Morphological and Physiological Responses of Sugarcane to Leifsonia xyli subsp. xyli Infection

Silicon application enhances resistance against sorghum mosaic virus infection in sugarcane

Mosaic disease is one of the predominant viral diseases affecting sugarcane plants worldwide. In China, it is mainly caused by the sorghum mosaic virus (SrMV). Exogenous silicon (ESi) has emerged as a promising inducer of plant disease resistance. This study aims to elucidate the mechanistic effects of exogenous silicon on enhancing the resistance to SrMV in sugarcane. ESi was applied to the soil at different rates (15 g per barrel-1, 30 g per barrel-1, and 45 g per barrel-1), leading to a significant reduction in disease incidence (26.66-82.22%) compared to non-silicon-treated plants. Silicon application (15 g ESi barrel-1, 30 g ESi barrel-1) mitigated SrMV inhibition of sugarcane growth, including plant height, stem diameter, and leaf area, while improving photosynthesis, including stomatal conductance, intercellular CO2 concentration, net photosynthetic rate, and transpiration rate. Additionally, silicon mitigates SrMV-induced degradation of chlorophyll a and b and carotenoid content, alongside heightened activities of superoxide dismutase, peroxidase, and catalase, and decreased content of malondialdehyde and hydrogen peroxide in sugarcane leaves. The ultrastructural analysis revealed silicon's capacity to reduce SrMV accumulation within sugarcane mesophyll cells while preserving chloroplast integrity. Additionally, silicon application increases SA content in sugarcane leaves and upregulates the expression of key SA pathway genes (PAL, PR1, NPR1). These findings suggest that silicon may contribute to sugarcane resistance to SrMV by potentially influencing antioxidant enzyme activity, ROS production, and SA pathway genes.

Screening of Sugarcane Proteins Associated with Defense against Leifsonia xyli subsp. xyli, Agent of Ratoon Stunting Disease

Sugarcane is the most important sugar crop and one of the leading energy-producing crops in the world. Ratoon stunting disease (RSD), caused by the bacterium Leifsonia xyli subsp. xyli, poses a huge threat to ratoon crops, causing a significant yield loss in sugarcane. Breeding resistant varieties is considered the most effective and fundamental approach to control RSD in sugarcane. The exploration of resistance genes forms the foundation for breeding resistant varieties through molecular technology. The pglA gene is a pathogenicity gene in L. xyli subsp. xyli, encoding an endopolygalacturonase. In this study, the pglA gene from L. xyli subsp. xyli and related microorganisms was analyzed. Then, a non-toxic, non-autoactivating pglA bait was successfully expressed in yeast cells. Simultaneously the yeast two-hybrid library was generated using RNA from the L. xyli subsp. xyli-infected sugarcane. Screening the library with the pglA bait uncovered proteins that interacted with pglA, primarily associated with ABA pathways and the plant immune system, suggesting that sugarcane employs these pathways to respond to L. xyli subsp. xyli, triggering pathogenicity or resistance. The expression of genes encoding these proteins was also investigated in L. xyli subsp. xyli-infected sugarcane, suggesting multiple layers of regulatory mechanisms in the interaction between sugarcane and L. xyli subsp. xyli. This work promotes the understanding of plant-pathogen interaction and provides target proteins/genes for molecular breeding to improve sugarcane resistance to L. xyli subsp. xyli.

Histopathology of the Shoot Apex of Sugarcane Colonized by Leifsonia xyli subsp. xyli

Colonization of the xylem of sugarcane by Leifsonia xyli subsp. xyli results in the occlusion of the vessels by a gum-like compound and compromises the elongation of the stalk leading to stunted plants. However, no study has been performed in the apical tissue where the elongation of the stalks initiates at the intercalary meristem (IM). Microscopic and histochemical analyses were performed in plants with lower and higher bacterial titers and revealed that in both cases L. xyli subsp. xyli is present in this tissue and colonizes the forming xylem vessels in a similar way as observed in developed internodes. In both cases, it was observed adhering to the secondary walls, but only in plants with higher titers were a mild degradation of the walls and a granular material filling the vessels observed. The mixed composition of lipids, proteins, and pectin indicates that the filling is not a bacterial extracellular polymeric substance. Plants with higher bacterial populations also presented lower starch content in the ground parenchyma at the node elements, possibly resulting from the reported downregulation of photosynthesis and increased accumulation of phenolics. Their second and third IMs presented fewer cells and reduced expression of genes related to the cell cycle and to the synthesis of ABA in the apical tissue. These results indicate that increased L. xyli subsp. xyli colonization affects the development of the IM, which ultimately would reduce the length of the internodes, resulting in the main symptom of the disease.

Targeted Metabolic Profiles of the Leaves and Xylem Sap of Two Sugarcane Genotypes Infected with the Vascular Bacterial Pathogen Leifsonia xyli subsp. xyli

Ratoon stunt (RS) is a worldwide disease that reduces biomass up to 80% and is caused by the xylem-dwelling bacterium Leifsonia xyli subsp. xyli. This study identified discriminant metabolites between a resistant (R) and a susceptible (S) sugarcane variety at the early stages of pathogen colonization (30 and 120 days after inoculation—DAI) by untargeted and targeted metabolomics of leaves and xylem sap using gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), respectively. Bacterial titers were quantified in sugarcane extracts at 180 DAI through real-time polymerase chain reaction. Bacterial titers were at least four times higher on the S variety than in the R one. Global profiling detected 514 features in the leaves and 68 in the sap, while 119 metabolites were quantified in the leaves and 28 in the sap by targeted metabolomics. Comparisons between mock-inoculated treatments indicated a greater abundance of amino acids in the leaves of the S variety and of phenolics, flavonoids, and salicylic acid in the R one. In the xylem sap, fewer differences were detected among phenolics and flavonoids, but also included higher abundances of the signaling molecule sorbitol and glycerol in R. Metabolic changes in the leaves following pathogen inoculation were detected earlier in R than in S and were mostly related to amino acids in R and to phosphorylated compounds in S. Differentially represented metabolites in the xylem sap included abscisic acid. The data represent a valuable resource of potential biomarkers for metabolite-assisted selection of resistant varieties to RS.

A transcriptomic analysis of sugarcane response to Leifsonia xyli subsp. xyli infection

Sugarcane ratoon stunting disease (RSD) caused by Leifsonia xyli subsp. xyli (Lxx) is a common destructive disease that occurs around the world. Lxx is an obligate pathogen of sugarcane, and previous studies have reported some physiological responses of RSD-affected sugarcane. However, the molecular understanding of sugarcane response to Lxx infection remains unclear. In the present study, transcriptomes of healthy and Lxx-infected sugarcane stalks and leaves were studied to gain more insights into the gene activity in sugarcane in response to Lxx infection. RNA-Seq analysis of healthy and diseased plants transcriptomes identified 107,750 unigenes. Analysis of these unigenes showed a large number of differentially expressed genes (DEGs) occurring mostly in leaves of infected plants. Sugarcane responds to Lxx infection mainly via alteration of metabolic pathways such as photosynthesis, phytohormone biosynthesis, phytohormone action-mediated regulation, and plant-pathogen interactions. It was also found that cell wall defense pathways and protein phosphorylation/dephosphorylation pathways may play important roles in Lxx pathogeneis. In Lxx-infected plants, significant inhibition in photosynthetic processes through large number of differentially expressed genes involved in energy capture, energy metabolism and chloroplast structure. Also, Lxx infection caused down-regulation of gibberellin response through an increased activity of DELLA and down-regulation of GID1 proteins. This alteration in gibberellic acid response combined with the inhibition of photosynthetic processes may account for the majority of growth retardation occurring in RSD-affected plants. A number of genes associated with plant-pathogen interactions were also differentially expressed in Lxx-infected plants. These include those involved in secondary metabolite biosynthesis, protein phosphorylation/dephosphorylation, cell wall biosynthesis, and phagosomes, implicating an active defense response to Lxx infection. Considering the fact that RSD occurs worldwide and a significant cause of sugarcane productivity, a better understanding of Lxx resistance-related processes may help develop tools and technologies for producing RSD-resistant sugarcane varieties through conventional and/or molecular breeding.

Comparative Transcriptome Profiling of Resistant and Susceptible Sugarcane Cultivars in Response to Infection by Xanthomonas albilineans

Sugarcane (Saccharum spp. hybrids) is a major source of sugar and renewable bioenergy crop worldwide and suffers serious yield losses due to many pathogen infections. Leaf scald caused by Xanthomonas albilineans is a major bacterial disease of sugarcane in most sugarcane-planting countries. The molecular mechanisms of resistance to leaf scald in this plant are, however, still unclear. We performed a comparative transcriptome analysis between resistant (LCP 85-384) and susceptible (ROC20) sugarcane cultivars infected by X. albilineans using the RNA-seq platform. 24 cDNA libraries were generated with RNA isolated at four time points (0, 24, 48, and 72 h post inoculation) from the two cultivars with three biological replicates. A total of 105,783 differentially expressed genes (DEGs) were identified in both cultivars and the most upregulated and downregulated DEGs were annotated for the processes of the metabolic and single-organism categories, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the 7612 DEGs showed that plant-pathogen interaction, spliceosome, glutathione metabolism, protein processing in endoplasmic reticulum, and plant hormone signal transduction contributed to sugarcane's response to X. albilineans infection. Subsequently, relative expression levels of ten DEGs determined by quantitative reverse transcription-PCR (qRT-PCR), in addition to RNA-Seq data, indicated that different plant hormone (auxin and ethylene) signal transduction pathways play essential roles in sugarcane infected by X. albilineans. In conclusion, our results provide, for the first time, valuable information regarding the transcriptome changes in sugarcane in response to infection by X. albilineans, which contribute to the understanding of the molecular mechanisms underlying the interactions between sugarcane and this pathogen and provide important clues for further characterization of leaf scald resistance in sugarcane.

Activities of key enzymes in the C4 pathway and anatomy of sugarcane infected by Leifsonia xyli subsp. xyli

AIMS

Ratoon stunting disease caused by Leifsonia xyli subsp. xyli (Lxx) is a bacterial disease that has plagued sugarcane-planting countries for a long time. This study mainly analysed Lxx localization and its effects on sugarcane leaf.

METHODS AND RESULTS

Badila were inocultated by bacteria of Lxx. It was noted that the number of Lxx cells were rapidly enriched in sugarcane leaves from the 150th to the 210th days of post inoculation (dpi). Lxx infection disrupted the integrity of vascular bundle sheath cells (BSC) in the 'Kranz anatomy' of leaves, resulting in irregular accumulation of starch in vascular BSC of leaves. In situ PCR showed that the Lxx localized in the xylem vessels, mesophyll cell (MC) and BSC as described before in sugarcane leaf, a new niche within the host tissues in the phloem of sugarcane stem. The gene expression and activities of phosphoenolpyruvate carboxylase (PEPC), pyruvate, orthophosphate dikinase (PPDK) and NADP-malic enzyme (NADP-ME) enzymes were lower in Lxx-inoculated sugarcane plants as compared to the MI group.

CONCLUSION

Lxx infection not only disrupted the structure of vascular BSC in the C4 'Kranz anatomy' of sugarcane leaves, but also affected the activities and gene expression of the key enzymes PEPC, PPDK and NADP-ME in the C4 cycle of sugarcane suggesting a reduction in CO₂ fixation.

SIGNIFICANCE AND IMPACT OF THE STUDY

The effect of Leifsonia xyli subsp. xyli (Lxx) infection on the photosynthetic physiology of sugarcane is currently limited to the evaluation of photosynthetic parameters. This study assessed the impact of Lxx infection on the mechanism of C4 cycle CO₂ fixation and to accompanying plant anatomy.

Sugarcane Omics: An Update on the Current Status of Research and Crop Improvement

Sugarcane is an important crop from Poaceae family, contributing about 80% of the total world's sucrose with an annual value of around US$150 billion. In addition, sugarcane is utilized as a raw material for the production of bioethanol, which is an alternate source of renewable energy. Moving towards sugarcane omics, a remarkable success has been achieved in gene transfer from a wide variety of plant and non-plant sources to sugarcane, with the accessibility of efficient transformation systems, selectable marker genes, and genetic engineering gears. Genetic engineering techniques make possible to clone and characterize useful genes and also to improve commercially important traits in elite sugarcane clones that subsequently lead to the development of an ideal cultivar. Sugarcane is a complex polyploidy crop, and hence no single technique has been found to be the best for the confirmation of polygenic and phenotypic characteristics. To better understand the application of basic omics in sugarcane regarding agronomic characters and industrial quality traits as well as responses to diverse biotic and abiotic stresses, it is important to explore the physiology, genome structure, functional integrity, and collinearity of sugarcane with other more or less similar crops/plants. Genetic improvements in this crop are hampered by its complex genome, low fertility ratio, longer production cycle, and susceptibility to several biotic and abiotic stresses. Biotechnology interventions are expected to pave the way for addressing these obstacles and improving sugarcane crop. Thus, this review article highlights up to date information with respect to how advanced data of omics (genomics, transcriptomic, proteomics and metabolomics) can be employed to improve sugarcane crops.

The Evolution, Ecology, and Mechanisms of Infection by Gram-Positive, Plant-Associated Bacteria

Gram-positive bacteria are prominent members of plant-associated microbial communities. Although many are hypothesized to be beneficial, some are causative agents of economically important diseases of crop plants. Because the features of Gram-positive bacteria are fundamentally different relative to those of Gram-negative bacteria, the evolution and ecology as well as the mechanisms used to colonize and infect plants also differ. Here, we discuss recent advances in our understanding of Gram-positive, plant-associated bacteria and provide a framework for future research directions on these important plant symbionts.

Novel Insights Into the Early Stages of Ratoon Stunting Disease of Sugarcane Inferred from Transcript and Protein Analysis

Despite of the importance of ratoon stunting disease, little is known on the responses of sugarcane to its causal agent, the vascular bacterial endophyte Leifsonia xyli subsp. xyli. The transcriptome and proteome of young plants of a susceptible cultivar with no symptoms of stunting but with relative low and high bacterial titers were compared at 30 and 60 days after inoculation. Increased bacterial titers were associated with alterations in the expression of 267 cDNAs and in the abundance of 150 proteins involved in plant growth, hormone metabolism, signal transduction and defense responses. Some alterations are predicted to benefit the pathogen, such as the up-regulation of genes involved in the synthesis of methionine. Also, genes and proteins of the cell division cycle were all down-regulated in plants with higher titers at both times. It is hypothesized that the negative effects on cell division related to increased bacterial titers is cumulative over time and its modulation by other host and environmental factors results in the stunting symptom.

Turning a Blind Eye to Ratoon Stunting Disease of Sugarcane in Australia

The Australian sugar industry has never pursued genetic resistance to ratoon stunting disease (RSD), despite it being widely considered to be one of the most important diseases of sugarcane (Saccharum interspecific hybrids). This is because of a prevailing view that the disease is economically managed, and that no further action needs to take place. However, there is a range of epidemiological evidence that suggests that RSD is having a more significant impact than what is generally recognized. This review traces the factors that have led to an industry stance that is apparently without any scientific justification, and which has tended to downplay the significance of RSD on Australian sugarcane productivity, and thus has led to significant lost production. The consequences of this position are that RSD may be influencing broad but poorly explained issues such as commercial ratooning performance of existing varieties and the "yield decline" that has been subject to much scrutiny, if not much success in resolving the issue. Based on the available information, this review calls on the Australian sugar industry to prioritize selection for RSD resistance in the plant improvement program.

Molecular Detection of Diverse Leifsonia Strains Associated With Sugarcane

Leifsonia xyli subsp. xyli, causal agent of ratoon stunting disease (RSD) of sugarcane (Saccharum interspecific hybrids), is the most well-known member of the Microbacteriaceae genus Leifsonia. However, the presence of other Leifsonia strains associated with sugarcane has not been reported. A total of 697 Australian and 40 Indonesian sugarcane fields were screened by leaf sheath biopsy (LSB) PCR using primers specific for L. xyli subsp. xyli, in addition to primers designed to amplify DNA from other members of the genus Leifsonia. While L. xyli subsp. xyli was detected in 126 fields, a total of 37 distinct and novel Leifsonia and non-Leifsonia strains were detected in 116 fields. Representatives of these strains were also detected in multiple samples of expressed xylem sap. Sequencing and phylogenetic analyses demonstrated the presence of a broad complex of novel Leifsonia strains, in addition to strains closely related to the recently erected Cnuibacter genus. Attempts to isolate Leifsonia strains were unsuccessful; however, one strain related to Cnuibacter was recovered from expressed xylem sap. Among the genetically diverse lineages discovered, identical genotypes were present in multiple sugarcane varieties growing in disparate regions in different years, strongly suggesting an ongoing association with sugarcane. The epidemiological significance of these strains is unknown, but there is evidence that they can interfere with serological and microscopic RSD diagnostics, and there is the potential that they may represent new and distinct pathologies of sugarcane.