Abstract
Insects and nematodes are the most diverse and abundant groups of multicellular animals feeding on plants on either side of the soil–air interface. Several herbivore-induced responses are systemic, and hence can influence the preference and performance of organisms in other plant organs. Recent studies show that plants mediate interactions between belowground plant parasitic nematodes (PPNs) and aboveground herbivorous insects. Based on the knowledge of plant responses to pathogens, we review the emerging insights on plant systemic responses against root-feeding nematodes and shoot-feeding insects. We discuss the potential mechanisms of plant-mediated indirect interactions between both groups of organisms and point to gaps in our knowledge. Root-feeding nematodes can positively or negatively affect shoot herbivorous insects, and vice versa. The outcomes of the interactions between these spatially separated herbivore communities appear to be influenced by the feeding strategy of the nematodes and the insects, as well as by host plant susceptibility to both herbivores. The potential mechanisms for these interactions include systemic induced plant defense, interference with the translocation and dynamics of locally induced secondary metabolites, and reallocation of plant nutritional reserves. During evolution, PPNs as well as herbivorous insects have acquired effectors that modify plant defense responses and resource allocation patterns to their advantage. However, it is also known that plants under herbivore attack change the allocation of their resources, e.g., for compensatory growth responses, which may affect the performance of other organisms feeding on the plant. Studying the chemical and molecular basis of these interactions will reveal the molecular mechanisms that are involved. Moreover, it will lead to a better understanding of the ecological relevance of aboveground–belowground interactions, as well as support the development of sustainable pest management technologies.
Highlights
Under natural conditions plants are constantly exposed to various herbivorous organisms feeding on above- and belowground parts
OUTLOOK Given the high abundance of both groups of herbivores in the field, plants will inevitably encounter both root-feeding nematodes and aboveground feeding insects in their lifetime. Both types of herbivores will elicit induced defenses responses, and possibly shifts in primary metabolites, that are systemic throughout the plant
It is likely that plant parasitic nematodes (PPNs) and aboveground insects interact with each other via systemic induced responses in the plant
Summary
Under natural conditions plants are constantly exposed to various herbivorous organisms feeding on above- and belowground parts. In response to the plant defenses, pathogens and herbivores have in turn evolved several mechanisms to evade or suppress the plant’s innate immune system (Jones and Dangl, 2006) This is accomplished by secretion of proteins, the socalled “effectors.” Besides evasion or suppression of the host defense system, effectors can be involved in manipulation of the host developmental program, for example when galls are www.frontiersin.org formed (Gheysen and Mitchum, 2011). Systemic induced defense signaling results in the activation of a wide range of different defensive traits These could be morphological changes (e.g., formation of trichomes), production of defensive proteins (e.g., chitinase, proteinase inhibitors) or toxins (phytoalexins, alkaloids, glucosinolates), or release of volatiles that either have a repellent effect or attract predators of the attacking herbivores (Bezemer and van Dam, 2005; Kaplan et al, 2008a; Dicke et al, 2009)
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