Location of major effect genes in sorghum (Sorghum bicolor (L.) Moench)

Major effect genes are often used for germplasm identification, for diversity analyses and as selection targets in breeding. To date, only a few morphological characters have been mapped as major effect genes across a range of genetic linkage maps based on different types of molecular markers in sorghum (Sorghum bicolor (L.) Moench). This study aims to integrate all available previously mapped major effect genes onto a complete genome map, linked to the whole genome sequence, allowing sorghum breeders and researchers to link this information to QTL studies and to be aware of the consequences of selection for major genes. This provides new opportunities for breeders to take advantage of readily scorable morphological traits and to develop more effective breeding strategies. We also provide examples of the impact of selection for major effect genes on quantitative traits in sorghum. The concepts described in this paper have particular application to breeding programmes in developing countries where molecular markers are expensive or impossible to access.     

DNA polymorphisms in grain sorghum (Sorghum bicolor (L.) Moench)

Molecular markers [random amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP)] were used to determine the frequency of DNA polymorphism in grain sorghum (Sorghum bicolor (L.) Moench). Twenty-nine oligonucleotide primers were employed for RAPDs, generating a total of 262 DNA fragments, of which 145 were polymorphic in at least one pairwise comparison between 36 genotypes. Individual primers differed significantly in their ability to detect genetic polymorphism in the species. The overall frequency of polymorphisms was low with a mean frequency of 0.117 polymorphisms per RAPD band being obtained from all pairwise comparisons between genotypes, with maximum and minimum values of 0.212 and 0.039, respectively. Results from phenetic analysis of bandsharing data were consistent with current sub-specific groupings of the species, with clusters of Durra, Zerazera, Caud-Nig, Caud-Kaura and Caffrorum being discernible. The results also indicated that individuals of a similar taxonomic grouping but different geographic origin may be genetically less identical than previously considered. Similar frequencies of polymorphism to that obtained with RAPDs were obtained with RFLPs. Results from these experiments indicated that a high level of genetic uniformity exists within S. bicolor.     

Mapping QTLs associated with drought resistance in sorghum (Sorghum bicolor L. Moench)

Drought is a major abiotic stress factor limiting crop production. Identification of genetic factors involved in plant responses to drought stress will provide a solid foundation to improve drought resistance. Sorghum is well adapted to hot dry environments and regarded as a model for studying drought resistance among the grasses. Significant progress in genome mapping of this crop has also been made. In sorghum, rapid premature leaf death generally occurs when water is limited during the grain filling period. Premature leaf senescence, in turn, leads to charcoal rot, stalk lodging, and significant yield loss. More than 80% of commercial sorghum hybrids in the United States are grown under non-irrigated conditions and although most of them have pre-flowering drought resistance, many do not have any significant post-flowering drought resistance. Stay-green is one form of drought resistance mechanism, which gives sorghum resistance to premature senescence under soil moisture stress during the post-flowering period. Quantitative trait locus (QTL) studies with recombinant inbred lines (RILs) and near-isogenic lines (NILs) identified several genomic regions associated with resistance to pre-flowering and post-flowering drought stress. We have identified four genomic regions associated with the stay-green trait using a RIL population developed from B35 × Tx7000. These four major stay-green QTLs were consistently identified in all field trials and accounted for 53.5% of the phenotypic variance. We review the progress in mapping stay-green QTLs as a component of drought resistance in sorghum. The molecular genetic dissection of the QTLs affecting stay-green will provide further opportunities to elucidate the underlying physiological mechanisms involved in drought resistance in sorghum and other grasses.     

Biotechnology: Genetic improvement of sorghum (Sorghum bicolor (L.) Moench)

Summary This report reviews the contributions to the improvement of sorghum (Sorghum bicolor (L.) Moench) through traditional approaches with emphasis on the application of biotechnological methods. Strategies include breeding for higher yield, improved grain quality, and biotic and abiotic stress tolerance. Hybrid development and polyploidy breeding are also discussed. Plant breeders, working in concert with biotechnologists, have developed new powerful tools for plant genetic manipulation and genotype evaluation that will significantly improve the efficiency of plant breeding. Improving sorghum through biotechnology is the latest in a long series of technologies that have been applied to this crop. Five basic tools of technology have been developed for sorghum improvement: (1) in vitro protocols for efficient plant regeneration; (2) molecular markers; (3) gene identification and cloning; (4) genetic engineering and gene transfer technology to integrate desirable traits into the sorghum genome; and (5) genomics and germplasm databases. Reports on studies involving the problems, progress, and prospects for utilizing the biotechnological methods for sorghum improvement are discussed.     

Antixenosis component of resistance to sorghum midge, Contarinia sorghicola Coq. in Sorghum bicolor (L.) Moench

Sorghum midge, Contarinia sorghicola Coq. (Diptera: Cecidomyiidae) is the most destructive pest of grain sorghum, and host-plant resistance is an effective method of controlling this insect. We studied the antixenosis component of resistance to sorghum midge using multi-, double- and no-choice cage tests, and under multi-choice field conditions to quantify and understand the nature of antixenosis component of resistance to this insect in Sorghum bicolor (L.) Moench. Midge response towards sorghum panicles was influenced by panicle size and cage type used to study the orientation behaviour. Maximum number of midges were recorded at 30 and 60 min after initiating the experiment. Antixenosis shown by C. sorghicola under multi-choice field conditions to ICSV 197 and TAM 2566 was not confirmed under cage tests, while DJ 6514, AF 28 and IS 3461 were non-preferred both under field and cage conditions. Midge-resistant female parents (PM 7061 and PM 7068) were less preferred than the midge susceptible (ICSA 42 and 296A) female parents. Male-sterility did not influence host finding and acceptance by the midge females, although in one out of two tests, the maintainer lines (B-lines) were preferred over the male-sterile lines (A-lines).     

Quantitative trait loci influencing drought tolerance in grain sorghum (Sorghum bicolor L. Moench)

Drought is a major constraint in sorghum production worldwide. Drought-stress in sorghum has been characterized at both pre-flowering and post-flowering stages resulting in a drastic reduction in grain yield. In the case of post-flowering drought stress, lodging further aggravates the problem resulting in total loss of crop yield in mechanized agriculture. The present study was conducted to identify quantitative trait loci (QTLs) controlling post-flowering drought tolerance (stay green), pre-flowering drought tolerance and lodging tolerance in sorghum using an F₇ recombinant inbred line (RIL) population derived from the cross SC56×Tx7000. The RIL lines, along with parents, were evaluated for the above traits in multiple environments. With the help of a restriction fragment length polymorphism (RFLP) map, which spans 1,355 cM and consists of 144 loci, nine QTLs, located over seven linkage groups were detected for stay green in several environments using the method of composite interval mapping. Comparison of the QTL locations with the published results indicated that three QTLs located on linkage groups A, G and J were consistent. This is considered significant since the stay green line SC56 used in our investigation is from a different source compared to B35 that was used in all the earlier investigations. Comparative mapping has shown that two stay green QTLs identified in this study corresponded to stay green QTL regions in maize. These genomic regions were also reported to be congruent with other drought-related agronomic and physiological traits in maize and rice, suggesting that these syntenic regions might be hosting a cluster of genes with pleiotropic effects implicated in several drought tolerance mechanisms in these grass species. In addition, three and four major QTLs responsible for lodging tolerance and pre-flowering drought tolerance, respectively, were detected. This investigation clearly revealed the important and consistent stay green QTLs in a different stay green source that can logically be targeted for positional cloning. The identification of QTLs and markers for pre-flowering drought tolerance and lodging tolerance will help plant breeders in manipulating and pyramiding those traits along with stay green to improve drought tolerance in sorghum. Received: 2 June 2000 / Accepted: 15 November 2000     

Modulation of kernel storage proteins in grain sorghum (Sorghum bicolor (L.) Moench)

Sorghum prolamins, termed kafirins, are categorized into subgroups α, β, and γ. The kafirins are co‐translationally translocated to the endoplasmic reticulum (ER) where they are assembled into discrete protein bodies that tend to be poorly digestible with low functionality in food and feed applications. As a means to address the issues surrounding functionality and digestibility in sorghum, we employed a biotechnology approach that is designed to alter protein body structure, with the concomitant synthesis of a co‐protein in the endosperm fraction of the grain. Wherein perturbation of protein body architecture may provide a route to impact digestibility by reducing disulphide bonds about the periphery of the body, while synthesis of a co‐protein, with known functionality attributes, theoretically could impact structure of the protein body through direct association and/or augment end‐use applications of sorghum flour by stabilizing ß‐sheet formation of the kafirins in sorghum dough preparations. This in turn may improve viscoelasticity of sorghum dough. To this end, we report here on the molecular and phenotypic characterizations of transgenic sorghum events that are down‐regulated in γ‐ and the 29‐kDa α‐kafirins and the expression of a wheat Dy10/Dx 5 hybrid high‐molecular weight glutenin protein. The results demonstrate that down‐regulation of γ‐kafirin alone does not alter protein body formation or impacts protein digestibility of cooked flour samples. However, reduction in accumulation of a predicted 29‐kDa α‐kafirin alters the morphology of protein body and enhances protein digestibility in both raw and cooked samples.     

Molecular mapping of QTLs conferring stay-green in grain sorghum (Sorghum bicolor L. Moench).

Drought resistance is of enormous importance in crop production. The identification of genetic factors involved in plant response to drought stress provides a strong foundation for improving drought tolerance. Stay-green is a drought resistance trait in sorghum (Sorghum bicolor L. Moench) that gives plants resistance to premature senescence under severe soil moisture stress during the post-flowering stage. The objective of this study was to map quantitative trait loci (QTLs) that control the stay-green and chlorophyll content in sorghum. By using a restriction fragment length polymorphism (RFLP) map, developed from a recombinant inbred line (RIL) population, we identified four stay-green QTLs, located on three linkage groups. The QTLs (Stg1 and Stg2) are on linkage group A, with the other two, Stg3 and Stg4, on linkage groups D and J, respectively. Two stay-green QTLs, Stg1 and Stg2, explaining 13-20% and 20-30% of the phenotypic variability, respectively, were consistently identified in all trials at different locations in two years. Three QTLs for chlorophyll content (Chl1, Chl2, and Chl3), explaining 25-30% of the phenotypic variability were also identified under post-flowering drought stress. All coincided with the three stay-green QTL regions (Stg1, Stg2, and Stg3) accounting for 46% of the phenotypic variation. The Stg1 and Stg2 regions also contain the genes for key photosynthetic enzymes, heat shock proteins, and an abscisic acid (ABA) responsive gene. Such spatial arrangement shows that linkage group A is important for drought- and heat-stress tolerance and yield production in sorghum. High-resolution mapping and cloning of the consistent stay-green QTLs may help to develop drought-resistant hybrids and to understand the mechanism of drought-induced senescence in plants.     

Phosphorus efficiency and phosphorus remobilization in two sorghum (Sorghum bicolor (L.) Moench) cultivars

With two sorghum cultivars differing in P efficiency a P uptake experiment (³²P/³³P labelling) was carried out followed by a period of P deficiency. The tendency of the total P distribution and redistribution pattern was rather similar in both sorghum cultivars. Although in the cultivar with a greater P absorbing capacity per unit root weight a higher proportion of the P was found in the inorganic P soluble fraction this is not necessarily an indication of a higher vacuolar affinity for P. Under P deficiency in both cultivars a rapid decrease of the TCA soluble P fraction in the leaves was observed. Before complete exhaustion of this fraction the TCA insoluble P fraction was also markedly reduced. In the roots the total P content was maintained fairly constant with a distinct shift in favour of the insoluble fraction occurring during the period of P deficiency. It is assumed that in the P efficient sorghum cultivar producing more dry matter per increment of P absorbed, rather inherent growth promoting factors contribute to the intraspecific P efficiency by a stimulation of the intensity of P redistribution and thus compensate for the lower P absorbing capacity of its roots.     

Chloroplast DNA polymorphism in fertile and male-sterile cytoplasms of sorghum (Sorghum bicolor (L.) Moench)

Summary Restriction endonuclease patterns of chloroplast DNA (cpDNA) were consistently distinguishable between fertile and male-sterile cytoplasms of sorghum [Sorghum bicolor (L.) Moench], whereas no differences in restriction patterns of cpDNA among male-sterile (A₁) lines, including six isocytoplasmic strains, were revealed in this study. It is suggested that chloroplast DNA may contribute to the male sterility of A₁ lines used currently in hybrid sorghum production.This research was supported by a research grant from Kansas Grain Sorghum Commission, Kansas Board of Agriculture. Contribution 90-293-J from the Kansas Agricultural Experiment Station     

Gene discovery and functional marker development for fragrance in sorghum (Sorghum bicolor (L.) Moench)

Key message

Sequence analysis and genetic mapping revealed that a 1,444 bp deletion causes a premature stop codon in SbBADH2 of sorghum IS19912. The non-function of SbBADH2 is responsible for fragrance in sorghum IS19912.

Abstract

2-acetyl-1-pyrroline (2AP) is a potent volatile compound causing fragrance in several plants and foods. Seeds of some varieties of rice, sorghum and soybean possess fragrance. The genes responsible for fragrance in rice and soybean are orthologs that correspond to betaine aldehyde dehydrogenase 2 (BADH2). Genotypes harboring fragrance in rice and soybean contain a premature stop codon in BADH2 which impairs the synthesis of full length functional BADH2 protein leading to the accumulation of 2AP. In this study, we reported an association between the BADH2 gene and fragrance in sorghum. An F₂ population of 187 plants developed from a cross between KU630 (non-fragrant) and IS19912 (fragrant) was used. Leaves of F₂ and F₃ progenies were evaluated for fragrance by organoleptic test, while seeds of F₂ plants were analyzed for 2AP. The tests consistently showed that the fragrance is controlled by a single recessive gene. Gene expression analysis of SbBADH1 and SbBADH2 in leaves of KU630 and IS19912 at various stages revealed that SbBADH1 and SbBADH2 were expressed in both accessions. Sequence comparison between KU630 and IS19912 revealed a continuous 1,444 bp deletion encompassing exon 12 to 15 of SbBADH2 in IS19912 which introduces a frameshift mutation and thus causes a premature stop codon. An indel marker was developed to detect polymorphism in SbBADH2. Bulk segregant and QTL analyses confirmed the association between SbBADH2 and fragrance.     

High Frequency Shoot Organogenesis in Sorghum bicolor (L) Moench

In vitro morphogenesis that includes enlargement of apical meristems and differentiation of shoot buds on the surface of enlarged meristemoids, have been observed in Sorghum bicolor. The enlargement of apical meristems occurred on the MS medium containing different auxins [2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichloro-phenoxyacetic acid (2,4,5-T) and parachlorophenoxyacetic acid (pCPA)] with or without 6-benzylaminopurine (BAP)]. Large number of shoot buds arose over the entire surface of enlarged, green, compact, nodular, hard and shiny meristemoids on transfer to medium containing BAP (2.0 mg l^?1) + (IAA) indole 3-acetic acid (0.5 mg l^-1). Histological observations revealed the origin of shoot apices from the surface of enlarged meristemoids. Efficient rooting was achieved onto half-strength MS medium supplemented with α-napthaleneacetic acid (NAA, 2.0 mg l^?1) and sucrose 2% (w/v). Plantlets were transferred to earthen pots under field conditions for the evalution of several agronomically important characters.     

Biochemical characterization of six trisomics of grain sorghum,Sorghum bicolor (L.) Moench

To determine protein differences of grain sorghum disomics and trisomics, we analyzed leaf extracts from six trisomics and a disomic control by disc gel, gel isoelectric focusing, and SDS gel electrophoresis. Based on the number and position of protein bands revealed by Commassie blue staining, the disomic control could be differentiated from the trisomics, and trisomics could be shown to differ among themselves in most cases. SDS gels revealed the most protein bands, followed by isoelectric focusing and disc gel. However, disc gel electrophoresis was the simplest technique of the three and was just as effective in identifying trisomics and differentiating trisomics from the disomic control.Contribution 1596-j, Department of Agronomy, and 182-j, Department of Biochemistry, Kansas State University, Manhattan, Kansas.     

Efficient regeneration of sorghum, Sorghum bicolor (L.) Moench, from shoot-tip explant

Novel protocols for production of multiple shoot-tip clumps and somatic embryos of Sorghum bicolor (L.) Moench were developed with long-term goal of crop improvement through genetic transformation. Multiple shoot-tip clumps were developed in vitro from shoot-tip explant of one-week old seedling, cultured on MS medium containing only BA (0.5, 1 or 2 mg/l) or both BA (1 or 2 mg/l) and 2,4-D (0.5 mg/l) with bi-weekly subculture. Somatic embryos were directly produced on the enlarged dome shaped growing structures that developed from the shoot-tips of one-week old seedling explants (without any callus formation) when cultured on MS medium supplemented with both 2,4-D (0.5 mg/l) and BA (0.5 mg/l). However, the supplementation of MS medium with only 2,4-D (0.5 mg/l) induced compact callus without any plantlet regeneration. Each multiple shoot-clump was capable of regenerating more than 80 shoots via an intensive differentiation of both axillary and adventitious shoot buds, the somatic embryos were capable of 90% germination, plant conversion and regeneration. The regenerated shoots could be efficiently rooted on MS medium containing indole-3-butyric acid (IBA 1 mg/l). The plants were successfully transplanted to glasshouse and grown to maturity with a survival rate of 98%. Morphogenetic response of the explants was found to be genotypically independent.     

Bioaccumulation and photosynthetic activity response of sweet sorghum seedling (Sorghum bicolor L. Moench) to cadmium stress

Photosynthetica - A hydroponic experiment was conducted to investigate bioaccumulation and photosynthetic activity response to Cd in sweet sorghum seedlings. The seedlings were treated with 0, 50,...     

Formulation of a liquid fertilizer for sorghum (Sorghum bicolor (L.) Moench) using vermicompost leachate

Leachate from vermicomposting contains large amounts of plant nutrients and can be used as liquid fertilizer, but normally diluted to avoid plant damage. The amount of nutrients applied is thus reduced so that an additional fertilizer is required. We investigated how dilution of vermicompost leachate combined with different concentrations of NPK triple 17 fertilizer, and polyoxyethylene tridecyl alcohol as dispersant and polyethylene nonylphenol as adherent to increase efficiency of fertilizer uptake, affected sorghum plant development. The vermicomposting leachate with pH 7.8 and electrolytic conductivity 2.6 dS m(-1), contained 834 mg K(+) l(-1), 247 mg NO(3)(-)l(-1) and 168 mg PO(4)(3-) l(-1), was free of pathogens and resulted in a 65 % germination index. Vermicompost leachate can be used as liquid fertilizer for the cultivation of sorghum without dilution and mixed with 140-170 g l(-1) of NPK triple 17 fertilizer and 2-3 ml(-1) of dispersant and 0-1 ml l(-1) adherent. It was found that vermicompost leachate stimulated plant development, but fertilization with NPK was required for maximum growth.     

Identification and expression analysis of group 3 LEA family genes in sorghum [Sorghum bicolor (L.) Moench]

Sorghum with its remarkable adaptability to drought and high temperature provides a model system for grass genomics and resource for gene discovery especially for abiotic stress tolerance. Group 3 LEA genes from barley and rice have been shown to play crucial role in abiotic stress tolerance. Here, we present a genome-wide analysis of LEA3 genes in sorghum. We identified four genes encoding LEA3 proteins in the sorghum genome and further classified them into LEA3A and LEA3B subgroups based on the conservation of LEA3 specific motifs. Further, expression pattern of these genes were analyzed in seeds during development and vegetative tissues under abiotic stresses. SbLEA3A group genes showed expression at early stage of seed development and increased significantly at maturity, while SbLEA3B group genes expressed only in matured seeds. Expression of SbLEA3 genes in response to abiotic stresses such as soil moisture deficit (drought), osmotic, salt, and temperature stresses, and exogenous ABA treatments was also studied in the leaves of 2-weeks-old seedlings. ABA and drought induced the expression of all LEA3 genes, while cold and heat stress induced none of them. Promoter analysis revealed the presence of multiple ABRE core cis-elements and a few low temperature response (LTRE)/drought responsive (DRE) cis-elements. This study suggests non-redundant function of LEA3 genes in seed development and stress tolerance in sorghum.     

Anther culture of Sorghum bicolor (L.) Moench

The sequence of pollen development from the tetrad stage to the mature tricellular grain was studied in freshly harvested anthers of Sorghum bicolor. This pattern of development was then compared with that occurring during panicle pretreatment and subsequent anther incubation in vitro. It was found that during pretreatment at 7° C mitoses of the vegetative cell were induced in up to 30% of the pollen. During anther incubation procallus development was highly polarised with contributions from both the generative and vegetative cells. After pretreatment at 14 or 20° C the generative cell became detached from the pollen wall and it was not possible to determine whether subsequent development involved only the vegetative cell or both the vegetative and generative cells.Although retarded pollen grains were observed both in vivo and in vitro, and were occasionally seen to divide in culture, they did not appear to be the source of the procalluses produced.     

Molecular characterization of allelic variation in spontaneous brown midrib mutants of sorghum (Sorghum bicolor (L.) Moench)

Modification of lignin composition and content are important to enhance the saccharification potential of lignocellulosic biomass. Brown midrib (bmr) mutants with altered lignin and enhanced glucose yields are a valuable resource for modification of the lignin biosynthetic pathway in sorghum (Sorghum bicolor (L.) Moench). Of the 38 bmr mutants reported in sorghum, some have been classified into four independent groups, namely bmr2, bmr6, bmr12 and bmr19, based on the allelic test, and a few have been characterized at the molecular level. The bmr2, bmr6 and bmr12 groups have mutations that impair 4-coumarate:coenzyme A ligase (4CL), cinnamyl alcohol dehydrogenase (CAD2) and caffeic O-methyltransferase (COMT), respectively. The molecular basis of bmr19 is unknown. In the present study, four spontaneous bmr mutants of sorghum were analyzed for allelic variation at two candidate gene loci. cDNAs of CAD2 and COMT genes were cloned and sequenced from these mutants. Sequence analysis revealed that two of these mutants, IS23789 and IS23253, share a new allele of CAD2. These mutants have a G-to-C transversion at position 3699 of the genomic sequence that leads to glycine-to-arginine (G191R) substitution in the CAD2 protein sequence. This mutation lies in the highly conserved glycine-rich motif ¹⁸⁸G(X)GGV(L)G¹⁹³ that participates in the binding of the pyrophosphate group of NADP⁺ cofactor and hence might impair the activity of CAD2. Phloroglucinol staining of midribs of these mutants also showed a dark wine-red color that is characteristic of the bmr6 group. These two mutants can be distinguished by an intron length polymorphic marker developed based on the COMT gene sequence in this study. Mutant IS23549, which has also been assigned to the bmr6 group, was found to have another new allele with alanine-to-valine (A164V) substitution in CAD2. Alanine-164 is highly conserved among MDR proteins in plants and hence may be necessary for the activity of the enzyme. In mutant IS11861, there was no mutation that led to a change in amino acid in CAD2, while a threonine-to-serine (T302S) substitution was found in COMT. This single nucleotide polymorphism (SNP) at position 2645 in the COMT gene was converted into a cleaved amplified polymorphic sequence marker that can be used for its identification. In addition, additional SNP- and/or indel-based markers were developed, which can be used for exploiting these alleles in the molecular breeding of sorghum for dedicated bioenergy feedstock.     

Developmental and genetic characterization of a short leaf mutant of sorghum (Sorghum bicolor L. (Moench.))

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.