Abstract
Herbivorous insects are responsible for large losses to agriculture through feeding damage itself but also by vectoring pathogens such as viruses. Plants have evolved various mechanisms to defend themselves against insects, including the production of specialised metabolites that act as natural insecticides. Throughout history, plants and plant extracts harbouring specialised metabolites have been exploited by man for their anti-insect properties. Also, commercial insecticides are often based on plant-specialised metabolites and adapted further e.g. to give them the physicochemical properties that render them systemic. The development and application of these synthetic pesticides has made it possible to scale up the production and to increase the yield of food crops worldwide. However, their adversary effects on the environment and on non-target organisms, first discussed in Rachel Carson’s Silent Spring, continue to cause debate and recently resulted in a ban on the use of certain pesticides in the European Union. The trait of insect defence has been largely overlooked during crop domestication and resulted in basically unarmed crop plants. Endogenous production of specialised metabolites could be re-introduced in modern crop varieties to retrieve self-defences. However, despite the fact that a plethora of specialised metabolites from plants has been described, studies that validate their deterrent or insecticidal effect in planta (crop), let alone in field situations, are sparse. To answer the question whether it is feasible to re-introduce endogenous insecticides to bring about insect resistance, the defence metabolites of plants, their synthesis and regulatory mechanisms underlying their production are examined.
Highlights
Herbivorous insects are responsible for large losses to agriculture through feeding damage itself and by vectoring pathogens such as viruses
Despite the fact that anthocyanin accumulation is enhanced by the herbivore-response hormone Jasmonic Acid (JA) (Shan et al 2009), flavonols and anthocyanins themselves appear not to have direct insecticidal properties
Over the past decade it has become clear that many transcription factors themselves are under control of yet another layer of regulation that involves endogenous small RNAs
Summary
In an attempt to battle the destructive behaviour of herbivores, many plants have evolved their own, broad or insect-specific, chemical defence. In addition to feeding on plants, insects can host otherwise rather immobile plant pathogens that use them as mode of transport and as a mechanical tool to enter the host plant (intracellularly) Such insect-transmitted pathogens include fungi and bacteria but the most reported examples, especially in crop production, are viruses (Whitfield et al 2015). Plant-produced defence compounds, first thought to be mere by-products of the primary metabolism, are extremely diverse in structure, shaped by millennia of herbivore-selection pressure (Mithöfer and Boland 2012). Nowadays these metabolites are no longer termed secondary, but rather specialised metabolites (Pichersky and Lewinsohn 2011). The controlled synthesis of a single bioactive compound has the advantage that both quantity and quality are less variable compared
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