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     

Quantitative trait loci for grain quality, productivity, morphological and agronomical traits in sorghum (Sorghum bicolor L. Moench)

 Quantitative trait loci (QTLs) for grain quality, yield components and other traits were investigated in two Sorghum caudatum×guinea recombinant inbred line (RIL) populations. A total of 16 traits were evaluated (plant height, panicle length, panicle compactness, number of kernels/panicle, thousand-kernel weight, kernel weight/panicle, threshing percentage, dehulling yield, kernel flouriness, kernel friability, kernel hardness, amylose content, protein content, lipid content, germination rate and molds during germination and after harvest) and related to two 113- and 100-point base genetic maps using simple (SIM) and composite (CIM) interval mapping. The number, effects and relative position of QTLs detected in both populations were generally in agreement with the distributions, heritabilities and correlations among traits. Several chromosomal segments markedly affected multiple traits and were suspected of harbouring major genes. The positions of these QTLs are discussed in relation to previously reported studies on sorghum and other grasses. Many QTLs, depending on their relative effects and position, could be used as targets for marker-assisted selection and provide an opportunity for accelerating breeding programmes. Received: 14 February 1998 / Accepted: 4 March 1998     

Quantitative trait loci associated with traits determining grain and stover yield in pearl millet under terminal drought-stress conditions

Drought stress during the reproductive stage is one of the most important environmental factors reducing the grain yield and yield stability of pearl millet. A QTL mapping approach has been used in this study to understand the genetic and physiological basis of drought tolerance in pearl millet and to provide a more-targeted approach to improving the drought tolerance and yield of this crop in water-limited environments. The aim was to identify specific genomic regions associated with the enhanced tolerance of pearl millet to drought stress during the flowering and grain-filling stages. Testcrosses of a set of mapping-population progenies, derived from a cross of two inbred pollinators that differed in their response to drought, were evaluated in a range of managed terminal drought-stress environments. A number of genomic regions were associated with drought tolerance in terms of both grain yield and its components. For example, a QTL associated with grain yield per se and for the drought tolerance of grain yield mapped on linkage group 2 and explained up to 23% of the phenotypic variation. Some of these QTLs were common across stress environments whereas others were specific to only a particular stress environment. All the QTLs that contributed to increased drought tolerance did so either through better than average maintenance (compared to non-stress environments) of harvest index, or harvest index and biomass productivity. It is concluded that there is considerable potential for marker-assisted backcross transfer of selected QTLs to the elite parent of the mapping population and for their general use in the improvement of pearl millet productivity in water-limited environments. Received: 15 November 2000 / Accepted: 12 April 2001     

Identification of genomic regions associated with stay green in sorghum by testing RILs in multiple environments

Stay green is an important drought resistance trait for sorghum production. QTLs for this trait with consistent effects across a set of environments would increase the efficiency of selection because of its relatively low heritability. One hundred and sixty recombinant inbreds, derived from a cross between QL39 and QL41, were used as a segregating population for genome mapping and stay green evaluation. Phenotypic data were collected in replicated field trials from five sites and in three growing seasons, and analysed by fitting appropriate models to account for spatial variability and to describe the genotype by environment interaction. Interval mapping and non-parametric mapping identified three regions, each in a separate linkage group, associated with stay green in more than one trial, and two regions in single trial. The regions on linkage groups B and I were both consistently identified from three trials. The multiple environment testing was very helpful for correctly identifying QTLs associated with the trait. The utilisation of molecular markers for stay green in sorghum breeding is also discussed. Received: 22 March 1999 / Accepted: 11 November 1999     

Quantitative trait dissection analysis in Eucalyptus using RAPD markers: 1. Detection of QTL in interspecific hybrid progeny, stability of QTL expression across different ages

 The objective of this study was to use random amplified polymorphic DNA (RAPD) to determine the genetic location and effects of genomic regions controlling wood density, stem growth and stem form in two species of Eucalyptus. Two hundred F₁ trees generated from an interspecific cross E. urophylla×E. grandis between two elite trees were used. Genetic maps were constructed for each parent with markers segregating in the 1:1 ratio in FS progeny. A total of 86 and 92 markers distributed among 11 linkage groups covered 1295 cM and 1312 cM for the E. urophylla and E. grandis parent, respectively. Traits were measured three times up to selection age (38 months). The magnitude of the phenotypic variation explained by the joint action of the segregating quantitative trait alleles indicated that genetic factors of large effect were involved in the control of the studied characters. Several regions controlling part of the variation for the studied traits were identified by interval mapping. Some regions of the genome exerted effects on more than one trait, providing a genetic explanation for at least some of the correlation between the traits. On the basis of an age-by-age analysis, a partial stability of QTL expression was observed with 68% of the QTL being expressed at two ages and 32% being age-specific. No QTL were significant for all three ages. Taking advantage of repeated measurements on the same material across different ages, we investigated with a maximum statistical power, the effect of marker genotype on traits, with age and QTL×age interaction effects being removed. A two-way analysis of variance made it possible to detect significant marker-trait associations over the period studied. Most of them had already been detected in the annual analysis. This result is very encouraging for the application of marker information to the early selection of hybrid trees to be vegetatively propagated for the production of clonal varieties. Received: 2 December 1996/Accepted: 4 April 1997     

Characteristics, linkage-map positions, and allelic differentiation of Sorghum bicolor (L.) Moench DNA simple-sequence repeats (SSRs)

Fifty one clones isolated from a size-fractionated genomic DNA library of Sorghum bicolor (L.) Moench, that had been probed with four radiolabeled di- and tri-nucleotide oligomers, were sequenced. Fifty of the clones contained one or more simple-sequence repeats (SSRs) [72% of which were (AG/TC) _n SSRs] and, following analysis of the clones, polymerase-chain-reaction primer sets that amplify 38 unique SSR loci were developed. Genotyping of the 38 loci in 18 sorghum accessions, including the parents of a recombinant inbred (RI) mapping population, revealed polymorphism at 36 of the loci among the 18 accessions and at 31 of the loci (not including null alleles at two loci) between the parents of the RI population. All of the latter 31 loci were mapped. The genotypes at 17-mapped SSR loci were assayed in 190 S. bicolor accessions in order to determine δ* _T , the estimated level of allelic differentiation (the estimated probability that two members of a population, chosen at random and without replacement, differ in allelic composition), at each of the loci. The mean δ* _T value determined for S. bicolor overall was 0.89, the range of mean δ* _T values for ten S. bicolor races was from 0.88 to 0.83, and the range of mean δ* _T values for ten working groups (= sub-races) of the race caudatum, with only two exceptions, was from 0.87 to 0.79. The lowest δ* _T values for six of the loci among the ten race-caudatum working groups ranged from 0.86 to 0.70; thus, the probability that different alleles will be present at one or more of these loci in two accessions chosen at random from a working group is > 0.996 when three of the loci are genotyped, and > 0.9999 when all six of the loci are genotyped. The results of this study confirm that most S. bicolor SSR loci are sufficiently polymorphic to be useful in marker- assisted selection programs and they indicate that the levels of polymorphism at some loci are high enough to allow the vast majority of S. bicolor accessions, even accessions within working groups, to be distinguished from one another by determining the genotypes at a small number, perhaps as few as a half-dozen, SSR loci. Received: 13 September 1999 / Accepted: 2 December 1999     

Genetic mapping of Sorghum bicolor (L.) Moench QTLs that control variation in tillering and other morphological characters

Grain yield of Sorghum bicolor (L.) Moench is significantly influenced by genetically controlled variation in the number of tillers, plant height, time of anthesis, and various other morphological and physiological characters. In this study, a minimum of 27 unique QTLs that control variation in nine morphological traits, including the presence versus the absence and the height of basal tillers, were mapped, and the percentage of additive genetic variance explained by the QTLs was determined in a population of 137 recombinant inbred lines in two environments. Four QTLs explained from 86.3% to 48.9% (depending upon the environment) of the additive genetic variance in the number of basal tillers with heads, and seven QTLs explained from 85.9% to 47.9% of the additive genetic variance in panicle width. It is unlikely that different alleles were segregating in the mapping population at any of the major dwarfing loci, but five QTLs that explained from 65.8% to 52.0% of the additive genetic variance in main-culm height were mapped. QTLs controlling variation in height of the tallest basal tiller, number of basal tillers per basal-tillered plant, panicle length, leaf angle, maturity, and awn length also were mapped. Three or more QTLs were mapped in linkage groups A, E, G, and I, while none were mapped in linkage groups B and D. Several of the QTLs mapped in this study are likely candidates for marker-assisted selection in breeding programs. Received: 20 September 2000 / Accepted: 26 October 2000     

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.     

Culture and regeneration of mesophyll-derived protoplasts of sorghum [Sorghum bicolor (L.) Moench]

 A protocol for plant regeneration from mesophyll/protoplasts of sorghum [Sorghum bicolor (L.) Moench] was developed. The yield of intact protoplasts, their subsequent divisions and regeneration were genotype-dependent. The genotype 296B was always more responsive than IS 32266. For 296B, the sixth leaf from 18-day-old plants kept in dark for 2 days before harvesting was found to be the most suitable source of viable protoplasts. The first division was observed 10–12 days after plating, and the second division after 12–14 days. The maximum plating efficiency was 4.8% in 296 B, followed by 2.48% in IS 32266. Microcolonies were visible after 25–30 days, and microcalli after 60–75 days. Whole plants were obtained after 6–8 weeks of culture of microcalli on MS medium containing 0.2 mg l^–1 kinetin and 2 mg l^–1 BAP. The frequency of regeneration in 296B and IS 32266 was 12.80% and 10.58%, respectively. Ten plants transferred to pots in the glasshouse established well. The seeds collected from glasshouse-grown plants were sown in the field where plants were grown to maturity. Received: 7 October 1998 / Revision received: 13 January 1999 / Accepted: 20 January 1999     

Quantitative trait loci for yield components in oil palm (Elaeis guineensis Jacq.)

The development of an oil palm RFLP marker map has enabled marker-based QTL mapping studies to be undertaken. Information from 153 RFLP markers was used in combination with phenotypic data from an F₂ population to estimate the position and effects of quantitative trait loci (QTLs) for traits including yield of fruit and its components and measures of vegetative growth. The mapping population consisted of 84 palms segregating for the major gene influencing shell thickness. Marker data were analysed to produce a linkage map consisting of 22 linkage groups. The QTL mapping analysis was carried out by interval mapping and single-marker analysis for the unlinked markers; significance thresholds were generated by permutation. Using both single-marker and interval-mapping analysis significant marker associated QTL effects were found for 11 of the 13 traits analysed. The results of interval-mapping analysis of fruit weight, petiole cross section and rachis length, and ratios of shell:fruit, mesocarp:fruit and kernel:fruit indicated significant (P<0.05) QTLs at the genome-wide threshold. The putative QTLs were associated with between 8.2% and 44.0% of the phenotypic variation, with an average of 27% for the single-marker analysis and 19% for the interval-mapping analysis. The higher percentage of phenotypic variation explained in the single-marker analysis, when compared to the interval-mapping analysis, is likely to be due to the lower stringency associated with the single-marker analysis. Large dominance deviations were associated with a sizeable proportion of the putative QTLs. The ultimate objective of mapping QTLs in commercial populations is to utilise novel breeding strategies such as marker-assisted selection (MAS). The potential impact of MAS in oil palm breeding programmes is discussed. Received: 26 June 2000 / Accepted: 24 October 2000     

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.     

Mapping of quantitative trait loci influencing frost tolerance in Eucalyptus nitens

 Regions of the genome influencing frost tolerance in an outbred family of Eucalyptus nitens have been identified. Two QTLs present on the same linkage group, but located 40 cM apart, were identified using single-factor analysis of variance. The QTLs explained between 7.7 and 10.8% of the phenotypic variation for frost tolerance in this family. Analysis of marker loci linked to the QTLs showed one of them to have a simple mode of action with the effect segregating from the male parent in the family. For the other QTL multiple alleles were identified. This QTL showed segregation from the female parent which gave a positive effect on frost tolerance; however, an allele segregating from the male parent was identified which showed a negative interaction with the allele for increased frost tolerance. Received: 10 May 1997 / Accepted: 2 June 1997     

Multiple methods for the identification of polymorphic simple sequence repeats (SSRs) in sorghum [Sorghum bicolor (L.) Moench]

Simple sequence repeats (SSRs), also known as microsatellites, are highly variable DNA sequences that can be used as markers for the genetic analysis of plants. Three approaches were followed for the development of PCR primers for the amplification of DNA fragments containing SSRs from sorghum [Sorghum bicolor (L.) Moench]: a search for sorghum SSRs in public DNA databases; the use of SSR-specific primers developed in the Poaceae species maize (Zea mays L.) and seashore paspalum grass (Paspalum vaginatum Swartz); and the screening of sorghum genomic libraries by hybridization with SSR oligonucleotides. A total of 49 sorghum SSR-specific PCR primer pairs (two designed from GenBank SSR-containing sequences and 47 from the sequences of genomic clones) were screened on a panel of 17 sorghum and one maize accession. Ten primer pairs from paspalum and 90 from maize were also screened for polymorphism in sorghum. Length polymorphisms among amplification products were detected with 15 of these primer pairs, yielding diversity values ranging from 0.2 to 0.8 with an average diversity of 0.56. These primer pairs are now available for use as markers in crop improvement and conservation efforts.     

Molecular mapping of genomic regions harbouring QTLs for root and yield traits in sorghum (Sorghum bicolor L. Moench)

Root system is a vital part of plants for absorbing soil moisture and nutrients and it influences the drought tolerance. Identification of the genomic regions harbouring quantitative trait loci (QTLs) for root and yield traits, and the linked markers can facilitate sorghum improvement through marker-assisted selection (MAS) besides the deeper understanding of the plant response to drought stress. A population of 184 recombinant inbred lines (RILs), derived from E36-1 × SPV570, along with parents were phenotyped for component traits of yield in field and root traits in an above ground rhizotron. High estimates of heritability and genetic advance for all the root traits and for most of the yield traits, presents high scope for improvement of these traits by simple selection. A linkage map constructed with 104 marker loci comprising 50 EST-SSRs, 34 non-genic nuclear SSRs and 20 SNPs, and QTL analysis was performed using composite interval mapping (CIM) approach. A total of eight and 20 QTLs were mapped for root and yield related traits respectively. The QTLs for root volume, root fresh weight and root dry weight were found co-localized on SBI-04, supported by a positive correlation among these traits. Hence, these traits can be improved using the same linked markers. The lack of overlap between the QTLs of component traits of root and yield suggested that these two sets of parameters are independent in their influence and the possibility of combining these two traits might enhance productivity of sorghum under receding moisture condition.     

Molecular mapping of the rf1 gene for pollen fertility restoration in sorghum (Sorghum bicolor L.)

We report the molecular mapping of a gene for pollen fertility in A1 (milo) type cytoplasm of sorghum using AFLP and SSR marker analysis. DNA from an F₂ population comprised of 84 individuals was screened with AFLP genetic markers to detect polymorphic DNAs linked to fertility restoration. Fifteen AFLP markers were linked to fertility restoration from the initial screening with 49 unique AFLP primer combinations (+3/+3 selective bases). As many of these AFLP markers had been previously mapped to a high-density genetic map of sorghum, the target gene (rf1) could be mapped to linkage group H. Confirmation of the map location of rf1 was obtained by demonstrating that additional linkage group-H markers (SSR, STS, AFLP) were linked to fertility restoration. The closest marker, AFLP Xtxa2582, mapped within 2.4 cM of the target loci while two SSRs, Xtxp18 and Xtxp250, flanked the rf1 locus at 12 cM and 10.8 cM, respectively. The availability of molecular markers will facilitate the selection of pollen fertility restoration in sorghum inbred-line development and provide the foundation for map-based gene isolation. Received: 22 August 2000 / Accepted: 18 October 2000     

Evaluation of genetic variability of sorghum (Sorghum bicolor L. Moench) in northwestern Morocco by ISSR and RAPD markers

The study of the genetic variability of the Moroccan landraces of sorghum constitutes a necessary step that can be exploited in the programs of improvement and valorisation of this marginalized species. The aim of this investigation is to evaluate the variability of sorghum populations and to establish their phylogenetic relations using RAPD and ISSR markers. Sampling was taken in 33 fields of northern regions where this species is most cultivated. Individual plants (398) were collected in 13, 11, 5, and 4 fields of Larache, Tangier, Chefchaouen, and Tetouan, respectively. Thirty-eight RAPD primers and four ISSR primers were used. The percentage of polymorphic fragments revealed with ISSR (98%) is higher than the one revealed with RAPD (85%). The level of the variability obtained through the two techniques is very high. Nevertheless, ISSR markers revealed more diversity than RAPD (0.995+/-0.006 against 0.946+/-0.031). The classification based on Jaccard's similarity index distinguished the totality of fields. Data analysis revealed a genetic structure that is closely related to the micro-geographical repartition of the different fields.     

Introgression of quantitative trait loci (QTLs) determining stripe rust resistance in barley: an example of marker-assisted line development

 Genome-analysis tools are useful for dissecting complex phenotypes and manipulating determinants of these phenotypes in breeding programs. Quantitative trait locus (QTL)-analysis tools were used to map QTLs conferring adult plant resistance to stripe rust (caused by Puccinia striiformis f.sp. hordei) in barley. The resistance QTLs were introgressed into a genetic background unrelated to the mapping population with one cycle of marker-assisted backcrossing. Doubled-haploid lines were derived from selected backcross lines, phenotyped for stripe-rust resistance, and genotyped with an array of molecular markers. The resistance QTLs that were introgressed were significant determinants of resistance in the new genetic background. Additional resistance QTLs were also detected. The susceptible parent contributed resistance alleles at two of these new QTLs. We hypothesize that favorable alleles were fixed at these new QTLs in the original mapping population. Genetic background may, therefore, have an important role in QTL-transfer experiments. A breeding system is described that integrates single-copy and multiplex markers with confirmation of the target phenotype in doubled-haploid lines phenotyped in field tests. This approach may be useful for simultaneously producing agronomically useful germplasm and contributing to an understanding of quantitatively inherited traits. Received: 6 May 1997 / Accepted: 1 September 1997     

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.     

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.     

Development, genetic mapping of candidate gene-based markers and their significant association with the shoot fly resistance quantitative trait loci in sorghum [Sorghum bicolor (L.) Moench]

The associations of candidate genes with quantitative trait loci (QTL) for insect resistance provide primary insight into the molecular mechanisms of resistance. The objectives of the present study were to genetically map the candidate genes and identify their association with shoot fly resistance, and update the genetic map with new markers to locate additional QTL. In this study, 80 candidate gene (CG)-based markers were developed, targeting the seven most important shoot fly resistance genomic regions reported in our previous study. Of the 17 polymorphic CGs, the allelic polymorphisms of seven genes were significantly associated with 18 major QTL for component traits of resistance in multiple QTL mapping (MQM), and two genes in the single-marker analysis. MQM with an updated map revealed 20 new QTL with LOD and R ² (%) values ranging from 2.6 to 15.6 and 5.5 to 34.5?%, respectively. The susceptible parent 296B contributed resistance at 10 QTL. Interestingly, an orthologous insect resistance gene Cysteine protease-Mir1 (XnhsbmSFC34/SBI-10), previously presumed to be a CG based on synteny with maize, was significantly associated with major QTL for all traits (except seedling vigor) explaining 22.1?% of the phenotypic variation for deadhearts%, a direct measure of shoot fly resistance. Similarly, a NBS?CLRR gene (XnhsbmSFCILP2/SBI-10), involved in rice brown planthopper resistance, was associated with deadhearts% and number of eggs per plant. Beta-1,3-glucanase (XnhsbmSFC4/SBI-10), involved in aphid and brown planthopper resistance, was associated with deadhearts% and leaf glossiness. Comparative QTL analysis revealed the existence of common QTL for shoot fly and other important sorghum insect pests such as greenbug, head bug, and midge. Finally, the associated CGs should aid in elucidating the molecular basis of resistance, high-resolution mapping, and map-based cloning of major QTL, besides providing powerful gene tags for marker-assisted selection of shoot fly resistance.