Sequential Delivery of Host-Induced Virulence Effectors by Appressoria and Intracellular Hyphae of the Phytopathogen Colletotrichum higginsianum

Phytopathogens secrete effector proteins to manipulate their hosts for effective colonization. Hemibiotrophic fungi must maintain host viability during initial biotrophic growth and elicit host death for subsequent necrotrophic growth. To identify effectors mediating these opposing processes, we deeply sequenced the transcriptome of Colletotrichum higginsianum infecting Arabidopsis. Most effector genes are host-induced and expressed in consecutive waves associated with pathogenic transitions, indicating distinct effector suites are deployed at each stage. Using fluorescent protein tagging and transmission electron microscopy-immunogold labelling, we found effectors localised to stage-specific compartments at the host-pathogen interface. In particular, we show effectors are focally secreted from appressorial penetration pores before host invasion, revealing new levels of functional complexity for this fungal organ. Furthermore, we demonstrate that antagonistic effectors either induce or suppress plant cell death. Based on these results we conclude that hemibiotrophy in Colletotrichum is orchestrated through the coordinated expression of antagonistic effectors supporting either cell viability or cell death

The stress kinase gene MtSK1 in Medicago truncatula with particular reference to somatic embryogenesis

Medicago truncatula, a model for legume genomics, can be regenerated by somatic embryogensis by the use of a suitable genotype and an auxin plus cytokinin. The stress response induced by explant wounding and culture is increasingly recognized as an important component of somatic embryo induction. We have cloned and investigated the stress kinase gene MtSK1 in relation to somatic embryogenesis in M. truncatula, using the highly embryogenic mutant Jemalong 2HA (2HA) and its progenitor Jemalong. The main features of the MtSK1 protein of 351 amino acids are an N-terminal kinase domain and a C-terminal glutamic acid-rich region, which is predicted to be a coiled-coil. MtSK1 is a member of the SnRK2 subgroup of the SnRK group of plant kinases. Members of the SnRK2 kinases play a role in stress responses of plants. MtSKI expression is induced by wounding in the cultured tissue independent of auxin or cytokinin. However, in both 2HA and Jemalong, as the callus develops in response to auxin plus cytokinin, MtSK1 expression continues to increase. MtSK1 responds to salt stress in vivo, consistent with its role as a stress kinase. The likely role of MtSK1 in stress-induced signaling will facilitate the relating of stress–response pathways to auxin and cytokinin-induced signaling in the understanding of the molecular mechanisms involved in the induction of somatic embryogenesis in M. truncatula