Sumary Citriculture is one of the branches of fruit growing with greater production, being Spain the fourth producer and first world-wide exporting country of citrus for fresh consumption (FAO, 2005). To continue being competitive, obtaining new varieties adapted to market exigencies is mandatory. The reduction of precipitations in many agricultural zones makes drought one of the abiotic stresses that most limit crop production. Many molecular studies have been performed in model species to decipher the molecular response of plants to water stress. However, virtually nothing is known about this response in citrus plants. The objective of this work is the development of genomic tools for citrus research, and use them to gain insight into the response of these plants to water stress, opening the way to the generation of new citrus varieties more adapted to low precipitations conditions. A global transcriptomic analysis from citrus plants under water stress have been performed, using two approaches: ESTs generation and cDNA microarray hybridization. The work have allowed the identification of a plethora of candidates genes putatively involved in the response of citrus to this kind of stress. EST generation has been performed by partial sequencing of 2296 clones isolated from two cDNA substraction libraries, generated from roots and leaves of water stressed plants. ESTs have been added to the Citrus Functional Genomics Project EST repository. The comparison of our ESTs with the rest of the repository, that included ESTs generated from many developmental stages and stress conditions, reveals that root response to water stress appears to be more specific that that of leaves. It is relevant to note also that, as in other plant species, a common response to cold, salinity and drought stress exists also in citrus. Microarray experiments have been performed using the first generation cDNA microarray generated by the Citrus Functional Genomics Project. Transcriptomic profiling of root and leaves from water stress plants suggests an interesting component of tissue specificity in the response to the stress. Moreover, clustering of data from 5, 10 and 24 hours after stress initial reveals two distinct behaviors. The first one, named “early response cluster”, included genes that are already induced or repressed 5h after application of the stress and that maintain this level of expression along the 24h treatment. The second one, named “early transient response cluster” included genes that are already induced or repressed 5h after application of the stress, but this level of expression decays to basal levels along the 24h treatment. Functional classification of candidate genes obtained using both strategies has allowed a comprehensive overview of the transcriptional regulation of water stress response in citrus. It is remarkable the existence of a group of genes with unknown function, that could be part of stress response pathways without determining. Finally, three gene families (proteinases inhibitors, LEA proteins and osmolite sinthesis enzymes), have been selected for further studies, and the first steps towards the understanding of the role of these genes in the response of plants to water stress have been iniciated. XIII