata also facilitate symplastic migration of a number of signaling biomolecules. With the advent of molecular genetic studies on Arabidopsis thaliana in the current era of plant biology research, there has been a flood of information on signal perception and transduction mechanisms in plants. A variety of receptors for various plant hormones have been identified and characterized. Mutant analysis has facilitated the identification of many new signal transduction components which act downstream of receptors for various environmental and internal signals perceived by plants. Thus, an entirely new level of understanding of the complexities of signaling mechanisms in plants has been unfolded. New signaling molecules continue to be discovered, and sophisticated signaling mechanisms are being explained through the development of models which explain interactions between signaling pathways and modulation of various signaling networks (Fig. 23.2). The application of knowledge thus acquired on signaling mechanisms using model plant systems such as Arabidopsis thaliana, to agriculturally significant species, is likely to provide practical benefits in understanding plant responses to varied environmental stresses.

Multiple-Choice Questions

• 1. Which of the following is NOT a non-cell autonomous response?

  • (a) Radial patterning of Arabidopsis primary root regulated by transcription regulator-SHR

  • (b) Auxin gradient in the root by rapid redistribution of PIN3 (auxin efflux protein)

  • (c) Opening of stomata in response to blue light

  • (d) Transition from vegetative to reproductive phase (floral induction)

• 2. Signal perception via symplast regulates the movement of which of the molecules listed below?

  • (a) Lipophilic molecules

  • (b) Chemicals like signal peptides

  • (c) Mechanical forces like touch and blue light

  • (d) RNAs and transcription factors

• 3. In GPCRs, the Gα subunit of G protein is active when bound with _____. Inactivation of Gα subunit is mediated via the action of _________.

  • (a) GTP, GTPase-activating protein (GAP)

  • (b) GDP, Guanine nucleotide exchange factors (GEFs)

  • (c) GDP, GTPase-activating protein (GAP)

  • (d) GTP, Guanine nucleotide exchange factors (GEFs)

• 4. Tip growth and polar extension in plants is regulated by:

  • (a) GSNO

  • (b) Phospholipase A (PLA)

  • (c) Rho-like GTPase (ROP1)

  • (d) Phospholipase D (PLD)

• 5. In peroxisomes, S-nitrosylation has the ability to inhibit the activity of:

  • (a) Superoxide dismutase (SOD)

  • (b) Catalase

  • (c) Glutathione peroxidase

  • (d) b and c

• 6. Small lipid molecules consisting of a single carbon chain and a polar head group are known as:

  • (a) Sphingolipids

  • (b) Phosphoglycerolipids

  • (c) Lysophospholipids

  • (d) None of these

• 7. Calcium channels—glutamate-like receptors (GLRs) and cyclic nucleotide-gated channels (CNGCs)—are located in:

  • (a) Tonoplast

  • (b) Plasma membrane

  • (c) Endoplasmic reticulum

  • (d) All of these

• 8. Activated _____ cleaves PIP 2 to yield secondary messengers- ______ and _______.

  • (a) PLD, IP 5, and DAG

  • (b) PLC, IP 3, and DAG

  • (c) PLC, IP 2, and DAHA

  • (d) PLD, IP 3, and DAG

Answers

1. c

2. d

3. a

4. c

5. b

6. c

7. b

8. b