A Bacterial Type Three Secretion-Based Delivery System for Functional Characterization ofSporisorium scitamineumPlant Immune Suppressing Effector Proteins

Abstract

The facultative biotrophic basidiomycete Sporisorium scitamineum causes smut disease in sugarcane. This study applied an assay to identify S. scitamineum candidate effectors (CEs) with plant immunity suppression activities by delivering them into Nicotiana benthamiana cells via the type-three secretion system of Pseudomonas fluorescens EtHAn. Six CEs were individually cloned into the pEDV6 vector and expressed by P. fluorescens EtHAn for translocation into the plant cells. Three CEs (g1052, g3890, and g5159) could suppress pattern-triggered immunity (PTI) responses with high reproducibility in different co-infiltration experiments with Pseudomo-nas syringae pv. tomato DC3000. In addition, three CEs (g1052, g4549, and g5159) were also found to be AvrB-induced suppressors of effector-triggered immunity (ETI), demonstrating for the first time that S. scitamineum can defeat both PTI and ETI responses. A transcriptomic analy-sis at different stages of infection by the smut fungus of three sugarcane cultivars with con-trasting responses to the pathogen revealed that suppressors g1052, g3890, g4549, and g5159 were induced at the early stage of infection. By contrast, the two CEs (g2666 and g6610) that did not exhibit suppression activities expressed only at the late stage of infection. Moreover, ge-nomic structures of the CEs and searches for orthologs in other smut species suggested duplica-tion events and further divergence in CEs evolution of S. scitamineum. Thus, the transient assay applied here demonstrated the potential of pEDV6 and P. fluorescens EtHAn as biological tools for identifying plant immune suppressors from S. scitamineum.

Identification of Gene Modules and Hub Genes Associated with Sporisorium scitamineum Infection Using Weighted Gene Co-Expression Network Analysis

“…A total of 68 effector candidates have been identified in the S. scitamineum genome [ 6 ]; while little is known about their roles in plants, only SsPele1 and SsPEP1 have been functionally characterized to date [ 7 ]. Additionally, the other five candidates, which contain characteristics of signal peptides, low molecular mass, and upregulation in planta , could affect programmed cell death (PCD) by using transient expression on Nicotiana benthamiana [ 8 , 9 ]. A lack of sufficient knowledge on effectors has prevented a complete understanding of the pathogenesis of sugarcane smut.…”