Pinus Susceptibility to Pitch Canker Triggers Specific Physiological Responses in Symptomatic Plants: An Integrated Approach

Abstract

Fusarium circinatum , the causal agent of pine pitch canker (PPC), is an emergent and still understudied risk that threatens Pinus forests worldwide, with potential production and sustainability losses. In order to explore the response of pine species with distinct levels of susceptibility to PPC, we investigated changes in physiology, hormones, specific gene transcripts, and primary metabolism occurring in symptomatic Pinus pinea , Pinus pinaster , and Pinus radiata upon inoculation with F. circinatum . Pinus radiata and P. pinaster exhibiting high and intermediate susceptibility to PPC, respectively, suffered changes in plant water status and photosynthetic impairment. This was associated with sink metabolism induction, a general accumulation of amino acids and overexpression of pathogenesis-related genes. On the other hand, P. pinea exhibited the greatest resistance to PPC and stomatal opening, transpiration increase, and glycerol accumulation were observed in inoculated plants. A stronger induction of pyruvate decarboxylase transcripts and differential hormones regulation were also found for inoculated P. pinea in comparison with the susceptible Pinus species studied. The specific physiological changes reported herein are the first steps to understand the complex Pinus–Fusarium interaction and create tools for the selection of resistant genotypes thus contributing to disease mitigation.

Effect of Trichoderma viride pre‐inoculation in pine species with different levels of susceptibility to Fusarium circinatum: physiological and hormonal responses

“…Regardless of T. viride pre‐inoculation, the pine species studied reflected the levels of susceptibility to PPC previously reported (Amaral et al , ). Fusarium circinatum inoculation induced similar physiological changes in P. radiata and P. pinaster plants with symptoms affecting plant water status, stomatal aperture and photosynthesis, and led to the accumulation of proline and ABA, as reported by Amaral et al (). Besides water stress response, the ABA signalling pathway has been previously shown to be hijacked by Fusarium spp.…”

“…Treatments without T. viride received the same volume of sterile distilled water. Fusarium circinatum inoculations were carried out by the stem inoculation technique (Amaral et al , ) 2 weeks after T. viride inoculations. The timing between BCA treatment and pathogen challenge was based on a previous study (Martín‐García et al , ).…”

“…All plants were evaluated daily throughout the experiment by visual observation to assess disease symptoms (tip dieback, needle wilting and chlorosis and resin formation). As Pinus with different levels of susceptibility to F. circinatum were studied, independent sampling points were carried out for each species when at least 50% of the plants of one of the groups inoculated with F. circinatum expressed disease symptoms (Cerqueira et al , ; Amaral et al , ): the first sampling point (A1) was performed for P. radiata , the most susceptible species; the second sampling point (A2) corresponded to when P. pinaster disease symptoms were established; and the third sampling point (A3) was performed when the relatively resistant P. pinea showed disease symptoms.…”

“…Needles were collected, frozen in liquid nitrogen and kept at −80 °C for further quantification of pigments, proline and hormones. Pigments (total chlorophyll, carotenoids and anthocyanins) and hormones (ABA, JA and SA) were quantified as described by Amaral et al (), while proline concentration was assessed following the procedure described by Cerqueira et al ().…”

“…Changes in antioxidant activity and carbohydrates have also been reported for P. pinaster after F. circinatum inoculation (Vivas et al , , ). In addition, Amaral et al () reported that PPC affects P. radiata and P. pinaster water status and photosynthesis, induces sink metabolism (i.e. consumption and allocation of photoassimilates), and is associated with amino acid accumulation and overexpression of pathogenesis‐related genes, contrasting with the responses found for the relatively resistant P. pinea .…”