Physiological Mechanisms of Improved Smut Resistance in Sugarcane Through Application of Silicon

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.

SsCyp86 modulates sporisorium scitamineum mating/filamentation and pathogenicity through regulating fatty acid metabolism

BACKGROUND

Fatty acid metabolism constitutes a significant and intricate biochemical process within microorganisms. Cytochrome P450 (CYP450) enzymes are found in most organisms and occupy a pivotal position in the metabolism of fatty acids. However, the role of CYP450 enzyme mediated fatty acid metabolism in the pathogenicity of pathogenic fungi remains unclear.

METHODS

In this study, a CYP450 enzyme-encoding gene, SsCYP86, was identified in the sugarcane smut fungus Sporisorium scitamineum and its functions were characterized using a target gene homologous recombination strategy and metabonomics.

RESULTS

We found that the expression of SsCYP86 was induced by or sugarcane wax or under the condition of mating/filamentation. Sexual reproduction assay demonstrated that the SsCYP86 deletion mutant was defective in mating/filamentation and significantly reduced its pathogenicity. Further fatty acid metabolomic analysis unravelled the levels of fatty acid metabolites were reduced in the SsCYP86 deletion mutant. Exogenous addition of fatty acid metabolites cis-11-eicosenoic acid (C20:1N9), pentadecanoic acid (C15:0), and linolenic acid (C18:3N3) partially restored the mating/filamentation ability of the SsCYP86 deletion mutant and restored the transcriptional level of the SsPRF1, a pheromone response transcription factor that is typically down-regulated in the absence of SsCYP86. Moreover, the constitutive expression of SsPRF1 in the SsCYP86 deletion mutant restored its mating/filamentation.

CONCLUSION

Our results indicated that SsCyp86 modulates the SsPRF1 transcription by fatty acid metabolism, and thereby regulate the sexual reproduction of S. scitamineum. These findings provide insights into how CYPs regulate sexual reproduction in S. scitamineum.

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.

Silica nanoparticles conferring resistance to bacterial wilt in peanut (Arachis hypogaea L.)

Peanut bacterial wilt (PBW) caused by the pathogen Ralstonia solanacearum severely affects the growth and yield potential of peanut crop. In this study, we synthesized silica nanoparticles (SiO2 NPs), a prospective efficient management approach to control PBW, and conducted a hydroponic experiment to investigate the effects of different SiO2 NPs treatments (i.e., 0, 100, and 500 mg L-1 as NP0, NP100, and NP500, respectively) on promoting plant growth and resistance to R. solanacearum. Results indicated that the disease indices of NP100 and NP500 decreased by 51.5 % and 55.4 % as compared with NP0 under R. solanacearum inoculation, respectively, while the fresh and dry weights and shoot length of NP100 and NP500 increased by 7.62-42.05 %, 9.45-32.06 %, and 2.37-17.83 %, respectively. Furthermore, SiO2 NPs induced an improvement in physio-biochemical enzymes (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, and lipoxygenase) which eliminated the excess production of hydrogen peroxide, superoxide anions, and malondialdehyde to alleviate PBW stress. Notably, the targeted metabolomic analysis indicated that SiO2 NPs enhanced salicylic acid (SA) contents, which involved the induction of systemic acquired resistance (SAR). Moreover, the transcriptomic analysis revealed that SiO2 NPs modulated the expression of multiple transcription factors (TFs) involved in the hormone pathway, such as AHLs, and the identification of hormone pathways related to plant defense responses, such as the SA pathway, which activated SA-dependent defense mechanisms. Meanwhile, the up-regulated expression of the SA-metabolism gene, salicylate carboxymethyltransferase (SAMT), initiated SAR to promote PBW resistance. Overall, our findings revealed that SiO2 NPs, functioning as a plant elicitor, could effectively modulate physiological enzyme activities and enhance SA contents through the regulation of SA-metabolism genes to confer the PBW resistance in peanuts, which highlighted the potential of SiO2 NPs for sustainable agricultural practices.

Potential of silicon-rich biochar (Sichar) amendment to control crop pests and pathogens in agroecosystems: A review

We reviewed the potential of silicon (Si)-rich biochars (sichars) as crop amendments for pest and pathogen control. The main pathosystems that emerged from our systematic literature search were bacterial wilt on solanaceous crops (mainly tomato, pepper, tobacco and eggplant), piercing-sucking hemipteran pests and soil-borne fungi on gramineous crops (mainly rice and wheat), and parasitic nematodes on other crops. The major pest and pathogen mitigation pathways identified were: i) Si-based physical barriers; ii) Induction of plant defenses; iii) Enhancement of plant-beneficial/pathogen-antagonistic soil microflora in the case of root nematodes; iv) Alteration of soil physical-chemical properties resulting in Eh-pH conditions unfavorable to root nematodes; v) Alteration of soil physical-chemical properties resulting in Eh-pH, bulk density and/or water holding capacity favorable to plant growth and resulting tolerance to necrotrophic pathogens; vi) Increased Si uptake resulting in reduced plant quality, owing to reduced nitrogen intake towards some hemi-biotrophic pests or pathogens. Our review highlighted synergies between pathways and tradeoffs between others, depending, inter alia, on: i) crop type (notably whether Si-accumulating or not); ii) pest/pathogen type (e.g. below-ground/root-damaging vs above-ground/aerial part-damaging; "biotrophic" vs "necrotrophic" sensu lato, and corresponding systemic resistance pathways; thriving Eh-pH spectrum; etc.); iii) soil type. Our review also stressed the need for further research on: i) the contribution of Si and other physical-chemical characteristics of biochars (including potential antagonistic effects); ii) the pyrolysis process to a) optimize Si availability in the soil and its uptake by the crop and b) to minimize formation of harmful compounds e.g. cristobalite; iii) on the optimal form of biochar, e.g. Si-nano particles on the surface of the biochar, micron-sized biochar-based compound fertilizer vs larger biochar porous matrices.

Ultrasonic treatment alleviated cadmium stress in sugarcane via improving antioxidant activity and physiological and biochemical status

Cadmium (Cd) is a toxic element that endangers crop growth and affects food safety and human health. Therefore, the study of Cd mitigation technology is important. Ultrasonic treatment can improve crop growth and enhance their ability to resist various abiotic stresses. In this study, the effect of ultrasonic treatment on alleviating sugarcane Cd stress was studied in a barrel experiment using sugarcane varieties 'ROC22' and 'LC05-136' as test materials. Sugarcane buds without ultrasonic treatment and with ultrasonic treatment (20-40 kHz mixed frequency ultrasound for 2 min, dry treatment) were planted in soil with Cd contents of 0, 50, 100, 250, and 500 mg·kg-1. Compared with non-ultrasonic treatment, Ultrasonic treatment significantly increased the activities of antioxidant enzymes in sugarcane, significantly increased the content of osmoregulation substances, significantly reduced the content of superoxide anion (the highest decreases reached 11.55%) and malondialdehyde (the highest decreases reached 20.59%), and significantly increased the expression level of metallothionein (MT)-related genes, with the expression of ScMT1 increased by 8.80-37.49% and the expression of ScMT2-1-5 increased by 1.55-69.33%. In addition, ultrasonic treatment significantly reduced the Cd contents in sugarcane roots, stems, leaves, bagasse, and juice (the highest reduction in Cd content was 49.18%). In general, ultrasonic treatment regulated the metabolism of reactive oxygen species and MT-related gene expression in sugarcane, increased the Cd tolerance of sugarcane, promoted photosynthesis in sugarcane leaves, improved root morphology, enhanced sugarcane growth, and increased cane and sugar yield.

Silicon Application for the Modulation of Rhizosphere Soil Bacterial Community Structures and Metabolite Profiles in Peanut under Ralstonia solanacearum Inoculation

Silicon (Si) has been shown to promote peanut growth and yield, but whether Si can enhance the resistance against peanut bacterial wilt (PBW) caused by Ralstonia solanacearum, identified as a soil-borne pathogen, is still unclear. A question regarding whether Si enhances the resistance of PBW is still unclear. Here, an in vitro R. solanacearum inoculation experiment was conducted to study the effects of Si application on the disease severity and phenotype of peanuts, as well as the microbial ecology of the rhizosphere. Results revealed that Si treatment significantly reduced the disease rate, with a decrement PBW severity of 37.50% as compared to non-Si treatment. The soil available Si (ASi) significantly increased by 13.62-44.87%, and catalase activity improved by 3.01-3.10%, which displayed obvious discrimination between non-Si and Si treatments. Furthermore, the rhizosphere soil bacterial community structures and metabolite profiles dramatically changed under Si treatment. Three significantly changed bacterial taxa were observed, which showed significant abundance under Si treatment, whereas the genus Ralstonia genus was significantly suppressed by Si. Similarly, nine differential metabolites were identified to involve into unsaturated fatty acids via a biosynthesis pathway. Significant correlations were also displayed between soil physiochemical properties and enzymes, the bacterial community, and the differential metabolites by pairwise comparisons. Overall, this study reports that Si application mediated the evolution of soil physicochemical properties, the bacterial community, and metabolite profiles in the soil rhizosphere, which significantly affects the colonization of the Ralstonia genus and provides a new theoretical basis for Si application in PBW prevention.

MIR390 Is Involved in Regulating Anthracnose Resistance in Apple

As an important cash crop in China, apple has a good flavor and is rich in nutrients. Fungal attacks have become a major obstacle in apple cultivation. Colletotrichum gloeosporioides is one of the most devastating fungal pathogens in apple. Thus, discovering resistance genes in response to C. gloeosporioides may aid in designing safer control strategies and facilitate the development of apple resistance breeding. A previous study reported that 'Hanfu' autotetraploid apple displayed higher C. gloeosporioides resistance than 'Hanfu' apple, and the expression level of mdm-MIR390b was significantly upregulated in autotetraploid plants compared to that in 'Hanfu' plants, as demonstrated by digital gene expression (DGE) analysis. It is still unclear, however, whether mdm-MIR390b regulates apple anthracnose resistance. Apple MIR390b was transformed into apple 'GL-3' plants to identify the functions of mdm-MIR390b in anthracnose resistance. C. gloeosporioides treatment analysis indicated that the overexpression of mdm-MIR390b reduced fungal damage to apple leaves and fruit. Physiology analysis showed that mdm-MIR390b increased C. gloeosporioides resistance by improving superoxide dismutase (SOD) and peroxidase (POD) activity to alleviate the damage caused by O₂^- and H₂O₂. Our results demonstrate that mdm-MIR390b can improve apple plants' anthracnose resistance.

Astragalus sinicus Incorporated as Green Manure for Weed Control in Corn

Astragalus sinicus L. (milk vetch), one of the most widespread green manure species, is widely planted in the temperate zone. Eleusine indica L. (goosegrass), a serious annual weed in the world, has evolved resistance to some non-selective herbicides. The use of milk vetch as green manure for weed control in paddy fields was proposed. Aqueous extracts of milk vetch are known to exert a different level of phytotoxicity on weeds and crops. Phytotoxic substances contained in green manure were released into the soil by leaching at the initial stage and decomposition at the later stage after the return of green manure. Considering the need for searching new sustainable strategies for weed control, a question arises: "if milk vetch could be applied in goosegrass control, which stage is the most important to control goosegrass after milk vetch returned to the field, and at the same time, will the subsequent crop, corn (Zea mays L.), be affected by the side effects from milk vetch phytotoxicity?" In this study, the potential of milk vetch for goosegrass control was approached by repeated laboratory experiments, which include the aqueous extract experiment, decomposed experiment, and pot experiment. The effects of milk vetch returning to the field on maize were simulated by a pot experiment. The extract of milk vetch could significantly inhibit the germination of goosegrass at 2% concentration, and the inhibition enhanced with the increase of concentration. In the decomposed liquid experiment, decay time within 15 days, with the increase of decay days or concentration, goosegrass inhibition effect of decomposed liquid was enhanced. When decay time was more than 15 days, the inhibition ability of the decomposed liquid to goosegrass decreased. According to the RI accumulated value, aqueous extract and decomposed liquid have a "hormesis effect" on the germination and growth of goosegrass. Pot experiment proved that the addition of 1-10% (w/w) of milk vetch significantly reduced the germination and growth of goosegrass. On the contrary, the comprehensive analysis showed that the participation of milk vetch was conducive to the growth of corn. Our results constitute evidence that the incorporation of milk vetch into the soil could be a feasible practice to reduce weed infarctions in the corn-based cropping system.

Ovarian Tumor Domain-Containing Proteases-Deubiquitylation Enzyme Gene SsCI33130 Involved in the Regulation of Mating/Filamentation and Pathogenicity in Sporisorium scitamineum

Sugarcane is an important sugar crop. Sugarcane smut, caused by Sporisorium scitamineum, is a worldwide sugarcane disease with serious economic losses and lack of effective control measures. Revealing the molecular pathogenesis of S. scitamineum is very helpful to the development of effective prevention and control technology. Deubiquitinase removes ubiquitin molecules from their binding substrates and participates in a variety of physiological activities in eukaryotes. Based on the transcriptome sequencing data of two isolates (Ss16 and Ss47) of S. scitamineum with different pathogenicities, SsCI33130, a gene encoding an OTU1-deubiquitin enzyme, was identified. The positive knockout mutants and complementary mutants of the SsCI33130 gene were successfully obtained through polyethylene glycol-mediated protoplast transformation technology. In order to study the possible function of this gene in pathogenicity, phenotypic comparison of the growth, morphology, abiotic stress, sexual mating, pathogenicity, and gene expression levels of the knockout mutants, complementary mutants, and their wild type strains were conducted. The results demonstrated that the gene had almost no effect on abiotic stress, cell wall integrity, growth, and morphology, but was related to the sexual mating and pathogenicity of S. scitamineum. The sexual mating ability and pathogenicity between the knockout mutants or between the knockout mutant and wild type were more significantly reduced than between the wild types, the complementary mutants, or the wild types and complementary mutants. The sexual mating between the knockout mutants or between the knockout mutant and wild type could be restored by the exogenous addition of small-molecule signaling substances such as 5 mM cyclic adenosine monophosphate (cAMP) or 0.02 mM tryptophol. In addition, during sexual mating, the expression levels of tryptophol and cAMP synthesis-related genes in the knockout mutant combinations were significantly lower than those in the wild type combinations, while the expression levels in the complementary mutant combinations were restored to the level of the wild type. It is speculated that the SsCI33130 gene may be involved in the development of sexual mating and pathogenicity in S. scitamineum by regulating the synthesis of the small-molecule signaling substances (cAMP or tryptophol) required during the sexual mating of S. scitamineum, thereby providing a molecular basis for the study of the pathogenic mechanisms of S. scitamineum.

Silicon enhancement for endorsement of Xanthomonas albilineans infection in sugarcane

Silicon (Si) is considered to be a plant growth and development regulator element as well as provide the regulatory response against various biotic stressors. However, the potential mechanism of Si enhancement to regulate plant disease resistance remains to be studied. Therefore, the current study evaluated the effects of Si application on the performance of sugarcane against Xanthomonas albilineans (Xa) infection. Si was applied exogenously (0, 3.85 and 7.70 g Si/kg soil) and the results show that plant height, stem circumference and leaf width of siliconized sugarcane have been improved, which effectively reduced the disease index (0.17-0.21) and incidence (58.2%-69.1%) after Xa infection. Lowest values of MDA (348.5 nmol g^-1 FW) and H₂O₂ (3539.4 mmol/L) were observed in 7.70 g Si/kg soil followed by in 3.85 g Si/kg soil (MDA: 392.6 nmol g^-1 FW and H₂O₂: 3134.6 mmol/L) than that of the control. Whereas, PAL enzyme activity (50.8 mmol/L), JA (230.2 mmol/L) and SA (2.7 ug mL^-1) contents were significantly higher in 7.70 g Si/kg soil followed by in 3.85 g Si/kg soil (PAL: 46.3 mmol/L, JA: 182.7 mmol/L and SA: 2.4 ug mL^-1) as compared to control. The lower MDA, H₂O₂ level and higher enzymatic activities were associated with the highest expression levels of their metabolic pathway associated genes i.e., ShMAPK1, ShLOX, ShPAL, ShAOS, ShAOC, ShC4H, ShCAT, Sh4CL and ShNPR1 (22.08, 15.56, 10.42, 3.35, 2.54, 2.14, 1.82, 1.67 and 1.22 folds, respectively) in 7.70 g Si/kg soil as compared to other experimental units and control. Overall, the results of current study indicates that siliconized sugarcane more actively regulates disease resistance through modulation of growth and MDA, H₂O₂, SA and JA associated metabolic pathways.

Silicon Application Modulates the Growth, Rhizosphere Soil Characteristics, and Bacterial Community Structure in Sugarcane

Silicon (Si) deficiency, caused by acidic soil and rainy climate, is a major constraint for sugarcane production in southern China. Si application generally improves sugarcane growth; however, there are few studies on the relationships between enhanced plant growth, changes in rhizosphere soil, and bacterial communities. A field experiment was conducted to measure sugarcane agronomic traits, plant nutrient contents, rhizosphere soil enzyme activities and chemical properties, and the rhizosphere bacterial community diversity and structure of three predominant sugarcane varieties under two Si treatments, i.e., 0 and 200 kg of silicon dioxide (SiO₂) ha^-1 regarded as Si0 and Si200, respectively. Results showed that Si application substantially improved the sugarcane stalk fresh weight and Si, phosphorus (P), and potassium (K) contents comparing to Si0, and had an obvious impact on rhizosphere soil pH, available Si (ASi), available P (AP), available K (AK), total phosphorus (TP), and the activity of acid phosphatase. Furthermore, the relative abundances of Proteobacteria showed a remarkable increase in Si200, which may be the dominant group in sugarcane growth under Si application. Interestingly, the AP was noticed as a major factor that caused bacterial community structure differences between the two Si treatments according to canonical correspondence analysis (CCA). In addition, the association network analysis indicated that Si application enriched the rhizosphere bacterial network, which could be beneficial to sugarcane growth. Overall, appropriate Si application, i.e., 200 kg SiO₂ ha^-1 promoted sugarcane growth, changed rhizosphere soil enzyme activities and chemical properties, and bacterial community structures.