Sugarcane smut: shedding light on the development of the whip-shaped sorus.

Background and Aims: Sugarcane smut is caused by the fungus Sporisorium scitamineum (Ustilaginales/Ustilaginomycotina/Basidiomycota), which is responsible for losses in sugarcane production worldwide. Infected plants show a profound metabolic modification resulting in the development of a whip-shaped structure (sorus) composed of a mixture of plant tissues and fungal hyphae. Within this structure, ustilospores develop and disseminate the disease. Despite the importance of this disease, a detailed histopathological analysis of the plant-pathogen interaction is lacking. Methods: The whip-shaped sorus was investigated using light microscopy, scanning and transmission electron microscopy, histochemical tests and epifluorescence microscopy coupled with deconvolution. Key Results: Sorus growth is mediated by intercalary meristem activity at the base of the sorus, where the fungus causes partial host cell wall degradation and formation of intercellular spaces. Sporogenesis in S. scitamineum is thallic, with ustilospore initials in intercalary or terminal positions, and mostly restricted to the base of the sorus. Ustilospore maturation is centrifugal in relation to the ground parenchyma and occurs throughout the sorus median region. At the apex of the sorus, the fungus produces sterile cells and promotes host cell detachment. Hyphae are present throughout the central axis of the sorus (columella). The plant cell produces callose around the intracellular hyphae as well as inside the papillae at the infection site. Conclusions: The ontogeny of the whip-shaped sorus suggests that the fungus can cause the acropetal growth in the intercalary meristem. The sporogenesis of S. scitamineum was described in detail, demonstrating that the spores are formed exclusively at the base of the whip. Light was also shed on the nature of the sterile cells. The presence of the fungus alters the host cell wall composition, promotes its degradation and causes the release of some peripheral cells of the sorus. Finally, callose was observed around fungal hyphae in infected cells, suggesting that deposition of callose by the host may act as a structural response to fungal infection.

Identification of smut-responsive genes in sugarcane using cDNA-SRAP.

Sugarcane smut, caused by the fungus Sporisorium scitamineum, is one of the main diseases that affect sugarcane worldwide. In the present study, the cDNA-SRAP technique was used to identify genes that are likely to be involved in the response of sugarcane to S. scitamineum infection. In total, 21 bands with significant differential expression during cDNA-SRAP analysis were cloned and sequenced. Real-time qPCR confirmation demonstrated that expression of 19 of these 21 differential bands was consistent with the expression observed during cDNA-SRAP analysis, with a deduced false positive rate of 9.5%. Sequence alignment indicated that 18 of 19 differentially expressed genes showed homologies from 19% to 100% to certain genes in GenBank, including the following genes: topoisomerase (EU048780), ethylene insensitive (EU048778), and tetraspanin (EU048770). A real-time qPCR assay showed that during 0-72 h after pathogen infection, expression of the topoisomerase and the ethylene insensitive genes was upregulated, whereas expression of the tetraspanin gene was downregulated, identical to the expression patterns observed under salicylic acid treatment. Therefore, all three genes are thought to play a role during S. scitamineum challenge, but with different functions. To our knowledge, this is the first report on the application of cDNA-SRAP in differential gene expression analysis of sugarcane during a sugarcane-S. scitamineum interaction. The results obtained also contribute to a better understanding of the molecular mechanisms associated with sugarcane-S. scitamineum interactions.