Elucidation of the biosynthetic pathways of timosaponins reveals the antifungal mechanisms in Anemarrhena asphodeloides

Abstract

Introduction

Timosaponins, as steroidal saponins, are the primary active constituents and quality biomarkers in Anemarrhena asphodeloides Bunge. Despite their significance, the biosynthetic pathways of timosaponins have not been thoroughly investigated.

Objectives

This study aims to delineate the biosynthetic pathway of timosaponins in A. asphodeloides, elucidate the catalytic mechanisms of the key cycloartenol synthase (CAS), and investigate the antifungal properties of timosaponins.

Methods

Genes were cloned from A. asphodeloides and heterologous expressed in yeast, tobacco or bacillus coli. Site-directed mutagenesis and molecular docking were used to elucidate the catalytic mechanism of CAS. Antifungal assays were conducted to evaluate the antifungal activities of timosaponins.

Results

In this study, we elucidated the biochemical functions of seven genes involved in timosaponins biosynthesis in A. asphodeloides. Among three candidate OSC genes, AaOSCR12 was identified as the gene encoding cycloartenol synthase, which is responsible for the skeleton cyclization in timosaponin biosynthesis. Six residues (257H, 369N, 448T, 507V, 558P, 616Y) were identified as the critical catalytic active sites of CAS (AaOSCR12). Sterol methyltransferase (AaSMT1) and sterol side-chain reductase (AaSSR2) were found to be the subsequent enzymes and the branching points leading to phytosterol and cholesterol biosynthesis, respectively. Two oxide reductase genes, AaCYP90B27 and AaCYP90B2, were responsible for post-modification of cholesterol, serving as a precursor of timosaponins. A key 26-O-β-glucosidase (AaF26G1) was identified as facilitating the conversion of furostanol-type timosaponins into spirostanol-type timosaponins. Antifungal assays revealed that spirostanol-type timosaponin AⅢ exhibits superior antifungal activity compared to furostanol-type timosaponin BⅡ, potentially linked to plant defense mechanisms involving AaF26G1.

Conclusion

This study utilized a multi-chassis cross-identification strategy, revealing key enzymes in the timosaponin biosynthetic pathway and offering novel insights into plant defense mechanisms against microbial pathogens.