Defense response of pine stem phloem to wounding and treatment with mycelial extracts from Ceratocystis laricicola

Abstract

Ophiostomatoid fungi colonize the conducting tissues of conifer stems, the phloem and the xylem. These pathogenic fungi penetrate into the stem through injuries made by xylophagous insects vectoring these pathogens. In this study the response of the phloem of Scotch pine (Pinus sylvestris L.) to wounding (treatment 1) was compared with the response to wounding combined with application of high-molecular-weight compounds isolated from the mycelium of the ophiostomatoid fungus Ceratocystis laricicola Redfern & Minter (treatment 2). Both treatments induced the appearance of necrosis in the inner bark, the formation of periderm separating living and dead tissues, and formation of the callus alongside the wound perimeter. In addition, the bark accumulated lignin, bound proanthocyanidins, and resins, with a parallel decrease in the content of free proanthocyanidins, low-molecular-weight carbohydrates, and non-lignin components of the cell wall (P > 0.95). The size of necrotic spots, as well as changes in the content of most substances, were significantly higher in the treatment 2 than in the treatment 1 (P > 0.95). The accumulation of lignin in cell walls of phloem sieve cells was delayed in the treatment 2 as compared with that in the treatment 1. This suggested that the mycelial extract temporarily inhibited lignification at the early stage of the wound response. This disturbance of the cell wall protective transformation led to the hypothesis that the fungal suppressors retard the repair of inner bark injured by insects, thereby favoring the invasion of conifer tissues by ophiostomatoid fungi.