Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 5, 1997
Publication Date: N/A
Interpretive Summary: Insects us enzymes, polygalacturonases (PG's), to break down pectin in plants. Plant resistance to insect herbivores has been achieved by changing the structure of the pectins such that the insect PG's are ineffective. Although plants have polygalacturonase inhibitor proteins (PGIP's) that are effective against fungal PG's, little is known about their effects on insect PG's. The reason for this lack of knowledge is because almost no research has been conducted on the insect PG's. Improved germplasm with insect resistance could result from plants with elevated PGIP levels. We have purified a PG from a citrus root weevil (Diaprepes abbreviatus) that causes ca. $70 million in losses to Florida citrus growers. We have characterized this enzyme and shown that it is easily inhibited by PGIP isolated from grapefruit peels. This work indicates that there is a good possibility of elevating PGIP in citrus to increase resistance to this pest and other pests such as the brown citrus aphid that was recently introduced into the U.S.
Technical Abstract: An endo-polygalacturonase (PG) (EC:18.104.22.168) with a pI of 9.4 and a molecular weight (Mr) of 44,500 was purified to electrophoretic homogeneity from the gut of West Indies sugarcane rootstock borer weevil (Diaprepes abbreviatus) larvae. Hydrolytic activity was maximal in 150 mM sodium acetate buffer, pH 5.5 at 30øC. Kinetic determinations yielded an apparent Km of 3.68 (mg polygalacturonic acid [PGA]/ml) and a Vmax of 283 ( mol galacturonic acid [GA]/min/mg protein) for polygalacturonic acid. Enzymatic activity was inhibited by a polygalacturonase inhibitor protein from Hamlin' orange flavedo. The purified protein does not appear to be glycosylated, and its N-terminal sequence showed no homology to any protein sequences in data banks.