Herbarium specimens as tools for exploring the evolution of fatty acid‐derived natural products in plants.

SUMMARY: Plants synthesize natural products via lineage‐specific offshoots of their core metabolic pathways, including fatty acid synthesis. Recent studies have shed light on new fatty acid‐derived natural products and their biosynthetic pathways in disparate plant species. Inspired by this progress, we set out to develop tools for exploring the evolution of fatty‐acid derived products. We sampled multiple species from all major clades of euphyllophytes, including ferns, gymnosperms, and angiosperms (monocots and eudicots), and we show that the compositional profiles (though not necessarily the total amounts) of fatty‐acid derived surface waxes from preserved plant specimens are consistent with those obtained from freshly collected tissue in a semi‐quantitative and sometimes quantitative manner. We then sampled herbarium specimens representing 57 monocot species to assess the phylogenetic distribution and evolution, of two fatty acid‐derived natural products found in that clade: beta‐diketones and alkylresorcinols. These chemical data, combined with analyses of 26 monocot genomes, revealed a co‐occurrence (though not necessarily a causal relationship) between whole genome duplication and the evolution of diketone synthases from an ancestral alkylresorcinol synthase‐like polyketide synthase. Limitations of using herbarium specimen wax profiles as proxies for those of fresh tissue seem likely to include effects from loss of epicuticular wax crystals, effects from preservation techniques, and variation in wax chemical profiles due to genotype or environment. Nevertheless, this work reinforces the widespread utility of herbarium specimens for studying leaf surface waxes (and possibly other chemical classes) and reveals some of the evolutionary history of fatty acid‐derived natural products within monocots. Significance Statement: Plant chemicals are key components in our food and medicine, and advances in genomic technologies are accelerating plant chemical research. However, access to tissue from specific plant species can still be rate limiting, especially for species that are difficult to cultivate, endangered, or inaccessible. Here, we demonstrate that herbarium specimens provide a semiquantitative proxy for the cuticular wax composition of their fresh counterparts, thus reducing the need to collect fresh tissue for studies of wax chemicals and suggesting the same may also be true of other plant chemical classes. We also demonstrate the utility of combining herbarium‐based plant chemical profiling with genomic analyses to understand the evolution of plant natural products. [ABSTRACT FROM AUTHOR]