Search

Grain size is a major determinant of grain yield in sorghum and other cereals. Over 100 quantitative trait loci (QTLs) of grain size have been identified in sorghum. However, no gene underlying any grain size QTL has been cloned. Here, we describe the fine mapping and cloning of one grain size QTL. From an F8 recombinant inbred line population derived from a cross between inbred lines 654 and LTR108, we identified 44 grain size QTLs. One QTL, qTGW1a, was detected consistently on the long arm of chromosome 1 in the span of 4 years. Using the extreme recombinants from an F2:3 fine-mapping population, qTGW1a was delimited within a ~33 kb region containing three predicted genes. One of them, SORBI_3001G341700, predicted to encode a G-protein γ subunit and homologous to GS3 in rice, is likely to be the causative gene for qTGW1a. qTGW1a appears to act as a negative regulator of grain size in sorghum. The functional allele of the putatively causative gene of qTGW1a from inbred line 654 decreased grain size, plant height, and grain yield in transgenic rice. Identification of the gene underlying qTGW1a advances our understanding of the regulatory mechanisms of grain size in sorghum and provides a target to manipulate grain size through genome editing.

Low soil moisture is a major abiotic stress wide spread in arid and semi-arid regions, limiting both seed germination and seedling establishment. Deep-seeding technique allows seeds to use the moisture in deep soil, however, most sorghum seeds cannot penetrate soil layer of 10 cm or deeper. To develop sorghum varieties tolerant to deep-seeding, it is necessary to identify promising accessions with good agronomic traits and tolerance to deep-seeding. A high throughput hydroponic method was developed to screen sorghum accessions with long mesocotyl under dark conditions. This method revealed large genetic variations for mesocotyl length in the panel of 105 sorghum accessions, which were validated by soil culture and deep-seeding experiments. The lines with long mesocotyl under hydroponic culture had good seedling establishments in deep-seeding experiments, while the lines with short mesocotyl length could not emerge. Compared to conventional sand or soil cultural methods, the hydroponic culture was highly efficient that requires less time, space, and labor. The mesocotyl lengths of screened sorghum accessions under the newly developed hydroponic method were highly correlated with soil culture and germination rate under deep-seeding in soil. As an efficient screening tool for the mesocotyl lengths under soil culture and the seedling emergence rates from deep soil layer, the hydroponic culture method can be easily integrated in breeding programs targeting deep-seeding tolerance. Breeding for long mesocotyl sorghum varieties may lead to good seedling establishments under arid and semi-arid regions.

The rice weevil Sitophilus oryzae (L.) is an important pest of stored sorghum ( Sorghum bicolor L. Moench). Quantitative trait loci (QTL) associated with rice weevil resistance was analyzed using a F 10 recombinant inbred line (RIL) population derived from the cross 654 (susceptible) × LTR108 (resistant). Grain weight losses (GWL), flour production (FP), and percentage kernels damaged (PKD) were evaluated across two trails. A total of 21 QTLs for GWL, FP, and PKD were identified. The phenotypic variation explained by individual QTL varied from 4.14% to 15.51%. Both parents contributed resistance alleles. Several main effect QTLs affecting GWL, FP, and PKD were mapped to the same regions on chromosome 2. The identification of genomic regions associated with rice weevil resistance will be useful to understand the resistance mechanism and improve rice weevil resistance of sorghum by marker-aided approaches.

Induced mutagenesis is a powerful approach to generate variations for elucidation of gene function and to create new traits for breeding. Here, we described a procedure to develop a pedigreed mutant library through chemical mutagenesis with ethylmethane sulfonate (EMS) treated seeds in sorghum and discussed its potential to generate new traits for sorghum improvement. Unlike random mutagenesis, a pedigreed mutant library, once properly established, can serve as a powerful resource to isolate and recover mutations of both agronomical and biological importance. With the development of affordable and high-throughput next-generation sequencing technologies, identification of causal mutations from a mutant library with a uniform genetic background becomes increasingly efficient and cost-effective. Fast causal gene discovery from mutant libraries combined with precise genome editing techniques will accelerate incorporation of new traits and revolutionize crop breeding.

Polyphenol oxidase (PPO) is believed to play a role in plant growth, reproduction, and resistance to pathogens and pests. PPO causes browning of grains in cereals. In this study, genetic mapping of sorghum grain for phenol color reaction (PHR) was performed using a recombinant inbred line population. Only one locus was detected between SSR markers SM06072 and Xtxp176 on chromosome 6. Two linked orthologous genes (Sb06PPO1 and Sb06PPO2) within the mapped region were discovered and cloned. Transformation experiments using Nipponbare (a PHR negative rice cultivar) showed that Sb06PPO1 from LTR108 and two Sb06PPO2 alleles from both varieties could complement Nipponbare, whereas Sb06PPO1 from 654 could not. Subsequent quantitative real-time PCR (qPCR) experiments showed that Sb06PPO1 and Sb06PPO2 functioned diversely, Sb06PPO1 was mainly expressed in young panicles before flowering. Sb06PPO2 was strongly expressed in flowering panicles, especially in hulls and branches at filling stage. Moreover, the expression of Sb06PPO1 was found to be significantly up-regulated by exogenous ABA and salt, whereas Sb06PPO2 was not changed significantly, further demonstrating functional differentiation between the two genes.

The productivity of sorghum is mainly determined by agronomically important traits. The genetic bases of these traits have historically been dissected and analysed through quantitative trait locus (QTL) mapping based on linkage maps with low-throughput molecular markers, which is one of the factors that hinder precise and complete information about the numbers and locations of the genes or QTLs controlling the traits. In this study, an ultra-high-density linkage map based on high-quality single nucleotide polymorphisms (SNPs) generated from low-coverage sequences (~0.07 genome sequence) in a sorghum recombinant inbred line (RIL) population was constructed through new sequencing technology. This map consisted of 3418 bin markers and spanned 1591.4 cM of genome size with an average distance of 0.5 cM between adjacent bins. QTL analysis was performed and a total of 57 major QTLs were detected for eight agronomically important traits under two contrasting photoperiods. The phenotypic variation explained by individual QTLs varied from 3.40% to 33.82%. The high accuracy and quality of this map was evidenced by the finding that genes underlying two cloned QTLs, Dw3 for plant height (chromosome 7) and Ma1 for flowering time (chromosome 6), were localized to the correct genomic regions. The close associations between two genomic regions on chromosomes 6 and 7 with multiple traits suggested the existence of pleiotropy or tight linkage. Several major QTLs for heading date, plant height, numbers of nodes, stem diameter, panicle neck length, and flag leaf width were detected consistently under both photoperiods, providing useful information for understanding the genetic mechanisms of the agronomically important traits responsible for the change of photoperiod.

KEY MESSAGE : The performance of callus induction and callus differentiation was evaluated by 9 indices for 140 RILs; 2 major QTLs associated with plant regeneration were identified. In order to investigate the genetic mechanisms of tissue culture response, 140 recombinant inbred lines (RILs) derived from 93-11 (Oryza sativa ssp. indica) × Nipponbare (Oryza sativa ssp. japonica) and a high quality genetic map based on the SNPs generated from deep sequencing of the RIL genomes, were used to identify the quantitative trait loci (QTLs) associated with in vitro tissue culture response (TCR) from mature seed in rice. The performance of callus induction was evaluated by indices of induced-callus color (ICC), induced-callus size (ICS), induced-callus friability (ICF) and callus induction rate (CIR), respectively, and the performance of callus differentiation was evaluated by indices of callus proliferation ability (CPA), callus browning tendency (CBT), callus greening ability (CGA), the average number of regenerated shoots per callus (NRS) and regeneration rate (%, RR), respectively. A total of 25 QTLs, 2 each for ICC, ICS, ICF, CIR and CBA, 3 for CPA, 4 each for CGA, NRS and RR, respectively, were detected and located on 8 rice chromosomes. Significant correlations were observed among the traits of CGA, NRS and RR, and QTLs identified for these three indices were co-located on chromosomes 3 and 7, and the additive effects came from both Nipponbare and 93-11, respectively. The results obtained from this study provide guidance for further fine mapping and gene cloning of the major QTL of TCR and the knowledge of the genes underlying the traits investigated would be very helpful for revealing the molecular bases of tissue culture response.

Rice seed size is an important agronomic trait in determining the yield potential, and four seed size related genes (GS3, GW2, qSW5/GW5 and GIF1) have been cloned in rice so far. However, the relationship among these four genes is still unclear, which will impede the process of gene pyramiding breeding program to some extent. To shade light on the relationship of above four genes, gene expression analysis was performed with GS3-RNAi, GW2-RNAi lines and CSSL of qSW5 at the transcriptional level. The results clearly showed that qSW5 and GW2 positively regulate the expression of GS3. Meanwhile, qSW5 can be down-regulated by repression of GW2 transcription. Additionally, GIF1 expression was found to be positively regulated by qSW5 but negatively by GW2 and GS3. Moreover, the allelic effects of qSW5 and GS3 were detailedly characterized based on a natural population consisting of 180 rice cultivars. It was indicated that mutual interactions exist between the two genes, in which, qSW5 affecting seed length is masked by GS3 alleles, and GS3 affecting seed width is masked by qSW5 alleles. These findings provide more insights into the molecular mechanisms underlying seed size development in rice and are likely to be useful for improving rice grain yield.