Functional Genomics of the Grape - Xylella Interaction : toward the Identification

Susceptible Vitis vinifera responds to Xylella infection with a massive re-direction of gene transcription involving >800 genes with strong statistical support. This number is increased from previous estimates based on use of a more sensitive and robust statistical method known as linear models for microarray data (LIMMA). The transcriptional response to Xylella infection is characterized by increased transcripts for phenlypropanoid and flavonoid biosynthesis, ethylene production, adaptation to oxidative stress, and homologs of pathogenesis related (PR) proteins, and decreased transcripts for genes related to photosynthesis. A survey of 22 transcripts by means of in situ hybridization reveals that a majority of transcriptional activity is associated with phloem and cortical tissues, consistent with the presence of the pathogen in adjacent xylem elements. DNA sequence analysis of regions 5' to the transcription site for ~200 differentially expressed genes provides a rich source of new gene promoters and the possibility of in silico analysis of regulatory cis-elements. In addition to highlighting potential metabolic and biochemical changes that are correlated with disease, the results suggest that susceptible genotypes respond to Xylella infection by induction of a limited, but apparently inadequate, defense response. We have also tested the hypothesis that Pierce's disease results from pathogen-induced drought stress. We compared the transcriptional and physiological response of plants treated by pathogen infection, low or moderate water deficit, or a combination of pathogen infection and water deficit. Although the transcriptional response of plants to Xylella infection was distinct from the response of healthy plants to moderate water stress, we observed synergy between water stress and disease. In particular, water stressed plants exhibit a stronger transcriptional response to the pathogen. This interaction was mirrored at the physiological level for aspects of water relations and photosynthesis, and in terms of the severity of disease symptoms and pathogen colonization, providing a molecular correlate of the classical concept of the disease triangle. INTRODUCTION All organisms adapt to external stressors by activating the expression of genes that confer adaptation to the particular stress. In the case of Pierce’s disease, such genes are likely to include those coding for resistance or susceptibility to Xylella fastidiosa (Xf). Genomics technology offers an opportunity to monitor gene expression changes on a massive scale (so-called "transcriptional profiling"), with the parallel analysis of thousands of host genes conducted in a single experiment. In the case of Pierce's disease of grapes, the resulting data can reveal aspects of the host response that are inaccessible by other experimental strategies. In May of 2004, the first Affymetrix gene chip was made available for public use, with ~15,700 Vitis genes represented. This gene chip has been developed based primarily on collaboration between the Cook laboratory and researchers at the University of Nevada-Reno (Goes da Silva et al., 2005). With the arrival of the Affymetrix gene chip, we are poised to make a quantum leap in the identification of host gene expression in response to Xf. In addition to enumerating differences between susceptible and resistant genotypes of Vitis, this research is testing a longstanding but largely untested hypothesis that pathogen-induced drought stress is one of the fundamental triggers of PD symptom development. The utility of this type of data will be to inform the PD research community about the genes and corresponding protein products that are produced in susceptible, tolerant and resistant interactions. Differences in the transcriptional profiles between these situations are expected to include host resistance and susceptibility genes, and thus provide the basis for new lines of experimental inquiry focused on testing the efficacy of specific host genes for PD resistance. It should be possible, for example, to determine the extent to which resistance responses in grapes are related to well-characterized defense responses in other plant species (e.g., Maleck et al 2002; Tao et al 2003; de Torres et al 2003).