SsPEP1, an Effector with Essential Cellular Functions in Sugarcane Smut Fungus

Exploring Potential Surrogate Systems for Studying the Early Steps of the Sporisorium scitamineum Pathogenesis

Despite its global importance as a primary source of table sugar and bioethanol, sugarcane faces a significant threat to its production due to diseases. One of these diseases, sugarcane smut, involves the emergence of a whip-like structure from the host apical shoot. The slow onset of this pathogenesis is the most substantial challenge for researchers to investigate the molecular events leading to resistance or susceptibility. In this study, we explored the early interaction between the smut fungus Sporisorium scitamineum and foliar tissues of the model plants Arabidopsis thaliana and Nicotiana benthamiana. Upon inoculation with the fungus, A. thaliana showed a compatible reaction, producing lesions during fungus colonization, whereas N. benthamiana showed signs of nonhost resistance. In addition, we propose a sugarcane detached leaf assay using plants cultivated in vitro to reveal sugarcane smut response outcomes. We used two sugarcane genotypes with known contrasting reactions to smut in the field. Although there is no evidence of sugarcane smut fungus infecting host leaves naturally, the sugarcane detached leaf assay enabled a rapid assessment of disease outcomes. Different symptoms in the detached leaves after inoculation distinguished smut-susceptible and smut-resistant sugarcane genotypes. Microscopic observations and gene expression analysis of S. scitamineum candidate effectors confirmed the fungal growth and its restriction on the compatible and incompatible interactions, respectively. These findings offer new prospects into the disease phenotyping of S. scitamineum, which could greatly expedite the comprehension of the initial stages of the pathogenesis and predict smut resistance in sugarcane genotypes.

An efficient inoculation method to evaluate virulence differentiation of field strains of sugarcane smut fungus

Sugarcane smut, caused by the fungal pathogen Sporisorium scitamineum, is a prominent threat to the sugarcane industry. The development of smut resistant varieties is the ultimate solution for controlling this disease, due to the lack of other efficient control methods. Artificial inoculation method is used to evaluate the virulence differentiation of pathogens. The mostly used artificial inoculation methods are soaking of the seed canes in the teliospore solution and injection of teliospores or haploid sporidia into the sugarcane sprouts. However, due to the infection nature of the pathogen that invades the sugarcane plant through meristem tissue of the sprout or shoot, the rate of successful infection is often low and fluctuated, resulting in low confidence of the assays. We recently reported a rapid and high-throughput inoculation method called plantlet soaking by using tissue culture-derived sugarcane plantlets as the test plants. Here, we compare different inoculation methods and report the characterization of parameters that may affect the sensitivity and efficiency of the plantlet soaking technique. The results showed that sugarcane plantlets were highly vulnerable to infection, even with the inoculum density at 6.0 × 105 basidial spores/ml, and this method could be applied to all varieties tested. Notably, varieties showing high smut resistance in the field exhibited high susceptibility when inoculated with the plantlet soaking method, suggesting that the plantlet soaking method is a good complement to the traditional methods for screening germplasms with internal resistance. In addition, this method could also be used to monitor the variation of cellular virulence of the smut pathogen strains in the field.

Identification of Gene Modules and Hub Genes Associated with Sporisorium scitamineum Infection Using Weighted Gene Co-Expression Network Analysis

Sporisorium scitamineum is a biotrophic fungus responsible for sugarcane smut disease. To investigate the key genes involved in S. scitamineum infection, we conducted RNA sequencing of sugarcane sprouts inoculated with S. scitamineum teliospores. A weighted gene co-expression network analysis (WGCNA) showed that two co-expressed gene modules, MEdarkturquoise and MEpurple—containing 66 and 208 genes, respectively—were associated with S. scitamineum infection. The genes in these two modules were further studied using Gene Ontology (GO) enrichment analysis, pathogen-host interaction (PHI) database BLASTp, and small secreted cysteine-rich proteins (SCRPs) prediction. The top ten hub genes in each module were identified using the Cytohubba plugin. The GO enrichment analysis found that endoplasmic reticulum-related and catabolism-related genes were expressed during S. scitamineum infection. A total of 83 genes had homologs in the PHI database, 62 of which correlated with pathogen virulence. A total of 21 proteins had the characteristics of small secreted cysteine-rich proteins (SCRPs), a common source of fungal effectors. The top ten hub genes in each module were identified, and seven were annotated as Mig1-Mig1 protein, glycosyl hydrolase, beta-N-acetylglucosaminidase, secreted chorismate mutase, collagen, mRNA export factor, and pleckstrin homology domain protein, while the remaining three were unknown. Two SCRPs—SPSC_06609 and SPSC_04676—and three proteins—SPSC_01958, SPSC_02155, and SPSC_00940—identified in the PHI database were also among the top ten hub genes in the MEdarkturquoise and MEpurple modules, suggesting that they may play important roles in S. scitamineum infection. A S. scitamineum infection model was postulated based on current findings. These findings help to deepen the current understanding of early events in S. scitamineum infection.

Sugarcane Smut: Current Knowledge and the Way Forward for Management

Whip smut of sugarcane is the most serious and widely spread disease of sugarcane and causes a significant reduction in cane quantity and quality. The severity of this disease often depends on the pathogen races, environmental conditions, cultivar genotype and the interaction among these three factors. Under optimum climatic conditions, this disease has the potential to cause total crop failure. Resistance screening is an ongoing process due to the variability among smut pathogen isolates. Multiple races and mutation ability of smut pathogen makes the breeding task more complex. A number of studies on various aspects of the disease epidemiology and management have been published. Due to many overlapping characteristics within the species complex, there is a dearth of information on early detection and strategies to control the smut pathogen. Furthermore, there is a need to coordinate these findings to expedite its research and control. In this paper, we summarize the disease etiology, especially disease impact on the qualitative and quantitative parameters of sugarcane. We also gathered research progress on molecular-based detection and available information on genetic variability in S.scitamineum. The research on the set of management options needed to effectively cope with the disease are reviewed herein. The present review is expected to be helpful for the further investigation on smut resistance in sugarcane.