Accumulation of Salicylic Acid and Related Metabolites in Selaginella moellendorffii

Salicylic acid (SA) is a phytohormone that plays manifold roles in plant growth, defense, and other aspects of plant physiology. The concentration of free SA in plants is fine-tuned by a variety of structural modifications. SA is produced by all land plants, yet it is not known whether its metabolism is conserved in all lineages. Selaginella moellendorffii is a lycophyte and thus a representative of an ancient clade of vascular plants. Here, we evaluated the accumulation of SA and related metabolites in aerial parts of S. moellendorffii. We found that SA is primarily stored as the 2-O-beta-glucoside. Hydroxylated derivatives of SA are also produced by S. moellendorffii and stored as beta-glycosides. A candidate signal for SA aspartate was also detected. Phenylpropanoic acids also occur in S. moellendorffii tissue. Only o-coumaric acid is stored as the beta-glycoside, while caffeic, p-coumaric, and ferulic acids accumulate as alkali-labile conjugates. An in silico search for enzymes involved in conjugation and catabolism of SA in the S. moellendorffii genome indicated that experimental characterization is necessary to clarify the physiological functions of the putative orthologs. This study sheds light on SA metabolism in an ancestral plant species and suggests directions towards elucidating the underlying mechanisms.

Automated summary

Salicylic acid (SA) is a key plant hormone involved in various physiological processes, with its concentration in plants finely regulated by structural modifications. The lycophyte Selaginella moellendorffii, representing an ancient clade of vascular plants, primarily stores SA and related metabolites as glucosides in its aerial parts. The study highlights the need for further experimental characterization of enzymes in SA metabolism to elucidate physiological functions and mechanisms in ancestral plant species.