Synthesis and accumulation of benzoic acid in Pinus radiata needles in response to tissue injury by dothistromin, and correlation with resistance of P. radiata families to Dothistroma pini

Abstract

Within 5 days of the introduction of dothistromin into Pinus radiata needles, red band symptoms typical of those produced by Dothistroma pini appeared. When injected into needles From different full-sib families, a continuous distribution of necrotic lesion length was obtained ranging from short ( 10 mm). The response was dependent on the age of tissue and, possibly, the season. The maximum response was obtained with currently emerging (harvested in early summer), and the minimum response with 1-year-old needles (harvested in winter), Injected dothistromin was metabolized by needle cells, or photolytically degraded, to a low but persistent level (e.g. 10–20% of the initial dose) within 24 h, concomitant with the formation of a small (approx. 2–5 mm) necrotic lesion. The detectable products of dothistromin metabolism were CO 2 and oxalic acid. Based on the formation of the metabolites, a mechanism involving peroxidase catalysed oxidation of dothistromin by hydrogen peroxide is proposed. Cells adjacent to those killed by dothistromin themselves die and the initial lesion rapidly expands over 2–3 days. These adjacent cells have been shown to synthesize and accumulate benzoic acid, which was toxic to P. radiata needle mesophyll cells and highly fungistatic to D. pini, in vitro . Yellow, necrotic needle tissue adjacent to the red band lesions, containing mycelium, of natural D. pini infections also accumulated benzoic acid, which prevents immediate colonisation of that tissue by the fungus. Benzoic acid is therefore proposed as a phytoalexin of P. radiata needles. The length of the dothistromin-induced lesion was proportional to the amount of benzoic acid isolated from the dead tissue, and a linear dose-response relationship was obtained for benzoic acid injected into P. radiata needles. The dothistromin-induced lesions were usually terminated by narrow (0·1–0·5 mm) areas of dark green tissue. The cells in this tissue were highly lignified, the lignin deposited being disproportionately high in 4-hydroxyphenyl residues compared with the guaiacyl residue content. The lignin polymer also contained bound benzoic acid. The magnitude of the response of needles to dothistromin has been correlated with observed field resistance to D. pini among 7-year-old control-cross progeny comprising 50 full-sib P. radiata families. This correlation supports the hypothesis that a rapid response of P. radiata needles to dothistromin accompanied by accumulation of the phytoalexin, benzoic acid, is a mechanism of resistance of P. radiata to D. pini .