Plant hormones play a crucial role in integrating endogenous and exogenous signals and in determining developmental responses to form the plant body throughout its life cycle. In citrus species, several economically important processes are controlled by phytohormones, including seed germination, secondary growth, fruit abscission and ripening. Integrative genomics is a powerful tool for linking newly researched organisms, such as tropical woody species, to functional studies already carried out on established model organisms. Based on gene orthology analyses and expression patterns, we searched the Citrus Genome Sequencing Consortium (CitEST) database for Expressed Sequence Tags (EST) consensus sequences sharing similarity to known components of hormone metabolism and signaling pathways in model species. More than 600 homologs of functionally characterized hormone metabolism and signal transduction members from model species were identified in citrus, allowing us to propose a framework for phytohormone signaling mechanisms in citrus. A number of components from hormone-related metabolic pathways were absent in citrus, suggesting the presence of distinct metabolic pathways. Our results demonstrated the power of comparative genomics between model systems and economically important crop species to elucidate several aspects of plant physiology and metabolism.

Water deficit is one of the most critical environmental stresses to which plants are submitted during their life cycle. The evolutionary and economic performance of the plant is affected directly by reducing its survival in the natural environment and its productivity in agriculture. Plants respond to water stress with biochemical and physiological modifications that may be involved in tolerance or adaptation mechanisms. A great number of genes have been identified as transcriptionally regulated for water deficit. EST sequencing projects provide a significant contribution to the discovery of expressed genes. The identification and determination of gene expression patterns is important not only to understand the molecular bases of plant responses but also to improve water stress tolerance. In our citrus transcriptome survey we have attempted to identify homologs to genes known to be induced and regulated under water stress conditions. We have identified 89 transcripts whose deduced amino acid sequences share similarities with proteins involved in uptake and transport of water and ion, 34 similar to components of the osmolyte metabolism, 67 involved in processes of membranes and proteins protection and 115 homologs of reactive oxygen species scavenger. Many drought-inducible genes identified are known to be regulated by development, salt, osmotic and low temperature. Their possible roles in specific or general mechanisms of water stress citrus responses are discussed.