Abstract
Summary A number of genes that confer resistance to coffee leaf rust (SH1–SH9) have been identified within the genus Coffea, but despite many years of research on this pathosystem, the complementary avirulence genes of Hemileia vastatrix have not been reported.After identification of H. vastatrix effector candidate genes (HvECs) expressed at different stages of its lifecycle, we established an assay to characterize HvEC proteins by delivering them into coffee cells via the type‐three secretion system (T3SS) of Pseudomonas syringae pv. garcae (Psgc).Employing a calmodulin‐dependent adenylate cyclase assay, we demonstrate that Psgc recognizes a heterologous P. syringae T3SS secretion signal which enables us to translocate HvECs into the cytoplasm of coffee cells. Using this Psgc‐adapted effector detector vector (EDV) system, we found that HvEC‐016 suppresses the growth of Psgc on coffee genotypes with the SH1 resistance gene. Suppression of bacterial blight symptoms in SH1 plants was associated with reduced bacterial multiplication. By contrast, HvEC‐016 enhanced bacterial multiplication in SH1‐lacking plants.Our findings suggest that HvEC‐016 may be recognized by the plant immune system in a SH1‐dependent manner. Thus, our experimental approach is an effective tool for the characterization of effector/avirulence proteins of this important pathogen.
Highlights
Coffee leaf rust, caused by the biotrophic pathogen Hemileia vastatrix, is the most important fungal disease affecting coffee production
Of the 146 predicted secreted proteins, 49 (33.6%) have no significant similarity (BLASTP E-value > 10À5) to sequences deposited in the nonredundant NCBI databases (Fig. 1a)
Two sequences (1.4%) were deposited in the nonredundant NCBI (nr-NCBI) databases already as H. vastatrix proteins; these correspond to H. vastatrix effector candidate genes (HvECs)-033 and HvEC-045 (Table S2)
Summary
Coffee leaf rust, caused by the biotrophic pathogen Hemileia vastatrix, is the most important fungal disease affecting coffee production. 49 races of the pathogen have been described (Gichuru et al, 2012) Such a large number of races is consistent with the high genetic variability reported for the fungus (Nunes et al, 2009; Maia et al, 2013). These various races provide an important framework for understanding the genetics of coffee resistance against rust
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