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Synthetic chemical elicitors, so called plant strengtheners, can protect plants from pests and pathogens. Most plant strengtheners act by modifying defense signaling pathways, and little is known about other mechanisms by which they may increase plant resistance. Moreover, whether plant strengtheners that enhance insect resistance actually enhance crop yields is often unclear. Here, we uncover how a mechanism by which 4-fluorophenoxyacetic acid (4-FPA) protects cereals from piercing-sucking insects and thereby increases rice yield in the field. Four-FPA does not stimulate hormonal signaling, but modulates the production of peroxidases, H 2 O 2 , and flavonoids and directly triggers the formation of flavonoid polymers. The increased deposition of phenolic polymers in rice parenchyma cells of 4-FPA-treated plants is associated with a decreased capacity of the white-backed planthopper (WBPH) Sogatella furcifera to reach the plant phloem. We demonstrate that application of 4-PFA in the field enhances rice yield by reducing the abundance of, and damage caused by, insect pests. We demonstrate that 4-FPA also increases the resistance of other major cereals such as wheat and barley to piercing-sucking insect pests. This study unravels a mode of action by which plant strengtheners can suppress herbivores and increase crop yield. We postulate that this represents a conserved defense mechanism of plants against piercing-sucking insect pests, at least in cereals.

Using prenal and prenol as precursors,3-methyl-2-butenal diprenyl acetal was obtained via an acetalization reaction using senecioic acid as catalyst.Then an elimination of 3-methyl-2-butenal diprenyl acetal catalyzed by ammonium dihydrogen phosphate yielded the cis/trans-prenyl-3-methyl-butadienyl ether.Further rearrangement of the elimination product through heating produced to the target citral.The conditions of acetalization reaction and elimination reaction were optimized.It was found that a conversion yield of 63%～64% for prenal could be achieved at 70～75 ℃ for 8 h with 0.3% senecioic acid as catalyst.After azeotropism treatment,the purity and yield of 3-methyl-2-butenal diprenyl acetal are 97.5% and 96.8%,respectively.Using 0.2%～0.5% ammonium dihydrogen phosphate as catalyst,the elimination reaction was carried out at 100～130 ℃ and 2.66 kPa.The cis/trans-prenyl-3-methylbutadienyl ether was distilled from the reaction batch and its purity and yield are 95.9% and 97.0%,respectively.The rearrangement reaction temperature was 120～130 ℃,the purity and yield of citral are 97.5% and 90.2%,respectively.