Effect of atmospheric carbon dioxide levels and nitrate fertilization on glucosinolate biosynthesis in mechanically damaged Arabidopsis plants.

BACKGROUND: Increased 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. RESULTS: Constitutive 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. CONCLUSIONS: This 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.

Allelopathic effects of glucosinolate breakdown products in Hanza [Boscia senegalensis (Pers.) Lam.] processing waste water.

Boscia senegalensis is a drought resistant shrub whose seeds are used in West Africa as food. However, the seeds, or hanza, taste bitter which can be cured by soaking them in water for 4-7 days. The waste water resulting from the processing takes up the bitter taste, which makes it unsuitable for consumption. When used for irrigation, allelopathic effects were observed. Glucosinolates and their breakdown products are the potential causes for both the bitter taste and the allelopathic effects. The objectives of this study are to identify and quantify the glucosinolates present in processed and unprocessed hanza as well as different organs of B. senegalensis, to analyze the chemical composition of the processing water, and to pinpoint the causal agent for the allelopathic properties of the waste water. Hanza (seeds without testa), leaves, branches, unripe, and ripe fruits were collected in three populations and subjected to glucosinolate analyses. Methylglucosinolates (MeGSL) were identified in all plant parts and populations, with the highest concentrations being found in the hanza. The levels of MeGSLs in the hanza reduced significantly during the soaking process. Waste water was collected for 6 days and contained large amounts of macro- and micronutrients, MeGSL as well as methylisothiocyanate (MeITC), resulting from the conversion of glucosinolates. Waste water from days 1-3 (High) and 4-6 (Low) was pooled and used to water seeds from 11 different crops to weeds. The High treatment significantly delayed or reduced germination of all the plant species tested. Using similar levels of MeITC as detected in the waste water, we found that germination of a subset of the plant species was inhibited equally to the waste water treatments. This confirmed that the levels of methylisiothiocyanate in the waste water were sufficient to cause the allelopathic effect. This leads to the possibility of using hanza waste water in weed control programs.

Isolation and identification of 4-α-rhamnosyloxy benzyl glucosinolate in Noccaea caerulescens showing intraspecific variation.

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.

DELLA proteins modulate Arabidopsis defences induced in response to caterpillar herbivory.

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.