Wound-associated responses in Sitka spruce root bark challenged with Phaeolus schweinitzii

Abstract

The sequence of events leading to the restoration of an intact periderm surface in Sitka spruce root bark following wounding and challenge with the weakly pathogenic wood decaying fungus Phaeolus schweinitzii has been determined. During the first 3–10 days after injury cells in the immediate vicinity of the wound became necrotic and accumulated oxidized polyphenolic compounds in association with their walls. By day 10, an accumulation of phloroglucinol-HCl-reactive material was detected in the walls of living cells adjacent to the necrotic region. These walls became thickened between 24 and 48 days. During this period a wound periderm differentiated below this region. In sections first treated with chlorine dioxide to remove wall-bound phenolic material, an early onset of suberization in pre-existing cells was evident by day 13. By day 48 this treatment revealed the development, by the wound periderm, of three or four cell layers of thin-walled phellem. In the absence of chlorine dioxide extraction, only a single layer of suberized wound phellem was evident in sections stained with Sudan dyes for suberin. Fungal colonization in all wounds (including controls, which were frequently contaminated by Penicillium spp.) was superficial; hyphae were confined to the necrotic tissues, and never seen more than a few cell layers below wound surfaces. The significance of early suberization and the overall role of these structural responses in the antimicrobial defence of Sitka spruce roots is discussed and a model for the resistance of this conifer to penetration by potentially pathogenic fungi is presented.