A large sequenced mutant library - valuable reverse genetic resource that covers 98% of sorghum genes.

Mutant populations are crucial for functional genomics and discovering novel traits for crop breeding. Sorghum, a drought and heat-tolerant C4 species, requires a vast, large-scale, annotated, and sequenced mutant resource to enhance crop improvement through functional genomics research. Here, we report a sorghum large-scale sequenced mutant population with 9.5 million ethyl methane sulfonate (EMS)-induced mutations that covered 98% of sorghum's annotated genes using inbred line BTx623. Remarkably, a total of 610 320 mutations within the promoter and enhancer regions of 18 000 and 11 790 genes, respectively, can be leveraged for novel research of cis-regulatory elements. A comparison of the distribution of mutations in the large-scale mutant library and sorghum association panel (SAP) provides insights into the influence of selection. EMS-induced mutations appeared to be random across different regions of the genome without significant enrichment in different sections of a gene, including the 5' UTR, gene body, and 3'-UTR. In contrast, there were low variation density in the coding and UTR regions in the SAP. Based on the Ka /Ks value, the mutant library (~1) experienced little selection, unlike the SAP (0.40), which has been strongly selected through breeding. All mutation data are publicly searchable through SorbMutDB (https://www.depts.ttu.edu/igcast/sorbmutdb.php) and SorghumBase (https://sorghumbase.org/). This current large-scale sequence-indexed sorghum mutant population is a crucial resource that enriched the sorghum gene pool with novel diversity and a highly valuable tool for the Poaceae family, that will advance plant biology research and crop breeding.

Leaf structural traits mediating pre-existing physical innate resistance to sorghum aphid in sorghum under uninfested conditions.

KEY MESSAGE: We found four indicative traits of innate immunity. Sorghum-resistant varieties had a greater trichome, stomatal and chloroplast density, and smaller mesophyll intercellular width than susceptible varieties. The sorghum aphid (SA), Melanaphis sorghi (Theobald), can severely reduce sorghum yield. The contribution of structural traits to SA resistance has not been extensively studied. Moreover, the current screening method for resistance is inherently subjective for resistance and requires infestation in plants. Quantifying the microanatomical basis of innate SA resistance is crucial for developing reliable screening tools requiring no infestation. The goal of this study was to identify structural traits linked to physical innate SA resistance in sorghum. We conducted controlled environment and field experiments under no SA infestation conditions, with two resistant (R. LBK1 and R. Tx2783) and two susceptible (R. Tx7000 and R. Tx430) varieties. Leaf tissues collected at the fifth leaf stage in the controlled environment experiment were analyzed for the epidermal and mesophyll traits using light and transmission electron microscopy. Leaf tissues collected at physiological maturity in the field experiment were analyzed for surface traits using scanning electron microscopy. Our results showed that stomatal density, trichome density, trichome length, and chloroplast density are key leaf structural traits indicative of physical innate SA resistance. We found that resistant varieties had a greater density of trichomes (39%), stomata (31%), and chloroplast (42%), and smaller mesophyll intercellular width (- 52%) than susceptible varieties. However, the chloroplast, mitochondria, and epidermal cell ultrastructural traits were ineffective indicators of SA resistance. Our findings provide the foundation for developing an objective high-throughput method for SA resistance screening. We suggest a follow-up validation experiment to confirm our outcomes under SA infestation conditions.

Evaluation of A3 cytoplasmic male sterile forage sorghum lines for resistance to sugarcane aphid.

MAIN CONCLUSION: Three known sugarcane aphid-resistant pollinator parents were sterilized in A3 cytoplasmic male sterility and were confirmed in this study to be resistant to sugarcane aphid allowing for the development of sugarcane aphid-resistant forage hybrids. We utilized A3 cytoplasmic male sterility and converted known sugarcane aphid-resistant sorghum TX 2783, and newly released R. LBK1 (Reg. No. GP-865, PI 687244) and R. LBK2 (Reg. No. GP-866, PI 687245) into A3 sterility to determine if the sterile counterparts would also equally express tolerance and or antibiosis to sugarcane aphid. Free-choice flat screen trials and life-table demographic studies were utilized and compared to know susceptible/fertile entries KS 585, and TX 7000, and known resistant/fertile entries TX 2783 and DKS 37-07. The R. LBK1 fertile entry was more tolerant than the known susceptible entries KS 585 and TX 7000, but was not as resistant as the other resistant entries, sustaining a damage rating of 6.0 across two different screen trials. The sterile A3 R. LBK2 showed a greater tolerance and expressed higher levels of antibiosis during aphid reproductive studies when compared to the known resistant and fertile TX 2783. All other fertile (R. LBK2, TX2783) and the A3 male sterile counterparts (A3 R. LBK2, A3 TX2783) were very similar in expression of high levels of tolerance and exhibited statistically similar damage ratings of 3.3-4.3 when exposed to sugarcane aphids. No entry, either fertile or sterile, was as tolerant as DKS 37-07, a known resistant commercial hybrid. Other plant measurements including percent loss in chlorophyll content, difference in plant height, and number of true leaves for sugarcane aphid infested versus non-infested were very consistent and highly correlated with damage ratings. Antibiosis was also exhibited in both fertile and sterile versions of the resistant lines. There was a 2 × reduction in fecundity between the R. LBK1 fertile and its sterile A3 R. LBK1 when compared to the susceptible KS 585 and TX 7000; however, the remaining fertile and sterile entries had 3.8 × to 5.8 × decrease in fecundity when compared to the susceptible KS 585 and TX 7000. Other measurements in life-table statistics such as nymphs produced/female/d, and the intrinsic rates of increased were significantly lower for all fertile and sterile lines, showing that antibiosis significantly affected sugarcane aphid reproduction. In conclusion, the A3 cytoplasmic male sterility shows consistency for maintaining the single dominant trait SCA-resistant trait of TX 2783 for expressing both antibiosis and tolerance, and great utility in the development of sugarcane aphid-resistant forage sorghums.

SorghumBase: a web-based portal for sorghum genetic information and community advancement.

MAIN CONCLUSION: SorghumBase provides a community portal that integrates genetic, genomic, and breeding resources for sorghum germplasm improvement. Public research and development in agriculture rely on proper data and resource sharing within stakeholder communities. For plant breeders, agronomists, molecular biologists, geneticists, and bioinformaticians, centralizing desirable data into a user-friendly hub for crop systems is essential for successful collaborations and breakthroughs in germplasm development. Here, we present the SorghumBase web portal ( https://www.sorghumbase.org ), a resource for the sorghum research community. SorghumBase hosts a wide range of sorghum genomic information in a modular framework, built with open-source software, to provide a sustainable platform. This initial release of SorghumBase includes: (1) five sorghum reference genome assemblies in a pan-genome browser; (2) genetic variant information for natural diversity panels and ethyl methanesulfonate (EMS)-induced mutant populations; (3) search interface and integrated views of various data types; (4) links supporting interconnectivity with other repositories including genebank, QTL, and gene expression databases; and (5) a content management system to support access to community news and training materials. SorghumBase offers sorghum investigators improved data collation and access that will facilitate the growth of a robust research community to support genomics-assisted breeding.

A Sorghum Mutant Resource as an Efficient Platform for Gene Discovery in Grasses.

Sorghum (Sorghum bicolor) is a versatile C4 crop and a model for research in family Poaceae. High-quality genome sequence is available for the elite inbred line BTx623, but functional validation of genes remains challenging due to the limited genomic and germplasm resources available for comprehensive analysis of induced mutations. In this study, we generated 6400 pedigreed M4 mutant pools from EMS-mutagenized BTx623 seeds through single-seed descent. Whole-genome sequencing of 256 phenotyped mutant lines revealed >1.8 million canonical EMS-induced mutations, affecting >95% of genes in the sorghum genome. The vast majority (97.5%) of the induced mutations were distinct from natural variations. To demonstrate the utility of the sequenced sorghum mutant resource, we performed reverse genetics to identify eight genes potentially affecting drought tolerance, three of which had allelic mutations and two of which exhibited exact cosegregation with the phenotype of interest. Our results establish that a large-scale resource of sequenced pedigreed mutants provides an efficient platform for functional validation of genes in sorghum, thereby accelerating sorghum breeding. Moreover, findings made in sorghum could be readily translated to other members of the Poaceae via integrated genomics approaches.

Sorghum MSD3 Encodes an ω-3 Fatty Acid Desaturase that Increases Grain Number by Reducing Jasmonic Acid Levels.

Grain number per panicle is an important component of grain yield in sorghum (Sorghum bicolor (L.)) and other cereal crops. Previously, we reported that mutations in multi-seeded 1 (MSD1) and MSD2 genes result in a two-fold increase in grain number per panicle due to the restoration of the fertility of the pedicellate spikelets, which invariably abort in natural sorghum accessions. Here, we report the identification of another gene, MSD3, which is also involved in the regulation of grain numbers in sorghum. Four bulked F2 populations from crosses between BTx623 and each of the independent msd mutants p6, p14, p21, and p24 were sequenced to 20× coverage of the whole genome on a HiSeq 2000 system. Bioinformatic analyses of the sequence data showed that one gene, Sorbi_3001G407600, harbored homozygous mutations in all four populations. This gene encodes a plastidial ω-3 fatty acid desaturase that catalyzes the conversion of linoleic acid (18:2) to linolenic acid (18:3), a substrate for jasmonic acid (JA) biosynthesis. The msd3 mutants had reduced levels of linolenic acid in both leaves and developing panicles that in turn decreased the levels of JA. Furthermore, the msd3 panicle phenotype was reversed by treatment with methyl-JA (MeJA). Our characterization of MSD1, MSD2, and now MSD3 demonstrates that JA-regulated processes are critical to the msd phenotype. The identification of the MSD3 gene reveals a new target that could be manipulated to increase grain number per panicle in sorghum, and potentially other cereal crops, through the genomic editing of MSD3 functional orthologs.

Fertility of Pedicellate Spikelets in Sorghum Is Controlled by a Jasmonic Acid Regulatory Module.

As in other cereal crops, the panicles of sorghum (Sorghum bicolor (L.) Moench) comprise two types of floral spikelets (grass flowers). Only sessile spikelets (SSs) are capable of producing viable grains, whereas pedicellate spikelets (PSs) cease development after initiation and eventually abort. Consequently, grain number per panicle (GNP) is lower than the total number of flowers produced per panicle. The mechanism underlying this differential fertility is not well understood. To investigate this issue, we isolated a series of ethyl methane sulfonate (EMS)-induced multiseeded (msd) mutants that result in full spikelet fertility, effectively doubling GNP. Previously, we showed that MSD1 is a TCP (Teosinte branched/Cycloidea/PCF) transcription factor that regulates jasmonic acid (JA) biosynthesis, and ultimately floral sex organ development. Here, we show that MSD2 encodes a lipoxygenase (LOX) that catalyzes the first committed step of JA biosynthesis. Further, we demonstrate that MSD1 binds to the promoters of MSD2 and other JA pathway genes. Together, these results show that a JA-induced module regulates sorghum panicle development and spikelet fertility. The findings advance our understanding of inflorescence development and could lead to new strategies for increasing GNP and grain yield in sorghum and other cereal crops.

Genomic characterization of a core set of the USDA-NPGS Ethiopian sorghum germplasm collection: implications for germplasm conservation, evaluation, and utilization in crop improvement.

BACKGROUND: The USDA Agriculture Research Service National Plant Germplasm System (NPGS) preserves the largest sorghum germplasm collection in the world, which includes 7,217 accessions from the center of diversity in Ethiopia. The characterization of this exotic germplasm at a genome-wide scale will improve conservation efforts and its utilization in research and breeding programs. Therefore, we phenotyped a representative core set of 374 Ethiopian accessions at two locations for agronomic traits and characterized the genomes. RESULTS: Using genotyping-by-sequencing, we identified 148,476 single-nucleotide polymorphism (SNP) markers distributed across the entire genome. Over half of the alleles were rare (frequency < 0.05). The genetic profile of each accession was unique (i.e., no duplicates), and the average genetic distance among accessions was 0.70. Based on population structure and cluster analyses, we separated the collection into 11 populations with pairwise F ST values ranging from 0.11 to 0.47. In total, 198 accessions (53%) were assigned to one of these populations with an ancestry membership coefficient of larger than 0.60; these covered 90% of the total genomic variation. We characterized these populations based on agronomic and seed compositional traits. We performed a cluster analysis with the sorghum association panel based on 26,026 SNPs and determined that nine of the Ethiopian populations expanded the genetic diversity in the panel. Genome-wide association analysis demonstrated that these low-coverage data and the observed population structure could be employed for the genomic dissection of important phenotypes in this core set of Ethiopian sorghum germplasm. CONCLUSIONS: The NPGS Ethiopian sorghum germplasm is a genetically and phenotypically diverse collection comprising 11 populations with high levels of admixture. Genetic associations with agronomic traits can be used to improve the screening of exotic germplasm for selection of specific populations. We detected many rare alleles, suggesting that this germplasm contains potentially useful undiscovered alleles, but their discovery and characterization will require extensive effort. The genotypic data available for these accessions provide a valuable resource for sorghum breeders and geneticists to effectively improve crops.

Transcriptome profiling and validation of gene based single nucleotide polymorphisms (SNPs) in sorghum genotypes with contrasting responses to cold stress.

BACKGROUND: Sorghum is a versatile cereal crop, with excellent heat and drought tolerance. However, it is susceptible to early-season cold stress (12-15 °C) which limits stand-establishment and seedling growth. To gain further insights on the molecular mechanism of cold tolerance in sorghum we performed transcriptome profiling between known cold sensitive and tolerant sorghum lines using RNA sequencing technology under control and cold stress treatments. RESULTS: Here we report on the identification of differentially expressed genes (DEGs) between contrasting sorghum genotypes, HongkeZi (cold tolerant) and BTx623 (cold sensitive) under cool and control temperatures using RNAseq approach to elucidate the molecular basis of sorghum response to cold stress. Furthermore, we validated bi-allelic variants in the form of single nucleotide polymorphism (SNPs) between the cold susceptible and tolerant lines of sorghum. An analysis of transcriptome profile showed that in response to cold, a total of 1910 DEGs were detected under cold and control temperatures in both genotypes. We identified a subset of genes under cold stress for downstream analysis, including transcription factors that exhibit differential abundance between the sensitive and tolerant genotypes. We identified transcription factors including Dehydration-responsive element-binding factors, C-repeat binding factors, and Ethylene responsive transcription factors as significantly upregulated during cold stress in cold tolerant HKZ. Additionally, specific genes such as plant cytochromes, glutathione s-transferases, and heat shock proteins were found differentially regulated under cold stress between cold tolerant and susceptible genotype of sorghum. A total of 41,603 SNP were identified between the cold sensitive and tolerant genotypes with minimum read of four. Approximately 89 % of the 114 SNP sites selected for evaluation were validated using endpoint genotyping technology. CONCLUSION: A new strategy which involved an integrated analysis of differential gene expression and identification of bi-allelic single nucleotide polymorphism (SNP) was conducted to determine and analyze differentially expressed genes and variation involved in cold stress response of sorghum. The results gathered provide an insight into the complex mechanisms associated with cold response in sorghum, which involve an array of transcription factors and genes which were previously related to abiotic stress response. This study also offers resource for gene based SNP that can be applied towards targeted genomic studies of cold tolerance in sorghum and other cereal crops.

Genomics and phenomics enabled prebreeding improved early-season chilling tolerance in Sorghum.

In temperate climates, earlier planting of tropical-origin crops can provide longer growing seasons, reduce water loss, suppress weeds, and escape post-flowering drought stress. However, chilling sensitivity of sorghum, a tropical-origin cereal crop, limits early planting, and over 50 years of conventional breeding has been stymied by coinheritance of chilling tolerance (CT) loci with undesirable tannin and dwarfing alleles. In this study, phenomics and genomics-enabled approaches were used for prebreeding of sorghum early-season CT. Uncrewed aircraft systems (UAS) high-throughput phenotyping platform tested for improving scalability showed moderate correlation between manual and UAS phenotyping. UAS normalized difference vegetation index values from the chilling nested association mapping population detected CT quantitative trait locus (QTL) that colocalized with manual phenotyping CT QTL. Two of the 4 first-generation Kompetitive Allele Specific PCR (KASP) molecular markers, generated using the peak QTL single nucleotide polymorphisms (SNPs), failed to function in an independent breeding program as the CT allele was common in diverse breeding lines. Population genomic fixation index analysis identified SNP CT alleles that were globally rare but common to the CT donors. Second-generation markers, generated using population genomics, were successful in tracking the donor CT allele in diverse breeding lines from 2 independent sorghum breeding programs. Marker-assisted breeding, effective in introgressing CT allele from Chinese sorghums into chilling-sensitive US elite sorghums, improved early-planted seedling performance ratings in lines with CT alleles by up to 13-24% compared to the negative control under natural chilling stress. These findings directly demonstrate the effectiveness of high-throughput phenotyping and population genomics in molecular breeding of complex adaptive traits.

Use of genomic prediction to screen sorghum B-lines in hybrid testcrosses.

Use of trifluoromethanesulfonamide (TFMSA), a male gametocide, increases the opportunities to identify promising B-lines because large quantities of F1 seed can be generated prior to the laborious task of B-line sterilization. Combining TFMSA technology with genomic selection could efficiently evaluate sorghum B-lines in hybrid combination to maximize the rates of genetic gain of the crop. This study used two recombinant inbred B-line populations, consisting of 217 lines, which were testcrossed to two R-lines to produce 434 hybrids. Each population of testcross hybrids were evaluated across five environments. Population-based genomic prediction models were assessed across environments using three different cross-validation (CV) schemes, each with 70% training and 30% validation sets. The validation schemes were as follows: CV1-hybrids chosen randomly for validation; CV2-B-lines were randomly chosen, and each chosen B-line had one of the two corresponding testcross hybrids randomly chosen for the validation; and CV3-B-lines were randomly chosen, and each chosen B-line had both corresponding testcross hybrids chosen for the validation. CV1 and CV2 presented the highest prediction accuracies; nonetheless, the prediction accuracies of the CV schemes were not statistically different in many environments. We determined that combining the B-line populations could improve prediction accuracies, and the genomic prediction models were able to effectively rank the poorest 70% of hybrids even when genomic prediction accuracies themselves were low. Results indicate that combining genomic prediction models and TFMSA technology can effectively aid breeders in predicting B-line hybrid performance in early generations prior to the laborious task of generating A/B-line pairs.

Genomic prediction can be used to screen sorghum B-lines for hybrid grain yield and days to mid-anthesis. Using genomic prediction and the chemical gametocide TFMSA can increase the rate of genetic gain in sorghum B-lines. Using testers to screen sorghum B-line populations is an effective method for screening with genomic prediction. Genomic prediction can effectively predict hybrid performance within and across populations of sorghum B-lines. The ability to accurately rank hybrid performance remained relatively consistent regardless of prediction accuracy.

Seed-to-seed early-season cold resiliency in sorghum.

Early planted sorghum usually experiences cooler day/night temperatures, which may result in delayed growth, floral initiation, and infertile pollen, limiting productivity in high altitudes and temperate regions. Genetic variability for cold tolerance in sorghum has been evaluated by characterizing germination, emergence, vigor, and seedling growth under sub-optimal temperatures. However, the compounded effect of early season cold on plant growth and development and subsequent variability in potential grain yield losses has not been evaluated. Agro-morphological and physiological responses of sorghum grown under early-, mid-, and standard planting dates in West Texas were characterized from seed-to-seed. A set of diverse lines and hybrids with two major sources of tolerance, and previously selected for seedling cold tolerance were used. These were evaluated with a standard commercial hybrid known for its seedling cold tolerance and some cold susceptible breeding lines as checks. Variabilities in assessed parameters at seedling, early vegetative, and maturity stages were observed across planting dates for genotypes and sources of cold tolerance. Panicle initiation was delayed, and panicle size reduced, resulting in decreased grain yields under early and mid-planting dates. Coupled with final germination percent, panicle width and area were significant unique predictors of yield under early and mid-planting dates. Significant variability in performance was observed not only between cold tolerant and susceptible checks, but noticeably between sources of cold tolerance, with the Ethiopian highland sources having lesser yield penalties than their Chinese counterparts. Thus, screening for cold tolerance should not be limited to early seedling characterization but should also consider agronomic traits that may affect yield penalties depending on the sources of tolerance.

Mapping QTLs and Identification of Genes Associated with Drought Resistance in Sorghum.

Water limits global agricultural production. Increases in global aridity, a growing human population, and the depletion of aquifers will only increase the scarcity of water for agriculture. Water is essential for plant growth and in areas that are prone to drought, the use of drought-resistant crops is a long-term solution for growing more food for more people with less water. Sorghum is well adapted to hot and dry environments and has been used as a dietary staple for millions of people. Increasing the drought resistance in sorghum hybrids with no impact on yield is a continual objective for sorghum breeders. In this review, we describe the loci, quantitative trait loci (QTLs), or genes that have been identified for traits involved in drought avoidance (water-use efficiency, cuticular wax synthesis, trichome development and morphology, root system architecture) and drought tolerance (compatible solutes, pre- and post-flowering drought tolerance). Many of these identified genes and QTL regions have not been tested in hybrids and the effect of these genes, or their interactions, on yield must be understood in normal and drought-stressed conditions to understand the strength and weaknesses of their utility.

Genetic Architecture of Chilling Tolerance in Sorghum Dissected with a Nested Association Mapping Population.

Dissecting the genetic architecture of stress tolerance in crops is critical to understand and improve adaptation. In temperate climates, early planting of chilling-tolerant varieties could provide longer growing seasons and drought escape, but chilling tolerance (<15°) is generally lacking in tropical-origin crops. Here we developed a nested association mapping (NAM) population to dissect the genetic architecture of early-season chilling tolerance in the tropical-origin cereal sorghum (Sorghum bicolor [L.] Moench). The NAM resource, developed from reference line BTx623 and three chilling-tolerant Chinese lines, is comprised of 771 recombinant inbred lines genotyped by sequencing at 43,320 single nucleotide polymorphisms. We phenotyped the NAM population for emergence, seedling vigor, and agronomic traits (>75,000 data points from ∼16,000 plots) in multi-environment field trials in Kansas under natural chilling stress (sown 30-45 days early) and normal growing conditions. Joint linkage mapping with early-planted field phenotypes revealed an oligogenic architecture, with 5-10 chilling tolerance loci explaining 20-41% of variation. Surprisingly, several of the major chilling tolerance loci co-localize precisely with the classical grain tannin (Tan1 and Tan2) and dwarfing genes (Dw1 and Dw3) that were under strong directional selection in the US during the 20th century. These findings suggest that chilling sensitivity was inadvertently selected due to coinheritance with desired nontannin and dwarfing alleles. The characterization of genetic architecture with NAM reveals why past chilling tolerance breeding was stymied and provides a path for genomics-enabled breeding of chilling tolerance.

MSD1 regulates pedicellate spikelet fertility in sorghum through the jasmonic acid pathway.

Grain number per panicle (GNP) is a major determinant of grain yield in cereals. However, the mechanisms that regulate GNP remain unclear. To address this issue, we isolate a series of sorghum [Sorghum bicolor (L.) Moench] multiseeded (msd) mutants that can double GNP by increasing panicle size and altering floral development so that all spikelets are fertile and set grain. Through bulk segregant analysis by next-generation sequencing, we identify MSD1 as a TCP (Teosinte branched/Cycloidea/PCF) transcription factor. Whole-genome expression profiling reveals that jasmonic acid (JA) biosynthetic enzymes are transiently activated in pedicellate spikelets. Young msd1 panicles have 50% less JA than wild-type (WT) panicles, and application of exogenous JA can rescue the msd1 phenotype. Our results reveal a new mechanism for increasing GNP, with the potential to boost grain yield, and provide insight into the regulation of plant inflorescence architecture and development.

Genome-Wide Association Study of Developing Leaves' Heat Tolerance during Vegetative Growth Stages in a Sorghum Association Panel.

Heat stress reduces grain yield and quality worldwide. Enhancing heat tolerance of crops at all developmental stages is one of the essential strategies required for sustaining agricultural production especially as frequency of temperature extremes escalates in response to climate change. Although heat tolerance mechanisms have been studied extensively in model plant species, little is known about the genetic control underlying heat stress responses of crop plants at the vegetative stage under field conditions. To dissect the genetic basis of heat tolerance in sorghum [ (L.) Moench], we performed a genome-wide association study (GWAS) for traits responsive to heat stress at the vegetative stage in an association panel. Natural variation in leaf firing (LF) and leaf blotching (LB) were evaluated separately for 3 yr in experimental fields at three locations where sporadic heat waves occurred throughout the sorghum growing season. We identified nine single-nucleotide polymorphisms (SNPs) that were significantly associated with LF and five SNPs that were associated with LB. Candidate genes near the SNPs were investigated and 14 were directly linked to biological pathways involved in plant stress responses including heat stress response. The findings of this study provide new knowledge on the genetic control of leaf traits responsive to heat stress in sorghum, which could aid in elucidating the genetic and molecular mechanisms of vegetative stage heat tolerance in crops. The results also provide candidate markers for molecular breeding of enhanced heat tolerance in cereal and bioenergy crops.

Natural Variation in Synthesis and Catabolism Genes Influences Dhurrin Content in Sorghum.

Cyanogenic glucosides are natural compounds found in more than 1000 species of angiosperms that produce HCN and are deemed undesirable for agricultural use. However, these compounds are important components of the primary defensive mechanisms of many plant species. One of the best-studied cyanogenic glucosides is dhurrin [(S)-p-hydroxymandelonitrile-ß-D-glucopyranoside], which is produced primarily in sorghum [Sorghum bicolor (L.) Moench]. The biochemical basis for dhurrin metabolism is well established; however, little information is available on its genetic control. Here, we dissect the genetic control of leaf dhurrin content through a genome-wide association study (GWAS) using a panel of 700 diverse converted sorghum lines (conversion panel) previously subjected to pre-breeding and selected for short stature (∼1 m in height) and photoperiod insensitivity. The conversion panel was grown for 2 yr in three environments. Wide variation for leaf dhurrin content was found in the sorghum conversion panel, with the Caudatum group exhibiting the highest dhurrin content and the Guinea group showing the lowest dhurrin content. A GWAS using a mixed linear model revealed significant associations (a false discovery rate [FDR] < 0.05) close to both UGT 185B1 in the canonical biosynthetic gene cluster on chromosome 1 and close to the catabolic dhurrinase loci on chromosome 8. Dhurrin content was associated consistently with biosynthetic genes in the two N-fertilized environments, while dhurrin content was associated with catabolic loci in the environment without supplemental N. These results suggest that genes for both biosynthesis and catabolism are important in determining natural variation for leaf dhurrin in sorghum in different environments.