Interactions between heavy metals and glucosinolates as defense mechanisms in Thlaspi caerulescens

Abstract

Hyperaccumulator plant species grow in metalliferous soils and accumulate exceedingly high concentrations of metals. They are increasingly studied because of their potential for cleaning up land contaminated with heavy metals, but another aspect of study relates to the reason for hyperaccumulation. The most accepted hypothesis over the last few decades is the ‘elemental defence’ hypothesis, which states that high levels of metals defend the plant against herbivores. Whilst some of the literature is contradictory, some is supportive. An added complication is that many hyperaccumulators belong to the Brassicaceae and produce glucosinolates as organic defences against herbivory. The question to be answered is whether metals or glucosinolates act as the primary defence in these plants and the most recent suggestion is the ‘joint effects’ hypothesis, which states that both classes of chemical work together to benefit the plant and protect it from herbivores. This study investigates these hypotheses and utilized three experimental systems. The hyperaccumulator studied was Thlaspi caerulescens (Gange ecotype) which hyperaccumulates zinc. Plants were grown in a series of glasshouse experiments at a range of soil zinc amendments. There was a positive relationship between soil and foliar zinc; optimum growth occurred at 2000 mg Zn kg-1 soil and this equated to approximately 8000 mg Zn kg-1 shoot, although plants took up as much as 14000 mg Zn kg-1 shoot tissue at higher levels of soil amendment. The herbivore systems studied were generalist thrips (Franklinella occidentalis) and the specialist cabbage whitefly (Aleyrodes proletella). In addition, artificial damage caused by clipping served as a positive control. Four aromatic glucosinolates were extracted from T. caerulescens and two were identified as benzyl and p-OH-benzyl. Glucosinolates were synthesized 32 hours after damage occurred and reached a maximum concentration after 48 hours. Generally, lower concentrations of glucosinolates were observed in plants with higher foliar Zn concentrations and vice versa. However, when plants were subjected to a sustained and heavy herbivore attack, as was the case when thrips infested the plants, glucosinolate production occurred irrespective of foliar Zn concentration. This observation supports the ‘joint effects’ hypothesis, which states that both defences work in tandem and enhance overall defence. Nitrogen was an important component that directed herbivore response. Thrip feeding damage was negatively correlated with foliar nitrogen whilst cabbage whitefly (CWF) benefitted from higher N. Nitrogen was positively correlated with glucosinolate concentrations and glucosinolate content negatively affected the generalist thrips but not the specialist CWF. Data were analysed by accumulated general linear regression and the explanatory model for thrip feeding was C/N ratio + GS + Zn whilst the explanatory model for CWFs was C/N ratio + Zn. Use of the specialist feeder (CWF) allowed for study of the effects of zinc without glucosinolates confounding the results since the CWF was unaffected by foliar glucosinolates. Zinc acted as a defence against CWF but only at high concentrations. The data taken together show that zinc acts as a defence against herbivores that are unaffected by glucosinolates, but only at high concentrations. Zinc also defends the plant against generalist thrips, but glucosinolates are more influential in this case. This might be because of the severe and sustained damage that these plants suffered and systemic effects (i.e. higher concentrations of glucosinolates in undamaged leaves relative to attacked leaves) suggests flexibility in the Zn-glucosinolate relationship. The overall conclusion is in support of the joint effects hypothesis