Flavonoids promote haustoria formation in the root parasite triphysaria versicolor

Abstract

Parasitic plants in the Scrophulariaceae develop infective root structures called haustoria in response to chemical signals released from host-plant roots. This study used a simple in vitro assay to characterize natural and synthetic molecules that induce haustoria in the facultative parasite Triphysaria versicolor. Several phenolic acids, flavonoids, and the quinone 2,6-dimethoxy-p-benzoquinone induced haustoria in T. versicolor root tips within hours after treatment. The concentration at which different molecules were active varied widely, the most active being 2, 6-dimethoxy-p-benzoquinone and the anthocyanidin peonidin. Maize (Zea mays) seeds are rich sources of molecules that induce T. versicolor haustoria in vitro, and chromatographic analyses indicated that the active molecules present in maize-seed rinses include anthocyanins, other flavonoids, and simple phenolics. The presence of different classes of inducing molecules in seed rinses was substantiated by the observation that maize kernels deficient in chalcone synthase, a key enzyme in flavonoid biosynthesis, released haustoria-inducing molecules, although at reduced levels compared with wild-type kernels. We discuss these results in light of existing models for host perception in the related parasitic plant Striga.

Figure 1

Secondary haustoria on T. versicolor roots. Haustoria were induced by a fraction (the 30-min eluant from Fig. 7B) of the purified methanolic rinse of maize seeds (0.04 g seed equivalents/mL) (A), by 10 μm peonidin (B), or by 50 μm DMBQ (C). Photographs were taken approximately 24 h after treatment. Scale bars = 100 μm.

Figure 2

Phenolics, phenolic acids, and quinones assayed for haustoria-inducing activity in T. versicolor.

Figure 3

Anthocyanidins assayed for haustoria-inducing activity in T. versicolor.

Figure 4

Flavonoids assayed for haustoria-inducing activity in T. versicolor.

Figure 5

Dosage responses to haustoria-inducing anthocyanidins and phenolics. Haustoria-inducing activity in T. versicolor of two anthocyanidins, peonidin (•) and pelargonidin (▴), and DMBQ (▪). Mean values associated with the same letter were not significantly different (P ≤ 0.05).

Figure 6

Haustoria-inducing activity of maize-seed rinses. Ten grams of B73 maize kernels was swirled overnight in 25 mL of water (•) or 50% methanol (▪). The seed rinse was diluted in water and 3 mL was applied to the roots of in vitro-grown T. versicolor. Data are averages ± sd of three experiments, with about 18 plants treated in each experiment.

Figure 7

HPLC characteristics and haustoria-inducing activity of maize-seed rinse. A, A_max (200–400 nm) of 100% methanolic maize-seed rinse fractionated on a C₁₈ column and eluted by a linear acetonitrile gradient of 5% to 100% for 2 to 60 min, followed by 100% acetonitrile for 60 to 90 min. B, Haustoria-inducing activity in T. versicolor for different HPLC fractions assayed at 0.9 g seed equivalents/mL. In the same assay, 50 μm DMBQ induced haustoria in 57% of the roots, but no haustoria were induced in the methanol-treated control.

Figure 8

Haustoria-inducing activity of CHS-deficient maize seeds. Rinses from yellow (c2/c2,whp1/whp1) or purple (C2/c2,whp1/whp1) maize seeds were assayed on roots of T. versicolor at 40 g seed equivalents/L. Each value represents the mean ± sd of three tests. Seed weights of the yellow and purple genotypes did not differ significantly.

Figure 9

Two structures of the anthocyanin peonidin. As the pH was increased, the flavylium cation (A) was converted into several quinone structures, including that shown in B (Cheminat and Brouillard, 1986).