Arabidopsis systemic immunity uses conserved defense signaling pathways and is mediated by jasmonates

Abstract

In the absence of adaptive immunity displayed by animals, plants respond locally to biotic challenge via inducible basal defense networks activated through recognition and response to conserved pathogen-associated molecular patterns. In addition, immunity can be induced in tissues remote from infection sites by systemic acquired resistance (SAR), initiated after gene-for-gene recognition between plant resistance proteins and microbial effectors. The nature of the mobile signal and remotely activated networks responsible for establishing SAR remain unclear. Salicylic acid (SA) participates in the local and systemic response, but SAR does not require long-distance translocation of SA. Here, we show that, despite the absence of pathogen-associated molecular pattern contact, systemically responding leaves rapidly activate a SAR transcriptional signature with strong similarity to local basal defense. We present several lines of evidence that suggest jasmonates are central to systemic defense, possibly acting as the initiating signal for classic SAR. Jasmonic acid (JA), but not SA, rapidly accumulates in phloem exudates of leaves challenged with an avirulent strain of Pseudomonas syringae. In systemically responding leaves, transcripts associated with jasmonate biosynthesis are up-regulated within 4 h, and JA increases transiently. SAR can be mimicked by foliar JA application and is abrogated in mutants impaired in jasmonate synthesis or response. We conclude that jasmonate signaling appears to mediate long-distance information transmission. Moreover, the systemic transcriptional response shares extraordinary overlap with local herbivory and wounding responses, indicating that jasmonates may be pivotal to an evolutionarily conserved signaling network that decodes multiple abiotic and biotic stress signals.

Fig. 1.

Rapid systemic induction of A70 depends on RPM1 recognition. RNA blots of A70 (At5g56980) accumulation 4 hpi in local or systemic tissues after virulent (D) or avirulent (avrRpm1) (R) P. syringae pv tomato DC3000 challenge. (A) A70 accumulation in RPM1-compromised rpm1 or rar1 mutants or in transgenic plants expressing a bacterial salicylic hydroxylase gene (NahG) (6). (B) Coinfiltration with the calcium channel blocker lanthanum chloride (La³⁺, 1.5 mM), but not the NADPH oxidase inhibitor diphenyl-iodonium (DPI, 7 μM), blocks systemic A70 accumulation. (C) A70 induction in systemic leaves 4 hpi with DC3000(avrRpm1) after inoculated leaves were removed at the time points indicated.

Fig. 2.

Overlap of transcriptional reprogramming associated with secondary metabolism in basal and systemic defense. (A) Schematic showing commonality and differences in transcriptional induction of secondary metabolism pathways associated with defense. Arrows represent enzymes corresponding to transcripts significantly induced by DC3000hrpA (blue arrows), determined 12 hpi, and the systemic transcriptional reprogramming 4 h after local challenge with DC3000(avrRpm1) (red arrows). Black arrows represent transcripts with no detectable change. PAL, phenylalanine ammonium lyase; 4CL, 4-coumarate ligase; CAD, cinnamyl alcohol dehydrogenase; C4H, cinnamate-4-hydroxylase. (B) Transcriptional induction of the JA biosynthetic pathway. Transcripts encoding JA biosynthesis enzymes were strongly induced systemically at 4 hpi with DC3000(avrRpm1). PLA, phospholipase A; AOS, allene oxide synthase; AOC, allene oxide cyclase; OPR, 12-oxophytodienoate reductase 3; PKT, 3-keto-acyl-CoA-thiolase; ACX, fatty acyl-CoA oxidase; JMT, jasmonic acid methyl transferase.

Fig. 3.

Competent JA signaling and synthesis are integral to effective SAR. (A) Comparison of JA and SA accumulation in systemic leaves of DC3000(avrRpm1)- or DC3000hrpA-challenged plants. Experiments were repeated three times with similar results. (B–D) Multiplication of virulent P. syringae 4 days after inoculation in SAR-induced plants. (B) SAR is diminished or abolished in the JA response or broad-spectrum defense mutants sgt1b and eds1, respectively. (C) Attenuation of SAR in the JA biosynthetic mutant opr3 and the JA signaling mutant jin1. Experiments were repeated twice with similar results. (D) Exogenous JA application (30 μM) restricts virulent bacteria, although not as extensively as DC3000(avrRpm1), immunization.

Fig. 4.

Evidence for JA as both a mobile and inducing signal in a conserved SAR initiation process. (A) RNA blot of A70 expression after DC3000(avrRps4) challenge. (B) VSP2::LUC-expressing plant challenged with DC3000(avrRps4) (a–c) or DC3000 (d). Photons are false colored from red (low emission) to green (high emission). Biophotons are detected only in challenged leaves (asterisks) following RPS4 recognition. Systemic induction of the LUC reporter in the petioles and vegetative meristem occurs rapidly after local biophoton generation. (a) Biophoton generation in challenged leaves (asterisk; 12.5–13.25 hpi). (b) Biophoton generation and LUC activity in petioles 13.75–14 hpi. (c) LUC activity in petioles of challenged and unchallenged leaves, 15–15.75 hpi. (d) VSP2::LUC plant imaged 15–15.75 hpi after DC3000 challenge. (C) AOS::GUS reporter activity is induced within 4 hpi after DC3000(avrRpm1) challenge and within 13 hpi after DC3000(avrRps4) inoculation. (D) JA accumulation in phloem exudates after a local incompatible interaction. SA and JA levels were determined in exudates collected until 5 hpi with DC3000(avrRpm1) or DC3000hrpA. This experiment was repeated three times with similar results.

Fig. 5.

Overlap of early systemic responses with expression patterns derived from local wounding, insect feeding, and exogenous MeJA application. A combined data set of 2,539 probe sets generated from the systemic response to avrRpm1 and those displaying a minimum 2-fold change in response to wounding, MeJA treatment, or insect feeding (see SI Fig. 10). These experiments were normalized by using RMA (44), replicate data sets were averaged and set relative to the most obvious control. Genes and experiments were first ordered into self-organizing maps then hierarchically clustered by using complete linkage clustering and an uncentered correlation as the similarity metric.