Identification of differentially expressed root genes upon rhizomania disease.

Rhizomania is one of the most devastating sugar beet diseases. It is caused by Beet necrotic yellow vein virus (BNYVV), which induces abnormal rootlet proliferation. To understand better the physiological and molecular basis of the disorder, transcriptome analysis was performed by restriction fragment differential display polymerase chain reaction (RFDD-PCR), which provided differential gene expression profiles between non-infected and infected sugar beet roots. Two distinct viral isolates were used to detect specific or general virus-induced genes. Differentially expressed genes were selected and identified by sequence analysis, followed by reverse Northern and reverse transcriptase PCR experiments. These latter analyses of different plants (Beta vulgaris and Beta macrocarpa) infected under distinct standardized conditions revealed specific and variable expressions. Candidate genes were linked to cell development, metabolism, defence signalling and oxidative stress. In addition, the expression of already characterized genes linked to defence response (pathogenesis-related protein genes), auxin signalling and cell elongation was also studied to further examine some aspects of the disease. Differential expression was retrieved in both B. vulgaris and B. macrocarpa. However, some candidate genes were found to be deregulated in only one plant species, suggesting differential response to BNYVV or specific responses to the BNYVV vector.

Thermal and chlorophyll-fluorescence imaging distinguish plant-pathogen interactions at an early stage.

Different biotic stresses yield specific symptoms, owing to their distinct influence on a plant's physiological status. To monitor early changes in a plant's physiological status upon pathogen attack, chlorophyll fluorescence imaging (Chl-FI) and thermography, which respectively visualize photosynthetic efficiency and transpiration, were carried out in parallel for two fundamentally different plant-pathogen interactions. These non-destructive imaging techniques were able to visualize infections at an early stage, before damage appeared. Under growth-room conditions, a robotized set-up captured time series of visual, thermal and chlorophyll fluorescence images from infected regions on attached leaves. As a first symptom of the plant-virus interaction between resistant tobacco and tobacco mosaic virus (TMV), thermal imaging detected a local rise in temperature while Chl-FI monitored a co-localized increase in fluorescence intensity. Chl-FI also revealed pre-symptomatic high-intensity spots for the plant-fungus system sugar beet-Cercospora beticola. Concomitantly, spots of lower temperature were monitored with thermography, in marked contrast with our observations on TMV-infection in tobacco. Knowledge of disease signatures for different plant-pathogen interactions could allow early identification of emerging biotic stresses in crops, facilitating the containment of disease outbreaks. Presymptomatic monitoring clearly opens perspectives for quantitative screening for disease resistance, either on excised leaf pieces or attached leaves.