Abstract
Aloe vera (Asphodeloideae) is a medicinal plant in which useful secondary metabolites are plentiful. Among the representative secondary metabolites of Aloe vera are the anthraquinones including aloe emodin and chrysophanol, which are tricyclic aromatic quinones synthesized via a plant-specific type III polyketide biosynthesis pathway. However, it is not yet clear which cellular responses can induce the pathway, leading to production of tricyclic aromatic quinones. In this study, we examined the effect of endogenous elicitors on the type III polyketide biosynthesis pathway and identified the metabolic changes induced in elicitor-treated Aloe vera adventitious roots. Salicylic acid, methyl jasmonate, and ethephon were used to treat Aloe vera adventitious roots cultured on MS liquid media with 0.3 mg/L IBA for 35 days. Aloe emodin and chrysophanol were remarkably increased by the SA treatment, more than 10–11 and 5–13 fold as compared with untreated control, respectively. Ultra-performance liquid chromatography-electrospray ionization mass spectrometry analysis identified a total of 37 SA-induced compounds, including aloe emodin and chrysophanol, and 3 of the compounds were tentatively identified as tricyclic aromatic quinones. Transcript accumulation analysis of polyketide synthase genes and gas chromatography mass spectrometry showed that these secondary metabolic changes resulted from increased expression of octaketide synthase genes and decreases in malonyl-CoA, which is the precursor for the tricyclic aromatic quinone biosynthesis pathway. In addition, anti-inflammatory activity was enhanced in extracts of SA-treated adventitious roots. Our results suggest that SA has an important role in activation of the plant specific-type III polyketide biosynthetic pathway, and therefore that the efficacy of Aloe vera as medicinal agent can be improved through SA treatment.
Highlights
Aloe vera (Asphodeloideae) is a medicinal plant in which useful secondary metabolites are abundant [1,2]
Time course analysis revealed that when adventitious roots were treated with various concentrations of salicylic acid (SA), methyl jasmonate (MJ), and ethephon, accumulation of aloe emodin and chrysophanol in adventitious roots was increased by more than 10–11 and 5–13 fold at 24 h, respectively, and that in the growth medium rose at 24–72 h of 2000 mM SA treatment (Figure 4A and 4B)
SA treatment showed the most marked effect on production of aloe emodin and chrysophanol, and we investigated the responses of the tricyclic aromatic quinone biosynthesis pathway to SA elicitation
Summary
Aloe vera (Asphodeloideae) is a medicinal plant in which useful secondary metabolites are abundant [1,2]. The heterologously expressed enzymes produced SEK and SEK4b, which have an octaketide structure, from eight malonyl-CoAs, but SEK and SEK4b were found to be shunt products of the type II polyketide biosynthesis pathway and have not been detected in plants [4,5] (Figure 1). This suggested that these novel plant enzymes might potentially be associated with biosynthesis of natural tricyclic aromatic quinones in aloe, but it remains unclear whether these enzymes produce end products such as aloe emodin and chrysophanol in vivo
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