title: The sorghum SWEET gene family: stem sucrose accumulation as revealed through transcriptome profiling.
abstract: BACKGROUND
  SWEET 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).
  
  
  RESULTS
  We 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.
  
  
  CONCLUSIONS
  We 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.
authors.affiliation: Agrogenomics Research Center, National Institute of Agrobiological Sciences (NIAS), 2-1-2, Kannondai, Tsukuba, Ibaraki 305-8602 Japan ; Institute of Crop Science (NICS), National Agriculture and Food Research Organization, 1-2, Owashi, Tsukuba, Ibaraki 305-8602 Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Mizuno
authors.affiliation: Faculty of Agriculture, Shinshu University, 8304 Minami-minowa, Nagano, 399-4598 Japan.
authors.forename: Shigemitsu
authors.initials: S
authors.lastname: Kasuga
authors.affiliation: Agrogenomics Research Center, National Institute of Agrobiological Sciences (NIAS), 2-1-2, Kannondai, Tsukuba, Ibaraki 305-8602 Japan ; Institute of Crop Science (NICS), National Agriculture and Food Research Organization, 1-2, Owashi, Tsukuba, Ibaraki 305-8602 Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Kawahigashi
keywords: Bioethanol; Phloem loading; Phloem unloading; Photosynthesis; RNA-seq; SNP; Sugar transporter