Introduction

Elicitins, a unique class of elicitors, make up a family of proteins secreted by members of the phytopathogenic Chromista, Phytophthora spp. and Pythium spp. (Kamoun et al. 1993; Panabieres et al. 1995; Vauthrin et al. 1999). Elicitins play an important role in the success of Phytophthora spp. infection and growth. In non-host species, elicitins can trigger a successful hypersensitive response (Pernollet et al. 1993; Vleeshouwers et al. 2000). In host species, the potential role of elicitins in disease development is incompletely understood. However, Phytophthora ramorum elicitins are linked to increased pathogen virulence (Manter et al. 2010), photosynthetic declines in tanoak, rhododendron, and bay laurel (Manter et al. 2007b), but not beech (Fleischmann et al. 2005), and ultra-structural changes in oak (Brummer et al. 2002) and pepper (Ivanova and Singh 2003).

One of the main biological functions of elicitins for Phytophthora spp. appears to be sterol binding and transfer through phospholipid membranes (Mikes et al. 1998; Vauthrin et al. 1999). Phytophthora spp. lack the ability to synthesize sterols, but sterols are required for sexual and asexual reproduction (Elliott et al. 1966; Haskins et al. 1964; Hendrix 1970; Leal et al. 1964; Nes and Stafford 1983). As a result, Phytophthora spp. take up and metabolize a variety of sterols from host plants (Gonzalez and Parks 1981; Grant et al. 1988; Nes and Stafford 1983, 1984). Sporulation rates differ depending upon the sterol acquired (Nes et al. 1980; Nes and Stafford 1983, 1984).

Due to the discrimination of Phytophthora spp. against various sterols and the associated changes in growth and spore formation, one might expect plants with quantitative and/or qualitative differences in plant sterol content to exhibit variable field resistance and/or to influence sporulation rates of Phytophthora spp. In a study with various potato cultivars, Hazel et al. (1988) examined P. infestans sporulation on both detached potato leaves and artificial media amended with a mixture of sterols designed to mimic the potato leaf sterol profiles. Although sterol treatment did increase sporangia production compared to non-sterol treatments, and sporangia production differed significantly between the potato cultivars, the in planta and in vitro treatments were not well correlated, suggesting that additional plant compound(s) influenced sterol acquisition.

Phytophthora ramorum is a highly virulent pathogen that infects diverse hosts including deciduous, evergreen, woody, and herbaceous plants (Goheen et al. 2006; Grunwald et al. 2008). The result of infection varies depending on the plant host species, tissue type, and isolate of P. ramorum (Goheen et al. 2006; Grunwald et al. 2008; Manter et al. 2010; Rizzo et al. 2005). Factors influencing host resistance are largely unknown, although various phytochemicals have antimicrobial activity against P. ramorum. For example, three terpenes from conifer heartwood had strong antimicrobial activity against P. ramorum (Manter et al. 2007a). A study of phloem tissue in coast live oaks found significant differences between infected and non-infected tissue for several phenolic compounds, including gallic acid, catechin, and ellagic acid (Nagle et al. 2011; Ockels et al. 2007).

One class of compounds that may influence sterol acquisition via elicitins is the group of polyphenolic compounds known as tannins. Tannins play a role in plant resistance by exhibiting direct toxicity against a wide variety of microbes (Latte and Kolodziej 2000; Nelson et al. 1997; Sivakumaran et al. 2004) although the mechanistic basis for this activity is not well described. One plausible mechanism, may be associated with the ability of tannins to bind and precipitate proteins (Hagerman and Butler 1981; Shahidi and Naczk 1995) that are necessary for pathogen growth and survival, such as the Phytophthora elicitin protein.

The goal of this in vitro study was to explore the potential effect sterols and/or tannin-amended artificial growth media may have on P. ramorum growth, sporulation, and elicitin production. The leaf tissue, sterol, and tannin extracts were obtained from three different plant species, California bay laurel, California black oak, and Oregon white oak (Umbellularia californica, Quercus kelloggii, and Q. garryana) in an effort to obtain extracts with differing quantities and/or compositions of sterols and tannins.

Methods and Materials

Pathogen Strain and Plant Material

A single North American isolate (PR-07-031, original name 15-WA-M isolated from soil in Washington 2006) of P. ramorum was grown as a starter culture for sporulation experiments on corn meal agar (Becton Dickinson, Rutherford, NJ, USA) without light at room temperature. A single 0.7 cm agar plug was taken from the outermost margins of 2–3 week-old starter cultures and used to inoculate agar (15 g l−1, Bacto agar, Becton Dickinson) plates containing the Phytophthora synthetic medium (PSM) formulated by Hoitink and Schmitthenner (1974) and amended with various amounts of ground leaf tissue, sterols, and/or tannin-enriched extracts in lieu of the recommended cholesterol (10 mg l−1). All treatments were replicated three times in each of three independent trials.

Leaf Tissue Preparations

Foliage for sterol extraction was collected from trees in Oregon between 29 September and 4 October, 2007. Leaves of Oregon white oak were gathered from randomly sampled branches of several native trees in Philomath, Benton County (44° 32′ 21.42″N, 123° 20′ 20.30″W, elevation 108 m). California black oak foliage was sampled from several trees growing near Jacksonville, Jackson County (42° 15′ 48″N, 122° 59′15″, elevation 824 m), and leaves of California bay laurel were gathered from a single ornamental tree in Albany, Linn County (44° 37′ 22.25″N, 123° 5′ 47.58″W, elevation 70 m). The leaves were removed from the stems, air dried in the laboratory, and stored at room temperature until needed.

Foliage used to amend culture media, or for tannin extraction, was gathered on 9 September, 2008 from the same Oregon white oak and bay laurel trees sampled in 2007. However, a new black oak tree was sampled near Eugene (Lane County, 44° 0′ 40.59″N, 123° 4′ 59.08″, elevation 144 m). After collection, foliage was allowed to air-dry at room temperature for 7 d before grinding with a Wiley-mill, sieving with a 5 mm mesh screen, and storing at 4 °C until needed. Ground leaf tissue was added directly to PSM prior to autoclaving at a final concentration of 0.1, 0.5, 1.0, or 5.0 mg ml−1.

Commercially Available Sterols

Several commercially available sterols—β-sitosterol, cholesterol, ergosterol, stigmasterol, and stigmastanol (Sigma-Aldrich, St. Louis, MO)—were selected for their variable properties. β-Sitosterol is the most common plant sterol, and is structurally similar to cholesterol except for the ethyl substitution at position 24. Although the quantity of cholesterol in plants typically is low in terms of total lipid content, it is a frequent component of plant membranes, and it may be the major sterol on leaf surfaces (Behrman and Gopalan 2005). Stigmasterol is the second most common plant sterol and is nearly identical to β-sitosterol except for the absence of a double bond. Ergosterol is similar to cholesterol but is almost exclusively found in fungi. All sterols were dissolved in ethanol and added directly to PSM prior to autoclaving at a final concentration of 0.1, 1, 10, 25, or 50 μM.

Plant Sterol Extracts

Leaf sterols were extracted according to the methods of Jeong and Lachance (2001). Air-dried, ground leaf tissue (10 g) was placed in a screw-cap 250 ml glass bottle before adding 25 ml of 50 % KOH and 100 ml 95 % ethanol and heating in an 80 °C water bath for 1 hr. The solution was transferred to a separatory funnel with the aid of 30 ml 95 % ethanol, 50 ml warm distilled water, followed by 50 ml cold water. The mixture was rinsed six times with 100 ml petroleum ether. The combined petroleum ether fraction was divided into two aliquots (300 ml) and each was washed 4 times with 100 ml distilled water. The petroleum ether fractions were combined and concentrated to less than 50 ml in a rotary evaporator at 40 °C. Anhydrous sodium sulfate (1.0 g) was added, and the solution was transferred to a weighed 50 ml round bottom flask with the aid of 5 ml methylene chloride, air-dried, and reweighed. Residual water in the air dried tissue was determined by mass using triplicate subsamples of ground tissue (250 mg) dried 16 hr at 102 °C. The sterol extract yield was calculated on a dry mass basis.

The concentrated bay laurel extract retained a strong odor from monoterpenes, and the presence of these contaminants was confirmed by gas chromatography. These volatiles were removed before testing the extract on P. ramorum, as they can be inhibitory. The extract was redissolved in methylene chloride, and a portion was transferred to a small round-bottom flask before removing 70 % of the solution on a rotary evaporator at 40 °C. Monoterpenes were removed by adding 20 ml of dH2O to the flask and evaporating to dryness in a 70 °C water bath. The removal of the monoterpenes was then verified by gas chromatography, as outlined below.

Sterol concentrations in the extracts were quantified on a Hewlett Packard (HP) 5890 Series II gas chromatograph with an Agilent (J&W Scientific, Inc.) DB-5 column (30 m × 0.25 mm, 0.25 μm film thickness) connected to a flame ionization detector. The helium carrier gas flow rate was 1.0 ml min−1 at 150 °C and a split of 1:20. The column temperature started at 150 °C and was increased 5 °C min−1 to 300 °C where it was held for 20 min. The injector and detector temperatures were 250 °C. Extracts were dissolved in hexanes, or 2:1 hexanes:methylene chloride (3 to 12 mg ml−1) containing isophytol as an internal standard, and 2 μl were injected. Compounds were quantified with three point standard curves using solutions of the commercial samples dissolved in methylene chloride with isophytol added as an internal standard. Compounds in leaf extracts were identified using the same conditions as above but with an Agilent (J&W Scientific, Inc.) DB-5MS column connected to an HP 5970 mass selective detector. The split was set at 1:10 and extract concentrations increased to about 20 mg ml−1.

The dry plant sterol residues were dissolved in ethanol and added directly to PSM media prior to autoclaving at a final concentration of 1, 5, 10, 50, or 100 μg extract ml−1. Due to the presence of non-sterol components in the extract (Table 1), the actual amount of sterols added to the PSM media differed among samples; therefore, all analyses and reported masses were based on the amount of α- and β-sitosterol in the extract (9.51, 5.08, and 6.92 % of the total sterol extract for bay laurel, black oak, and white oak samples, respectively).

Table 1 Foliar sterol and tannin concentrations
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Plant Tannin-enriched Extracts

Sub-samples (5 g) of the ground bay laurel, black oak, or white oak foliage were extracted × 3 with 100 ml of 70 % acetone for at least 4 hr at room temperature. Tannin fractions were purified after evaporating the extracts under nitrogen and re-dissolving each extract in 50 ml of ethanol. Ethanol samples were applied to 5 ml Sephadex LH-20 columns that were washed with at least 50 ml ethanol or until absorbance at 280 nm in the effluent was no longer detected, then eluted with 50 ml of 70 % acetone (Strumeyer and Malin 1975) and used directly as described below.

Total tannin content of the LH-20 column eluent was determined chemically and gravimetrically. The Folin-Denis assay (Folin and Denis 1912) was used to assess total phenols, using tannic acid (MP Biomedicals, Solon, OH, USA) as a standard. Condensed tannin was measured using the acid butanol assay (Porter et al. 1986) using procyanidin C1 (PHY89537, Cerriliant, Round Rock, TX, USA) as a standard. The concentration of galloyl-containing compounds (e.g., gallotannins, ellagitannins, and other galloyl esters) was measured using the rhodanine assay (Inoue and Hagerman 1988) with gallic acid as a standard. The remaining LH-20 column eluents were evaporated under nitrogen, weighed, and re-dissolved in ethanol at a final concentration of 10 mg extract ml−1. Tannins were added directly to PSM prior to autoclaving at a final concentration of 0.1, 1, 10, 25, and 50 μg dry mass ml−1.

Phytophthora ramorum Growth and Sporulation

Inoculated plates were incubated for 11 d at 18 °C in darkness before measuring average colony diameter (two perpendicular measurements per plate). Plates were then flooded with 8 ml of distilled H2O to induce sporangium formation, and incubated overnight at 18 °C in darkness. After 18 hr, a 200 μl aliquot of distilled H2O was removed from each plate and stored at 4 °C for elicitin determination using the ELISA assay described below. Plates were incubated for 2–4 hr at 4 °C before warming to room temperature to stimulate zoospore release. Zoospore solution was poured from each plate into a 17 × 100 mm, 14.0 ml culture tube, and vortexed two times for 20 sec to encyst swimming zoospores. The zoospores were quantified using a hemocytometer at 40× magnification.

ELISA Assay

Elicitin concentration was determined with a custom, indirect ELISA assay using rabbit anti-elicitin polyclonal antibodies (Covance Research, Denver, PA, USA) as described previously (Manter et al. 2010). Absorbance at 650 nm was recorded every 30 sec for 15 min with shaking using a Biotek ELx808 microplate reader (Winooski, VT, USA). Elicitin concentrations were determined using an external standard curve (100, 50, 25, 12.5, 6.3, 3.1, 1.6, 0.8, 0.4, 0.2 μM) using a purified recombinant ram-α2 protein (Manter et al. 2010).

Elicitin RT-qPCR

Liquid PSM medium, amended with either foliar sterols or tannins at the same concentrations used for the bioactivity assays, was inoculated with P. ramorum (PR-07-031) and cultured for 2 wk. Total RNA was extracted from the mycelium using the RNeasy Total RNA Extraction kit (Qiagen, Germantown, MD, USA), and cDNA was synthesized using the RETROscript kit (Applied Biosystems/Ambion, Austin, TX, USA) following the manufacturer’s recommendations. Elicitin (ram-α2) gene expression was determined using the primers, TaqMan probe, and procedures outlined in Manter et al. (2010). All reported values of ram-α2 gene expression are relativized to β-tubulin using the standard curve method.

Statistical Analysis

The effect of the various foliage, sterol, or tannin amendments on vegetative growth (i.e., colony diameter), zoospore production, ELISA-detected elicitin secretion, and elicitin gene expression were analyzed by nonlinear regression PROC NLIN (SAS, Vers 9.3). For each trial/treatment combination, response curves were fit in order to calculate 3 different parameters (X @ Y max , Y max , and Y i ) (see Tables 2, 3, 4 and 5). X @ Y max is the optimum amendment concentration that resulted in the maximum response (Y max ). For lognormal curves, values were determined from the 1st derivative of the response curve, such that f′(X) = 0. Y i is the calculated response at the highest amendment concentration tested. Significant differences in the calculated parameters (X @ Y max , Y max , and Y i ) between trial/treatment combinations were tested by ANOVA with Holm-Sidak post-hoc testing using PROC MIXED (SAS, Vers 9.3). Inhibition values reported throughout the text were calculated as the percent difference between Y i and Y max .

Table 2 Phytophthora ramorum colony diameter in response to various amendments. For each of three trials, response curves were fitted to each species/compound using PROC NLIN (SAS Vers 9.3). Values are the mean and SD of three independent trials. Values with different letters are significantly different (P < 0.05). X @ Y max units are the same as those defined in the source column, Y max and Y i units are mm
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Table 3 Phytophthora ramorum zoospore production in response to various amendments. For each of three trials, response curves were fitted to each species/compound using PROC NLIN (SAS Vers 9.3). Values are the mean and SD of three independent trials. Values with different letters are significantly different (P < 0.05). X @ Y max units are the same as those defined in the source column, Y max and Y i units are spores ml−1
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Table 4 ELISA-detectable Phytophthora ramorum elicitin production in response to various amendments. For each of three trials, response curves were fitted to each species/compound using PROC NLIN (SAS Vers 9.3). Values are the mean and SD of three independent trials. Values with different letters are significantly different (P < 0.05). X @ Y max units are the same as those defined in the source column, Y max and Y i units are nM.
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Table 5 Phytophthora ramorum ram-a2 elicitin gene expression in response to various amendments. For each of three trials, response curves were fitted to each species/compound using PROC NLIN (SAS Vers 9.3). Values are the mean and SD of three independent trials. Values with different letters are significantly different (P < 0.05). X @ Y max units are the same as those defined in the source column, Y max and Y i units are dimensionless
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Results

Leaf Tissue

Colony diameter exhibited a log-normal relationship in response to the leaf tissue amendment of the culture medium (Fig. 1a). The amount of leaf tissue required to reach maximum colony growth (X @ Y max ) was significantly lower for the oak tissue than for the bay laurel (Table 2). However, the maximum colony size did not differ significantly between the three plant-amended media (Table 2). Increasing the amount of leaf tissue inhibited colony growth, with the largest percent difference between Y i and Y max observed with the two oak species (Table 2).

Fig. 1
figure 1

Phytophthora ramorum colony diameter in response to a ground foliage, b foliar sterol extracts, c foliar tannin extracts, d β-sitosterol and cholesterol, and e ergosterol, stigmastanol and stigmasterol. Each point is the average (N = 9) value for all three trials

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Similar to colony diameter, zoospore production showed a log-normal response to leaf tissue amendment of the culture medium (Fig. 2a). However, maximum zoospore production differed between the bay laurel and oak amendments, with nearly five-fold more zoospore production with bay laurel-containing medium (Table 3). Inhibition was noted at the highest concentration tested for all three species, with about 40 % inhibition in zoospore production for the bay laurel treatment and nearly 100 % for the oaks (Table 3).

Fig. 2
figure 2

Phytophthora ramorum zoospore production in response to a ground foliage, b foliar sterol extracts, c foliar tannin extracts, d β-sitosterol and cholesterol, and e ergosterol, stigmastanol and stigmasterol. Each point is the average (N = 9) value for all three trials

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The amount of ELISA-detected elicitin in the samples amended with leaf tissue (Fig. 3a) was similar to the pattern observed for zoospore production. For example, maximum elicitin secretion was nearly 2.5-fold higher with bay laurel than with the oaks; and inhibition was only about 10 % with bay laurel, as compared to approximately 50 % with the oaks (Table 4).

Fig. 3
figure 3

ELISA-detectable Phytophthora ramorum elicitin secretion in response to a ground foliage, b foliar sterol extracts, c foliar tannin extracts, d β-sitosterol and cholesterol, and e ergosterol, stigmastanol and stigmasterol. Each point is the average (N = 9) value for all three trials

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Plant and Commercial Sterol Amendments

The foliage sterol extracts included a variety of sterol and non-sterol compounds: phytol, heptacosane, nonacosane, hentricontane, α-sitosterol, and β-sitosterol (Table 1). Although a statistical comparison among the three species is not possible due to a lack of replicate trees, the three extracts exhibited a range of sterol (α- and β-sitosterol) contents (1,745, 612, and 712 μg g−1 DW) for the bay laurel, black oak, and white oak extracts, respectively (Table 1). Similar to the leaf tissue amendments, there was a log-normal relationship between leaf sterol amendments and P. ramorum colony diameters (Fig. 1b), zoospore production (Fig. 2b), or elicitin secretion (Fig. 3b). For all three parameters, similar patterns were observed regardless of the origin of the plant sterol extract (Tables 1, 2 and 3). Similar to the leaf tissue amendments, inhibition was observed at the highest concentrations tested. For example, colony diameters were inhibited 15.3 % (Table 2), zoospore production 24.7 % (Table 3), and elicitin secretion 3.1 % (Table 4). The effect of plant sterols on elicitin gene expression, as measured by RT-qPCR, showed a log-normal response curve for all three plant species (Fig. 4a). The curves did not differ among species for any of the descriptors (X @ Y max , Y max , or Y i , Table 5). Similar to colony diameter and zoospore production, elicitin gene expression was inhibited at the highest concentration of foliar sterols (Table 5).

Fig. 4
figure 4

RT-qPCR Expression analysis of ramorum ram-α2 elicitin gene expression in response to a foliar sterol extracts and b foliar tannin extracts. Each point is the average (N = 9) value for all three trials

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Qualitative differences in the effect of sterols on P. ramorum growth and sporulation were tested using a variety of commercially available sterols (β-sitosterol, cholesterol, egrosterol, stigmastanol, and stigmasterol). For colony diameters, the response to sterol amendments was either log-normal (β-sitosterol, cholesterol) or exponential rise (ergosterol, stigmastanol, and stigmasterol) (Fig. 1d,e, Table 1). The only sterol with any inhibitory effect on colony size was β-sitosterol at high concentrations (Table 2). The different sterols had different effects on production of zoospores (Fig. 2d,e, Table 3). For example, a decline in zoospore production with high levels of sterols was observed for β-sitosterol (84.2 % inhibition) and cholesterol (37.8 % inhibition) (Table 3); whereas, zoospore production exhibited a positive linear response to ergosterol, stigmastanol, and stigmasterol. The effect of sterols on elicitin secretion was similar to the trends observed for zoospore production, except that β-sitosterol inhibited elicitin production by 54.3 %, while no inhibition was noted for any of the four other sterols (Fig. 3d,e, Table 4).

Tannins

Gravimetric estimates of leaf tannin content after Sephadex LH-20 purification indicated that the concentration of tannin was highest for black oak, moderate for white oak, and lowest for bay laurel (Table 1). Total phenolics (Folin-Denis), gallotannin (rhodanine), and condensed tannin (acid butanol) assays all showed similar patterns, with the highest concentrations present in black oak, followed by white oak, and the lowest in bay laurel (Table 1). Tannin amendments had an inhibitory effect on all parameters measured, except elicitin gene expression (Figs. 1c, 2c, 3c, 4b). With increasing tannin concentrations, a significant decline was seen for colony diameters (Table 2), zoospore production (Table 3), and ELISA-detected elicitin secretion (Table 4) across all three species. In contrast, P. ramorum ram-α2 gene expression remained constant at all levels of tannin amendments tested, suggesting that elicitin gene expression is not affected by tannin concentrations (Fig. 4, Table 5).

Colony diameter and zoospore production were highly correlated with the amount of ELISA-detected elicitin across all treatment and trials (Fig. 5). It is worth noting that these highly significant relationships hold true across the three plant species tested despite differing sterol and tannin concentrations and compositions. Furthermore, the amount of elicitin required to maximize colony diameter growth is much lower than is required for zoospore production (Fig. 5). Zoospore production did not appear to reach its maximal level in this study.

Fig. 5
figure 5

Relationship between ELISA-detectable Phytophthora ramorum elicitin secretion and a zoospore production or b colony diameter growth. Each point is the average (N = 9) value for all three trials. Symbol shapes denote amendment type—leaf tissue (circles), foliar sterol extracts (upward triangles), commercial sterols (downward triangles), foliar tannin extracts (squares).; symbol colors (gray intensity, from dark to light) denote plant species—bay (black), black oak (gray), white oak (no color) or commercial sterols—β-sitosterol (black), cholesterol (dark gray), ergosterol (gray), stigmastanol (light gray), stigmasterol (no color)

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Discussion

Phytophthora ramorum growth and sporulation exhibited two general trends relative to sterols: (i) the response curves differed depending upon the type of sterol present and (ii) high levels of sterols could lead to a decline in P. ramorum growth and sporulation, which is consistent with other Phytophthora spp. (Elliott et al. 1966; Haskins et al. 1964; Hendrix 1970; Leal et al. 1964; Nes and Stafford 1983). The mechanistic basis for the varying response curves, or sterol ‘discrimination’ is unknown; however, it may be dependent upon differences in sterol uptake and metabolism (Mikes et al. 1998; Nes and Stafford 1983, 1984) or gene expression (Yousef et al. 2009). Regardless of the mechanism, it is clear that optimal P. ramorum growth and sporulation was achieved at different levels for each of the sterols tested in this study. For example, P. ramorum zoospore production was maximized at ca. 14 μM for β-sitosterol and ca. 36 μM for cholesterol; whereas, a maximum was not reached for ergosterol, stigmastanol, or stgmasterol at the concentrations tested. The decline in P. ramorum growth and the amount of ELISA-detectable elicitins at high concentrations of either β-sitosterol or cholesterol is consistent with the ability of sterols to stimulate a down-regulation in elicitin gene expression in P. sojae (Yousef et al. 2009).

In this study, gene expression was not evaluated for each sterol individually; however, the foliar sterol extracts, comprised of mainly α- and β-sitosterol, did diminish elicitin gene expression at concentrations above ca. 4 μg ml−1. It is possible that the sterol-dependent down-regulation of Phytophthora spp. elicitin genes balances the need to evade plant detection, with the acquisition of sterols required for sporulation. For example, cholesterol is a major component of leaf surface lipids (Noda et al. 1988); whereas, β-sitosterol is often the major sterol component within plant tissues (Gunstone et al. 1994). Elicitin gene expression, and sterol acquisition, may be maximized on the leaf surface where sporulation occurs, and minimized within plant cells to avoid host defense responses. The strong relationship between sterol acquisition and Phytophthora growth and sporulation, and the apparent discrimination between sterols, suggests that plant sterol profiles may be useful predictors of plant susceptibility or sporulation of Phytophthora spp. However, in a study by Hazel et al. (1988), the sterol profiles of potato leaves were not suitable predictors of field resistance to P. infestans or its sporulation potential, indicating that other factors are involved.

Tannins may be one such factor, as evidenced by our data showing that tannin-enriched foliage extracts (Sephadex LH20 purified) negatively influence P. ramorum growth and colonization. The mechanistic basis for this inhibition, however, is unknown. One plausible mechanism may be the binding of elicitins by tannins, thus inhibiting sterol uptake and therefore inhibiting Phytophthora growth and colonization. Tannins, by definition, have the potential to bind and precipitate proteins; and the tannin-enriched extracts used in this study have the ability to bind and precipitate P. ramorum elicitins in solution (Manter, unpublished data). In the studies reported here, the tannin-enriched extracts reduced the amount of ELISA-detectable water-soluble elicitins (i.e., flooding of PSM agar plates) but did not influence elicitin gene expression. We hypothesize that tannin-elicitin binding may remove the elicitins from solution and/or interfere with the elicitin-antibody binding required for the ELISA-based detection. In either case, the amount of elicitin available for sterol binding (i.e., ‘functional’ elicitin) may be reduced leading to a decrease of P. ramorum growth and sporulation. We are unaware of any other studies that have examined the potential role of tannins on P. ramorum growth and sporulation; however, some simple phenolics (e.g., tyrosol, gallic acid) have direct antimicrobial activity against P. ramorum (Ockels et al. 2007). The mechanistic basis associated with their antimicrobial activity has not been studied; however, it is interesting to note that these simple phenolics are the precursors for the polyphenolic tannins (Bianco and Savolaninen 1997) and may be indicative of increased tannin production in infected tissue.

The studies reported here were conducted in vitro; therefore, it is unknown to what degree elicitins, tannins, and sterols interact in planta. However, there is some evidence that this interaction may occur. For example, both tannins (Evert 2006) and elicitins (Brummer et al. 2002; Osman et al. 2001) associate with plant cell walls suggesting that their interaction is spatially possible; although, whether this truly happens can only be found through further in planta research. Furthermore, additional work with other plant tissues, i.e., bole/stems, needs to be pursued, as P. ramorum colonization is often limited to the bole in a variety of oak species. Due to the limitations of the tannin assays used in this study, additional studies will also be required to determine the specific compound(s) in the tannin-enriched extract or any other plant compound(s) that interfere with elicitin activity and subsequent P. ramorum growth and sporulation. In summary, our data indicate that the amount of ‘functional’ elicitin (i.e., ELISA-detectable and presumably the amount of elicitin available to bind sterols), not necessarily elicitin gene expression, is influenced by both sterol and tannin contents and is highly correlated with P. ramorum growth and sporulation in vitro.