Glucosinolates are secondary plant compounds typically found in members of the Brassicaceae and a few other plant families. Usually each plant species contains a specific subset of the ∼ 130 different glucosinolates identified to date. However, intraspecific variation in glucosinolate profiles is commonly found. Sinalbin (4-hydroxybenzyl glucosinolate) so far has been identified as the main glucosinolate of the heavy metal accumulating plant species Noccaea caerulescens (Brassicaceae). However, a screening of 13 N. caerulescens populations revealed that in 10 populations a structurally related glucosinolate was found as the major component. Based on nuclear magnetic resonance (NMR) and mass spectrometry analyses of the intact glucosinolate as well as of the products formed after enzymatic conversion by sulfatase or myrosinase, this compound was identified as 4-α-rhamnosyloxy benzyl glucosinolate (glucomoringin). So far, glucomoringin had only been reported as the main glucosinolate of Moringa spp. (Moringaceae) which are tropical tree species. There was no apparent relation between the level of soil pollution at the location of origin, and the presence of glucomoringin. The isothiocyanate that is formed after conversion of glucomoringin is a potent antimicrobial and antitumor agent. It has yet to be established whether glucomoringin or its breakdown product have an added benefit to the plant in its natural habitat.

Summary Optimal defence allocation theory (ODT) is one of the most prominent theoretical frameworks to explain the allocation of defence compounds within plants. It predicts that the most valuable and vulnerable plant organs have the highest levels of chemical defence. The ODT has been well worked out and experimentally tested for shoot defences, but not for root defences. To assess if ODT principles apply similarly to roots and shoots, we examined glucosinolates in above‐ground and below‐ground organs of nine plant species belonging to two families. In order to evaluate whether ODT equally applies to shoot and root organs, we designed a conceptual model in which above‐ground and below‐ground organs were assigned to orders of importance to plant performance. We hypothesized that organs constituting the plant's core structure are better protected than more distal organs. The nine plant species were cultivated, and their roots and shoots were harvested and divided into three orders for glucosinolate analysis. Using a specialist (Delia radicum) and a generalist (Amphimallon solstitiale) root herbivore, we also experimentally tested the hypothesis that the generalist herbivore prefers to feed on fine roots (FRs) with a low glucosinolate concentration, while the specialist prefers taproots (TRs) with a high glucosinolate concentration. We found that both in roots and shoots, the higher ordered core structural organs (TRs and stems) had the highest levels of glucosinolates. Below‐ground, TRs and lateral roots were better protected than the more distal, and less costly, FRs in seven out of nine species tested. The specialist root herbivore preferred feeding on the highly defended TRs, which is in line with what has been found for above‐ground specialist herbivores. Moreover, the glucosinolate concentration in roots overall was significantly higher than that in shoots. Synthesis. These results support the hypothesis that Optimal defence allocation theory (ODT) generally applies to glucosinolate allocation in above‐ground and below‐ground organs and may mainly serve to maintain the integrity of the main plant structure. Moreover, it suggests that above‐ground and below‐ground insect herbivores independently exert similar selection pressures on defence allocation patterns in roots and shoots.

Upon insect herbivory, many plant species change the direction of metabolic flux from growth into defence. Two key pathways modulating these processes are the gibberellin (GA)/DELLA pathway and the jasmonate pathway. In this study, the effect of caterpillar herbivory on plant-induced responses was compared between wild-type Arabidopsis thaliana (L.) Heynh. and quad-della mutants that have constitutively elevated GA responses. The labial saliva (LS) of caterpillars of the beet armyworm, Spodoptera exigua, is known to influence induced plant defence responses. To determine the role of this herbivore cue in determining metabolic shifts, plants were subject to herbivory by caterpillars with intact or impaired LS secretions. In both wild-type and quad-della plants, a jasmonate burst is an early response to caterpillar herbivory. Negative growth regulator DELLA proteins are required for the LS-mediated suppression of hormone levels. Jasmonate-dependent marker genes are induced in response to herbivory independently of LS, with the exception of AtPDF1.2 that showed LS-dependent expression in the quad-della mutant. Early expression of the salicylic acid (SA)-marker gene, AtPR1, was not affected by herbivory which also reflected SA hormone levels; however, this gene showed LS-dependent expression in the quad-della mutant. DELLA proteins may positively regulate glucosinolate levels and suppress laccase-like multicopper oxidase activity in response to herbivory. The present results show a link between DELLA proteins and early, induced plant defences in response to insect herbivory; in particular, these proteins are necessary for caterpillar LS-associated attenuation of defence hormones.

BackgroundIncreased atmospheric carbon dioxide (CO2) levels predicted to occur before the end of the century will impact plant metabolism. In addition, nitrate availability will affect metabolism and levels of nitrogen-containing defense compounds, such as glucosinolates (GSLs). We compared Arabidopsis foliar metabolic profile in plants grown under two CO2 regimes (440 vs 880 ppm), nitrate fertilization (1 mM vs 10 mM) and in response to mechanical damage of rosette leaves.ResultsConstitutive foliar metabolites in nitrate-limited plants show distinct global patterns depending on atmospheric CO2 levels; in contrast, plants grown under higher nitrate fertilization under elevated atmospheric CO2 conditions have a unique metabolite signature. Nitrate fertilization dampens the jasmonate burst in response to wounding in plants grown at elevated CO2 levels. Leaf GSL profile mirrors the jasmonate burst; in particular, indole GSLs increase in response to damage in plants grown at ambient CO2 but only in nitrate-limited plants grown under elevated CO2 conditions.ConclusionsThis may reflect a reduced capacity of C3 plants grown under enriched CO2 and nitrate levels to signal changes in oxidative stress and has implications for future agricultural management practices.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0752-1) contains supplementary material, which is available to authorized users.