Transcriptome analysis reveals contrasting plant responses of sorghum upon colonization by two formae speciales of Sporisorium reilianum

The biotrophic fungus Sporisorium reilianum exists in two host-adapted formae speciales that cause head smut of the important cereal crops maize (S. reilianum f. sp. zeae; SRZ) and sorghum (S. reilianum f. sp. reilianum; SRS). Both formae speciales can penetrate and spread in the non-favored host but do not lead to smut symptoms. We started analyzing the plant responses to colonization with each of the formae speciales by determining the transcriptome of inoculated sorghum leaves. Each forma specialis induced a specific set of genes. SRZ-induced genes were involved in a large variety of processes that can be associated to plant defense: The molecular function-associated GO terms chitin binding, chitinase activity, abscisic acid binding, protein phosphatase inhibitor activity, terpene synthase activity and signaling receptor activity were more than 10 fold enriched in the dataset. In addition, we found upregulation of genes involved in phospholipid degradation and sphingolipid biosynthesis. In contrast, colonization of sorghum with SRS increased expression of genes involved in the unfolded protein response and the detoxification of cellular oxidants, as well as of genes modifying the cuticle wax and lipid composition. These results indicate that the two formae speciales of S. reilianum induce distinct plant responses that lead to either defence against (SRZ) or facilitation of (SRS) colonization of sorghum. Identification of sorghum genes involved in defense or susceptibility against S. reilianum could lead to development of S. reilianum-resistant sorghum lines.