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Mechanisms of drought-induced susceptibility of Austrian pine to Diplodia pinea
P. SHERWOOD (1), C. Villari (1), P. Capretti (2), P. Bonello (1). (1) The Ohio State Univ, Columbus, OH, U.S.A.; (2) Università degli Studi di Firenze, Firenze, Italy

Water limitation is an important abiotic factor that often predisposes plants to infection. However, the molecular mechanisms of this induced susceptibility are poorly understood, particularly in trees. Here we studied the molecular responses of <i>Pinus nigra </i>and its susceptibility to the pathogen <i>Diplodia pinea</i>. Drought increased host susceptibility, and resulted in the accumulation of proline and reactive oxygen species (ROS) in the shoots before inoculation. However, ROS levels were lower and proline concentrations higher around infection sites relative to non-inoculated shoots on droughted trees. Proline has several beneficial roles in stress responses including an ability to scavenge ROS and act as a nutrient source. We hypothesized these functions may be contributing to <i>D.</i> <i>pinea</i>’s success in stressed pines, because proline enhanced the <i>in vitro</i> growth of <i>D. pinea </i>by functioning as a preferred N source while also protecting the fungus from H2O2 damage. Furthermore, hyperactive proline metabolism can trigger apoptosis, which may facilitate <i>D.</i> <i>pinea</i>’s necrotrophic lifestyle. When challenged <i>in vitro </i>with H2O2, <i>D.</i> <i>pinea</i>’s catalase and peroxidase activities were induced, suggesting that the pathogen has substantial ROS detoxifying abilities. We propose that <i>D. pinea </i>is particularly successful in droughted trees because it is well adapted to water stress-induced shifts in host N and ROS metabolism that can lead to apoptosis.

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