MAGNETIC RESONANCE IMAGING: A NON-DESTRUCTIVE APPROACH FOR DETECTION OF XYLEM BLOCKAGES IN X. FASTIDIOSA-INFECTED GRAPEVINES

• Author(s): Labavitch, John; Shackel, Ken; Greve, Carl; Matthews, Mark; Perez, Alonso; Walton, Jeffrey;
• Abstract: Results from PD research programs led by Matthews, Rost and Labavitch (reported in 2001 and 2002 in San Diego) have provided substantial support for the idea that obstructions in the vine's water-transporting xylem tissue develop rapidly post-inoculation, before an appreciable bacterial population has been established. The results also strongly suggest that these obstructions, and likely other aspects of the Pierces disease (PD) "syndrome", result from the grapevine's active responses to the presence of X. fastidiosa (Xf), rather than to direct "action" by the bacterium. Thus, careful analysis of the timing of changes in xylem element anatomy and function relative to Xf introduction, as well as to external symptoms of disease development, is important for establishing reliable indicators of the "stage" of PD development. The analyses done thus far have been based on destructive tissue sampling. While the Matthews and Labavitch project continues to test a model of PD development, sampling of infected vines is essentially blind and must, therefore, be destructive. This is particularly so since it has become clear that important internal responses to the bacterium are correlated with the presence of rather few bacteria in the tissues, at a time, post-inoculation, that is well before external PD symptoms are in evidence. Perez and Walton have has carried out a number of pilot studies aimed at "observing" the development of xylem vessel obstructions in infected vines without damaging the plants. The value of the work is that periodic examinations can be done on each of several vines over a number of weeks (reported in 2002). The NMR images provided clear evidence of the progressive, localized deterioration of water movement capacity in treated vines. The data provide spatial information about where, around the circumference of a vine as well as along its length, obstructions are present. This information should provide specific guidance for eventual destructive sampling to assess the presence and nature of xylem obstructions. Optimization of the analytical approach (our Objective 1) in order to maximize the signal to noise ratio by reducing the noise component will add considerably to the value of the technique. This will lead to studies aimed at addressing whether the vessels cavitate (i.e., air embolisms develop) when the glassy-winged sharpshooter feeds on vines and non-destructive testing of the impacts of Xf cell wall-degrading enzymes, pectin-derived oligosaccharides, and ethylene on vine water transport, as discussed in the Matthews and Labavitch PD development model (Figure 2, below).
• Publication Date: Aug 2003
• Journal: 2003 Pierce's Disease Research Symposium