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

Joana Amaral,Carla António,Luis Valledor,Artur Alves,Barbara Correia,Robert D. Hancock,Aurelio Gómez-Cadenas,Glória Pinto,Ana Margarida Rodrigues

doi:10.3389/fpls.2019.00509

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.

Highlights

Pine pitch canker (PPC), caused by Fusarium circinatum, affects Pinus species and Pseudotsuga menziesii worldwide (Wingfield et al, 2008; European Food Safety Authority [EFSA], 2010)
P. radiata and P. pinaster presented significantly higher percentages of relative internal stem necrosis than controls when inoculated with F. circinatum (Figure 1B), as represented on the stereomicroscope images (Figure 1C)
The timing and intensity of disease symptoms observed after F. circinatum inoculation in the Pinus species tested agrees with the levels of susceptibility described by Bragança et al (2009)

Summary

Introduction

Pine pitch canker (PPC), caused by Fusarium circinatum, affects Pinus species and Pseudotsuga menziesii worldwide (Wingfield et al, 2008; European Food Safety Authority [EFSA], 2010). In nurseries its symptoms include damping-off and wilting of seedlings and, on mature trees, branch die-back, stem cankers, pitch formation and mortality (Wingfield et al, 2008). Unravelling Pinus–Fusarium circinatum Interaction ha are potentially threatened by PPC in Europe (European Food Safety Authority [EFSA], 2010), where it is recommended as a quarantine pathogen (Decision 2007/433/EC of 18 June 2007). Climate change may shift F. circinatum distribution toward Europe (Watt et al, 2011), endangering Pinaceae presently in pathogen-free areas such as Picea abies (Martín-García et al, 2017). Given the significance of conifer forests worldwide, the threat posed by F. circinatum should be urgently considered

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