Less Is More: a Mutation in the Chemical Defense Pathway of Erysimum cheiranthoides (Brassicaceae) Reduces Total Cardenolide Abundance but Increases Resistance to Insect Herbivores

Abstract

Many plants produce structurally related defensive metabolites with the same target sites in insect herbivores. Two possible drivers of this chemical diversity are: (i) interacting effects of structurally related compounds increase resistance against individual herbivores, and (ii) variants of the same chemical structures differentially affect diverse herbivore species or feeding guilds. Erysimum cheiranthoides L (Brassicaceae; wormseed wallflower) produces abundant and diverse cardenolide toxins, which are derived from digitoxigenin, cannogenol, and strophanthidin, all of which inhibit Na + /K +-ATPases in animal cells. Here we describe an E. cheiranthoides mutant with 66% lower cardenolide content, resulting from greatly decreased cannogenol-and strophanthidin-derived cardenolides, partially compensated for by increases in digitoxigenin-derived cardenolides. This compositional change created a more even cardenolide distribution, decreased the average cardenolide polarity, but did not impact glucosinolates, a different class of chemical defenses. Growth of generalist herbivores from two feeding guilds, Myzus persicae Sulzer (Hemiptera: Aphididae; green peach aphid) and Trichoplusia ni Hübner (Lepidoptera: Noctuidae; cabbage looper), was decreased on the mutant line compared to wildtype. Both herbivores accumulated cardenolides in proportion to plant content, with T. ni accumulating higher total concentrations than M. persicae. Helveticoside, an abundant cardenolide in E. cheiranthoides, was absent in M. persicae, suggesting that this compound is not present in the phloem. Our results support the hypothesis that cardenolide diversity protects plants against different herbivores, with digitoxigenin-derived compounds providing better protection against insects like M. persicae and T. ni, whereas cannogenol and strophanthidin provide better protection against other herbivores of E. cheiranthoides.

Understanding the phytochemical diversity of plants: Quantification, variation and ecological function

“…Among the few studies that have quantified phytochemical evenness, there is evidence of variation in evenness of (classes of) compounds between bryophyte species (Peters et al 2019(Peters et al , 2021, differences in cardenolide evenness between wild type and mutant Erysimum cheiranthoides (Mirzaei et al 2020), and differences between maize plant types (Bernal et al 2023). Pais et al (2018) found that an increased evenness of leaf metabolites in Cornus florida was associated with a higher probability of plants being diseased.…”

“…The copyright holder for this preprint this version posted March 25, 2023. ; https://doi.org/10.1101/2023.03.23.533415 doi: bioRxiv preprint include new compounds produced in existing biosynthetic pathways, or, potentially more efficient but less common, compounds produced in new biosynthetic pathways (Becerra et al 2009). Overall, phylogenetic patterns are often complex and might differ for different aspects of the phytochemical phenotype, or different classes of compounds (Courtois et al 2016, Züst et al 2020, Forrister et al 2022. Therefore, the disparity component of chemodiversity, based on biosynthetic classifications, should be taken into account in studies examining macroevolutionary patterns of phytochemicals.…”

“…Phytochemical richness is a frequently reported measure. Variation in it has been documented on all levels of biological organization, including between tissues (Whitehead et al 2013, Elser et al 2022, individuals (Ziaja and Müller 2022), populations (Zeng et al 2022, Eisen et al 2022, species (Macel et al 2014, Züst et al 2020, orders (Courtois et al 2009), and communities (Peguero et al 2021), as well as across herbivory treatments (Agrawal 2000), phylogenies (Becerra et al 2009, Cacho et al 2015 and landscapes (Defossez et al 2021).…”

Overexpression and RNAi-mediated Knockdown of Two 3β-hydroxy-Δ5-steroid dehydrogenase Genes in Digitalis lanata Shoot Cultures Reveal Their Role in Cardenolide Biosynthesis

“…The variation in muta-chemotypes was analysed randomly for more than two years and their cardenolide content was also shown to be stable. Mutatype-specific, stable cardenolide content was also seen in EMS-induced mutants of Erysimum cheiranthoides, were several mutachemotypes were established [31].…”

Quantifying chemodiversity considering biochemical and structural properties of compounds with the R package chemodiv

“…However, quantifying such diversity is not straightforward, as, for example, Shannon's diversity do not consider that these two groups of compounds contain structurally different molecules produced in different biosynthetic pathways. To demonstrate the applicability of calculating measures of phytochemical diversity and dissimilarity that take such difference into account, we used a randomly selected set of eight glucosinolates and eight cardenolides that have been found in Erysimum cheiranthoides L. in previous studies (Mirzaei et al ., 2020; Züst et al ., 2020). Thereafter, by sampling from different normal distributions, we simulated data on the relative concentration of these compounds in three groups of 16 individual plants each: (1) plants with a high concentration of glucosinolates and a low concentration of cardenolides; (2) plants with a low concentration of glucosinolates and a high concentration of cardenolides; and (3) plants with a high concentration of four glucosinolates and four cardenolides, and a low concentration of the remaining glucosinolates and cardenolides.…”