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

A 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.