Response to cadmium in higher plants

The paper summarizes present knowledge in the field of higher plant respons es to cadmium, an important environmental pollutant. The principal mechanis ms reviewed here include phytochelatin-based sequestration and compartmenta lization processes, as well as additional defense mechanisms, based on cell wall immobilization, plasma membrane exclusion, stress proteins, stress et hylene, peroxidases, metallothioneins, etc. An analysis of data taken from the international literature has been carried out, in order to highlight po ssible 'qualitative' and 'quantitative' differences in the response of wild -type (non-tolerant) plants to chronic and acute cadmium stress. The dose-r esponse relationships indicate that plant response to low and high cadmium level exposures is a very complex phenomenon indeed: cadmium evokes a numbe r of parallel and/or consecutive events at molecular, physiological and mor phological levels. We propose that, above all in response to acute cadmium stress, various mechanisms might operate both in an additive and in a poten tiating way. Thus, a holistic and integrated approach seems to be necessary in the study of the response of higher plants to cadmium. This multi-compo nent model, which we would call 'fan-shaped' response, may accord with the Selyean 'general adaptation syndrome' hypothesis. While cadmium detoxificat ion is a complex phenomenon, probably under polygenic control, cadmium 'rea l' tolerance-found in mine plants or in plant systems artificially grown un der long-term selection pressure, exposed to high levels of cadmium-seems t o be a simpler phenomenon, possibly involving only monogenic/oligogenic con trol. We conclude that, following a 'pyramidal' model, (adaptive) tolerance is supported by (constitutive) detoxification mechanisms, which in turn re ly on (constitutive) homeostatic processes. The shift between homeostasis a nd 'fan-shaped' response can be rapid and involve quick changes in (poly)ge ne expression. Differently, the slow shift from 'fan-shaped' response to 'r eal' cadmium tolerance is caused and affected by long-term selection pressu re, which may increase the frequency land promote the expression) of one or a few tolerance gene(s). (C) 1999 Elsevier Science B.V. All rights reserve d.