Roots under attack: contrasting plant responses to below‐ and aboveground insect herbivory

Summary The distinctive ecology of root herbivores, the complexity and diversity of root–microbe interactions, and the physical nature of the soil matrix mean that plant responses to root herbivory extrapolate poorly from our understanding of responses to aboveground herbivores. For example, root attack induces different changes in phytohormones to those in damaged leaves, including a lower but more potent burst of jasmonates in several plant species. Root secondary metabolite responses also differ markedly, although patterns between roots and shoots are harder to discern. Root defences must therefore be investigated in their own ecophysiological and evolutionary context, specifically one which incorporates root microbial symbionts and antagonists, if we are to better understand the battle between plants and their hidden herbivores. Contents Summary 413 I. Introduction 413 II. What makes root herbivores different from shoot herbivores? 414 III. Plant perception of attack and induced signalling 414 IV. Growth, photosynthetic and primary metabolite responses 415 V. Secondary metabolite responses 416 VI. Plant defence theories – their applicability belowground 417 VII. Where next? 417 Acknowledgements 417 References 417