Chlorophyll loss in a greenbug-susceptible sorghum due to pectinases and pectin fragments

Leaf discoloration of greenbug-susceptible sorghum NC+ 159 due to plant cell wall-degrading enzymes and pectin fragments was measured using a hand-held chlorophyll meter (SPAD-502, Minolta Corporation, Ramsey, NJ). Polygalacturonase from biotype E greenbug, Schizaphis graminum (Rondani), caused local discoloration when the enzyme was micro-injected into the plant leaves. Pectinases from two plant pathogens, Aspergillus niger and Erwinia carotovora subsp. carotovora, caused a weaker leaf discoloration when compared to that of greenbug polygalacturonase. A significant reduction in plant chlorophyll content around the enzyme-treated areas was detected by the chlorophyll meter. Damage symptoms caused by the enzyme treatments were very similar to greenbug feeding damage. Surface treatment of sorghum leaves with pectic fragments caused damage symptoms similar to those induced by pectic enzymes. The results demonstrated the ability of pectinases and pectic fragments to elicit a response similar to that due to greenbug feeding damage in greenbug-susceptible sorghum plants, separate and apart from any feeding greenbugs. The potential roles of greenbug pectinases and pectic fragments in insect-plant interactions are discussed.