Silicon Supply Improves Leaf Gas Exchange, Antioxidant Defense System and Growth in Saccharum officinarum Responsive to Water Limitation

Silicon (Si) is not categorized as a biologically essential element for plants, yet a great number of scientific reports have shown its significant effects in various crop plants and environmental variables. Plant Si plays biologically active role in plant life cycle, and the significant impact depends on its bioaccumulation in plant tissues or parts. In particular, it has been investigated for its involvement in limited irrigation management. Therefore, this experiment was conducted to examine the effect of Si application in eco-physiological, enzymatic and non-enzymatic activities of sugarcane plants against water stress. Four irrigation levels, i.e., normal (100-95% of soil moisture), 80-75, 55-50, and 35-30% of soil moisture were treated for the sugarcane cultivar GT 42 plants supplied with 0, 100, 200, 300, 400 and 500 mg Si L-1 and exposed for 60 days after Si application. Under stress, reduction in plant length (~26-67%), leaf area-expansion (~7-51%), relative water content (~18-57%), leaf greenness (~12-35%), photosynthetic pigments (~12-67%), physiological responses such as photosynthesis (22-63%), stomatal conductance (~25-61%), and transpiration rate (~32-63%), and biomass production were observed in the plants without Si application. The drought condition also inhibited the activities of antioxidant enzymes like catalase (~10-52%), peroxidase (ca. 4-35), superoxide dismutase (10-44%) and enhanced proline (~73-410%), and malondialdehyde content (ca. 15-158%), respectively. However, addition of Si ameliorated drought induced damage in sugarcane plants. The findings suggest that the active involvement of Si in sugarcane responsive to water stress ranges from plant performance and physiological processes, to antioxidant defense systems.

Hormetic effect of glyphosate persists during the entire growth period and increases sugarcane yield

BACKGROUND

Stimulation of plant growth by low doses of a toxic compound is defined as a hormetic effect. Exposure of plants to low doses of glyphosate can cause stimulatory effects on growth or other variables. Sugarcane is the major biofuel and sugar-production crop cultivated in Brazil, but its expansion to new areas is limited; therefore, there is a demand for new technologies to improve sugarcane production per unit area. The use of pesticides to stimulate growth through the hormetic effect might be a suitable strategy to increase sugarcane yields. The purpose of this research was to investigate the effect of a low dose of glyphosate on metabolic compound accumulation, leaf phosphorus (P) concentration, and morphological variables across a one-year sugarcane cycle, as well as to determine whether the glyphosate effect was sustained and effective in improving the yield and technological quality of the sugarcane at harvest.

RESULTS

The application of a low dose of glyphosate led to higher concentrations of shikimic acid and quinic acid, higher leaf P concentrations, and improved plant growth, yield, and technological quality of the sugarcane, by increasing the Brix% juice, pol% cane, total recoverable sugar, tons of culms per hectare, and tons of pol per hectare, relative to the results for an untreated control.

CONCLUSIONS

The increased growth stimuli, observed through several variables, promoted an improvement in sugarcane yield. Therefore, the application of a low dose of glyphosate to sugarcane is a promising practice for crop management. © 2020 Society of Chemical Industry.

Epigenetic diversity of Saccharum spp. accessions assessed by methylation-sensitive amplification polymorphism (MSAP)

The epigenetic diversity of six genotype groups (commercial cultivars, S. officinarum, S. spontaneum, S. robustum, S. barberi, and Erianthus sp.) was assessed through methylation-sensitive amplification polymorphism (MSAP). A total of 1341 MSAP loci were analyzed, of which 1117 (83.29%) were susceptible to cytosine methylation and responsible for a higher proportion of overall diversity among genotypes. The MSAP selective primer combinations captured different proportions of internal and external cytosine methylation loci across genotype groups, while the average external cytosine frequency was higher for all genotype groups. The genotypes were divided into two subpopulations with a high differentiation index (φst = 0.086) based on epigenetic loci. The genotypes were clustered in three subgroups for both methylated and unmethylated loci, considering dissimilarity values. Four methylated fragments (MFs) were randomly selected and subsequently sequenced and compared with sugarcane public databases using BLASTN. MF alignments suggest that cytosine methylation occurs in sugarcane near CpG islands and tandem repeats within transcribed regions and putative cis-regulatory sequences, which assigned functions are associated with stress adaptation. These results provide the first insights about the distribution of this epigenetic mark in sugarcane genome, and suggest a biological relevance of methylated loci.

Transcription Factors in Plant Stress Responses: Challenges and Potential for Sugarcane Improvement

Increasing vulnerability of crops to a wide range of abiotic and biotic stresses can have a marked influence on the growth and yield of major crops, especially sugarcane (Saccharum spp.). In response to various stresses, plants have evolved a variety of complex defense systems of signal perception and transduction networks. Transcription factors (TFs) that are activated by different pathways of signal transduction and can directly or indirectly combine with cis-acting elements to modulate the transcription efficiency of target genes, which play key regulators for crop genetic improvement. Over the past decade, significant progresses have been made in deciphering the role of plant TFs as key regulators of environmental responses in particular important cereal crops; however, a limited amount of studies have focused on sugarcane. This review summarizes the potential functions of major TF families, such as WRKY, NAC, MYB and AP2/ERF, in regulating gene expression in the response of plants to abiotic and biotic stresses, which provides important clues for the engineering of stress-tolerant cultivars in sugarcane.

Expression levels of vacuolar ion homeostasis-related genes, Na+ enrichment, and their physiological responses to salt stress in sugarcane genotypes

Sugarcane is a sugar-producing crop widely grown in tropical regions in over 120 countries of the world. Salt-affected soil is one of the most significant abiotic constraints that inhibit growth and crop productivity, and, consequently, reduce sucrose concentration in the stalk. The present study investigated vacuolar ion homeostasis, Na⁺ accumulation, and physiological and morphological adaptations under salt stress in two different sugarcane genotypes (salt-tolerant K88-92 and salt-sensitive K92-80) under greenhouse conditions. Na⁺ was rapidly absorbed by the root tissues of both sugarcane genotypes within 3-7 days of 150 mM NaCl treatment, as confirmed by the results of CoroNa Green fluorescence staining. In addition, the rate of Na⁺ translocation from roots to shoots was evidently reduced, leading to lower amount of Na⁺ in the leaf tissues. At the cellular level, expression of ShNHX1 (vacuolar Na⁺/H⁺ antiporter), ShV-PPase (vacuolar H⁺-pyrophosphatase), and ShV-ATPase (vacuolar H⁺-ATPase) was upregulated in salt-stressed plants for the compartmentation of Na⁺ into the vacuoles of root cells. Interestingly, sucrose, glucose, and fructose in root tissues of salt-stressed sugarcane cv. K88-92 were increased by 10.61, 5.58, and 1.81 folds, respectively, over the control. Total soluble sugars in the roots and free proline in the leaves of sugarcane cv. K88-92 (salt-tolerant) were enriched by 3.08 and 1.99 folds, respectively, when plants were exposed to 150 mM NaCl, leading to maintain better photosynthetic abilities, net photosynthetic rate (P_n), stomatal conductance (g_s), transpiration rate (E), and water use efficiency (WUE) in sugarcane cv. K88-92 than those in cv. K92-80. The study concludes that Na⁺ compartmentation in the root tissue acts as a major defense mechanism in sugarcane, especially in salt-tolerant genotype.

Identification of Differentially Expressed Proteins in Sugarcane in Response to Infection by Xanthomonas albilineans Using iTRAQ Quantitative Proteomics

Sugarcane can suffer severe yield losses when affected by leaf scald, a disease caused by Xanthomonas albilineans. This bacterial pathogen colonizes the vascular system of sugarcane, which can result in reduced plant growth and plant death. In order to better understand the molecular mechanisms involved in the resistance of sugarcane to leaf scald, a comparative proteomic study was performed with two sugarcane cultivars inoculated with X. albilineans: one resistant (LCP 85-384) and one susceptible (ROC20) to leaf scald. The iTRAQ (isobaric tags for relative and absolute quantification) approach at 0 and 48 h post-inoculation (hpi) was used to identify and annotate differentially expressed proteins (DEPs). A total of 4295 proteins were associated with 1099 gene ontology (GO) terms by GO analysis. Among those, 285 were DEPs during X. albilineans infection in cultivars LCP 85-384 and ROC20. One hundred seventy-two DEPs were identified in resistant cultivar LCP 85-384, and 113 of these proteins were upregulated and 59 were downregulated. One hundred ninety-two DEPs were found in susceptible cultivar ROC20 and half of these (92) were upregulated, whereas the other half corresponded to downregulated proteins. The significantly upregulated DEPs in LCP 85-384 were involved in metabolic pathways, the biosynthesis of secondary metabolites, and the phenylpropanoid biosynthesis pathway. Additionally, the expression of seven candidate genes related to photosynthesis and glycolytic pathways, plant innate immune system, glycosylation process, plant cytochrome P450, and non-specific lipid transfer protein was verified based on transcription levels in sugarcane during infection by X. albilineans. Our findings shed new light on the differential expression of proteins in sugarcane cultivars in response to infection by X. albilineans. The identification of these genes provides important information for sugarcane variety improvement programs using molecular breeding strategies.

Global Gene Responses of Resistant and Susceptible Sugarcane Cultivars to Acidovorax avenae subsp. avenae Identified Using Comparative Transcriptome Analysis

Red stripe disease in sugarcane caused by Acidovorax avenae subsp. avenae (Aaa) is related to serious global losses in yield. However, the underlying molecular mechanisms associated with responses of sugarcane plants to infection by this pathogen remain largely unknown. Here, we used Illumina RNA-sequencing (RNA-seq) to perform large-scale transcriptome sequencing of two sugarcane cultivars to contrast gene expression patterns of plants between Aaa and mock inoculations, and identify key genes and pathways involved in sugarcane defense responses to Aaa infection. At 0–72 hours post-inoculation (hpi) of the red stripe disease-resistant cultivar ROC22, a total of 18,689 genes were differentially expressed between Aaa-inoculated and mock-inoculated samples. Of these, 8498 and 10,196 genes were up- and downregulated, respectively. In MT11-610, which is susceptible to red stripe disease, 15,782 genes were differentially expressed between Aaa-inoculated and mock-inoculated samples and 8807 and 6984 genes were up- and downregulated, respectively. The genes that were differentially expressed following Aaa inoculation were mainly involved in photosynthesis and carbon metabolism, phenylpropanoid biosynthesis, plant hormone signal transduction, and plant–pathogen interaction pathways. Further, qRT-PCR and RNA-seq used for additional validation of 12 differentially expressed genes (DEGs) showed that eight genes in particular were highly expressed in ROC22. These eight genes participated in the biosynthesis of lignin and coumarin, as well as signal transduction by salicylic acid, jasmonic acid, ethylene, and mitogen-activated protein kinase (MAPK), suggesting that they play essential roles in sugarcane resistance to Aaa. Collectively, our results characterized the sugarcane transcriptome during early infection with Aaa, thereby providing insights into the molecular mechanisms responsible for bacterial tolerance.

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.

Influence of nitrate - ammonium ratio on the growth, nutrition, and metabolism of sugarcane

Although ammonium (NH₄⁺) has been claimed as the preferential N source for sugarcane (Saccharum spp.), the intense uptake of this mineral form by plants can impair metabolic processes and crop yield. We aimed to assess the growth, nutrition, and metabolic responses of sugarcane grown under different amounts of nitrate (NO₃^-) and NH₄⁺. Sugarcane setts were grown in nutrient solution at a total concentration of 15 mM N using different NO₃^-/NH₄⁺ ratios (100/0, 75/25, 50/50, 25/75, and 0/100, respectively) for 163 d under controlled conditions. The pH of the medium was daily adjusted to 5.8 ± 0.1, with replacement of the hydroponic solution every 10 d. NH₄⁺-only fed plants showed lower dry biomass yield, nutrient content, leaf surface area, and leaf gas exchange than those under sole NO₃^- supply, in addition to favoring the development of brown rust (Puccinia melanocephala). However, there was no indication that NH₄⁺ is directly related to oxidative stress in sugarcane. On the other hand, the highest N utilization efficiency was obtained with NO₃^--only fed plants, which also resulted in the highest biomass yield, leaf surface area, nutrient content, leaf gas exchange, and root growth. Since NO₃^- was not stored in plant tissues, we therefore suggested that most of this N form is assimilated following its uptake. Despite the well-known preference of the crop for NH₄⁺, the optimal growth response of sugarcane plants to NO₃^-/NH₄⁺ ratios was observed under NO₃^- supply.

Characterization of an AP endonuclease from sugarcane - ScARP1

Plants are sessile organisms that need to cope with different conditions. The Base Excision Repair (BER) pathway is an important mechanism protecting the genome from DNA lesions. Apurinic/apyrimidinic (AP) endonucleases are key BER enzymes that process AP sites arising either spontaneously or as BER intermediates. In Arabidopsis there are three AP endonucleases: AtARP1, AtAPE1L, and AtAPE2, and in sugarcane two AtARP1 homologues have been identified: ScARP1 and ScARP3. ScARP1 shares 59% sequence identity with Arabidopsis AtARP. Protein modeling of ScARP1 and AtARP1 revealed conserved active sites and metal binding sites. For biochemical characterisation, recombinant ScARP1 protein displayed AP endonuclease activity both in the presence of MnCl₂ or MgCl₂ and the optimal temperature for its activity was 37 °C. Under these conditions, 3'-exonuclease, 3'-phosphatase, and 3'-phosphodiesteterase activities were not detectable. We also show that ScARP1 protein is able to complement mutant atarp^-/- cell extracts deficient in AP endonuclease activity. These results suggest that AP endonucleases from different plant species preserve AP endonuclease activity. The biochemical characterisation of ScARP1 extends our knowledge of the BER pathway to a monocot crop plant group.

Plant Cell Wall Proteomics: A Focus on Monocot Species, Brachypodium distachyon, Saccharum spp. and Oryza sativa

Plant cell walls mostly comprise polysaccharides and proteins. The composition of monocots' primary cell walls differs from that of dicots walls with respect to the type of hemicelluloses, the reduction of pectin abundance and the presence of aromatic molecules. Cell wall proteins (CWPs) differ among plant species, and their distribution within functional classes varies according to cell types, organs, developmental stages and/or environmental conditions. In this review, we go deeper into the findings of cell wall proteomics in monocot species and make a comparative analysis of the CWPs identified, considering their predicted functions, the organs analyzed, the plant developmental stage and their possible use as targets for biofuel production. Arabidopsis thaliana CWPs were considered as a reference to allow comparisons among different monocots, i.e., Brachypodium distachyon, Saccharum spp. and Oryza sativa. Altogether, 1159 CWPs have been acknowledged, and specificities and similarities are discussed. In particular, a search for A. thaliana homologs of CWPs identified so far in monocots allows the definition of monocot CWPs characteristics. Finally, the analysis of monocot CWPs appears to be a powerful tool for identifying candidate proteins of interest for tailoring cell walls to increase biomass yield of transformation for second-generation biofuels production.

Hemipteran Pests of Sugarcane in North America

Piercing-sucking herbivores (Insecta: Hemiptera) represent one of the greatest threats to agricultural production worldwide. Hemipteran pests directly injure plants as well as vector disease-causing plant pathogens. Production of sugarcane (Saccharum spp.) in North America is impacted by a complex of Hemiptera including the sugarcane aphid, Melanaphis sacchari Zehntner (Aphididae); yellow sugarcane aphid, Sipha flava (Forbes) (Aphididae); West Indian canefly, Saccharosydne saccharivora (Westwood) (Delphacidae); sugarcane delphacid, Perkinsiella saccharicida Kirkaldy (Delphacidae); and sugarcane lace bug, Leptodictya tabida (Herric-Schaeffer) (Tingidae). None of these pests is consistently damaging to large amounts of sugarcane acreage, but regional outbreaks are common. The biology, ecology, and pest management of these insects are discussed with emphasis on North America sugarcane production.

Reference genes for gene expression studies targeting sugarcane infected with Sugarcane mosaic virus (SCMV)

Objective

The selection of reference genes in sugarcane under Sugarcane mosaic virus (SCMV) infection has not been reported and is indispensable to get reliable reverse transcription quantitative PCR (RT-qPCR) results for validation of transcriptome analysis. In this regard, seven potential reference genes were tested by RT-qPCR and ranked according to their stability using BestKeeper, NormFinder and GeNorm algorithms, and RefFinder WEB-based software in an experiment performed with samples from two sugarcane cultivars contrasting for SCMV resistance, when mechanically inoculated with a severe SCMV strain and using mock inoculated plant controls.

Results

The genes Uridylate kinase (UK) and Ubiquitin-conjugating enzyme 18 (UBC18) were the most stable according to GeNorm algorithm and the Pearson correlation coefficients with the BestKeeper index. On the other hand, ribosomal protein L35-4 (RPL1), Actin (ACT) and Ubiquitin1 (UBQ1) were the least stable genes for all algorithms tested.

Electronic supplementary material

The online version of this article (10.1186/s13104-019-4168-5) contains supplementary material, which is available to authorized users.

Screening for Sugarcane Root Phenes Reveals That Reducing Tillering Does Not Lead to an Increased Root Mass Fraction

Sugarcane root systems are poorly studied and understood due to the perennial nature, tall stature, and the long cropping cycle. Whilst some field studies gave insights into sugarcane root traits, there is no detailed description of root and root system traits available. The objectives of our work were to establish a baseline of sugarcane root trait values that will serve for future studies, and to characterize the degree of root system resilience when restricting tiller number. We first conducted an initial screening for root trait diversity on a collection of twenty cultivars representative of sugarcane breeding from 1930 to now. Then we investigated the effect of reduced tillering, via manual de-tillering, on the plant root and root system traits of five varieties grown under optimal conditions in a glasshouse for 1700°Cd. In addition to establishing baseline means and variation for sugarcane root trait values that could serve as a reference for crop models, we demonstrated that the sugarcane root mass fraction was extremely resilient to drastic reduction in tiller number. Restricted plants were effectively maintaining their root system configuration (opening angle) by dramatically increasing the number of nodal roots produced per tiller as well as maximizing total root length by increasing the specific root length. Using this knowledge of sugarcane root traits in combination with the specific agronomic constraints for sugarcane will now underpin the development of a root system ideotype for sugarcane to enable targeted root trait selection for improving crop productivity.

Target enrichment sequencing of 307 germplasm accessions identified ancestry of ancient and modern hybrids and signatures of adaptation and selection in sugarcane (Saccharum spp.), a 'sweet' crop with 'bitter' genomes

Sugarcane (Saccharum spp.) is a highly energy-efficient crop primarily for sugar and bio-ethanol production. Sugarcane genetics and cultivar improvement have been extremely challenging largely due to its complex genomes with high polyploidy levels. In this study, we deeply sequenced the coding regions of 307 sugarcane germplasm accessions. Nearly five million sequence variations were catalogued. The average of 98× sequence depth enabled different allele dosages of sequence variation to be differentiated in this polyploid collection. With selected high-quality genome-wide SNPs, we performed population genomic studies and environmental association analysis. Results illustrated that the ancient sugarcane hybrids, S. barberi and S. sinense, and modern sugarcane hybrids are significantly different in terms of genomic compositions, hybridization processes and their potential ancestry contributors. Linkage disequilibrium (LD) analysis showed a large extent of LD in sugarcane, with 962.4 Kbp, 2739.2 Kbp and 3573.6 Kbp for S. spontaneum, S. officinarum and modern S. hybrids respectively. Candidate selective sweep regions and genes were identified during domestication and historical selection processes of sugarcane in addition to genes associated with environmental variables at the original locations of the collection. This research provided an extensive amount of genomic resources for sugarcane community and the in-depth population genomic analyses shed light on the breeding and evolution history of sugarcane, a highly polyploid species.

Sugarcane yield gap analysis in Brazil - A multi-model approach for determining magnitudes and causes

Brazil is the largest sugarcane producer in the world playing a pivotal role on global ethanol production. The sugarcane yield levels across the producing regions of the country vary substantially, resulting in yield gaps of different magnitudes, which represent a huge opportunity for increasing sugarcane and ethanol production. According to that, the present study aimed to investigate the sugarcane yield gap in Brazil, their magnitude and causes (water deficit or crop management), considering a multi-model approach. Three different sugarcane simulation models, FAO-Agroecological Zone, DSSAT/CANEGRO and APSIM-Sugarcane, properly calibrated and validated for sugarcane in Brazil, were used to estimate potential and water-limited yields and yield gaps for 30 locations across the country. The average of total yield gap for the 30 locations was 124 t ha^-1. The main proportion of the total yield gap was caused by water deficit (about 89 t ha^-1, representing 73% of the total), followed by sub-optimal crop management (about 35 t ha^-1, representing 27% of the total). The highest yield gap by water deficit was found in the Northeastern region, whereas São Paulo State showed the lowest yield gap by crop management. The main causes of yield gap by crop management are possibly related to the production system based on long-term monoculture and soil compaction due to intense crop mechanization in recent years. Reducing sugarcane yield gap caused by crop management by 20 to 100% would allow to diminish the cultivated area with this crop, respectively, from 9 to 32%. Possible solutions to mitigate the yield gaps, such as use of irrigation, adoption of drought tolerant cultivars, better traffic control, periodical crop rotation, among others, were also discussed.

Revisiting Meiosis in Sugarcane: Chromosomal Irregularities and the Prevalence of Bivalent Configurations

Traditional sugarcane cultivars (Saccharum officinarum) proved highly susceptible to diseases, and this led breeders to progress to interspecific crosses resulting in disease resistance. A backcrossing program to S. officinarum was then required to boost sucrose content. Clonal selection across generations and incorporation of other germplasm into cultivated backgrounds established the (narrow) genetic base of modern cultivars (Saccharum spp.), which have a man-made genome. The genome complexity has inspired several molecular studies that have elucidated aspects of sugarcane genome constitution, architecture, and cytogenetics. However, there is a critical shortage of information on chromosome behavior throughout meiosis in modern cultivars. In this study, we examined the microsporogenesis of a contemporary variety, providing a detailed analysis of the meiotic process and chromosome association at diakinesis, using FISH with centromeric probes. Chromosomal abnormalities were documented by examining high quality preparations of pollen mother cells (700 in total). Approximately 70% of the cells showed abnormalities, such as metaphase chromosomes not lined up at the plate, lagging chromosomes and chromosomal bridges, and tetrad cells with micronuclei. Some dyads with asynchronous behavior were also observed. Due to the hybrid composition of the sugarcane genome, we suggest that bivalent incomplete pairing may occur in the first prophase leading to univalency. The presence of rod bivalents showing the lagging tendency is consistent with a reduction in chiasma frequency. Finally, the presence of chromatin bridges indicates the indirect occurrence of chromosomal inversions, although chromosome fragments were not clearly recognized. Possible reasons for such meiotic abnormalities and the large prevalence of bivalent formation are discussed.

Low doses of glyphosate enhance growth, CO2 assimilation, stomatal conductance and transpiration in sugarcane and eucalyptus

INTRODUCTION

Sublethal doses of herbicides can enhance plant growth and stimulate other process, an effect known as hormesis. The magnitude of hormesis is dependent on the plant species, the herbicide and its dose, plant development stage and environmental parameters. Glyphosate hormesis is well established, but relatively little is known of the mechanism of this phenomenon. The objective of this study was to determine if low doses of glyphosate that cause growth stimulation in sugarcane and eucalyptus concomitantly stimulate CO2 assimilation.

RESULTS

Shoot dry weight in both species increased at both 40 and 60 days after application of 6.2 to 20.2 g a.e. ha-1 glyphosate. The level of enhanced shoot dry weight was 11 to 37%, depending on the time after treatment and the species. Concomitantly, CO2 assimilation, stomatal conductance and transpiration were increased by glyphosate doses similar to those that caused growth increases.

CONCLUSION

Glyphosate applied at low doses increased the dry weight of sugarcane and eucalyptus plants in all experiments. This hormetic effect was related to low dose effects on CO2 assimilation rate, stomatal conductance and transpiration rate, indicating that low glyphosate doses enhance photosynthesis of plants. © 2017 Society of Chemical Industry.

Identifying Quantitative Trait Loci (QTLs) and Developing Diagnostic Markers Linked to Orange Rust Resistance in Sugarcane (Saccharum spp.)

Sugarcane (Saccharum spp.) is an important economic crop, contributing up to 80% of table sugar used in the world and has become a promising feedstock for biofuel production. Sugarcane production has been threatened by many diseases, and fungicide applications for disease control have been opted out for sustainable agriculture. Orange rust is one of the major diseases impacting sugarcane production worldwide. Identifying quantitative trait loci (QTLs) and developing diagnostic markers are valuable for breeding programs to expedite release of superior sugarcane cultivars for disease control. In this study, an F1 segregating population derived from a cross between two hybrid sugarcane clones, CP95-1039 and CP88-1762, was evaluated for orange rust resistance in replicated trails. Three QTLs controlling orange rust resistance in sugarcane (qORR109, qORR4 and qORR102) were identified for the first time ever, which can explain 58, 12 and 8% of the phenotypic variation, separately. We also characterized 1,574 sugarcane putative resistance (R) genes. These sugarcane putative R genes and simple sequence repeats in the QTL intervals were further used to develop diagnostic markers for marker-assisted selection of orange rust resistance. A PCR-based Resistance gene-derived maker, G1 was developed, which showed significant association with orange rust resistance. The putative QTLs and marker developed in this study can be effectively utilized in sugarcane breeding programs to facilitate the selection process, thus contributing to the sustainable agriculture for orange rust disease control.

Physiological Plasticity Is Important for Maintaining Sugarcane Growth under Water Deficit

The water availability at early phenological stages is critical for crop establishment and sugarcane varieties show differential performance under drought. Herein, we evaluated the relative importance of morphological and physiological plasticity of young sugarcane plants grown under water deficit, testing the hypothesis that high phenotypic plasticity is associated with drought tolerance. IACSP95-5000 is a high yielding genotype and IACSP94-2094 has good performance under water limiting environments. Plants were grown in rhizotrons for 35 days under three water availabilities: high (soil water matric potential [Ψ_m] higher than -20 kPa); intermediate (Ψ_m reached -65 and -90 kPa at the end of experimental period) and low (Ψ_m reached values lower than -150 kPa). Our data revealed that morphological and physiological responses of sugarcane to drought are dependent on genotype and intensity of water deficit. In general, IACSP95-5000 showed higher physiological plasticity given by leaf gas exchange and photochemical traits, whereas IACSP94-2094 showed higher morphological plasticity determined by changes in leaf area (LA) and specific LA. As IACSP94-2094 accumulated less biomass than IACSP95-5000 under varying water availability, it is suggested that high morphological plasticity does not always represent an effective advantage to maintain plant growth under water deficit. In addition, our results revealed that sugarcane varieties face water deficit using distinct strategies based on physiological or morphological changes. When the effectiveness of those changes in maintaining plant growth under low water availability is taken into account, our results indicate that the physiological plasticity is more important than the morphological one in young sugarcane plants.

Physiological Plasticity Is Important for Maintaining Sugarcane Growth under Water Deficit

The water availability at early phenological stages is critical for crop establishment and sugarcane varieties show differential performance under drought. Herein, we evaluated the relative importance of morphological and physiological plasticity of young sugarcane plants grown under water deficit, testing the hypothesis that high phenotypic plasticity is associated with drought tolerance. IACSP95-5000 is a high yielding genotype and IACSP94-2094 has good performance under water limiting environments. Plants were grown in rhizotrons for 35 days under three water availabilities: high (soil water matric potential [Ψ_m] higher than -20 kPa); intermediate (Ψ_m reached -65 and -90 kPa at the end of experimental period) and low (Ψ_m reached values lower than -150 kPa). Our data revealed that morphological and physiological responses of sugarcane to drought are dependent on genotype and intensity of water deficit. In general, IACSP95-5000 showed higher physiological plasticity given by leaf gas exchange and photochemical traits, whereas IACSP94-2094 showed higher morphological plasticity determined by changes in leaf area (LA) and specific LA. As IACSP94-2094 accumulated less biomass than IACSP95-5000 under varying water availability, it is suggested that high morphological plasticity does not always represent an effective advantage to maintain plant growth under water deficit. In addition, our results revealed that sugarcane varieties face water deficit using distinct strategies based on physiological or morphological changes. When the effectiveness of those changes in maintaining plant growth under low water availability is taken into account, our results indicate that the physiological plasticity is more important than the morphological one in young sugarcane plants.

miRNA alteration is an important mechanism in sugarcane response to low-temperature environment

BACKGROUND

Cold is a major abiotic stress limiting the production of tropical and subtropical crops in new production areas. Sugarcane (Saccharum spp.) originates from the tropics but is cultivated primarily in the sub-tropics where it frequently encounters cold stress. Besides regulating plant growth, miRNAs play an important role in environmental adaption.

RESULTS

In this study, a total of 412 sugarcane miRNAs, including 261 known and 151 novel miRNAs, were obtained from 4 small RNA libraries through the Illumina sequencing method. Among them, 62 exhibited significant differential expression under cold stress, with 34 being upregulated and 28 being downregulated. The expression of 13 miRNAs and 12 corresponding targets was validated by RT-qPCR, with the majority being consistent with the sequencing data. GO and KEGG analysis indicated that these miRNAs were involved in stress-related biological pathways. To further investigate the involvement of these miRNAs in tolerance to abiotic stresses, sugarcane miR156 was selected for functional analysis. RT-qPCR revealed that miR156 levels increased in sugarcane during cold, salt and drought stress treatments. Nicotiana benthamiana plants transiently overexpressing miR156 exhibited better growth status, lower ROS levels, higher anthocyanin contents as well as the induction of some cold-responsive genes, suggesting its positive role in the plant cold stress response.

CONCLUSIONS

This study provides a global view of the association of miRNA expression with the sugarcane response to cold stress. The findings have enriched the present miRNA resource and have made an attempt to verify the involvement of miR156 in plant response to cold stress.

Genotype-dependent regulation of drought-responsive genes in tolerant and sensitive sugarcane cultivars

Drought is the most damaging among the major abiotic stresses. Transcriptomic studies allow a global overview of expressed genes, providing the basis for molecular markers development. Here, the HT-SuperSAGE technique allowed the evaluation of four drought-tolerant cultivars and four-sensitive cultivars, after 24h of irrigation suppression. We identified 9831 induced unitags from roots of the tolerant cultivars with different regulations by the -sensitive cultivars after the applied stress. These unitags allowed a proposal of 15 genes, whose expressed profiles were validated by RT-qPCR, evaluating each cultivar independently. These genes covered broad metabolic processes: ethylene stress attenuation (ACCD); root growth (β-EXP8); protein degradation [ubiquitination pathway (E2, 20SPβ4); plant proteases (AP, C13)]; oxidative detoxification (TRX); fatty acid synthesis (ACC); amino acid transport (AAT), and carbohydrate metabolism [glycolysis (PFK, TPI, FBA); TCA cycle (LDP, MDH); pentose phosphate pathway (TKT)]. The expressed profiles showed a genotype-dependent regulation of the target genes. Two drought-tolerant cultivars (SP83-2847; CTC6) presented each one, nine of the induced genes. Among the -sensitive cultivars, CTC13 induced only one, while SP90-1636 induced two genes. These genes should help breeders to identify accessions managing drought stress tolerance responses, showing better ethylene stress attenuation, energy allocation, amino acid transport, and protein homeostasis.

Oviposition Preference and Survival of the Mexican Rice Borer (Lepidoptera: Crambidae) in Bioenergy and Conventional Sugarcane and Sorghum

Oviposition preference and host suitability of the Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), on bioenergy and conventional cultivars of sugarcane, Saccharum spp., and sorghum, Sorghum spp., were examined in a series of greenhouse experiments. Two energycane cultivars, two sugarcane cultivars, two high-biomass sorghum cultivars, and one sweet sorghum cultivar were assessed at two phenological stages (immature and mature). Mature plants possessed greater availability of dry leaf material compared with immature plants, and all E. loftini eggs were observed exclusively on dry leaves. Oviposition did not vary among host combinations (cultivar by phenological stage); however, eggs per plant and eggs per oviposition event were numerically greater on mature plants than immature plants. In a no-choice experiment, survival from egg to adult did not vary among host combinations, with <2.0% of E. loftini larvae surviving to adulthood. Failed establishment by neonates on plants was 13.4- to 53.9-fold greater than successful establishment across all host combinations. Results from this study suggest that plant physical characteristics continue to play an important role in host selection, but further evaluations will be needed to quantify other characteristics which influence host suitability.

Occurrence of a novel mastrevirus in sugarcane germplasm collections in Florida, Guadeloupe and Réunion

BACKGROUND

In Africa and Asia, sugarcane is the host of at least seven different virus species in the genus Mastrevirus of the family Geminiviridae. However, with the exception of Sugarcane white streak virus in Barbados, no other sugarcane-infecting mastrevirus has been reported in the New World. Conservation and exchange of sugarcane germplasm using stalk cuttings facilitates the spread of sugarcane-infecting viruses.

METHODS

A virion-associated nucleic acids (VANA)-based metagenomics approach was used to detect mastrevirus sequences in 717 sugarcane samples from Florida (USA), Guadeloupe (French West Indies), and Réunion (Mascarene Islands). Contig assembly was performed using CAP3 and sequence searches using BLASTn and BLASTx. Mastrevirus full genomes were enriched from total DNA by rolling circle amplification, cloned and sequenced. Nucleotide and amino acid sequence identities were determined using SDT v1.2. Phylogenetic analyses were conducted using MEGA6 and PHYML3.

RESULTS

We identified a new sugarcane-infecting mastrevirus in six plants sampled from germplasm collections in Florida and Guadeloupe. Full genome sequences were determined and analyzed for three virus isolates from Florida, and three from Guadeloupe. These six genomes share >88% genome-wide pairwise identity with one another and between 89 and 97% identity with a recently identified mastrevirus (KR150789) from a sugarcane plant sampled in China. Sequences similar to these were also identified in sugarcane plants in Réunion.

CONCLUSIONS

As these virus isolates share <64% genome-wide identity with all other known mastreviruses, we propose classifying them within a new mastrevirus species named Sugarcane striate virus. This is the first report of sugarcane striate virus (SCStV) in the Western Hemisphere, a virus that most likely originated in Asia. The distribution, vector, and impact of SCStV on sugarcane production remains to be determined.