Abstract
Salicinoids form a specific class of phenolic glycosides characteristic of the Salicaceae. Although salicinoids accumulate in large amounts and have been shown to be involved in plant defense, their biosynthesis is unclear. We identified two sulfated salicinoids, salicin-7-sulfate and salirepin-7-sulfate, in black cottonwood (Populus trichocarpa). Both compounds accumulated in high amounts in above-ground tissues including leaves, petioles, and stems, but were also found at lower concentrations in roots. A survey of salicin-7-sulfate and salirepin-7-sulfate in a subset of poplar (Populus sp.) and willow (Salix sp.) species revealed a broader distribution within the Salicaceae. To elucidate the formation of these compounds, we studied the sulfotransferase (SOT) gene family in P trichocarpa (PtSOT). One of the identified genes, PtSOT1, was shown to encode an enzyme able to convert salicin and salirepin into salicin-7-sulfate and salirepin-7-sulfate, respectively. The expression of PtSOT1 in different organs of P trichocarpa matched the accumulation of sulfated salicinoids in planta. Moreover, RNA interference-mediated knockdown of SOT1 in gray poplar (Populus × canescens) resulted in decreased levels of sulfated salicinoids in comparison to wild-type plants, indicating that SOT1 is responsible for their formation in planta. The presence of a nonfunctional SOT1 allele in black poplar (Populus nigra) was shown to correlate with the absence of salicin-7-sulfate and salirepin-7-sulfate in this species. Food choice experiments with leaves from wild-type and SOT1 knockdown trees suggest that sulfated salicinoids do not affect the feeding preference of the generalist caterpillar Lymantria dispar A potential role of the sulfated salicinoids in sulfur storage and homeostasis is discussed.
Highlights
79 In response to biotic and abiotic stresses, plants produce a multitude of specialized metabolites, with more than 200,000 structures known to date (Pichersky and Lewinsohn, 2011)
The compound was purified from P. x canescens leaves and identified by nuclear magnetic resonance (NMR) spectroscopy as salicin-7-sulfate (Supplemental Fig. S3), which was recently reported in willows (Noleto-Dias et al, 2018)
In order to screen for further sulfated salicinoids, we performed a precursor ion scan experiment with the sulfate (m/z 97) as the target fragment using a MeOH extract made from P. trichocarpa leaves
Summary
79 In response to biotic and abiotic stresses, plants produce a multitude of specialized metabolites, with more than 200,000 structures known to date (Pichersky and Lewinsohn, 2011). The high number and the structural diversity of specialized metabolites are mainly achieved by modifications of a limited number of core structures Such modifications include glycosylation, methylation, acetylation, or sulfation Benzoxazinoids, for example, are mainly restricted to the Poaceae, while glucosinolates are exclusively formed in the Brassicales (Halkier and Gershenzon, 2006; Niemeyer, 2009) Salicinoids represent another example of family-specific plant specialized metabolites, being restricted to the Salicaceae (Böckler et al, 2011; Philippe and Bohlmann, 2007; Chen et al., 2009). Sulfotransferases catalyze the transfer of a sulfuryl group from the cofactor 3’-
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