THE CONTRIBUTION OF THE PECTIN-DEGRADING ENZYME POLYGALACTURONASE (PG) IN TRANSMISSION OF XYLELLA FASTIDIOSA TO GRAPE AND USE OF PG-INHIBITOR PROTEINS FOR TRANSGENIC RESISTANCE TO PIERCES DISEASE

• Author(s): Backus, Elaine; Labavitch, John; Morgan, David; Aguero, Cecilia; Dandekar, Abhaya; Greve, Carl; Kirkpatrick, Bruce; Perez, Alonso; Roper, Caroline; Shackel, Ken;
• Abstract: Pierces disease (PD) develops because (1) inoculative glassy-winged sharpshooters (GWSS) feeding on grapevines transfer Xylella fastidiosa (Xf) bacteria into the vine, and (2) the Xf population in the vines water-conducting cells increases and spreads throughout the vine, triggering a set of responses that result in vine collapse and death. Our work on PD development thus far has focused on the spread of the bacteria once they have been introduced into the vine. The cell wall polysaccharide fabric of the pit membranes that separate xylem vessels from one another has interpolymer gaps (referred to as cell wall porosity) that are substantially smaller than Xf cells. Thus, the systemic spread of Xf is likely to be facilitated by the action of enzymes that digest some of the pit membranes constituent polysaccharides. Plant cell wall digestion is a common aspect of the biochemistry of most plant interactions with fungal and bacterial pathogens (Powell et al., 2000). In the report describing our continuing work to test a hypothetical model of PD development (Labavitch et al., 2001 & 2002,; Labavitch and Matthews, 2003 and Labavitch et al. in these Proceedings), we have described studies to determine whether Xf genes presumed to encode cell wall-degrading enzymes actually do encode the polygalacturonase (PG) and -1,4-glucanase that their sequences predict. Apparently they do. The work of Dr. Cecilia Aguero (Meredith and Dandekar, 2002 & 2003; Aguero et al., 2004) shows that transgenic grapevines that express the pear fruit gene that encodes a PG-Inhibiting Protein (PGIP) show slower and reduced symptom development, following needle inoculation, than do untransformed grapes. We presume that this is a consequence of the PGIPs inhibition of an Xf PG that is crucial for bacterial spread through the vine As a follow up to work we are doing on plant-insect interactions, we have identified glucanase and PG activity in protein extracts of homogenized GWSS heads. We presume that the enzymes were located in the insects salivary apparatus and represent some of the proteins in GWSS salivary secretions. If GWSS penetrates grapevine tissues and inserts its stylets in the water-conducting cells of the vine using only mechanical force, why should the saliva of the insect contain PG and other cell wall-degrading enzymes? Dr. Elaine Backus, co-PI on this proposal suggests that the salivary enzymes are important contributors to the insects feeding success, both in penetration and in correct stylet placement. If this is correct, and if the pear PGIP that has been introduced into transgenic grapevines inhibits the GWSS PG, then the transgenics should also be less susceptible to Xf transfer from the insect than untransformed vines. The Objectives of our work in this proposal are to obtain PG enzyme from both GWSS and Xf, and determine the extent to which PGIP inhibits the PGs from the bacteria and insect. Several PGIPs with differing abilities to inhibit PGs from various fungal plant pathogen sources are known (Stotz et al., 2000). If we find that pear PG inhibits either the Xf or GWSS PG, or both, continuing research will screen PGIPs from other sources with the intent of identifying an inhibitor with maximal ability to slow infection and disease development in grapevines.
• Publication Date: Dec 2004
• Journal: 2004 Pierce's Disease Research Symposium