title: Molecular and cellular characteristics of hybrid vigour in a commercial hybrid of Chinese cabbage.
abstract: BACKGROUND
  Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior performance compared to their parental inbred lines. Most commercial Chinese cabbage cultivars are F1 hybrids and their level of hybrid vigour is of critical importance and is a key selection criterion in the breeding system.
  
  
  RESULTS
  We have characterized the heterotic phenotype of one F1 hybrid cultivar of Chinese cabbage and its parental lines from early- to late-developmental stages of the plants. Hybrid cotyledons are larger than those of the parents at 4 days after sowing and biomass in the hybrid, determined by the fresh weight of leaves, is greater than that of the larger parent line by approximately 20% at 14 days after sowing. The final yield of the hybrid harvested at 63 days after sowing is 25% greater than the yield of the better parent. The larger leaves of the hybrid are a consequence of increased cell size and number of the photosynthetic palisade mesophyll cells and other leaf cells. The accumulation of plant hormones in the F1 was within the range of the parental levels at both 2 and 10 days after sowing. Two days after sowing, the expression levels of chloroplast-targeted genes in the cotyledon cells were upregulated in the F1 hybrid relative to their mid parent values. Shutdown of chlorophyll biosynthesis in the cotyledon by norflurazon prevented the increased leaf area in the F1 hybrid.
  
  
  CONCLUSIONS
  In the cotyledons of F1 hybrids, chloroplast-targeted genes were upregulated at 2 days after sowing. The increased activity levels of this group of genes suggested that their differential transcription levels could be important for establishing early heterosis but the increased transcription levels were transient. Inhibition of the photosynthetic process in the cotyledon reduced heterosis in later seedling stages. These observations suggest early developmental events in the germinating seedling of the hybrid may be important for later developmental vigour and yield advantage.
authors.affiliation: Graduate School of Science and Technology, Niigata University, Ikarashi-ninocho, Niigata, 950-2181, Japan. f14m012c@mail.cc.niigata-u.ac.jp.
authors.forename: Natsumi
authors.initials: N
authors.lastname: Saeki
authors.affiliation: Graduate School of Agricultural Science, Kobe University, Rokkodai, Nada-ku, Kobe, 657-8501, Japan. k5802@cc.kyoto-su.ac.jp.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Kawanabe
authors.affiliation: CSIRO Agriculture, Canberra, ACT 2601, Australia. Hua.Ying@anu.edu.au.
authors.forename: Hua
authors.initials: H
authors.lastname: Ying
authors.affiliation: Graduate School of Science and Technology, Niigata University, Ikarashi-ninocho, Niigata, 950-2181, Japan. m-shimizu@ibrc.or.jp.
authors.forename: Motoki
authors.initials: M
authors.lastname: Shimizu
authors.affiliation: Center for Sustainable Resource Science, 1-7-22, Suehiro, Tsurumi, Yokohama, 230-0045, Japan. mikiko@psc.riken.go.jp.
authors.forename: Mikiko
authors.initials: M
authors.lastname: Kojima
authors.affiliation: Experimental Plant Division, RIKEN BioResource Center, Tsukuba, 305-0074, Japan. ahiroshi@rtc.riken.jp.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Abe
authors.affiliation: Graduate School of Science and Technology, Niigata University, Ikarashi-ninocho, Niigata, 950-2181, Japan. okazaki@agr.niigata-u.ac.jp.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Okazaki
authors.affiliation: Watanabe Seed Co., Ltd, Machiyashiki, Misato-cho, Miyagi, 987-0003, Japan. mktkaji@themis.ocn.ne.jp.
authors.forename: Makoto
authors.initials: M
authors.lastname: Kaji
authors.affiliation: CSIRO Agriculture, Canberra, ACT 2601, Australia. Jen.Taylor@csiro.au.
authors.forename: Jennifer M
authors.initials: JM
authors.lastname: Taylor
authors.affiliation: Center for Sustainable Resource Science, 1-7-22, Suehiro, Tsurumi, Yokohama, 230-0045, Japan. sakaki@riken.jp.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Sakakibara
authors.affiliation: CSIRO Agriculture, Canberra, ACT 2601, Australia. Jim.Peacock@csiro.au.|University of Technology, Broadway, Sydney, PO Box 123, NSW, 2007, Australia. Jim.Peacock@csiro.au.
authors.forename: W James
authors.initials: WJ
authors.lastname: Peacock
authors.affiliation: CSIRO Agriculture, Canberra, ACT 2601, Australia. Liz.Dennis@csiro.au.|University of Technology, Broadway, Sydney, PO Box 123, NSW, 2007, Australia. Liz.Dennis@csiro.au.
authors.forename: Elizabeth S
authors.initials: ES
authors.lastname: Dennis
authors.affiliation: Graduate School of Agricultural Science, Kobe University, Rokkodai, Nada-ku, Kobe, 657-8501, Japan. leo@people.kobe-u.ac.jp.|Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO), Saitama, 332-0012, Japan. leo@people.kobe-u.ac.jp.
authors.forename: Ryo
authors.initials: R
authors.lastname: Fujimoto
mesh_terms: D001937:Brassica / Q000235:genetics*; D018556:Crops, Agricultural / Q000235:genetics*; D020869:Gene Expression Profiling; D017343:Genes, Plant; D006823:Hybrid Vigor; D010937:Plant Growth Regulators / Q000235:genetics; D018749:RNA, Plant
chemical_list: D010937:Plant Growth Regulators; D018749:RNA, Plant