Abstract
Valine menthyl ester (ment-Val) acts as a plant defense potentiator for several crop species including soybean. Terpenoids, including menthol, exhibit potent abilities as plant defense potentiators in agriculture and horticulture. In the current study, we developed new terpene derivatives that consisted of menthol and various amino acids and that were expected to act as powerful plant defense potentiators. We used 6 amino acids possessing low-reactive sidechains to synthesize an array of amino acid ester of menthol (ment-aa) compounds. Transcript levels of two defense genes (pathogenesis-related protein 1 [PR1] and trypsin inhibitor [TI]) were evaluated in leaves of soybean plants 24h after application of aquatic solution of menthol or menthol-aa, and revealed that the valine menthyl ester (ment-Val) alone elevated the transcript level of defense genes, and it did so only at the low dose of 1 µM, not at higher or lower doses tested. Moreover, it appeared that histone acetylation was involved in this effect. Application of ment-Val enabled soybean plants to sustain the increased transcript levels in their leaves for up to 3 days. Moreover, when ment-Val was additionally applied at day 4, at which time the transcript level had declined to the basal level, the transcript level was re-elevated, indicating the possibility that ment-Val could be repeatedly used to sustain pest control. Ment-Val was found to be chemically stable and effective for defense of several crop species. Collectively, these data show that terpenoid conjugates are useful for pest control instead of or in addition to pesticides.
Highlights
The substances known as plant specialized metabolites (PSMs, ~200,000 metabolites in nature), including terpenoids, phenols, alkaloids, fatty acids, etc., play a wide spectrum of roles in plant growth/development and environmental adaptation (Arimura and Maffei 2016)
We focused on 6 amino acids with low-reactive sidechains to synthesize an array of ment-aa compounds (Fig. 1a)
It should be noted that D-Valine menthyl ester was able to induce the transcript levels of the defense genes to the L-Valine menthyl ester
Summary
The substances known as plant specialized metabolites (PSMs, ~200,000 metabolites in nature), including terpenoids, phenols, alkaloids, fatty acids, etc., play a wide spectrum of roles in plant growth/development and environmental adaptation (Arimura and Maffei 2016). PSMs play important roles in defense against herbivores and pathogens. Limonene, a representative monoterpenoid, functions in the peel of citrus fruit as a defense substance against fungal and bacterial pathogens and a repellent of the citrus pest medfly (Rodríguez et al 2011). In addition to these compounds with insecticide-like efficacy, vaporable PSMs (volatile organic compounds [VOCs]), including terpenoids and green leaf volatiles, and benzenoids/phenylpropanoids (phenols), are well known to act as airborne cues in repelling herbivores, in attracting pollinators and herbivore enemies, and in between/within plant communications (Arimura et al 2009)
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