Metabolodynamic analysis of metal ions as elicitors and modulators of saponins biosynthesis in Agave salmiana in vitro cultures

Abstract

Agave salmiana is a Mexican domestic Agave originally used for alcoholic beverages production, but recently studied as a potential source of steroidal saponins, a key raw material for steroidal drugs synthesis. Multiple studies have demonstrated the viability of metal ions as elicitors of saponins biosynthesis in Agave, but approaches oriented to understand the dynamic behavior of such adaptive response have not yet been explored. To bridge this gap, the saponin profile and morphophysiological characteristics of in vitro cultures of A. salmiana under elicitation treatments with 15, 60, and 150 mM of Ca2+ and Mg2+, as well as 25, 100 and 250 μM of Co2+, Cu2+ and Cd2+ were tracked during five months. Saponins and flavonols were quantified using HPLC-ELSD-DAD, while chlorophyll A/B ratio, biomass, and leaf and root number and length were measured for health monitoring. Five previously identified saponins: Chlorogenin G1 and G2 (CG1 and CG2), hecogenin G1 and G2 (HG1 and HG2) and tigogenin G1 (TG1), 8 other compounds likely to be saponins (UIS 1-7,9), four glycosides of quercetin (QG1-4) and three glycosides of kaempferol (KG1-3) were found in the samples. Saponin modulation was observed as a fine-tuned process targeted to CG2, HG2 and UIS3, while CG1, HG1 and UIS4 remained constitutive for all treatments, and all others suffered variable effects. Chlorophyll ratio remained below typical values for most treatments, excepting those containing Co2+, which generated plants overall similar to control in terms of saponin content and morphophysiological characteristics. 150 mM of Ca2+ and Mg2+ were observed to inhibit root growth, hinder plant development, and impair plant SM for the whole duration of the experiment. Treatments containing Cu2+ hindered plant health and elicited saponin accumulation in all months. Treatments containing Cd2+ elicited the greatest total saponin accumulation of all the experiment, and were observed to induce flavonol accumulation and promote root growth, especially towards the last months of treatment. This study demonstrates that the modulatory effect of metal ions on saponin profiles of A. salmiana in vitro cultures depends conjointly on ion identity, concentration, and exposure time to the treatments.