Polygalacturonase isozymes in Lygus hesperus salivary glands
1Department of Plant Sciences, University of California, Davis, CA, USA and
2USDA ARS Biological Control of Pests Research Unit, Stoneville, MS, USA
Correspondence: mpcelorio@ucdavis.edu
The feeding strategy of mirids has been referred to as “lacerate or macerate and flush feeding” which supports high rates of food intake. In other words, plant bugs digest the plant tissue extra-orally, producing a liquefied brew rich in simple nutrient molecules. The insect's salivary polygalacturonase (PG), amylase and protease presumably facilitate this feeding strategy. Some authors have suggested that hemipteran phytopathogenicity might depend principally on the proteins secreted from the salivary glands. Fungal PG has been considered an important virulence factor in the development of plant diseases, degrading the pectin component of the cell wall and facilitating pathogen ingress and colonization of host tissues. A plant's pathogen defenses include a role for the interaction of the plant's PG-inhibiting proteins (PGIPs) with pathogen PGs. Our previous research has provided evidence for cotton and alfalfa proteins that inhibit Lygus hesperus PG. Therefore, our immediate goal is the purification of the interacting proteins. Protein separation techniques yielded the partial purification of a number of Lygus PG isozymes based on their biochemical characteristics such as glycosylation and charge. The PG isozymes were isolated from salivary glands of wild insects collected at alfalfa field sites. Products of the enzymatic reaction catalyzed by the different partially purified isozymes were analyzed by high performance liquid chromatography (HPLC). Although further biochemical characterization of the isolated PGs is needed, the HPLC data suggest the presence of at least one endoacting PG and one exo-PG. Partial nucleotide sequences with high homology to L. lineolaris PGs have been obtained from L. hesperus salivary gland genomic DNA and cDNA, providing evidence for the presence of more than one pg gene, consistent with recent reports describing a PG gene family in L. lineolaris.