title: Identification of a GCC transcription factor responding to fruit colour change events in citrus through the transcriptomic analyses of two mutants.
abstract: BACKGROUND
  External ripening in Citrus fruits is morphologically characterized by a colour shift from green to orange due to the degradation of chlorophylls and the accumulation of carotenoid pigments. Although numerous genes coding for enzymes involved in such biochemical pathways have been identified, the molecular control of this process has been scarcely studied. In this work we used the Citrus clementina mutants 39B3 and 39E7, showing delayed colour break, to isolate genes potentially related to the regulation of peel ripening and its physiological or biochemical effects.
  
  
  RESULTS
  Pigment analyses revealed different profiles of carotenoid and chlorophyll modification in 39B3 and 39E7 mutants. Flavedo from 39B3 fruits showed an overall delay in carotenoid accumulation and chlorophyll degradation, while the flavedo of 39E7 was devoid of the apocarotenoid β-citraurin among other carotenoid alterations. A Citrus microarray containing about 20,000 cDNA fragments was used to identify genes that were differentially expressed during colour change in the flavedo of 39B3 and 39E7 mutants respect to the parental variety. The results highlighted 73 and 90 genes that were respectively up- and down-regulated in both mutants. CcGCC1 gene, coding for a GCC type transcriptional factor, was found to be down-regulated. CcGCC1 expression was strongly induced at the onset of colour change in the flavedo of parental clementine fruit. Moreover, treatment of fruits with gibberellins, a retardant of external ripening, delayed both colour break and CcGCC1 overexpression.
  
  
  CONCLUSIONS
  In this work, the citrus fruit ripening mutants 39B3 and 39E7 have been characterized at the phenotypic, biochemical and transcriptomic level. A defective synthesis of the apocarotenoid β-citraurin has been proposed to cause the yellowish colour of fully ripe 39E7 flavedo. The analyses of the mutant transcriptomes revealed that colour change during peel ripening was strongly associated with a major mobilization of mineral elements and with other previously known metabolic and photosynthetic changes. The expression of CcGCC1 was associated with peel ripening since CcGCC1 down-regulation correlated with a delay in colour break induced by genetic, developmental and hormonal causes.
authors.affiliation: Centro de Genómica, Instituto Valenciano de Investigaciones Agrarias, Carretera Moncada-Náquera km 4.5, 46113 Moncada, Valencia, Spain.
authors.forename: Gabino
authors.initials: G
authors.lastname: Ríos
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Naranjo
authors.forename: María-Jesús
authors.initials: MJ
authors.lastname: Rodrigo
authors.forename: Enriqueta
authors.initials: E
authors.lastname: Alós
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Zacarías
authors.forename: Manuel
authors.initials: M
authors.lastname: Cercós
authors.forename: Manuel
authors.initials: M
authors.lastname: Talón
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002338:Carotenoids / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D002957:Citrus / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D005638:Fruit / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D020869:Gene Expression Profiling; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D005875:Gibberellins / Q000494:pharmacology; D008969:Molecular Sequence Data; D009154:Mutation; D020411:Oligonucleotide Array Sequence Analysis; D010860:Pigments, Biological / Q000378:metabolism*; D010937:Plant Growth Regulators / Q000494:pharmacology; D010940:Plant Proteins / Q000145:classification / Q000235:genetics* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014157:Transcription Factors / Q000145:classification / Q000235:genetics* / Q000378:metabolism
chemical_list: D005875:Gibberellins; D010860:Pigments, Biological; D010937:Plant Growth Regulators; D010940:Plant Proteins; D014157:Transcription Factors; D002734:Chlorophyll; D002338:Carotenoids