ABSTRACT

	Citrus are the most important fruit tree crop in the world and Spain is the fifth producer and the first world-wide exporter of citrus for fresh consumption. A main objective in citrus research is the improvement of citrus quality and production. Vegetative development is important in determining the shape and the general performance of trees, and has a decisive influence in the amount and quality of fruit production, as well as in culture practices. However, very little is known about the regulation of vegetative development in citrus. In this work, we have developed new genomic tools for large-scale studies of citrus vegetative growth, including the generation of an EST collection and the construction of cDNA microarrays. Genes involved in photosynthesis were identified by functional analysis of an EST collection generated from vegetative tissues. In addition, we have analyzed 85,965 ESTs from the Citrus Functional Genomic Project to characterize the isoprenoid and flavonoid metabolic pathways in citrus. Genes involved in gibberellin (GA), brassinosteroid and abscisic acid biosynthesis were identified for the first time in citrus. Identification of monoterpene and sesquiterpene synthases showed that citrus terpene synthase genes represents one of the largest and diverse terpene synthase family in plants. Expression maps of vegetative and reproductive organs were generated using cDNA microarrays. Major functions of each organ were characterized using these expression maps. The effect of GA on internode transcriptome was investigated in transgenic Carrizo citrange plants (a citrus hybrid widely used as a rootstock) overexpressing endogenous CcGA20ox1 (encoding a GA biosynthetic enzyme), using a citrus cDNA microarray. Substantial modulation of gene expression was found in these plants. Extensive upregulation of genes involved in photosynthesis and carbon utilization, and downregulation of those involved in protein synthesis and ribosome biogenesis was shown for the first time in plants with higher GA content, suggesting that these processes are regulated by GA. Importantly, increase of net photosynthesis in attached leaves was also demonstrated. In addition, the expression of other genes belonging to functional groups not reported previously to be regulated by GA (diverse abiotic and biotic stresses, and cuticle biosynthesis), as well as genes involved in cell wall architecture were also differentially expressed. Functional genomic tools developed in this work, with their potential to: i) isolate genes of agronomical relevance, ii) generate molecular markers and iii) characterize germplasm collections, will be of great help in the development of varieties that are well adapted to environmental conditions and produce fruits exhibiting those characteristics desired by most consumers.