Quillaja lancifolia Immunoadjuvant Saponins Show Toxicity to Herbivores and Pathogenic Fungi

Abstract

Saponins from leaves of Quillaja lancifolia, a native species from southern Brazil, show potent immunoadjuvant activity in experimental vaccine formulations. The accumulation of the immunoadjuvant saponin fraction QB-90 is induced in cultured leaf disks and seedlings by several stresses and stress signaling molecules, such as osmotic agents, salicylic acid, jasmonic acid, mechanical damage, ultrasound, UV-C radiation, and high light irradiance. These observations suggest a role in plant defense. To further examine this possibility, an investigation of the potential inhibitory role of Q. lancifolia saponins on plant and human pathogenic fungi and two herbivore models was carried out. The screening tests showed that saponin-rich fractions, particularly QB-90, were able to significantly inhibit the growth of Bipolaris micropus, Curvularia inaequalis, Fusarium incarnatum, and Cryptococcus gattii R265. The same metabolites acted as deterrents against the generalist mollusk and insect herbivores Helix aspersa and Spodoptera frugiperda, respectively. Significant reductions in consumption of leaf area and larvae body weight were recorded. Taken together, these data indicate a role for Q. lancifolia saponins in plant defense against diverse herbivores and fungi, having potential as a natural pest control agent and/or as a molecular platform for the development of new environmentally friendly biocide molecules.

Keywords: Quillaja; antifungal; antiherbivore; saponin; specialized metabolism.

Figure 1

Growth radius of the fungus B. micropus after 7 days of incubation in agar containing AE or saponin fractions from Q. lancifolia. Treatments were formulated with fractions (m/v) QB-90 2% or 1%, QB-80 2% or 1%, AE 40% or 4%, Maxim^® XL 1 ppm (positive control), or water (negative control). Bars represent mean ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).

Figure 2

Growth radius of the fungus C. inaequais after 7 days of incubation in agar containing AE or saponin fractions from Q. lancifolia. Treatments were formulated with fractions (m/v) QB-90 2% or 1%, QB-80 2% or 1%, AE 40% or 4%, Maxim^® XL 1 ppm (positive control), or water (negative control). Bars represent mean ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).

Figure 3

Growth radius of the fungus F. incarnatum after 7 days of incubation in agar containing AE or saponin fractions from Q. lancifolia. Treatments were formulated with fractions (m/v) QB-90 2% or 1%, QB-80 2% or 1%, AE 40% or 4%, Maxim^® XL 1 ppm (positive control), or water (negative control). Bars represent mean ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).

Figure 4

Fungi growth radius inhibition for C. gattii R265 after 24 h of incubation with treatment-impregnated disks containing EA or saponin fractions from Q. lancifolia. Treatments were formulated with fractions (m/v) QB-90 1% or 2%, QB-80 1% or 2%, AE 4% or 40%, amphotericin B 0.2% (positive control), or water (negative control). Absence of bars indicates no inhibition. Bars represent mean ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).

Figure 5

Number of colony forming units (CFUs). After 24 h incubation of C. gattii R265 in liquid medium containing EA or saponin fractions from Q. lancifolia, samples were diluted to 1 × 10⁴ cells. An aliquot of 100 µL of each of the suspensions was separately applied to solid growth medium and counting was carried out after 48 h. Treatments were formulated with fractions (m/v) QB-90 1% or 2%, QB-80 1% or 2%, AE 4% or 40%, amphotericin B 0.2% (positive control), or water (negative control). Absence of bars indicates full inhibition. Bars represent mean ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).

Figure 6

Leaf area consumed by H. aspersa. Lettuce disks were treated with methanol (negative control), tannic acid 88.17 mM (positive control), QB-90 fraction 2 mg.mL⁻¹, or AE 40 mg.mL⁻¹ from Q. lancifolia. Bars represent the means ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).

Figure 7

Weight of S. frugiperda larvae feeding for three days on a diet containing AE or saponin fractions of Q. lancifolia. Diet preparation was formulated with saponin fractions QB-80 2 mg.mL⁻¹ or QB-90 1 or 2 mg.mL⁻¹, AE 40 mg.mL⁻¹, Deltamethrin 0.1 mg.mL⁻¹ (positive control), or methanol 30% (negative control). Larval weights did not differ significantly at the start of the experiment. Bars represent the means ± standard error. Different letters indicate significant difference by Tukey test (p ≤ 0.05).