Proteome changes induced by Pyrenophora tritici-repentis ToxA in both insensitive and sensitive wheat indicate senescence-like signaling

Abstract

Background: Pyrenophora tritici-repentis is a phytopathogenic fungus which causes tan spot on wheat. Some races of P. tritici-repentis produce host-specific toxins which present symptoms of chlorosis or necrosis on susceptible wheat cultivars. One such toxin is Ptr ToxA, which enters mesophyll cells through a putative toxin-receptor and localizes with chloroplasts, ultimately causing damage and necrosis on leaves. These symptoms can occur even in the absence of the pathogen. Insensitive cultivars lack the receptor and Ptr ToxA cannot enter cells. The molecular mechanisms surrounding this plant-pathogen interaction are still largely unknown, although some details have begun to emerge.

Results: Using 2-D electrophoresis, fifteen protein changes were identified reproducibly in the leaf proteomes of a sensitive and an insensitive cultivar over three days after inoculation of purified Ptr ToxA. Functional analysis of the proteins indicated that senescence signals may be induced in the sensitive cultivar. In the insensitive cultivar proteins involved in some features of senescence inhibition were seen. Complementary responses at the biochemical level may be actively promoting a localized senescence-like response in sensitive wheat cultivars whilst actively inhibiting this response in insensitive cultivars.

Conclusion: This is the first report of a biochemical response in an insensitive cultivar in this plant-pathogen interaction. Findings support the involvement of ethylene, and the activation of complementary pathways in sensitive versus insensitive wheat cultivars responding to Ptr ToxA. The nature of the system permits using purified toxin to mimic disease, which eliminates the pathogen proteome and ensures a synchronous response in inoculated leaves.

Keywords: Proteomics; Ptr ToxA; Tan spot.

Figure 1

Toxin isolation and infiltration. A: selected fractions from CM cellulose column separated on 12.5% Tris-tricine SDS polyacrylamide gels and stained with Coomassie brilliant blue. B: corresponding fractions 5 days post infiltration in sensitive wheat leaves.

Figure 2

Necrosis progression. Wheat leaves were infiltrated with a 0.9 ng.μl⁻¹ solution of Ptr ToxA and collected at day 1, day 2, and day 3 post infiltration.

Figure 3

Proteome changes in sensitive wheat leaves infiltrated with Ptr ToxA or control buffer.

Figure 4

Protein changes in 2D electrophoresis of Amazon wheat leaves infiltrated with Ptr ToxA or control buffer.