The Eucalyptus Cuticular Waxes Contribute in Preformed Defense Against Austropuccinia psidii

Abstract

<i>Austropuccinia psidii</i>, the causal agent of myrtle rust, is a biotrophic pathogen whose growth and development depends on the host tissues. The uredospores of <i>A. psidii</i> infect <i>Eucalyptus</i> by engaging in close contact with the host surface and interacting with the leaf cuticle that provides important chemical and physical signals to trigger the infection process. In this study, the cuticular waxes of <i>Eucalyptus</i> spp. were analyzed to determine their composition or structure and correlation with susceptibility/resistance to <i>A. psidii</i>. Twenty-one <i>Eucalyptus</i> spp. in the field were classified as resistant or susceptible. The resistance/susceptibility level of six <i>Eucalyptus</i> spp. were validated in controlled conditions using qPCR, revealing that the pathogen can germinate on the eucalyptus surface of some species without multiplying in the host. CG-TOF-MS analysis detected 26 compounds in the <i>Eucalyptus</i> spp. cuticle and led to the discovery of the role of hexadecanoic acid in the susceptibility of <i>Eucalyptus grandis</i> and <i>Eucalyptus phaeotricha</i> to <i>A. psidii</i>. We characterized the epicuticular wax morphology of the six previously selected <i>Eucalyptus</i> spp. using scanning electron microscopy and observed different behavior in <i>A. psidii</i> germination during host infection. It was found a correlation of epicuticular morphology on the resistance to <i>A. psidii</i>. However, in this study, we provide the first report of considerable interspecific variation in <i>Eucalyptus</i> spp. on the susceptibility to <i>A. psidii</i> and its correlation with cuticular waxes chemical compounds that seem to play a synergistic role as a preformed defense mechanism.