Sporisorium scitamineum Infection Alters Transcription Profile of Genes Related to Hormone Synthesis and Signaling in Sugarcane

Sugarcane smut disease is caused by the biotrophic fungus Sporisorium scitamineum. The most prominent disease symptom is the development of a structure like a whip in host primary meristems at the end of the disease cycle, which is associated with a reduction in sugar content and lignification. To study the role of plant hormones in smut disease, Illumina paired-end sequencing was used to obtain transcriptional profiles of a susceptible variety in early interaction (5 days after infection -DAI) and after whip emission (200 DAI). Enrichment analysis of differentially expressed genes (DEs) indicates that early infection reduces JA biosynthesis and SA-mediated signaling pathways, which can be directly related to an inefficient SAR response, leading to plant susceptibility. Genes up-regulated at 5 DAI were enriched in GO terms related to the auxin signaling pathway, which may be related to the meristem reprogramming occurring very early in susceptible plants. Late interaction is characterized by regulation in expression of genes involved in synthesis and signalization of five main hormones: auxin, cytokinin, ethylene, brassinosteroid and gibberellin, whose cross-talk could contribute to whip development, tillering and poor cane formation. Auxin is the hormone with the highest number of responsive genes up-regulated at 200 DAI, including those involved in auxin influx/efflux and Aux/IAA. This scenario suggests that increased auxin signaling is necessary to whip emission, also contributing to the loss of apical dominance leading to growth of secondary buds.