Chapter 5 - The importance of system complexity in understanding plant responses to engineered nanoparticles: direct versus indirect effects

Abstract

A review of the literature shows that when plants are exposed to engineered nanoparticles (ENPs), responses have been positive, negative, or in some cases have shown no effect. Recent review articles often attempt to explain these highly variable responses based on particle size, shape, and composition for providing insight into the underlying mechanisms of plant response. Far less attention has been given to the effects that may be occurring external to the plant that also may influence plant response to ENPs, as well as the longer-term ecological consequences of ENP exposure. For example, it is almost certain that ENP effects on soil and rhizosphere organisms can affect plant nutrition, growth, and development, even when the ENP is not taken up. As a result of these complex interactions, it is often difficult to determine whether observed plant effects are due to direct uptake of the ENP, are due to indirect effects on plant growth and development external to the plant, or are the results of both direct and indirect effects. Studies have shown changes in competition, reproductive output, and carry-over effects from one generation to the next even when ENP uptake has not been observed. Without verification of cellular uptake or a molecular perturbation, it complicates the development of Adverse Outcome Pathway’s and prediction at scales relevant in ecological risk assessment. Although the longer-term effects of ENPs on ecosystems have been recognized, there has been insufficient focus on establishing whether initial responses are direct or indirect, and how these responses translate to ecological changes at larger scales.