Plant Immunity Mechanisms and Systemic Defense Responses

Plants possess a multilayered immune system that integrates local and systemic signals to ward off pathogens. The first line of defence, pattern-triggered immunity (PTI), is activated upon recognition of conserved microbial signatures by cell-surface receptors. Successful pathogens may overcome PTI by delivering effector proteins, which in turn are detected by intracellular nucleotide-binding leucine-rich repeat receptors, triggering effector-triggered immunity (ETI) often accompanied by a localized hypersensitive response. Beyond these local defences, plants deploy systemic acquired resistance (SAR) and induced systemic resistance (ISR) to protect distal tissues. SAR is typically mediated by salicylic acid (SA) and its derivatives, together with pipecolic acid and its N-hydroxylated form, which act as mobile signals coordinating long-distance defence priming. ISR, commonly elicited by beneficial soil microbes, relies on jasmonate and ethylene signalling and modulates root-shoot communication. Reactive oxygen species and lipid-derived compounds further amplify systemic cues, while transcriptional master regulators ensure the timely expression of defence genes. Collectively, these mechanisms form a dynamic network that balances growth and immunity, underpins durable disease resistance in crops, and offers sustainable strategies for global agriculture.

Research from Nature Portfolio

Recent studies have revealed that N-hydroxypipecolic acid triggers systemic acquired resistance by inducing reactive oxygen species and accumulation of extracellular NAD(P), which activate a lectin receptor kinase-BAK1 module in distal tissues. Complementary work has shown that monoterpene volatiles released by infected leaves engage feed-forward loops involving pipecolic acid, glycerol-3-phosphate and a legume lectin-like protein to propagate immunity within and between plants. Foundational research on lipid transfer proteins has demonstrated that AZI1 and its paralog EARLI1 localise at membrane contact sites to mobilise the azelaic acid signal, thereby priming systemic defences.

Plant Immunity Mechanisms and Systemic Defense Responses publication trend

The graph below shows the total number of articles in plant immunity mechanisms and systemic defense responses across all publications each year (not limited to Nature Index journals).

Technical terms

Pattern-triggered immunity (PTI): A basal defence activated by recognition of conserved microbial patterns at the cell surface.

Effector-triggered immunity (ETI): A robust defence response elicited by intracellular detection of pathogen effectors, often leading to cell death.

Systemic acquired resistance (SAR): A long-lasting, broad-spectrum defence state established in uninfected tissues following local pathogen challenge.

Induced systemic resistance (ISR): A primed defence state induced by beneficial microbes, involving jasmonate and ethylene signalling.

N-hydroxypipecolic acid (NHP): A lysine-derived signal molecule that amplifies and transmits systemic acquired resistance.

Lipid transfer protein (LTP): A small protein that facilitates movement of lipid-derived defence signals between cellular compartments.

Monoterpenes: Volatile organic compounds that can act as airborne signals to propagate immunity within and between plants.

References

1. N-hydroxypipecolic acid triggers systemic acquired resistance through extracellular NAD(P). Nature Communications (2023).
2. Small size, big impact: Small molecules in plant systemic immune signaling. Current Opinion in Plant Biology (2024).
3. UDP-glycosyltransferase PpUGT74F2 is involved in fruit immunity via modulating salicylic acid metabolism. Horticulture Research (2025).
4. The glycosyltransferase UGT76B1 modulates N-hydroxy-pipecolic acid homeostasis and plant immunity. The Plant Cell (2021).
5. Arabidopsis AZI1 family proteins mediate signal mobilization for systemic defence priming. Nature Communications (2015).
6. Systemic acquired resistance networks amplify airborne defense cues. Nature Communications (2019).