Response to cadmium in higher plants

The paper summarizes present knowledge in the field of higher plant responses to cadmium, an important environmental pollutant. The principal mechanisms reviewed here include phytochelatin-based sequestration and compartmentalization processes, as well as additional defense mechanisms, based on cell wall immobilization, plasma membrane exclusion, stress proteins, stress ethylene, peroxidases, metallothioneins, etc. An analysis of data taken from the international literature has been carried out, in order to highlight possible 'qualitative' and 'quantitative' differences in the response of wild-type (non-tolerant) plants to chronic and acute cadmium stress. The dose-response relationships indicate that plant response to low and high cadmium level exposures is a very complex phenomenon indeed: cadmium evokes a number of parallel and/or consecutive events at molecular, physiological and morphological levels. We propose that, above all in response to acute cadmium stress, various mechanisms might operate both in an additive and in a potentiating way. Thus, a holistic and integrated approach seems to be necessary in the study of the response of higher plants to cadmium. This multi-component model, which we would call 'fan-shaped' response, may accord with the Selyean 'general adaptation syndrome' hypothesis. While cadmium detoxification is a complex phenomenon, probably under polygenic control, cadmium 'real' tolerance--found in mine plants or in plant systems artificially grown under long-term selection pressure, exposed to high levels of cadmium--seems to be a simpler phenomenon, possibly involving only monogenic/oligogenic control. We conclude that, following a 'pyramidal' model, (adaptive) tolerance is supported by (constitutive) detoxification mechanisms, which in turn rely on (constitutive) homeostatic processes. The shift between homeostasis and 'fan-shaped' response can be rapid and involve quick changes in (poly)gene expression. Differently, the slow shift from 'fan-shaped' response to 'real' cadmium tolerance is caused and affected by long-term selection pressure, which may increase the frequency (and promote the expression) of one or a few tolerance gene(s).