Abstract
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-β-glucoside. Hydroxylated derivatives of SA are also produced by S. moellendorffii and stored as β-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 β-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.
Highlights
The genus Selaginella belongs to the class Lycopodiopsida, a clade of non-seed vascular plants that diverged from euphyllophytes about 400 million years ago [1,2]
Even prior to genome sequencing, S. moellendorffii was subject to comparative studies of its lignin biosynthesis, which revealed some examples of lineage-specific processes [9,10]
There is no information regarding the accumulation of Salicylic acid (SA) and its metabolites in the genus Selaginella. To close this gap in knowledge, we evaluated the occurrence of SA and its derivatives, selected related benzenoids, and phenylpropanoic acids in aqueous-methanolic extracts from the aerial tissue of S. moellendorffii
Summary
The genus Selaginella belongs to the class Lycopodiopsida, a clade of non-seed vascular plants that diverged from euphyllophytes about 400 million years ago [1,2]. Salicylic acid (SA) is a multi-faceted phytohormone with a prominent role in plant defense [11,12]. Numerous studies indicate it is critical for establishing systemic acquired resistance [13,14]. Arabidopsis lines with altered levels of these DHBAs exhibit pronounced phenotypes such as changed susceptibility to downy mildew and early senescence. These phenotypes are primarily attributed to the changed SA levels in those lines [23]. There is evidence that glycosylated DHBAs modulate the immune responses in Arabidopsis [25,27], tomato, and cucumber [26]
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