Plant Defences against Fungal Attack: Perception and Signal Transduction


Plants utilise specialised proteins to perceive fungal invaders as ‘nonself’ by ‘recognising’ specific fungus‐derived molecules. This either results in the onset of massive physical and chemical responses that counter‐invasion by phytopathogenic fungi or enables symbioses with beneficial fungi. Induction of defence occurs when pathogen‐derived molecules such as pathogen‐associated molecular patterns (PAMP) or effectors (AVR) are respectively recognised by specialised plant receptors called pattern recognition receptors (PRR) or resistance proteins (R). Signalling induced downstream of PRR or R activation involves several common events though these may differ in their timing and amplitude.

Key Concepts

  • Plants defend themselves from microbial invasions by recognising the pathogen as ‘nonself’.
  • The perception of ‘nonself’ occurs when plant factors interact with microbial molecules that are either present on the surface of the organism or extruded by them.
  • Pathogen recognition and the subsequent defence response is driven by a two‐tiered innate immune system involving PAMP‐triggered immunity (PTI) and effector‐triggered immunity (ETI).
  • The strength and timing of downstream defence‐related events determine the outcome (resistance or susceptibility) of the host–pathogen interaction.
  • These downstream responses involve reactive oxygen species, ion influxes, MAPK signalling cascades, phytohormones and transcription factors.

Keywords: plant defence; fungal resistance; R gene signalling; plant–fungus interaction; innate immunity

Figure 1. Growth of Hyalopernospora arabidopsidis on resistant (R) and susceptible (S) Arabidopsis thaliana hosts. Upper panel shows prolific growth of fungal mycelia on the susceptible ecotype Nossen (right) as opposed to the resistant ecotype Di‐17 (left). Lower panel shows trypan blue stained leaves exhibiting mycelial growth and sporulation in the susceptible but not in the resistant ecotype. Source: Figure courtesy of Dr. Pradeep Kachroo.
Figure 2. Types of local defence responses induced against fungi. Recognition of pathogen‐/microbe‐associated molecular patterns (PAMP) by plant pattern recognition receptors (PRR) or pathogen effectors (AVR) by plant resistance (R) proteins (directly or indirectly via gaurdee/decoy proteins), induces signalling involving the accumulation of reactive oxygen species (ROS), induction of calcium (Ca2+) signalling and other ion flux, which activates mitogen‐associated protein kinase (MAPK) cascades, resulting in nuclear gene induction via transcription factors. This eventually induces PAMP‐triggered immunity (PTI) upon PAMP recognition or effector‐triggered immunity (ETI) upon AVR recognition in the infected tissue. AVR proteins can interfere with induction of PTI. Induction of ETI is often associated with the onset of the hypersensitive response (HR).
Figure 3. LysM activation by chitin. The LysM domains of Arabidopsis LysM receptor CERK1 bind chitin to activate defence signalling. This involves dimerisation of CERK1 and interactions with the CERK1 orthologue, LYK5. Chitin binding results in phosphorylation of CERK1, which in turn phosphorylates the receptor‐like cytoplasmic kinase PBL27. PBL27, in turn, phosphorylates MAPKKK5. Chitin perception results in dissociation of MAPKKK5 from PBL27 and activates downstream signalling involving the MAPK cascade and induction of transcription factor expression eventually leading to defence activation. The various domains of LysM receptors are depicted in the boxed inset.
Figure 4. Structures of sample R proteins against fungal/oomycete pathogens. (a) Structural depiction of known resistance proteins (R) against fungi (modified and reprinted from Kachroo et al., Signalling mechanisms underlying resistance responses: what have we learned, and how is it being applied? Phytopathology 107:1452–1461). The various structural domains are depicted at the bottom. Subcellular locations are indicated in italics. Although some NLR proteins (containing nucleotide binding and leucine‐rich repeats) are shown to be cytoplasmic, many tend to be attached to the plasma membrane (PM) and can change localisation from the cytoplasm to the nucleus during resistance signalling. (b) Table listing various R proteins shown in a, and the fungal/oomycete pathogen against which they impart resistance.
Figure 5. LOV1 activation in response to the fungal toxin victorin. LOV1 is a CC (coiled coil)‐NB (nucleotide binding)‐LRR (leucine‐rich repeat) type of protein, which imparts disease susceptibility to Cochliobolus victoriae in Arabidopsis thaliana. Binding of the fungal toxin victorin, to the plant thioredoxin TRX‐h5, results in activation of the LOV1 protein. LOV1 activation induces cell death which is proposed to facilitate necrotrophic growth of C. victoriae resulting in disease susceptibility.


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Further Reading

Lai Z and Mengiste T (2013) Genetic and cellular mechanisms regulating plant responses to necrotrophic pathogens. Current Opinion in Plant Biology 16: 505–512.

Lo Presti L and Kahmann R (2017) How filamentous plant pathogen effectors are translocated to host cells. Current Opinion in Plant Biology 38: 19–24.

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Xiao, Xueqiong, and Kachroo, Aardra(Jan 2019) Plant Defences against Fungal Attack: Perception and Signal Transduction. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003438.pub3]