PHYSIOLOGICAL COMPENSATION IN ANTISENSE TRANSFORMANTS - SPECIFIC INDUCTION OF AN ERSATZ GLUCAN ENDO-1,3-BETA-GLUCOSIDASE IN PLANTS INFECTEDWITH NECROTIZING VIRUSES

Plant class I glucan endo-1,3-beta-glucosidases (beta-1,3-glucanase; 1 ,3-beta-D-glucan glucanohydrolase, EC 3.2.1.39) have been implicated i n development and defense against pathogen attack. Nevertheless, beta- 1,3-glucanase deficiencies generated by antisense transformation of Ni cotiana sylvestris and tobacco have little biological effect. We repor t here that another beta-1,3-glucanase activity is induced in these de ficient mutants after infection with necrotizing viruses. Induction of class I beta-1,3-glucanase was markedly inhibited in leaves of N. syl vestris and tobacco antisense transformants infected with tobacco necr osis virus and tobacco mosaic virus, respectively. A serologically dis tinct beta-1,3-glucanase activity was present in the infected antisens e transformants but was absent in both healthy and infected control pl ants and in antisense transformants treated with the stress hormone et hylene. Immunoblot analyses, localization studies, and measurements of antibody specificity indicate that this compensatory beta-1,3-glucana se activity is an intracellular enzyme different from known tobacco be ta-1,3-glucanases. Therefore, plants can compensate for a deficiency i n enzyme activity by producing a functionally equivalent replacement-i .e., ''ersatz''-protein or proteins. The fact that compensation for be ta-1,3-glucanase activity occurs in response to infection argues stron gly for an important role of these enzymes in pathogenesis.