In order to understand the genetic responses resulting from physiological changes that occur in plants displaying citrus variegated chlorosis (CVC) symptoms, we adopted a strategy of comparing two EST libraries from sweet orange [Citrus sinensis (L.) Osbeck]. One of them was prepared with plants showing typical CVC symptoms caused by Xylella fastidiosa and the other with non-inoculated plants. We obtained 15,944 ESTs by sequencing the two cDNA libraries. Using an in silico hybridization strategy, 37 genes were found to have significant variation at the transcriptional level. Within this subset, 21 were up-regulated and 16 were down-regulated in plants with CVC. The main functional categories of the down-regulated transcripts in plants with CVC were associated with metabolism, protein modification, energy and transport facilitation. The majority of the up-regulated transcripts were associated with metabolism and defense response. Some transcripts associated with adaptation to stress conditions were up-regulated in plants with CVC and could explain why plants remain alive even under severe water and nutritional stress. Others of the up-regulated transcripts are related to defense response suggesting that sweet orange plants activate their defense machinery. The genes associated with stress response might be expressed as part of a secondary response related to physiological alterations caused by the infection.

Citrus species are known by their high content of phenolic compounds, including a wide range of flavonoids. In plants, these compounds are involved in protection against biotic and abiotic stresses, cell structure, UV protection, attraction of pollinators and seed dispersal. In humans, flavonoid consumption has been related to increasing overall health and fighting some important diseases. The goals of this study were to identify expressed sequence tags (EST) in Citrus sinensis (L.) Osbeck corresponding to genes involved in general phenylpropanoid biosynthesis and the key genes involved in the main flavonoids pathways (flavanones, flavones, flavonols, leucoanthocyanidins, anthocyanins and isoflavonoids). A thorough analysis of all related putative genes from the Citrus EST (CitEST) database revealed several interesting aspects associated to these pathways and brought novel information with promising usefulness for both basic and biotechnological applications.