Genes expressed in grapevine leaves reveal latent wood infection by the fungal pathogen Neofusicoccum parvum

Authors: CZEMMEL, STEFAN - University Of Nevada; Galarneau, Erin; TRAVADON, RENAUD - University Of California; McElrone, Andrew; CRAMER, GRANT - University Of Nevada; Baumgartner, Kendra

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2015
Publication Date: 3/23/2015
Citation: Czemmel, S., Galarneau, E.R., Travadon, R., Mcelrone, A.J., Cramer, G., Baumgartner, K. 2015. Genes expressed in grapevine leaves reveal latent wood infection by the fungal pathogen Neofusicoccum parvum. PLoS One. doi: 10.1371/journal.pone.0121828.

Interpretive Summary: Fungal pathogens that live inside the wood of plants are difficult to detect as the disease is building. For diseases caused by such pathogens, it is typically not until the leaves start to show symptoms or the plant dies that we know the pathogen is present. Detection of the early stage of infection, before the plant is suffering from the disease, would benefit from identification of genes in the plant’s leaves that signal the presence of the pathogen. The fungus Neofusicoccum parvum (causal agent of Botryosphaeria dieback) infects the wood of grapevine and other horticultural crops (e.g., pistachio, almond). We determined when the early stage of infection occurs, in terms of time after inoculation, the spread of the pathogen in the woody stem, and microscopic changes in the wood cells. From the leaves, we compared grape genes expressed in inoculated vs. non-inoculated plants. The early stage of infection occurred between 0 and 1.5 months post-inoculation (MPI), after which point the average size of stem lesions increased 3-fold. This coincided, at 2 MPI, microscopic changes in wood cells, namely the formation of gels that physically block the water-conducting elements (xylem vessels) and a decrease in the starch content. From the leaves, we identified 20 candidate genes activated by infection between 0 and 1.5 MPI. The best candidates were a galactinol synthase, an abscisic acid-induced wheat plasma membrane polypeptide-19 ortholog, an embryonic cell protein 63, and a BURP domain-containing protein. By screening the results of other similar gene-expression studies on grape, we found that these three candidate genes and several others from our group fo 20 were not affected by a range of abiotic or biotic stresses (e.g., foliar pathogen powdery mildew, wood-infecting pathogen Eutypa lata).

Technical Abstract: Detection of wood-infecting pathogens is often limited to the late stage of infection, when disease symptoms are obvious. Detection of the early stage of infection would benefit from identification of host-based markers in asymptomatic leaves. The fungus Neofusicoccum parvum (Botryosphaeria dieback) infects the wood of grapevine and other horticultural crops. The early stage of wood infection was characterized, in terms of the spatio-temporal relationship between pathogen colonization and anatomical changes, using light microscopy and high-resolution computed tomography. To identify host-based markers of infection, leaves from inoculated vs. non-inoculated plants were examined for differentially-expressed genes using RNA-Seq. The early stage of infection occurred between 0 and 1.5 months post-inoculation (MPI), after which stem lesions increased 3-fold. This coincided, at 2 MPI, with an increase in gel-occluded xylem vessels and a decrease in starch content in xylem fibers and rays. RNA-Seq identified 20 candidate genes transcriptionally-activated by infection, but not wounding, between 0 and 1.5 MPI. Screening 21 grape expression datasets confirmed the best candidates (galactinol synthase, abscisic acid-induced wheat plasma membrane polypeptide-19 ortholog, embryonic cell protein 63, BURP domain-containing protein) were not affected by a range of abiotic or biotic stresses (e.g., foliar pathogen powdery mildew, wood-infecting pathogen Eutypa lata).