Chemical Ecology & Phytochemistry-2006

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16 PHENOLICS SCHMIDT, et al. Another large group of secondary metabolites that often has defensive roles is phenolics. 46 In extensive anatomical studies of P. abies stems, Franceschi, Krekling and coworkers showed that wounding or fungal infection not only induced the formation of traumatic ducts with terpene resin, but also changes in certain cells believed to produce phenolics. They described a cell type found in secondary phloem referred to as polyphenolic parenchyma (PP) cells 24,47-50 (Fig. 1.1). These occur in concentric rings, 1-2 cells thick, surrounded by sieve cells. One ring of PP cells is formed per year. 48 The vacuoles of these cells harbor a material that appears to be phenolic, based on its intense fluorescence under 450-490 nm light 47 and strong staining with the periodic acid-Schiff procedure. 24 In addition, phenylalanine ammonia lyase, a major enzyme in plant phenolic formation has been localized to the PP cells by immunolocalization. 47 Upon wounding or fungal infection, the PP cells increase in size with a strong increase in periodic acid-Schiff’s staining, and the phenolic material appears to be released to the wall of surrounding cells 24,49 (Fig. 1.1). Are these changes associated with alterations in phenolic chemistry? In recent years, phenolic compounds have been identified in spruce bark, including stilbenes, flavonoids, and tannins. 51-54 Studies have looked for changes in phenolic quantity and composition after wounding or fungal infection. 55-58 However, the changes observed were unremarkable (increase or decrease of 2-fold or less) or poorly replicated. Thus, it is still unclear what changes in phenolic chemistry are associated with the dramatic changes observed in the anatomy of the PP cells. We looked for changes in the soluble phenolic content of P. abies saplings and mature trees after methyl jasmonate spraying by HPLC analysis of methanol bark extracts (Fig. 1.9). However, as in previous work, none of the major stilbenes or flavonoids showed any substantial changes over 4 weeks after treatment (Fig. 1.10), although the PP cells showed similar anatomical changes as after fungal infection. Perhaps other phenolics that have not yet been measured in P. abies bark are the ones associated with the anatomical changes in the PP cells, such as high molecular weight condensed tannins 59,60 or cell wall-bound substances. 61 These other phenolics may also be responsible for conifer protection against pathogens and bark beetles since the defensive role of stilbenes and flavonoids and simple phenylpropanoids described is ambiguous. In vitro tests showed that these compounds have antifungal properties against certain pathogens, 62,63 but not others, 57 and they did not affect bark beetle feeding. 64 Moreover, crude methanol extracts (which would be expected to contain nearly all stilbenes, flavonoids, and simple phenolic conjugates present) exhibited little or no inhibition of fungal growth. 57 Perhaps the situation in vitro is different due to the presence of activating enzymes or other factors. To clarify their importance in conifer defense, attempts must be made to carry out more extensive phenolic analyses and to manipulate their levels in vivo.
INDUCED CHEMICAL DEFENSES IN CONIFERS 17 2500 OH OH OH OH OMe OH 2000 Response (mAU) 1500 1000 OH OH Catechin O OH OH OH O-Glucose E-Astringin OH O-Glucose E-Piceid OH O-Glucose E-Isorhapontin 500 12 13 14 15 16 17 18 19 20 Retention time (min) Fig. 1.9: HPLC chromatogram of methanol extract of two year-old Norway spruce saplings showing the presence of soluble phenolic compounds. The chemical structures of the major compounds are shown. Content (mg/g FW) 3.6 3.0 2.4 1.8 1.2 0.6 0.0 Control MeJA Catechin Astringin Isorhapontin Treatment Fig. 1.10: Effect of methyl jasmonate (MeJA) treatment on the levels of the major soluble phenolic compounds in two year-old P. abies saplings. No significant differences were observed between treated and control plants.
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