Abstract
There is no argument to the fact that insect herbivores cause significant losses to plant productivity in both natural and agricultural ecosystems. To counter this continuous onslaught, plants have evolved a suite of direct and indirect, constitutive and induced, chemical and physical defenses, and secondary metabolites are a key group that facilitates these defenses. Polyphenols—widely distributed in flowering plants—are the major group of such biologically active secondary metabolites. Recent advances in analytical chemistry and metabolomics have provided an opportunity to dig deep into extraction and quantification of plant-based natural products with insecticidal/insect deterrent activity, a potential sustainable pest management strategy. However, we currently lack an updated review of their multifunctional roles in insect-plant interactions, especially focusing on their insect deterrent or antifeedant properties. This review focuses on the role of polyphenols in plant-insect interactions and plant defenses including their structure, induction, regulation, and their anti-feeding and toxicity effects. Details on mechanisms underlying these interactions and localization of these compounds are discussed in the context of insect-plant interactions, current findings, and potential avenues for future research in this area.
Highlights
The millions of years of dynamic co-existence and relentless competition for survival has led plants to evolve complex strategies to survive against the onslaught of damaging insect herbivores [1,2,3], primarily mediated through tolerance and resistance mechanisms [4,5]
The other various enzymes involved in pathway are chalcone isomerase (CHI), flavonoid 3-hydroxylase (F3H), flavanol synthase (FLS), dihydroflavonol reductase (DFR), anthocyanin synthase (ANS)/glucose transferase (UGTS) leading to the formation of naringenin, dihydroflavonols, flavanols, leucoanthocyanidins and anthocyanins, respectively [75,76,77] (Figure 2)
The regurgitant of caterpillars or salivary secretions contain polyphenol oxidase, peroxidase and reductase which activate plant responses and signaling pathways leading to the production of polyphenolic compounds which are either toxic or repellent to herbivory [112,113] (Figure 3). These include oral secretions isolated from lepidopteran species, oviposition fluid contains elicitors in the form of long chain diols known as bruchins and the fatty-acid-aminoacid conjugates (FACs) found in the regurgitant of larvae of Sphingidae (Hawk moths), Noctuidae and Geometridae [114]
Summary
The millions of years of dynamic co-existence and relentless competition for survival has led plants to evolve complex strategies to survive against the onslaught of damaging insect herbivores [1,2,3], primarily mediated through tolerance and resistance mechanisms [4,5]. Selection pressure for survival in this never ending co-evolutionary arms race has led to the development of complex, biochemically based, and tightly regulated second line of defenses These include the production of toxins that deter herbivores from feeding, reduce the palatability/digestibility of plant tissue, and compounds that can negatively affect herbivore growth and development [8,9]. We showed that polyphenol-rich pericarp extract of purple corn (Zea mays) negatively affected growth, development and adult fitness traits in tobacco hornworm (Manduca sexta L.), a specialist herbivore on Solanaceae [8,22] Consistent with these observations, it has been well documented that different groups of polyphenols collectively protect most plant species against wide range of attackers.
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