Evidence from stable-isotope labeling that catechol is an intermediate in salicylic acid catabolism in the flowers of Silene latifolia (white campion)

Kristen Van Gelder,Taylor Forrester,Tariq A Akhtar

doi:10.1007/s00425-020-03410-5

Abstract

Main conclusionA stable isotope-assisted mass spectrometry-based platform was utilized to demonstrate that the plant hormone, salicylic acid, is catabolized to catechol, a widespread secondary plant compound.The phytohormone salicylic acid (SA) plays a central role in the overall plant defense program, as well as various other aspects of plant growth and development. Although the biosynthetic steps toward SA are well documented, how SA is catabolized in plants remains poorly understood. Accordingly, in this study a series of stable isotope feeding experiments were performed with Silene latifolia (white campion) to explore possible routes of SA breakdown. S. latifolia flowers that were fed a solution of [2H6]-salicylic acid emitted the volatile and potent pollinator attractant, 1,2-dimethoxybenzene (veratrole), which contained the benzene ring-bound deuterium atoms. Extracts from these S. latifolia flowers revealed labeled catechol as a possible intermediate. After feeding flowers with [2H6]-catechol, the stable isotope was recovered in veratrole as well as its precursor, guaiacol. Addition of a trapping pool of guaiacol in combination with [2H6]-salicylic acid resulted in the accumulation of the label into catechol. Finally, we provide evidence for catechol O-methyltransferase enzyme activity in a population of S. latifolia that synthesizes veratrole from guaiacol. This activity was absent in non-veratrole emitting flowers. Taken together, these results imply the conversion of salicylic acid to veratrole in the following reaction sequence: salicylic acid > catechol > guaiacol > veratrole. This catabolic pathway for SA may also be embedded in other lineages of the plant kingdom, particularly those species which are known to accumulate catechol.

Highlights

The plant hormone, salicylic acid (SA), plays a central role in the overall plant defense response against pathogens and mediates several other aspects of plant growth and development (Vlot et al 2009; Dempsey et al 2011; Klessig et al 2018; Zhang and Li 2019)
Mass spectral data were acquired in electron impact (EI)-positive ionization mode at 70 eV with selected reaction monitoring (SRM) using 2mTorr collision pressure and 30 eV for precursor ion fragmentation mode
Using flowers from S. latifolia, a series of stable isotope feeding and isotope trapping experiments were employed to demonstrate that SA can be converted into catechol, a widespread phenolic compound that appears to accumulate in several species throughout the plant kingdom—examples of catechol accumulation have been noted in several monocots and among members of the core eudicots, as well as in some basal angiosperms (Tomazsewski 1960; Vázqueza et al 1968; Yoshi-Stark et al 2003; Yang et al 2004; Morse et al 2007; Mageroy et al 2012)

Summary

Introduction

The plant hormone, salicylic acid (SA), plays a central role in the overall plant defense response against pathogens and mediates several other aspects of plant growth and development (Vlot et al 2009; Dempsey et al 2011; Klessig et al 2018; Zhang and Li 2019). While the precursors and intermediates of each pathway have been well defined through a combination of genetic and isotopeassisted labeling approaches in several plants (Yalpani et al 1993; Ribnicky et al 1998; Jarvis et al 2000; Chong et al 2001; Wildermuth 2006; Strawn et al 2007), the genes encoding the final enzymatic steps for SA biosynthesis via the IC pathway were only recently identified in Arabidopsis (Rekhter et al 2019; Torrens-Spence et al 2019). The genetic basis for SA biosynthesis via the PAL pathway is less well understood; isotopic labeling studies have demonstrated that it is derived from phenylalanine through transcinnamic acid and benzoic acid intermediates (El-Basyouni et al 1964; Wildermuth et al 2001; Chen et al 2009; Dempsey et al 2011)

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Conclusion