Production of defensive metabolites by Pinus patula X Pinus tecunumanii hybrids in response to Fusarium circinatum infection

Abstract

The fungal pathogen Fusarium circinatum, causal agent of pitch canker disease, is currently one of the biggest threats to pine health worldwide. Symptomatic infection is associated with high mortality rates and reduced growth volume, resulting in significant annual losses for the forestry industry. Pines respond to insect damage and fungal infection by forming traumatic resin ducts, as well as significantly upregulating the production of defence compounds. These phytochemicals include terpenes, the main chemical constituents of pine resin, and phenolics, produced in specialized cells of the secondary phloem. Many of the compounds belonging to these two phytochemical groups are known to have inhibitory or lethal effects on pine pests and pathogens.

Although most Pinus species are susceptible to F. circinatum infection, there is significant variation in susceptibility to this pathogen among the different species and their interspecific hybrids. Resistance of a species to the pitch canker fungus is a major determining factor in its value to the pine industry, however, the underlying mechanisms of this phenomenon are poorly understood. Research in the role of phytochemicals in pine defence against infections by F. circinatum could aid in breeding resistant trees for commercial exploitation.

Therefore, the aim of this MSc study was to explore chemical defence response of susceptible and resistant Pinus hybrid crosses after inoculation with F. circinatum. Gas- and liquid-chromatography coupled to mass-spectrometry were used to characterize the phytochemical changes in young P. patula X P. tecunumanii hybrid clones in response to F. circinatum infection. A significant increase was observed in terpene and phenolic production in infected saplings between five and 14 days post-inoculation, compared to mechanically wounded plants. However, more resistant hybrid plants with less severe disease symptoms produced significantly lower concentrations of defensive phytochemicals, both in response to wounding and to F. circinatum infection. These findings suggest that increased concentrations of terpenoid oleoresin and phenolics are not part of the defence strategy of pine against infection by F. circinatum.