title: Influence of crop load on the expression patterns of starch metabolism genes in alternate-bearing citrus trees.
abstract: The fruit is the main sink organ in Citrus and captures almost all available photoassimilates during its development. Consequently, carbohydrate partitioning and starch content depend on the crop load of Citrus trees. Nevertheless, little is known about the mechanisms controlling the starch metabolism at the tree level in relation to presence of fruit. The aim of this study was to find the relation between the seasonal variation of expression and activity of the genes involved in carbon metabolism and the partition and allocation of carbohydrates in 'Salustiana' sweet orange trees with different crop loads. Metabolisable carbohydrates, and the expression and activity of the enzymes involved in sucrose and starch metabolism, including sucrose transport, were determined during the year in the roots and leaves of 40-year-old trees bearing heavy crop loads ('on' trees) and trees with almost no fruits ('off' trees). Fruit altered photoassimilate partitioning in trees. Sucrose content tended to be constant in roots and leaves, and surplus fixed carbon is channeled to starch production. Differences between 'on' and 'off' trees in starch content can be explained by differences in ADP-glucose pyrophosphorylase (AGPP) expression/activity and α-amylase activity which varies depending on crop load. The observed relation of AGPP and UGPP (UDP-glucose pyrophosphorylase) is noteworthy and indicates a direct link between sucrose and starch synthesis. Furthermore, different roles for sucrose transporter SUT1 and SUT2 have been proposed. Variation in soluble sugars content cannot explain the differences in gene expression between the 'on' and 'off' trees. A still unknown signal from fruit should be responsible for this control.
authors.affiliation: Departamento de Producción Vegetal, Universitat Politécnica de València, Edificio 3K-2 planta, Camino de vera s.n., 46022 Valencia, Spain. Electronic address: sergonne@bvg.upv.es.
authors.forename: Sergio G
authors.initials: SG
authors.lastname: Nebauer
authors.affiliation: Departamento de Producción Vegetal, Universitat Politécnica de València, Edificio 3K-2 planta, Camino de vera s.n., 46022 Valencia, Spain.
authors.forename: Begoña
authors.initials: B
authors.lastname: Renau-Morata
authors.affiliation: Departamento de Producción Vegetal, Universitat Politécnica de València, Edificio 3K-2 planta, Camino de vera s.n., 46022 Valencia, Spain.
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Lluch
authors.affiliation: Instituto de Agrobiotecnología, Universidad Pública/CSIC/Gobierno de Navarra, C. Mutilva Baja s.n., 31192 Mutilva Baja, Spain.
authors.forename: Edurne
authors.initials: E
authors.lastname: Baroja-Fernández
authors.affiliation: Instituto de Agrobiotecnología, Universidad Pública/CSIC/Gobierno de Navarra, C. Mutilva Baja s.n., 31192 Mutilva Baja, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Pozueta-Romero
authors.affiliation: Departamento de Producción Vegetal, Universitat Politécnica de València, Edificio 3K-2 planta, Camino de vera s.n., 46022 Valencia, Spain.
authors.forename: Rosa-Victoria
authors.initials: RV
authors.lastname: Molina
mesh_terms: D050260:Carbohydrate Metabolism / Q000235:genetics / Q000502:physiology; D002957:Citrus / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D005638:Fruit / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D051599:Glucose-1-Phosphate Adenylyltransferase / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D012621:Seasons; D013395:Sucrose / Q000378:metabolism*; D000516:alpha-Amylases / Q000235:genetics / Q000378:metabolism
chemical_list: D010940:Plant Proteins; D013395:Sucrose; D051599:Glucose-1-Phosphate Adenylyltransferase; D000516:alpha-Amylases
keywords: Alternate bearer; Citrus; Enzyme activities; Fruit load; Gene expression; Partition; Sinks