University of Warwick researchers discover rapid, jasmonate-driven, early immune response in plants using breakthrough live-imaging tool.
Plants mobilise their immune defences far earlier than scientists have believed for decades - and through a previously overlooked early signalling mechanism - according to a new study published in Nature Plants.
Unlike animals, plants are literally rooted to the spot and cannot deploy specialised immune cells or antibodies, nor run away. Instead, every cell must be capable of responding to attack from pathogenic viruses, bacteria, fungi, or insect pests. When attacked plants quickly initiate defence responses at the site of challenge, but they can also activate immune responses in distant, not yet infected tissues to protect the rest of the plant, a process known as Systemic Acquired Resistance (SAR).
For decades, SAR has been understood to rely on the signalling molecule salicylic acid - supported by N-hydroxypipecolic acid - to execute and maintain long-lasting immune protection throughout the plant. These molecules are synthesised following infection and gradually accumulate in distant uninfected tissues.
The Warwick team now shows that before this salicylic acid-centred defence is established, plants deploy a much faster communication system: a wave of jasmonate-dependent immune signals that spreads through the plant within just a few hours, initiating SAR well before classical measures of activated SAR.
"What we show here is that whole-plant immunity is activated much faster than we ever realised," said Professor Murray Grant, Elizabeth Creak Chair in Food Security at the University of Warwick and senior author of the study. "Classic salicylic acid-based SAR is still vital, but our work reveals a new early-warning system powered by jasmonates - hormones previously thought to suppress salicylic acid based immune response."
"Whereas salicylic acid accumulation can take more than 24 hours, the jasmonate-dependent signal appeared within three to four hours of infection, moving rapidly through the plant's epidermal and vascular tissues to the uninfected leaves. It is a fundamental shift in our understanding of how plant immunity works."
Watching immunity spread in real time
To uncover this hidden early SAR phase, the researchers developed a novel jasmonate-linked SAR reporter, JISS1:LUC, which functions as a molecular tracker for this early immune activation. This tool allowed them to visualise immune signals moving out of infected leaves and across into uninfected leaves in real time.
This very early signalling phase has remained hidden until now because most traditional approaches detect immune responses during or after systemic defences are fully established, measuring classical molecular markers or SA itself, well after these jasmonate-driven signals are developed.
The results point to a multi-phase SAR strategy. "Jasmonates sound the alarm," explained Dr Erin Stroud, Research Fellow in the School of Life Sciences at Warwick and joint first author. "They coordinate a fast, mobile immune signal, alerting the entire plant that trouble is coming. Classic signalling compounds such as salicylic acid and N-hydroxypipecolic acid then strengthen and stabilises these defences to ensure long-lasting protection."
This study showed that even in plants unable to produce or perceive salicylic acid, the early wave of signalling occurred - but SAR disappeared when jasmonate biosynthesis was disrupted. Those plants lacking jasmonate signalling mounted normal local immune responses to infection, but failed to protect distant leaves, making them vulnerable to secondary infections.
New possibilities for crop protection
Unexpectedly, the team also found that the jasmonate signalling is required to underpin plant-wide electrical signalling, similar to signals previously linked to wound and herbivore responses.
"These electrical signals are similar to those elicited by herbivory and require functional jasmonate signalling to allow this rapid long-distance communication," said Dr Emily Breeze, Assistant Professor at Warwick and joint first author. "Our JISS1:LUC reporter system is an excellent tool for visualising early jasmonate-based SAR initiation in real time, within hours of local attack, which gives us a unique method to explore how plants integrate hormones, calcium fluxes and bioelectricity signals to ultimately protect themselves against invaders."
The discovery that both jasmonate and electrical signalling are elaborated during early systemic immunity opens new possibilities for engineering crops that respond to infection more quickly, limiting disease spread and yield loss, particularly under conditions where pathogens spread quickly or plants face multiple pathogen threats simultaneously.
Professor Grant concluded: "This work not only reshapes our understanding of systemic plant immunity but understanding common SAR signalling mechanisms gives us a unique lead to design strategies for bioengineering defence systems that provide broad spectrum, rather than pathogen specific crop resistance.
"Specifically, activation of systemic immunity via conditional activation of early jasmonate signalling could provide a novel approach to mitigate crop losses to devastating diseases such as rusts, blights and mildews without the needs for environmentally damaging chemical control."
