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Changes in plant architecture, specifically conversion to compact canopy for cereal crops, have resulted in significant increases in grain yield for wheat (Triticum aestivum) and rice (Oryza sativa). For sorghum (Sorghum bicolor L. Moench.) a versatile crop with an open canopy, plant architecture is an important feature that merits strong consideration for modification. Here, we report the genetic, developmental, and physiological characterization of a sorghum genetic stock, KFS2061, a stable mutant (in the Western Black Hull Kafir background) which exhibit short and erect leaves resulting in compact plant architecture. Genetic study of an F2 population derived from the cross of KFS2061 to BTx623 showed that the short leaf is recessive and appeared to be controlled by a single gene. The expression of the short leaf trait commenced with the 3rd leaf and is propagated through the entire leaf hierarchy of the canopy. The short leaf mutant exhibited consistent steep leaf angle, 43° (with the main culm as reference), and greener leaves than wild type. Biochemical analyses indicated significantly higher chlorophyll and cellulose content per leaf area in the mutant than wild type. Histological studies revealed reduction in cell length along the longitudinal axis and enlargement of bulliform cells in the adaxial surface of the mutant leaf. Further evaluation of agronomic traits indicated that this mutation could increase harvest index. This study provides information on a short leaf genetic stock that could serve as a vital resource in understanding how to manipulate plant canopy architecture of sorghum.

Genetic diversity was characterized in a collection of 159 sorghum (Sorghum bicolor L. Moench) landraces gathered from the colder region (primarily the northeastern region) of China. A set of 41 microsatellites or simple sequence repeat (SSR) markers distributed throughout the 10 chromosomes of sorghum was utilized in the analysis. Majority (40 out of 41 SSRs, 98%) of the SSR markers were polymorphic and highly informative with polymorphism information content (PIC) value ranging from 0.05 to 0.92. An estimation of genetic similarity coefficients (GSC, with values ranging from 0.42 to 0.96) revealed a range of variability in this collection of sorghum landraces. Eight clusters of accessions were identified at a cut off at 0.50 GSC and these groupings were supported by the analysis of subpopulations using Structure. This study provides evidence that Chinese landraces of sorghum exhibit moderate genetic diversity and results from cluster analysis may well aid in identification of diverse accessions that can serve as parental lines for efficient utilization and application of these germplasm into sorghum breeding programs.

The BTx623/IS3620C sorghum [Sorghum bicolor (L.) Moench.] mapping population (Reg. No. MP-1, NSL 469802 MAP) is a set of 430 F 7 and F 9 recombinant inbred lines (RILs) (F 7 lines: PI 658758 through PI 659060; F 9 lines: PI 659144 through PI 659271) from a cross between the inbred lines BTx623 (PI 659985 MAP) and IS3620C (PI 659986 MAP). This sorghum mapping population was released jointly by the Plant Stress and the Germplasm Development Unit, Cropping Systems Research Laboratory, USDA-ARS, Lubbock, TX and the Crop Germplasm Research Unit, USDA-ARS, College Station, TX in 2009. The original 137 RILs of this mapping population served as the basis for the construction of landmark high-density genetic and physical maps, with the integrated genome map subsequently used to aid in assembly of the sorghum genome sequence. This population has been genotyped by a number of investigators with a total of 3348 markers—including amplifi ed fragment length polymorphisms, DArT (Diversity Arrays Technology, Yarralumla, Australia), restriction fragment length polymorphisms, simple sequence repeats, and insertion-deletions—mapped to ten linkage groups corresponding to the ten sorghum chromosomes. The population has been utilized to map quantitative trait loci that underlie various agronomic traits, including tillering, fl oral architecture, plant height, fl owering date, grain size, and abiotic stress tolerance. The registration of this recombinant inbred mapping population will allow public access to the genomic and germplasm resources that can facilitate mapping and gene discovery for complex traits in sorghum and related cereal species.

Soil temperatures at 15°C or below limit germination and seedling establishment for warm season cereal crops such as sorghum (Sorghum bicolor (L.) Moench) during early-season planting. To better understand the genetics of early-season cold tolerance in sorghum, mapping of quantitative trait loci (QTL) associated with germination, emergence and vigor using a recombinant inbred mapping population was carried out. A mapping population consisting of 171 F7–F8 recombinant inbred lines (RILs) derived from the cross between RTx430 (cold-sensitive) and PI610727 (cold-tolerant) was developed and a genetic map was constructed using 141 microsatellites or simple sequence repeat (SSR) markers. The RILs were evaluated for cold and optimal temperature germinability in the laboratory, field emergence, and seedling vigor in two locations during early-season planting. Two or more QTL were detected for all traits, except for seedling vigor, with only one QTL was detected in the population. A QTL for cold germinability (Germ 12-2.1) showed the highest LOD value and was also associated with optimal germinability. One of the QTL for field emergence, Fearlygerm-9.3, a contribution from PI610727, was found significant in both locations used for the study. This study showed alignment of QTL in SBi1 (Fearlygerm-1.2 and FGerm30-1.2) with previously reported QTL associated with late field emergence identified from a different mapping population. This indicates that PI617027 shares some common loci with other known early-season cold-tolerant sorghum germplasm but also harbors novel QTL that could be useful in introgression of enhanced laboratory germination and early-season field emergence.

Post-fl owering drought tolerance is an essential trait for increasing the production of sorghum [Sorghum bicolor (L.) Moench] and other cereals in Mediterranean and semiarid tropical climates. Current methodologies for identifying the nonsenescent (stay-green) trait require the right intensity of drought stress at the right developmental stage to visually evaluate lines in the fi eld. Field-based evaluations of drought tolerance are notoriously diffi cult to manage, and oft en require growing lines in multiple locations across several years to acquire a meaningful assessment of the stay-green trait. By means of a 30-min high-temperature challenge to leaf tissue during fl owering of well-watered sorghum and a 30-min room temperature recovery, we show that stay-green lines can be readily identifi ed. Using chlorophyll fl uorescence to monitor tissue injury, we found that tissue with higher intercellular sucrose concentrations exhibited higher chlorophyll fl uorescence yield following the temperature challenge. Stay-green lines evaluated in this study maintained higher dawn leaf sucrose levels than the senescent lines among the fi ve youngest leaf positions. Evaluation of 10 known stay-green and senescent sorghum lines, previously reported in the literature, with this bioassay allowed us to separate the two classes of sorghum from well-watered fl owering plants. Th e stay-green lines can also be separated from senescent lines under well-watered greenhouse conditions from the boot stage onward. Th is technology will greatly reduce the selection time needed to identify drought tolerant sorghum.

Sorghum is distinct from other cereal crops due to its ability to produce profuse amount of epicuticular wax (EW or bloom) on its culm and leaves along with less permeable cuticle which are considered to be important traits contributing to abiotic stress tolerance. Here, we report the molecular mapping and characterization of BL OO M-C UTICLE (BLMC), a locus associated with production of profuse wax, using a mutant mapping population developed from a cross between BTx623 (wild type with profuse wax) and KFS2021 (a mutant with greatly reduced wax). The F2 progenies were genotyped using known and newly developed microsatellite markers to establish a molecular map of BLMC. The locus mapped to a 3.6-centimorgans (cM) interval in the terminal end of sorghum chromosome 10 with flanking markers Xsbarslbk10.47 and Xcup42. Targeted mapping delimited BLMC to as small as 0.7 cM region and facilitated identification of three cosegregating markers with the trait. The BLMC region corresponds to approximately 153,000 bp and candidate genes identified include among others an acyl CoA oxidase (a gene involved in lipid and wax biosynthesis) and seven other putative transcripts. Phenotypic characterization showed that in addition to disrupting the EW production, BLMC mutation reduced culm and leaf cuticle, increased plant death rating in the field at anthesis and significantly reduced the C:28 to C:30 free fatty acid fractions of culm and leaf EW. These results clearly support the important role of BLMC in the expression of profuse wax and enhanced cuticular features of sorghum. Genetic mapping of BLMC opened avenues for identification of genes involved in the cuticle/wax pathway of sorghum and their application for improvement of abiotic stress tolerance.

Of the four major dwarfing genes described in sorghum, only Dw3 has been cloned. We used association mapping to characterize the phenotypic effects of the dw3 mutation and to fine map a second, epistatic dwarfing QTL on sorghum chromosome 9 (Sb-HT9.1). Our panel of 378 sorghum inbreds includes 230 sorghum conversion (SC) lines, which are exotic lines that have been introgressed with dwarfing quantitative trait loci (QTL) from a common parent. The causal mutation in dw3 associates with reduced lower internode length and an elongation of the apex, consistent with its role as an auxin efflux carrier. Lines carrying the dw3 mutation display high haplotype homozygosity over several megabases in the Dw3 region, but most markers linked to Dw3 do not associate significantly with plant height due to allele sharing between Dw3 and dw3 individuals. Using markers with a high mutation rate and the dw3 mutation as an interaction term, significant trait associations were detected across a 7-Mb region around Sb-HT9.1, largely due to higher detection power in the SC lines. Conversely, the likely QTL interval for Sb-HT9.1 was reduced to ∼100 kb, demonstrating that the unique structure of this association panel provides both power and resolution for a genomewide scan.

An irradiation-induced bloomless mutant of sorghum [Sorghum bicolor (L.) Moench.], KFS2021, which visually exhibits an absence of white fl uffy epicuticular wax in leaf and sheath, was characterized using a combination of genetic and physiological approaches. Study of the phenotypic segregation for the bloomless trait in F 2 and F 2:3 populations from a cross between KFS2021 and BTx623 (a cultivar with bloom showing profuse deposition of white epicuticular wax) suggests that bloomless is controlled by a single nuclear recessive gene. The bloomless parent (KFS2021) and F 2 individuals had lower frequency of guttation, leakier epidermal layer (based on percentage of chlorophyll leaching), and higher rate of seedling water loss than the BTx623 and F 2 bloom individuals. Bloomless F 2 individuals showed 3- to 6-fold higher nighttime transpiration rates relative to F 2 bloom individuals based on nighttime conductance. Correlation analysis showed signifi cant negative associations between leaf epicuticular wax load with epidermal permeability and nighttime conductance, which indicate the important role of epicuticular wax in these traits. These results suggest that epicuticular wax may enhance water use effi ciency of sorghum by regulating nighttime water loss.

Early season cold tolerance in grain sorghum [Sorghum bicolor (L.) Moench] is a desirable trait for extending its production range and minimizing risks associated with early spring plantings. Ten Chinese Kaoliang accessions were compared with 10 U.S. inbred parental lines and 10 U.S. commercial hybrids for a range of cold tolerance traits under laboratory, growth chamber, and field settings. Chinese lines were superior to both the inbred and hybrid classes in laboratory germination rates and field-based rates of emergence. In the growth chamber assays, Kaoliangs were not significantly different than U.S. hybrids for most traits measured at either of the two temperature treatments (12 and 24C), with the exception of shoot length, for which the Chinese germplasm was higher. At the cooler temperature, Kaoliangs were significantly greater than U.S. inbreds for only fresh shoot weight; when tested at the warmer temperature, Kaoliangs had higher dry root weight, fresh and dry shoot weights, and fresh and dry whole plant weights, relative to the U.S. inbred class. The U.S. hybrids had greater total plot weight and final stand counts in the field than Kaoliangs, which were likewise higher than U.S. inbreds for both of these traits. Chinese accessions from this working group would serve as a source of favorable genes primarily for tolerance to low temperatures during the germination and emergence phase of growth in the breeding of cold tolerance sorghum lines. I N RECENT YEARS, there has been increased interest in the development of grain sorghum germplasm adapted to U.S. production systems with heightened levels of tolerance to early season cold temperatures. This germplasm could serve both to expand the geographical range of grain sorghum cultivation and minimize the inherent risks involved in early season planting of grain sorghum within sorghum production regions. Additionally, an earlier sowing date could offer growers the option of capitalizing on higher levels of available soil moisture in the early spring and lower evapotranspirative demands, and thus, could potentially serve as a drought avoidance strategy. The initial phase of any plant improvement program is the identification of superior germplasm for the trait of interest (Stoskopf, 1993). With respect to early season cold tolerance in sorghum, Chinese sorghum germplasm of the working group Nervosum-Kaoliang has historically been reported as being superior in this regard. There is ample anecdotal evidence of this class of germplasm possessing heightened levels of early season cold tolerance, but specific measures of the germination and seedling vigor qualities of this germplasm as a class are somewhat limited. Stickler et al. (1962), in comparing a small set of Chinese and U.S. germplasm, found that the Kaoliangs were typically superior in terms of germination and emergence under suboptimal temperatures and indicated that these lines might also possess some advantage in terms of their seedling growth rate across all temperatures tested. Qingshan and Dahlberg (2001) examined a large number of Chinese accessions and noted that several accessions within the Chinese germplasm collection showed high levels of both germination under cold temperatures and seedling phase cold tolerance. It has been postulated, in both sorghum and maize (Zea mays L.), that the germination–emergence and early growth stage phases of seedling development are each under the control of entirely different suites of genes (Cisse and Ejeta, 2003; Hodges et al., 1997; Nordquist, 1971). In breeding for overall early season cold tolerance, therefore, it may be necessary to evaluate germplasm sources for tolerance in each phase. This study was undertaken to examine a set of Chinese sorghum lines in laboratory, growth chamber, and field settings for both cold temperature germination and seedling vigor traits and to compare the response of these lines with a representative set of U.S. parental lines and commercial hybrids.

We describe a recombinant inbred line (RIL) population of 161 F5 genotypes for the widest euploid cross that can be made to cultivated sorghum (Sorghum bicolor) using conventional techniques, S. bicolor × Sorghum propinquum, that segregates for many traits related to plant architecture, growth and development, reproduction, and life history. The genetic map of the S. bicolor × S. propinquum RILs contains 141 loci on 10 linkage groups collectively spanning 773.1 cM. Although the genetic map has DNA marker density well-suited to quantitative trait loci mapping and samples most of the genome, our previous observations that sorghum pericentromeric heterochromatin is recalcitrant to recombination is highlighted by the finding that the vast majority of recombination in sorghum is concentrated in small regions of euchromatin that are distal to most chromosomes. The advancement of the RIL population in an environment to which the S. bicolor parent was well adapted (indeed bred for) but the S. propinquum parent was not largely eliminated an allele for short-day flowering that confounded many other traits, for example, permitting us to map new quantitative trait loci for flowering that previously eluded detection. Additional recombination that has accrued in the development of this RIL population also may have improved resolution of apices of heterozygote excess, accounting for their greater abundance in the F5 than the F2 generation. The S. bicolor × S. propinquum RIL population offers advantages over early-generation populations that will shed new light on genetic, environmental, and physiological/biochemical factors that regulate plant growth and development.

Graphical genotypes have been generated for a set of sorghum [Sorghum bicolor (L.) Moench] germplasm, which includes selected public inbreds, germplasm from the world collection, and ancestral lines central to the early breeding efforts of sorghum. We have focused our present examination on sorghum chromosome SBI-06, which encodes mal and dw(2), two genes critical to sorghum improvement dating to the original introduction of tropical sorghums into the United States. Utilizing the pedigree relationship between sorghum cultivars, the patterns of genetic variation were detailed within segmental chromosomal blocks of SBI-06. Segmental genomic blocks were traced back through multiple generations of a pedigree, often back to founder tropical accessions. The graphical genotypes reveal genomic signatures of historical breeding decisions, especially evidence of directional selection during the conversion of tropical accessions to temperate adaptation. This information is central to our efforts to understand those crop improvement processes that have shaped the genomic diversity of elite sorghum cultivars.

Background Sorghum [Sorghum bicolor (L.) Moench] is ranked as the fifth most important grain crop and serves as a major food staple and fodder resource for much of the world, especially in arid and semi-arid regions. The recent surge in sorghum research is driven by its tolerance to drought/heat stresses and its strong potential as a bioenergy feedstock. Completion of the sorghum genome sequence has opened new avenues for sorghum functional genomics. However, the availability of genetic resources, specifically mutant lines, is limited. Chemical mutagenesis of sorghum germplasm, followed by screening for mutants altered in important agronomic traits, represents a rapid and effective means of addressing this limitation. Induced mutations in novel genes of interest can be efficiently assessed using the technique known as Targeting Induced Local Lesion IN Genomes (TILLING). Results A sorghum mutant population consisting of 1,600 lines was generated from the inbred line BTx623 by treatment with the chemical agent ethyl methanesulfonate (EMS). Numerous phenotypes with altered morphological and agronomic traits were observed from M2 and M3 lines in the field. A subset of 768 mutant lines was analyzed by TILLING using four target genes. A total of five mutations were identified resulting in a calculated mutation density of 1/526 kb. Two of the mutations identified by TILLING and verified by sequencing were detected in the gene encoding caffeic acid O-methyltransferase (COMT) in two independent mutant lines. The two mutant lines segregated for the expected brown midrib (bmr) phenotype, a trait associated with altered lignin content and increased digestibility. Conclusion TILLING as a reverse genetic approach has been successfully applied to sorghum. The diversity of the mutant phenotypes observed in the field, and the density of induced mutations calculated from TILLING indicate that this mutant population represents a useful resource for members of the sorghum research community. Moreover, TILLING has been demonstrated to be applicable for sorghum functional genomics by evaluating a small subset of the EMS-induced mutant lines.