Expression profiles of Mycosphaerella graminicola candidate effector genes

Mycosphaerella graminicola causes one of the most damaging diseases of wheat. M. graminicola has a large arsenal of secreted proteins expressed during infection that includes Candidate Effectors (CE, n ≥ 171)1. However, it remains unclear how CEs facilitate the initial symptomless phase, support fungal invasion and trigger the formation of necrotic sporulating lesions. Using the annotated IPO323 reference genome2, we selected 27 genes encoding small secreted proteins (i.e., MgSSPs) likely specifically expressed during infection according to EST data. We explored the temporal expression of these genes by quantitative RT-PCR during compatible and incompatible interactions. We have confirmed that most of the selected MgSSPs (75%) are over-expressed (10 to 8000 fold) during plant infection compared to mycelium grown in vitro. Among these MgSSPs, we observed groups of CEs specifically expressed to each step of the infectious process. The peak of expression of most of these MgSSPs (75%) was observed during the late symptomless phase (7-10 days postinoculation, dpi), with a dramatic decrease at early stages of lesion formation (≥ 14 dpi). This expression pattern was also observed for the known chitin-binding effector ECP6 required for infection3. Such effectors might be involved in endophytic growth or plant defense suppression during the symptomless phase. Few other MgSSPs (15%) have their peak of expression during the early necrotic symptom phase associated with exponential fungal growth (≥ 12-14 dpi, n = 1) or later during the necrotic phase (15-21 dpi, n = 2). These effectors might play a role in triggering either necrotic symptoms, massive colonization of host tissues or pycnidia formation. The functional analysis of some of these genes is underway to validate their role during infection.