Metal Uptake and Its Utility for Insect Herbivore Defense In Noccaea Caerulescens.

Elucidating how metals, in specialized metal hyperaccumulating plant tissues, influence plant physiology and insect feeding behavior is fundamental to understanding the influence these plants may have on ecosystem structure and function. The elemental defense hypothesis (EDH) states that metals taken up and stored are effective at protecting plants from insect herbivores due to the inherent toxicity, or unpalatability, of the metals. A concomitant hypothesis known as the trade-off hypothesis, states that plants will trade more physiologically costly chemical defenses (e.g. glucosinolates (GL’s)) for the less costly metal defenses.    The objectives of this study were to investigate these hypotheses by examining the spatial and temporal relationship between Zn and GL concentration and speciation in leaves of the model Zn hyperaccumulator Noccaea caerulescens (formerly Thlaspi caerulescens), and assess how these patterns influence insect feeding behavior. A six-week greenhouse study was conducted with N. caerulescens grown in soil receiving 25, 250 and 500 µM Zn treatments. At the end of the experiment, whole plant metal concentration was determined in addition to the analysis of young, medium and old leaves for total metal content (ICP-MS) and total GL’s (as glucose). Metal distribution was also determined in young, medium and old leaves using synchrotron based x-ray fluorescence (SXRF) mapping and the spatial distribution of specific GL’s determined using matrix assisted time of flight mass spectrometry (MALDI TOF/TOF MS) chemical mapping. Effects of these patterns on insect feeding behavior will be assessed using the generalist herbivore Trichoplusia ni (Cabbage looper) in choice and no-choice feeding experiments.  Preliminary results have indicated that younger leaves have a higher GL content than older leaves.