Endogenous plant metabolites against insects

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 antiinsect 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 reintroduced 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 reintroduce endogenous insecticides to bring about insect resistance, the defence metabolites of plants, their synthesis and regulatory mechanisms underlying their production are examined.

The role of volatiles in plant communication

“…On top of regulation by classical TFs, specialized metabolite biosynthesis is likely to involve transcriptional and post‐transcriptional silencing, that is regulation via endogenous small non‐coding RNAs (sRNAs). Although a substantial number of sRNAs has been computationally predicted to target genes in specialized metabolic pathways, very few have so far been experimentally validated (Kortbeek et al ., ). microRNAs (miRNAs), although quantitatively by far a minor type amongst the total of small RNAs encountered, have been studied most extensively.…”

“…A plethora of natural specialized metabolites is described in the literature for their potential in replacing chemicals. There are, however, only a handful of examples of such endogenously produced metabolites validated in planta , let alone in a field situation (Isman and Grieneisen, ; Kortbeek et al ., ).…”

Larval development and proteolytic activity of Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae) exposed to different soybean protease inhibitors

“…The exploration of endogenous plant resistance mechanisms against herbivory is a promising and effective alternative that can reduce the use of chemical pesticides (Kortbeek, van der Gragt, & Bleeker, ; Paixão et al, ). An important resistance mechanism of plants against herbivores is mediated by the lipoxygenase pathway, which produces intermediates used by the octadecanoid pathway to synthesize jasmonic acid, a phytohorme that controls the expression of defensive metabolites, such as protease inhibitors (Meriño‐Cabrera et al, ; Woldemariam, Ahern, Jander, & Tzin, ; Zhou et al, ).…”

Bayogenin 3-O-Cellobioside is a novel non-cultivar specific anti-blast metabolite produced in rice in response toPyricularia oryzaeinfection

“…To ascertain whether other saponins would similarly inhibit conidia germination and appressorium formation, we used the in vitro assays to test two closely related saponins, Hedegeranin, and Oleanolic acid. Despite their reported insecticidal effects (23, 24), we found that washing conidia with 5-100 nM/L of Hedegeranin or Oleanolic acid did not adversely affect conidia germination and appressorium morphogenesis of P oryzae in vitro (Figure.3 E, F, and G). Together, these findings suggest that glycosylated Bayogenin, such as the Bayogenin 3-O-cellobioside, and Bayogenin 3-O-β-D-glucopyranoside are potent phytochemicals that specifically inhibit infectious development of P oryzae .…”