Localized Response of De Novo Terpenoid Emissions Through the Jasmonate Signaling Cascade in Two Main European Tree Species

Abstract

The systemically induced production of volatile organic compounds (VOCs) in undamaged tissues of plants under herbivore attack is still not fully understood, particularly with respect to below- and aboveground signaling. Here, we test the hypotheses that treatment of trees with jasmonic acid (JA) to simulate local herbivory (i) systemically induces VOC emissions in leaves and roots by signal propagation via the vascular bundle system and (ii) that bidirectional signaling occurs between below- and aboveground organs. We applied JA to roots and branches of Fagus sylvatica and Picea abies in a controlled experiment and shielded untreated tissues from volatile cues. VOC emissions and gas exchange were measured continuously over 6-8 days and complemented by quantification of tissue terpenoid storage pools. In contrast to the strong increase in terpenoid emissions from directly treated leaves and needles, which were mainly composed of sesquiterpenes, no systemically induced terpenoid emissions were found. Direct JA treatment of shoots reduced net photosynthesis and stomatal conductance in P. abies by ~50%, while the gas exchange of F. sylvatica remained unaffected. In the root system of P. abies, terpenoid contents increased both locally and systemically in response to belowground JA treatment. Overall, our results challenge the concept of systemically induced terpenoid emissions through vascular JA signaling, which is commonly induced in trees in response to insect herbivory. Instead, our data point toward a possible role of volatile cues in intra-plant signaling.

Keywords: herbivore induced plant volatiles (HIPVs); roots; terpenoid storage pools; volatile organic compounds; within‐plant signaling.

FIGURE 1

Experimental set‐up. The local and induced systemic resistance (ISR) in response to exogenous jasmonic acid (JA) application (5 mM) was analyzed in a controlled pot‐experiment on 3‐year‐old Picea abies (depicted) and Fagus sylvatica saplings. Plants were either treated with JA aboveground (shoots) or belowground (roots). The VOC emissions and gas exchange parameters (net photosynthesis, stomatal conductance, root respiration) were measured in real‐time over a time span of 6–8 days (3 days before and 3–5 days after JA application). For this, glass cuvettes (indicated as rectangles) were connected to an automated measurement system in two walk‐in climate chambers. A total of six replicates were analyzed for each of the two tree species and treatments.

FIGURE 2

Effects of JA application on leaf/needle and root gas exchange. Branch net photosynthetic rates (A), stomatal conductance (Gs) and root respiration rates (R) were calculated from cuvette measurements of CO₂ and H₂O concentrations in a controlled climate chamber experiment. To investigate the signal propagation after a local stress, JA solution was applied either to the shoot or to the roots of 3‐year‐old Picea abies (spruce, panel A) and Fagus sylvatica (beech, panel B) trees (n = 6, each) and the local and induced systemic resistance (ISR) was analyzed in separate glass cuvettes. Daily mean values of the light phase ± standard error are shown. Comparison letters indicating similarities and differences between days were generated with paired t‐tests, using plant individuals as pairing variable and a significance level of p < 0.05.

FIGURE 3

Mean diurnal terpenoid emission rates from Picea abies (spruce, panel A) and Fagus sylvatica (beech, panel B) in response to jasmonic acid (JA) application. JA was applied on day zero either to the shoot or to the root; the local and the induced systemic resistance (ISR) of leaves/needles and roots of the same plant individuals were analyzed (n = 6 per treatment and species). Total terpenoid emissions from day zero (7–10 a.m., i.e., before JA application started) and day 1 (7–10 a.m., for comparability) were statistically evaluated with pairwise t‐tests and significant differences are marked with asterisks. Significance levels of *p < 0.05, **p < 0.01, and ***p < 0.001 were used.

FIGURE 4

Terpenoid content in roots and needles/leaves of Picea abies and Fagus sylvatica in response to jasmonic acid (JA) treatment (n = 3–6). JA was applied on day zero of the experiment either to the shoot or to the roots. Controls were collected 4 days prior to the application of JA and were employed for pairwise t‐tests with local and induced systemic resistance (ISR) samples collected 2 days after JA application from the same plant individuals. For statistical analysis, total terpenoid contents were used, as well as sums of mono‐, sesqui‐, and diterpenes, and significance levels of *p < 0.05, **p < 0.01 and ***p < 0.001 were applied. Note the difference scales in (A) and (B) for the different species and plant organs. DT, diterpenes; MT, monoterpenes; SQT, sesquiterpenes.

FIGURE 5

Correlation matrix with Pearson's correlation coefficients of all compounds identified in the roots of Picea abies with GC–MS analysis. DT, diterpenes; MT, monoterpenes; SQT, sesquiterpenes.