BackgroundSWEET is a newly identified family of sugar transporters. Although SWEET transporters have been characterized by using Arabidopsis and rice, very little knowledge of sucrose accumulation in the stem region is available, as these model plants accumulate little sucrose in their stems. To elucidate the expression of key SWEET genes involved in sucrose accumulation of sorghum, we performed transcriptome profiling by RNA-seq, categorization using phylogenetic trees, analysis of chromosomal synteny, and comparison of amino acid sequences between SIL-05 (a sweet sorghum) and BTx623 (a grain sorghum).ResultsWe identified 23 SWEET genes in the sorghum genome. In the leaf, SbSWEET8-1 was highly expressed and was grouped in the same clade as AtSWEET11 and AtSWEET12 that play a role in the efflux of photosynthesized sucrose. The key genes in sucrose synthesis (SPS3) and that in another step of sugar transport (SbSUT1 and SbSUT2) were also highly expressed, suggesting that sucrose is newly synthesized and actively exported from the leaf. In the stem, SbSWEET4-3 was uniquely highly expressed. SbSWEET4-1, SbSWEET4-2, and SbSWEET4-3 were categorized into the same clade, but their tissue specificities were different, suggesting that SbSWEET4-3 is a sugar transporter with specific roles in the stem. We found a putative SWEET4-3 ortholog in the corresponding region of the maize chromosome, but not the rice chromosome, suggesting that SbSWEET4-3 was copied after the branching of sorghum and maize from rice. In the panicle from the heading through to 36 days afterward, SbSWEET2-1 and SbSWEET7-1 were expressed and grouped in the same clade as rice OsSWEET11/Xa13 that is essential for seed development. SbSWEET9-3 was highly expressed in the panicle only just after heading and was grouped into the same clade as AtSWEET8/RPG1 that is essential for pollen viability. Five of 23 SWEET genes had SNPs that caused nonsynonymous amino acid substitutions between SIL-05 and BTx623.ConclusionsWe determined the key SWEET genes for technological improvement of sorghum in the production of biofuels: SbSWEET8-1 for efflux of sucrose from the leaf; SbSWEET4-3 for unloading sucrose from the phloem in the stem; SbSWEET2-1 and SbSWEET7-1 for seed development; SbSWEET9-3 for pollen nutrition.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0546-6) contains supplementary material, which is available to authorized users.

Semi-dwarfing genes have contributed to enhanced lodging resistance, resulting in increased crop productivity. In the history of grain sorghum breeding, the spontaneous mutation, dw1 found in Memphis in 1905, was the first widely used semi-dwarfing gene. Here, we report the identification and characterization of Dw1. We performed quantitative trait locus (QTL) analysis and cloning, and revealed that Dw1 encodes a novel uncharacterized protein. Knockdown or T-DNA insertion lines of orthologous genes in rice and Arabidopsis also showed semi-dwarfism similar to that of a nearly isogenic line (NIL) carrying dw1 (NIL-dw1) of sorghum. A histological analysis of the NIL-dw1 revealed that the longitudinal parenchymal cell lengths of the internode were almost the same between NIL-dw1 and wildtype, while the number of cells per internode was significantly reduced in NIL-dw1. NIL-dw1dw3, carrying both dw1 and dw3 (involved in auxin transport), showed a synergistic phenotype. These observations demonstrate that the dw1 reduced the cell proliferation activity in the internodes, and the synergistic effect of dw1 and dw3 contributes to improved lodging resistance and mechanical harvesting.

Simple sequence repeat (SSR) markers with a high degree of polymorphism contribute to the molecular dissection of agriculturally important traits in sorghum (Sorghum bicolor (L.) Moench). We designed 5599 non-redundant SSR markers, including regions flanking the SSRs, in whole-genome shotgun sequences of sorghum line ATx623. (AT/TA)n repeats constituted 26.1% of all SSRs, followed by (AG/TC)n at 20.5%, (AC/TG)n at 13.7% and (CG/GC)n at 11.8%. The chromosomal locations of 5012 SSR markers were determined by comparing the locations identified by means of electronic PCR with the predicted positions of 34 008 gene loci. Most SSR markers had a similar distribution to the gene loci. Among 970 markers validated by fragment analysis, 67.8% (658 of 970) markers successfully provided PCR amplification in sorghum line BTx623, with a mean polymorphism rate of 45.1% (297 of 658) for all SSR loci in combinations of 11 sorghum lines and one sudangrass (Sorghum sudanense (Piper) Stapf) line. The product of 5012 and 0.678 suggests that ∼3400 SSR markers could be used to detect SSR polymorphisms and that more than 1500 (45.1% of 3400) markers could reveal SSR polymorphisms in combinations of Sorghum lines.

Upon wounding or pathogen invasion, leaves of sorghum [Sorghum bicolor (L.) Moench] plants with the P gene turn purple, whereas leaves with the recessive allele turn brown or tan. This purple phenotype is determined by the production of two 3-deoxyanthocyanidins, apigeninidin and luteolinidin, which are not produced by the tan-phenotype plants. Using map-based cloning in progeny from a cross between purple Nakei-MS3B (PP) and tan Greenleaf (pp) cultivars, we isolated this gene, which was located in a 27-kb genomic region around the 58.1 Mb position on chromosome 6. Four candidate genes identified in this region were similar to the maize leucoanthocyanidin reductase gene. None of them was expressed before wounding, and only the Sb06g029550 gene was induced in both cultivars after wounding. The Sb06g029550 protein was detected in Nakei-MS3B, but only slightly in Greenleaf, in which it may be unstable because of a Cys252Tyr substitution. A recombinant Sb06g029550 protein had a specific flavanone 4-reductase activity, and converted flavanones (naringenin or eriodictyol) to flavan-4-ols (apiforol or luteoforol) in vitro. Our data indicate that the Sb06g029550 gene is involved in the 3-deoxyanthocyanidin synthesis pathway.

Semi-dwarf traits have been widely introgressed into cereal crops to improve lodging resistance. In sorghum (Sorghum bicolor L. Moench), four major unlinked dwarfing genes, Dw1-Dw4, have been introduced to reduce plant height, and among them, Dw3 and Dw1 have been cloned. Dw3 encodes a gene involved in auxin transport, whereas, Dw1 was recently isolated and identified as a gene encoding a protein of unknown function. In this study, we show that DW1 is a novel component of brassinosteroid (BR) signaling. Sorghum possessing the mutated allele of Dw1 (dw1), showed similar phenotypes to rice BR-deficient mutants, such as reduced lamina joint bending, attenuated skotomorphogenesis, and insensitivity against feedback regulation of BR-related genes. Furthermore, DW1 interacted with a negative regulator of BR signaling, BRASSINOSTEROID INSENSITIVE 2 (BIN2), and inhibited its nuclear localization, indicating that DW1 positively regulates BR signaling by inhibiting the function of BIN2. In contrast to rice and wheat breeding which used gibberellin (GA) deficiency to reduce plant height, sorghum breeding modified auxin and BR signaling. This difference may result from GA deficiency in rice and wheat does not cause deleterious side effects on plant morphology, whereas in sorghum it leads to abnormal culm bending.In crop breeding, breeders have significantly changed plant stature during the selection of improved grain crops. One famous example is the introduction of a semi-dwarf trait into rice and wheat in the 1960s. Compared to normal plants, semi-dwarf plants have lower center of gravity, which increases lodging resistance and thus enables plant to sustain high grain yield. This phenomenon was later referred as the 'green revolution' . The mechanism of semi-dwarfism that contributed to the 'green revolution' was the introduction of mutated alleles of gibberellin (GA) 20-oxidase (semidwarf 1; sd1) in rice 1, 2 and DELLA (Rht) in wheat 3 , encoding a GA biosynthesis enzyme and a dominant repressor of GA signal transduction, respectively. Furthermore, it has also been reported that the semi-dwarfism of barley, caused by the introgression of semi-dwarf 1 (sdw1/denso) into cultivars grown in Europe, probably depends on a defect in an ortholog of rice SD1 4-6 . Such wide usage of GA-related mutations to produce semi-dwarf plants has been possible due to a unique feature of GA. That is, GA deficiency specifically causes a decrease in plant height without deleterious side effects on other morphologies or physiologies, whereas dwarfism caused by other mechanisms often induces undesired phenotypes, such as abnormal leaf structure, abnormal internode elongation, and stunted seeds [7][8][9][10] . Therefore, it is rare that mutations involved in mechanisms other than GA deficiency were used to improve lodging resistance in crop breeding. One rare exception is a semi-dwarf barley mutant containing semi-brachytic 1 (uzu), a weak allele of the brassinosteroid (BR) receptor, which is grown in a limited region of East Asia, including areas...

Sorghum accumulates sugars (sucrose, glucose and fructose) in the stem after anthesis. Brix is commonly used to indicate total sugar content; however, the relationship between Brix and specific sugar components has not been sufficiently investigated in sorghum juice in Japan. In this study, we measured the sugar components of sorghum juices from 109 varieties using capillary electrophoresis, which can quantify each sugar component in crude juice without further purification. The results indicated that the Brix of sorghum juice was proportional to the total sugar and sucrose concentrations (r = 0.900 and r = 0.894, P < 0.01, respectively). Glucose concentration had a significant positive correlation with fructose concentration (r = 0.964, P < 0.01), but no correlation was detected between Brix and the two hexose sugars, glucose and fructose. Our results showed that sucrose comprised approximately 75% of the total sugar in varieties with Brix values greater than 15. These findings may be applied in sorghum breeding efforts to develop varieties that accumulate high levels of sugars in the juicy stems of sorghum plants.

Regulation of symmetrical cell growth in the culm is important for proper culm development. So far, the involvement of gibberellin (GA) in this process has not yet been demonstrated in sorghum. Here, we show that GA deficiency resulting from any loss-of-function mutation in four genes (SbCPS1, SbKS1, SbKO1, SbKAO1) involved in the early steps of GA biosynthesis, not only results in severe dwarfism but also in abnormal culm bending. Histological analysis of the bent culm revealed that the intrinsic bending was due to an uneven cell proliferation between the lower and upper sides of culm internodes. GA treatment alleviated the bending and dwarfism in mutants, whereas the GA biosynthesis inhibitor, uniconazole, induced such phenotypes in wild-type plants— both in a concentration-dependent manner, indicating an important role of GA in controlling erectness of the sorghum culm. Finally, we propose that because of the tight relationship between GA deficiency-induced dwarfism and culm bending in sorghum, GA-related mutations have unlikely been selected in the history of sorghum breeding, as could be inferred from previous QTL and association studies on sorghum plant height that did not pinpoint GA-related genes.

Pith parenchyma cells store water in various plant organs. These cells are especially important for producing sugar and ethanol from the sugar juice of grass stems. In many plants, the death of pith parenchyma cells reduces their stem water content. Previous studies proposed that a hypothetical gene might be responsible for the death of stem pith parenchyma cells in, a promising energy grass, although its identity and molecular function are unknown. Here, we identify the gene and note that it is located on chromosome 6 in agreement with previous predictions. Sorghum varieties with a functional allele had stems enriched with dry, dead pith parenchyma cells, whereas those with each of six independent nonfunctional alleles had stems enriched with juicy, living pith parenchyma cells. expression was spatiotemporally coupled with the appearance of dead, air-filled pith parenchyma cells in sorghum stems. Among homologs that are present in flowering plants, also is required for the death of stem pith parenchyma cells. and encode previously uncharacterized NAC transcription factors and are sufficient to ectopically induce programmed death of culture cells via the activation of autolytic enzymes. Taken together, these results indicate that and its ortholog,, are master transcriptional switches that induce programmed death of stem pith parenchyma cells. Thus, targeting the gene will provide an approach to breeding crops for sugar and ethanol production.