Apoplastic effector candidates of a foliar forest pathogen trigger cell death in host and non-host plants

Abstract

Forests are under threat from pests, pathogens, and changing climate. A major forest pathogen worldwide is the hemibiotroph Dothistroma septosporum, which causes dothistroma needle blight (DNB) of pines. While D. septosporum uses effector proteins to facilitate host infection, it is currently unclear whether any of these effectors are recognised by immune receptors to activate the host immune system. Such information is needed to identify and select disease resistance against D. septosporum in pines. We predicted and investigated apoplastic D. septosporum candidate effectors (DsCEs) using bioinformatics and plant-based experiments. We discovered DsCEs that trigger cell death in the angiosperm Nicotiana spp., indicative of a hypersensitive defence response and suggesting their recognition by immune receptors in non-host plants. In a first for foliar forest pathogens, we developed a novel protein infiltration method to show that tissue-cultured pine shoots can respond with a cell death response to a DsCE, as well as to a reference cell death-inducing protein. The conservation of responses across plant taxa suggests that knowledge of pathogen–angiosperm interactions may also be relevant to pathogen–gymnosperm interactions. These results contribute to our understanding of forest pathogens and may ultimately provide clues to disease immunity in both commercial and natural forests.

Characterization of two conserved cell death elicitor families from the Dothideomycete fungal pathogens Dothistroma septosporum and Fulvia fulva (syn. Cladosporium fulvum)

“…In our previous work, two small, secreted candidate effector proteins from D. septosporum NZE10, Ds70694 and Ds70057, triggered cell death in the non-host species N. tabacum and N. benthamiana (Hunziker et al, 2021). Both proteins are homologs of Ecps from F. fulva and are part of two different protein families, Ecp20 and Ecp32, respectively (Mesarich et al, 2018).…”

“…Following on from a previous study in which DsEcp20-3 and DsEcp32-3 were shown to elicit cell death in the non-host plants N. benthamiana and N. tabacum using ATTAs (Hunziker et al, 2021), we set out to determine whether other family members of both D. septosporum and F. fulva could elicit the same response. To achieve this, all Ecp20 and Ecp32 family members were transiently expressed in N. benthamiana and N. tabacum using ATTAs, and the percentages of plant responses in three different categories recorded: strong cell death, weak cell death and no cell death (Figure 5; Supplementary Figure S6).…”

“…The close phylogenetic relationship between D. septosporum and F. fulva, however, provides a unique opportunity for comparative analysis of core effector functions across angiosperm and gymnosperm pathosystems. Recently, a set of candidate effector proteins from D. septosporum was transiently expressed in the non-host plants, Nicotiana benthamiana and N. tabacum, to identify those that are involved in triggering plant defense responses (Hunziker et al, 2021). Several of these proteins, all with orthologs in F. fulva, elicited a plant cell death response, suggesting that they are recognized by extracellular immune receptors in these non-host plants, triggering non-host resistance.…”

“…Several of these proteins, all with orthologs in F. fulva, elicited a plant cell death response, suggesting that they are recognized by extracellular immune receptors in these non-host plants, triggering non-host resistance. Amongst these we found two proteins, Ds70057 and Ds70694, that were of particular interest, as they are orthologs of Ecps from F. fulva (Hunziker et al, 2021; Mesarich et al, 2018). Both proteins appear to be part of protein families – the Ecp20 family and the Ecp32 family (Hunziker et al, 2021).…”

“…Amongst these we found two proteins, Ds70057 and Ds70694, that were of particular interest, as they are orthologs of Ecps from F. fulva (Hunziker et al, 2021; Mesarich et al, 2018). Both proteins appear to be part of protein families – the Ecp20 family and the Ecp32 family (Hunziker et al, 2021). The Ecp20 family is made up of four members in both species, while the Ecp32 family is made up of four members in D. septosporum and five members in F. fulva.…”