Differentially expressed stress-related genes in the compatible citrus-Citrus leprosis virus interaction

Leprosis, caused by Citrus leprosis virus, cytoplasmic type (CiLV-C), is the main viral disease in the Brazilian citrus industry. This occurs because of the widespread source of inoculum and the year-round presence of the vector, the tenuipalpid mite Brevipalpus phoenicis, in citrus plants. In addition, while some Citrus species are resistant to CiLV-C, C. sinensis, the main cultivated species in the country, is extremely susceptible to the disease. The main objective of this work was to identify genes in C. sinensis cv. Pêra plants that were differentially expressed after the host was challenged with CiLV-C. In order to accomplish that, cDNA libraries were constructed from healthy and CiLV-inoculated sweet orange leaves. Two hundred and fifty-four genes were found to differ significantly in terms of expression, with 193 of them induced and 61 repressed after inoculation. Here we discuss the possible roles of a sub-set of these genes involved in metabolism, energy, signaling and cell rescue, defense and virulence, and indicate which kind of response may take place in the initial steps of the disease. Although the symptoms induced by CiLV-C in its compatible interaction with sweet orange resemble those of hypersensitive response (HR) in incompatible interactions, our data indicate that, apparently, the manifestation of leprosis symptoms should not be considered HR

CitEST libraries

In order to obtain a better understanding of what is citrus, 33 cDNA libraries were constructed from different citrus species and genera. Total RNA was extracted from fruits, leaves, flowers, bark, seeds and roots, and subjected or not to different biotic and abiotic stresses (pathogens and drought) and at several developmental stages. To identify putative promoter sequences, as well as molecular markers that could be useful for breeding programs, one shotgun library was prepared from sweet orange (Citrus sinensis var. Olimpia). In addition, EST libraries were also constructed for a citrus pathogen, the oomycete Phythophthora parasitica in either virulent or avirulent form. A total of 286,559 cDNA clones from citrus were sequenced from their 5’ end, generating 242,790 valid reads of citrus. A total of 9,504 sequences were produced in the shotgun library and the valid reads were assembled using CAP3. In this procedure, we obtained 1,131 contigs and 4,083 singletons. A total of 19,200 cDNA clones from P. parasitica were sequenced, resulting in 16,400 valid reads. The number of ESTs generated in this project is, to our knowledge, the largest citrus sequence database in the world

Analysis of expressed sequence tags from Citrus sinensis L. Osbeck infected with Xylella fastidiosa

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