Transcriptome analysis reveals pathways facilitating the growth of tobacco powdery mildew in Arabidopsis

Abstract

Powdery mildew (PM) fungi are biotrophic pathogens that rely on living hosts to survive and thrive. However, their colonization is restricted by host defenses at both the penetration and post-penetration stages. The tobacco PM strain Golovinomyces cichoracearum (Gc) SICAU1 has overcome penetration resistance of Arabidopsis but its growth is arrested by post-penetration resistance. While Gc SICAU1 only poorly grows in Arabidopsis Col-0 wild-type plants, it can sustainably grow for more than 20 days on the same infected leaves of the double mutant pad4-1 sid2-1 that is defective in both the synthesis and signaling of salicylic acid (SA). To understand the underlying molecular mechanisms, we conducted a comparative transcriptome analysis between Col-0 and pad4-1 sid2-1 in response to Gc SICAU1. We found that 4811 genes were differentially expressed more than four-fold between any two of the measured seven time points (0, 1, 3, 6, 8, 10 and 12 days post-inoculation). Gene expression pattern analysis suggests that differential expression of 348 genes and 190 genes may explain resistance in Col-0 and susceptibility in pad4-1 sid2-1, respectively. Gene Ontology (GO) analysis suggests that Gc SICAU1 might be arrested in Col-0 by both pattern-triggered immunity and SA-dependent defense. By contrast, its sustained growth in pad4-1 sid2-1 may be attributable to the activation of a detoxification pathway that is normally repressed by the SA-signaling pathway. Taken together, our results suggest that multiple distinct, yet interconnected pathways control the growth of tobacco powdery mildew in Arabidopsis.