Sugarcane Cell Wall-Associated Defense Responses to Infection by Sporisorium scitamineum

Analysis of spatiotemporal changes mechanism of cell wall biopolymers and monosaccharide components in kiwifruit during Botryosphaeria dothidea infection

To further reveal the interaction mechanism between plants and pathogens, this study used confocal Raman microscopy spectroscopy (CRM) combined with chemometrics to visualize the biopolymers distribution of kiwifruit cell walls at different infection stages at the cellular micro level. Simultaneously, the changes in the content of various monosaccharides in fruit were studied at the molecular level using high-performance liquid chromatography (HPLC). There were significant differences in the composition of various nutrient components in the cell wall structure of kiwifruit at different infection times after infection by Botryosphaeria dothidea. PCA could cluster samples with infection time of 0-9 d into different infection stages, and SVM was used to predict the PCA classification results, the accuracy >96 %. Multivariate curve resolution-alternating least squares (MCR-ALS) helped to identify single substance spectra and concentration signals from mixed spectral signals. The pure substance chemical imaging maps of low methylated pectin (LMP), high methylated pectin (HMP), cellulose, hemicellulose, and lignin were obtained by analyzing the resolved concentration data. The imaging results showed that the lignin content in the kiwifruit cell wall increased significantly to resist pathogens infection after the infection of B. dothidea. With the development of infection, B. dothidea decomposed various substances in the host cell walls, allowing them to penetrate the interior of fruit cells. This caused significant changes in the form, structure, and distribution of various chemicals on the fruit cell walls in time and space. HPLC showed that glucose was the main carbon source and energy substance obtained by pathogens from kiwifruit during infection. The contents of galactose and arabinose, which maintained the structure and function of the fruit cell walls, decreased significantly and the cell wall structure was destroyed in the late stage of pathogens infection. This study provided a new perspective on the cellular structure changes caused by pathogenic infection of fruit and the defense response process of fruit and provided effective references for further research on the mechanisms of host-pathogen interactions in fruit infected by pathogens.

Two Sugarcane Expansin Protein-Coding Genes Contribute to Stomatal Aperture Associated with Structural Resistance to Sugarcane Smut

Sporisorium scitamineum is a biotrophic fungus responsible for inducing sugarcane smut disease that results in significant reductions in sugarcane yield. Resistance mechanisms against sugarcane smut can be categorized into structural, biochemical, and physiological resistance. However, structural resistance has been relatively understudied. This study found that sugarcane variety ZZ9 displayed structural resistance compared to variety GT42 when subjected to different inoculation methods for assessing resistance to smut disease. Furthermore, the stomatal aperture and density of smut-susceptible varieties (ROC22 and GT42) were significantly higher than those of smut-resistant varieties (ZZ1, ZZ6, and ZZ9). Notably, S. scitamineum was found to be capable of entering sugarcane through the stomata on buds. According to the RNA sequencing of the buds of GT42 and ZZ9, seven Expansin protein-encoding genes were identified, of which six were significantly upregulated in GT42. The two genes c111037.graph_c0 and c113583.graph_c0, belonging to the α-Expansin and β-Expansin families, respectively, were functionally characterized, revealing their role in increasing the stomatal aperture. Therefore, these two sugarcane Expansin protein-coding genes contribute to the stomatal aperture, implying their potential roles in structural resistance to sugarcane smut. Our findings deepen the understanding of the role of the stomata in structural resistance to sugarcane smut and highlight their potential in sugarcane breeding for disease resistance.

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.

Elevated atmospheric carbon dioxide and plant immunity to fungal pathogens: do the risks outweigh the benefits?

Anthropogenic emissions have caused atmospheric carbon dioxide (CO2) concentrations to double since the industrial revolution. Although this could benefit plant growth from the 'CO2 fertilisation' effect, recent studies report conflicting impacts of elevated CO2 (eCO2) on plant-pathogen interactions. Fungal pathogens are the leading cause of plant disease. Since climate change has been shown to affect the distribution and virulence of these pathogens, it is important to understand how their plant hosts may also respond. This review assesses existing reports of positive, negative, and neutral effects of eCO2 on plant immune responses to fungal pathogen infection. The interaction between eCO2 and immunity appears specific to individual pathosystems, dependent on environmental context and driven by the interactions between plant defence mechanisms, suggesting no universal effect can be predicted for the future. This research is vital for assessing how plants may become more at risk under climate change and could help to guide biotechnological efforts to enhance resistance in vulnerable species. Despite the importance of understanding the effects of eCO2 on plant immunity for protecting global food security, biodiversity, and forests in a changing climate, many plant-pathogen interactions are yet to be investigated. In addition, further research into the effects of eCO2 in combination with other environmental factors associated with climate change is needed. In this review, we highlight the risks of eCO2 to plants and point to the research required to address current unknowns.

The Resistance of Maize to Ustilago maydis Infection Is Correlated with the Degree of Methyl Esterification of Pectin in the Cell Wall

Common smut caused by Ustilago maydis is one of the dominant fungal diseases in plants. The resistance mechanism to U. maydis infection involving alterations in the cell wall is poorly studied. In this study, the resistant single segment substitution line (SSSL) R445 and its susceptible recurrent parent line Ye478 of maize were infected with U. maydis, and the changes in cell wall components and structure were studied at 0, 2, 4, 8, and 12 days postinfection. In R445 and Ye478, the contents of cellulose, hemicellulose, pectin, and lignin increased by varying degrees, and pectin methylesterase (PME) activity increased. The changes in hemicellulose and pectin in the cell wall after U. maydis infection were analyzed via immunolabeling using monoclonal antibodies against hemicellulsic xylans and high/low-methylated pectin. U. maydis infection altered methyl esterification of pectin, and the degree of methyl esterification was correlated with the resistance of maize to U. maydis. Furthermore, the relationship between methyl esterification of pectin and host resistance was validated using 15 maize inbred lines with different resistance levels. The results revealed that cell wall components, particularly pectin, were important factors affecting the colonization and propagation of U. maydis in maize, and methyl esterification of pectin played a role in the resistance of maize to U. maydis infection.

Investigating Biochemical and Histopathological Responses between Raspberries and Aculeastrum americanum

Late leaf rust is a fungal disease in raspberries caused by Aculeastrum americanum (Farl.) M. Scholler U. Braun (syn. Thekopsora americana (Farl.) Aime McTaggart) leading to early defoliation and yield losses. Red raspberries (Rubus idaeus L.) are susceptible to this pathogen, although this susceptibility varies among cultivars. In contrast, black raspberries were previously reported to be more resistant (Rubus occidentalis L.) and immune (Rubus niveus Thunb.) to this pathogen, raising their importance in plant breeding programs. However, what features make them respond differently to the same pathogen? In this study, we characterize for the first time the pre- and post-formed structural and biochemical defense mechanisms of R. idaeus cv. Autumn Bliss, R. occidentalis and R. niveus. Ultrastructural and histopathological analyses were used to uncover the interactions between these raspberries and A. americanum. The ultrastructural results indicate that the pathogen germinates on both leaf surfaces but can only form appressoria on the stomata. Although the three raspberry species were infected and colonized by A. americanum, a clear difference in susceptibility was observed between them. A compact mesophyll, pre- and post-formed phenolic compounds, and post-formed pectic compounds were the main plant defense mechanisms against fungal colonization. These findings provide new information about raspberries’ defense mechanisms in response to A. americanum and elucidate the interactions occurring in these pathosystems.

Variety-Specific Flowering of Sugarcane Induced by the Smut Fungus Sporisorium scitamineum

Sugarcane smut is the most severe sugarcane disease in China. The typical symptom is the emerging of a long, black whip from the top of the plant cane. However, in 2018, for the first time we observed the floral structures of sugarcane infected by smut fungus in the planting fields of China. Such smut-associated inflorescence in sugarcane was generally curved and short, with small black whips emerging from glumes of a single floret on the cane stalk. Compatible haploid strains, named Ssf1-7 (MAT-1) and Ssf1-8 (MAT-2), isolated from teliospores that formed black whips in inflorescence of sugarcane were selected for sexual mating assay, ITS DNA sequencing analysis and pathogenicity assessment. The isolates Ssf1-7 and Ssf1-8 showed stronger sexual mating capability than the reported Sporisorium scitamineum strains Ss17 and Ss18. The ITS DNA sequence of the isolates Ssf1-7 and Ssf1-8 reached 100% similarity to the isolates of S. scitamineum strains available in GenBank. Inoculating Ssf1-7 + Ssf1-8 to six sugarcane varieties, i.e., GT42, GT44, GT49, GT55, LC05-136 and ROC22, resulted in different smut morphological modifications. The symptoms of floral structure only occurred in LC05-136, indicating that the flowering induction by S. scitamineum is variety-specific. Furthermore, six selected flowering-related genes were found to be differentially expressed in infected Ssf1-7 + Ssf1-8 LC05-13 plantlets compared to uninfected ones. It is concluded that the flowering induction by S. scitamineum depends on specific fungal race and sugarcane variety, suggesting a specific pathogen-host interaction and expression of some flowering-related genes.

Transcriptomics Integrated with Changes in Cell Wall Material of Chestnut (Castanea mollissima Blume) during Storage Provides a New Insight into the “Calcification” Process

Chestnut “calcification” is the result of a series of physiological and biochemical changes during postharvest storage; however, the associated mechanisms are unclear. In this study, several potential calcification-related physicochemical parameters in chestnut, including moisture, cell wall materials, cellulose, lignin, and pectin, were measured. Transcriptome analysis was performed on chestnut seeds during different stages of storage. The results showed that the degree of calcification in the chestnut seeds was significantly negatively correlated with the moisture content (r = −0.961) at room temperature (20–25 °C) and a relative humidity of 50–60%. The accumulation of cell wall material in completely calcified seeds was 5.3 times higher than that of fresh seeds. The total content of cellulose and lignin increased during the storage process. Transcriptome analysis of 0% and 50% calcified chestnut was performed; a total of 1801 differentially expressed genes consisting of 805 up-regulated and 996 down-regulated genes were identified during the calcification process. Furthermore, response to water, water deprivation, and salt stress were most enriched by gene ontology (GO) and gene set enrichment analysis (GSEA). The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways related to chestnut calcification included purine metabolism, RNA degradation, the mRNA surveillance pathway, starch and sucrose metabolism, arginine and proline metabolism, and fatty acid metabolism, and were detected. Most of the genes involved in cellulose synthase, lignin catabolism, and pectin catabolism were down-regulated, while only two important genes, scaffold11300 and scaffold0412, were up-regulated, which were annotated as cellulose and pectin synthase genes, respectively. These two genes may contribute to the increase of total cell wall material accumulation during chestnut calcification. The results provided new insights into chestnut calcification process and laid a foundation for further chestnut preservation.

A Plant-Derived Alkanol Induces Teliospore Germination in Sporisorium scitamineum

Sugarcane smut caused by the basidiomycetes fungus Sporisorium scitamineum is a devastating disease for the sugarcane industry worldwide. As the initial step, the smut teliospores germinate on sugarcane buds, and subsequently, the mycelium infects the bud tissues. However, chemical signals that induce spore germination are still unknown. By comparison of the behavior of the teliospores on the buds of both resistant and susceptible varieties, we found that spore germination rates were significantly lower on the buds of resistant cultivars ZZ1, ZZ6, and ZZ9 than on the susceptible varieties GT42 and ROC22. It was found that the levels of hexacosanol and octacosanol were higher on the buds of smut-susceptible varieties than on the smut-resistant varieties. These observations were extended to the smut-resistant and smut-susceptible sub-genetic populations derived from the cross of ROC25 and YZ89-7. In artificial surface assays, we found that hexacosanol and octacosanol promoted smut teliospore germination. Transcriptome analysis of smut teliospores under the induction by octacosanol revealed that genes in the MAPK signaling pathway and fatty acid metabolism were significantly differentially expressed. Overall, our results provide evidence that alkanol plays important roles in smut teliospore germination and thus could be used as a potential marker for smut resistance in sugarcane breeding programs.

A sugarcane smut fungus effector simulates the host endogenous elicitor peptide to suppress plant immunity

The smut fungus Sporisorium scitamineum causes the most prevalent disease on sugarcane. The mechanism of its pathogenesis, especially the functions and host targets of its effector proteins, are unknown. In order to identify putative effectors involving in S. scitamineum infection, a weighted gene co-expression network analysis was conducted based on the transcriptome profiles of both smut fungus and sugarcane using a customized microarray. A smut effector gene, termed SsPele1, showed strong co-expression with sugarcane PLANT ELICITOR PEPTIDE RECEPTOR1 (ScPEPR1), which encodes a receptor like kinase for perception of plant elicitor peptide1 (ScPep1). The relationship between SsPele1 and ScPEPR1, and the biological function of SsPele1 were characterized in this study. The SsPele1 C-terminus contains a plant elicitor peptide-like motif, by which SsPele1 interacts strongly with ScPEPR1. Strikingly, the perception of ScPep1 on ScPEPR1 is competed by SsPele1 association, leading to the suppression of ScPEPR1-mediated immune responses. Moreover, the Ustilago maydis effector UmPele1, an ortholog of SsPele1, promotes fungal virulence using the same strategy. This study reveals a novel strategy by which a fungal effector can mimic the plant elicitor peptide to complete its perception and attenuate receptor-activated immunity.

Resistance mechanisms and expression of disease resistance-related genes in sugarcane (Sacchrum officinarum) to Sporisorium scitamineum infection

Resistance of sugarcane (Saccharum officinarum L.) to smut disease (caused by Sporisorium scitamineum) is driven by two separate mechanisms, external and internal resistance. Two progenies generated from an introgression cross, with contrasting responses to smut infection were used to investigate this interaction. Histopathological screening at different stages of the plant growth was used to determine the extent of mycelium growth within sugarcane tissues. Ten disease resistance-related genes were selected, and the relative expression determined using quantitative real-time reverse transcription PCR (real-time RT-qPCR). The results revealed that PR10, HCT1 and ScChi were down-regulated in the susceptible progeny and up-regulated in the resistant progeny early infection process. This may reflect an early attempt to halt pathogen development by increasing the lignin deposition at the infection site. At 8 weeks post-inoculation, they were highly up-regulated in the susceptible progeny coincided with whip development. This reveals a major role for these genes to whip development in the susceptible progeny and indicates that while PR10 is involved in the resistance mechanism of resistant progeny early infection process it also has a role in susceptibility. These results on genetically related progeny with different responses to smut infection reveal a complex interaction of genes and gene networks being induced in response to fungal invasion.

Sugarcane Smut, Caused by Sporisorium scitamineum, a Major Disease of Sugarcane: A Contemporary Review

Sugarcane smut caused by the fungus Sporisorium scitamineum is one of the major diseases of sugarcane worldwide, causing significant losses in productivity and profitability of this perennial crop. Teliospores of this fungus are airborne, can travel long distances, and remain viable in hot and dry conditions for >6 months. The disease is easily recognized by its long whiplike sorus produced on the apex or side shoots of sugarcane stalks. Each sorus can release ≤100 million teliospores in a day; the spores are small (≤7.5 µ) and light and can survive in harsh environmental conditions. The airborne teliospores are the primary mode of smut spread around the world and across cane-growing regions. The most effective method of managing this disease is via resistant varieties. Because of the complex genomic makeup of sugarcane, selection for resistant traits is difficult in sugarcane breeding programs. In recent times, the application of molecular markers as a rapid tool of discarding susceptible genotypes early in the selection program has been investigated. Large effect resistance loci have been identified and have the potential to be used for marker-assisted selection to increase the frequency of resistant breeding lines in breeding programs. Recent developments in omics technologies (genomics, transcriptomics, proteomics, and metabolomics) have contributed to our understanding and provided insights into the mechanism of resistance and susceptibility. This knowledge will further our understanding of smut and its interactions with sugarcane genotypes and aid in the development of durable resistant varieties.

Detection of a major QTL related to smut disease resistance inherited from a Japanese wild sugarcane using GRAS-Di technology

Smut disease of sugarcane causes considerable yield losses and the use of resistant varieties is the best control practice. Our group identified a Japanese wild sugarcane with highly smut disease resistance named 'Iriomote8'. In this study, we conducted QTL analysis for smut disease resistance using a mapping population derived from a resistant variety 'Yaenoushie', in which resistance is inherited from 'Iriomote8'. We identified 4813 non-redundant markers using GRAS-Di technology and developed a linkage map of mapping parents. We evaluated smut disease resistance of the mapping population by the inoculation test. Consequently, a large number of clones did not show the disease symptoms and the distribution of smut disease incidence tended to be "L shaped". Composite interval mapping detected an identical QTL for indices of smut disease incidence with a markedly high LOD score (26.6~45.6) at the end of linkage group 8 of 'Yaenoushie'. This QTL explained approximately 50% of the cases of smut disease incidence. In the mapping population, there were no correlations between the indices of smut disease incidence and other agronomic traits. In conclusion, this QTL could be used for marker-assisted selection to significantly improve smut disease resistance without negative effects on other agronomic traits.

Time-series expression profiling of sugarcane leaves infected with Puccinia kuehnii reveals an ineffective defense system leading to susceptibility

Successful orange rust development on sugarcane can potentially be explained as suppression of the plant immune system by the pathogen or delayed plant signaling to trigger defense responses. Puccinia kuehnii is an obligate biotrophic fungus that infects sugarcane leaves causing a disease called orange rust. It spread out to other countries resulting in reduction of crop yield since its first outbreak. One of the knowledge gaps of that pathosystem is to understand the molecular mechanisms altered in susceptible plants by this biotic stress. Here, we investigated the changes in temporal expression of transcripts in pathways associated with the immune system. To achieve this purpose, we used RNA-Seq to analyze infected leaf samples collected at five time points after inoculation. Differential expression analyses of adjacent time points revealed substantial changes at 12, 48 h after inoculation and 12 days after inoculation, coinciding with the events of spore germination, haustoria post-penetration and post-sporulation, respectively. During the first 24 h, a lack of transcripts involved with resistance mechanisms was revealed by underrepresentation of hypersensitive and defense response related genes. However, two days after inoculation, upregulation of genes involved with immune response regulation provided evidence of some potential defense response. Events related to biotic stress responses were predominantly downregulated in the initial time points, but expression was later restored to basal levels. Genes involved in carbohydrate metabolism showed evidence of repression followed by upregulation, possibly to ensure the pathogen nutritional requirements were met. Our results support the hypothesis that P. kuehnii initially suppressed sugarcane genes involved in plant defense systems. Late overexpression of specific regulatory pathways also suggests the possibility of an inefficient recognition system by a susceptible sugarcane genotype.

Persistent Calyxes in Postbloom Fruit Drop: A Microscopy and Microanalysis Perspective

Citrus postbloom fruit drop, caused by Colletotrichum spp., is an important disease in the Americas. The pathogen infects citrus flowers, produces orange-brown lesions on petals, and may cause the abscission of young fruit. In diseased flowers, the calyxes remain attached to the peduncle after the young fruit drop. No anatomical and microanalysis studies have been conducted to determine whether calyx tissues can be infected by Colletotrichum spp. and why calyxes remain attached to the peduncle. Based on light microscopy, we demonstrate that the ovary abscission zone exhibits a separation region composed of layers of thickened lignified walled cells, indicating that abscission involves the disruption of cell walls. The first layers of the protective zone (PZ) are composed of densely packed cells with suberized walls produced by the wound meristem. Beneath the PZ, there is a compact mass of small cells that accumulate starch grains. X-ray fluorescence microanalysis (µ-XRF) confirmed the increased accumulation of calcium in the receptacle of the persistent calyxes compared to non-inoculated citrus flowers. Moreover, the peduncle pith and the receptacle exhibit hypertrophied cells with thick walls that may be related to calyx retention. Fungal structures are not observed inside the persistent calyx tissues.

Smut fungal strategies for the successful infection

The smut fungi include a large number of plant pathogens that establish obligate biotrophic relationships with their host. Throughout the whole life inside plant tissue, smut fungi keep plant cells alive and acquire nutrients via biotrophic interfaces. This mini-review mainly summarizes the interactions between smut fungi and their host plants during the infection process. Despite various strategies recruited by plants to defense invading pathogens, smut fungi successfully evolved an arsenal for colonization. Mating of two compatible haploids gives rise to parasitic mycelium, which can sense plant surface cues such as fatty acids and hydrophobic surface, and induce the formation of appressoria for surface penetration. Plants can recognize fungal invading and activate defense response, including callose and lignin deposition, programmed cell death, and SA signaling pathway. To suppress plant immunity and alter the metabolic pathway of host plants, a cocktail of effectors is secreted by smut fungi depending on the plant organ and cell type that is infected.

Microscopic analysis of colonization of Colletotrichum abscissum in citrus tissues

Postbloom fruit drop (PFD), caused mainly by Colletotrichum abscissum, is one of the most severe citrus diseases and can causes up to 80% fruit loss in favorable climatic conditions. According to the literature, other Colletotrichum species colonize hosts using distinct strategies: intracellular hemibiotrophic or subcuticular intramural necrotrophic colonization. However, so far, for C. abscissum only the necrotrophic stage has been described and some aspects remain unclear in PFD disease cycle. To better understand the disease cycle, microscopy studies could be applied. However, even using eGFP strains (expressing green fluorescent protein), the results are unclear due to the autofluorescence of citrus leaves. To eliminate this problem and to study the interaction between C. abscissum-citrus we used a destaining and staining methodologies, and we observed that in leaves, even applying injury before inoculation, C. abscissum does not colonize adjacent tissues. Apparently, in the leaves the fungus only uses the nutrients exposed in the artificial lesions for growth, and then produces large amount of spores. However, in flowers, C. abscissum penetrated and colonized the tissues of the petals 12 h after inoculation. In the early stages of infection, we observed the development of primary biotrophic hyphae, suggesting this species as a hemibiotrophic fungus, with a short biotrophic phase during flower colonization followed by dominant necrotrophic colonization. In conclusion, the use of an eGFP strain of C. abscissum and a different methodology of destaining and staining allowed a better understanding of the morphology and mechanisms used by this citrus pathogen to colonize the host.

Polyamines levels increase in smut teliospores after contact with sugarcane glycoproteins as a plant defensive mechanism

Previous studies have already highlighted the correlation between Sporisorium scitamineum pathogenicity and sugarcane polyamine accumulation. It was shown that high infectivity correlates with an increase in the amount of spermidine, spermine and cadaverine conjugated to phenols in the sensitive cultivars whereas resistant plants mainly produce free putrescine. However, these previous studies did not clarify the role of these polyamides in the disorders caused to the plant. Therefore, the purpose of this research is to clarify the effect of polyamines on the development of smut disease. In this paper, commercial polyamines were firstly assayed on smut teliospores germination. Secondly, effects were correlated to changes in endogenous polyamines after contact with defense sugarcane glycoproteins. Low concentrations of spermidine significantly activated teliospore germination, while putrescine had no activating effect on germination. Interestingly, it was observed that the diamine caused nuclear decondensation and breakage of the teliospore cell wall whereas the treatment of teliospores with spermidine did not induce nuclear decondensation or cell wall breakdown. Moreover, the number of polymerized microtubules increased in the presence of 7.5 mM spermidine but it decreased with putrescine which indicates that polyamines effects on Sporisorium scitamineum teliospore germination could be mediated through microtubules interaction. An increased production of polyamines in smut teliospores has been related to sugarcane resistance to the disease. Teliospores incubation with high molecular mass glycoproteins (HMMG) from the uninoculated resistant variety of sugarcane, Mayari 55-14, caused an increase of the insoluble fraction of putrescine, spermidine and spermine inside the teliospore cells. Moreover, the level of the soluble fraction of spermidine (S fraction) increased inside teliospores and the excess was released to the medium. The HMMG glycoproteins purified from Mayarí 55-14 plants previously inoculated with the pathogen significantly increased the levels of both retained and secreted soluble putrescine and spermidine. Polyamines levels did not increase in teliospores after incubation with HMMG produced by non resistant variety Barbados 42231 which could be related to the incapacity of these plants to defend themselves against smut disease. Thus, a hypothesis about the role of polyamines in sugarcane-smut interaction is explained.

An actin-depolymerizing factor from the halophyte smooth cordgrass, Spartina alterniflora (SaADF2), is superior to its rice homolog (OsADF2) in conferring drought and salt tolerance when constitutively overexpressed in rice

Actin-depolymerizing factors (ADFs) maintain the cellular actin network dynamics by regulating severing and disassembly of actin filaments in response to environmental cues. An ADF isolated from a monocot halophyte, Spartina alterniflora (SaADF2), imparted significantly higher level of drought and salinity tolerance when expressed in rice than its rice homologue OsADF2. SaADF2 differs from OsADF2 by a few amino acid residues, including a substitution in the regulatory phosphorylation site serine-6, which accounted for its weak interaction with OsCDPK6 (calcium-dependent protein kinase), thus resulting in an increased efficacy of SaADF2 and enhanced cellular actin dynamics. SaADF2 overexpression preserved the actin filament organization better in rice protoplasts under desiccation stress. The predicted tertiary structure of SaADF2 showed a longer F-loop than OsADF2 that could have contributed to higher actin-binding affinity and rapid F-actin depolymerization in vitro by SaADF2. Rice transgenics constitutively overexpressing SaADF2 (SaADF2-OE) showed better growth, relative water content, and photosynthetic and agronomic yield under drought conditions than wild-type (WT) and OsADF2 overexpressers (OsADF2-OE). SaADF2-OE preserved intact grana structure after prolonged drought stress, whereas WT and OsADF2-OE presented highly damaged and disorganized grana stacking. The possible role of ADF2 in transactivation was hypothesized from the comparative transcriptome analyses, which showed significant differential expression of stress-related genes including interacting partners of ADF2 in overexpressers. Identification of a complex, differential interactome decorating or regulating stress-modulated cytoskeleton driven by ADF isoforms will lead us to key pathways that could be potential target for genome engineering to improve abiotic stress tolerance in agricultural crops.