Resistance to disease in the hybrid Nicotiana glutinosa x Nicotiana debneyi is associated with high constitutive levels of β-1,3-glucanase, chitinase, peroxidase and polyphenoloxidase

Abstract

The tobacco hybrid Nicotiana glutinosa × Nicotiana debneyi is much more resistant than either parental species to tobacco mosaic virus, tobacco necrosis virus, Pseudomonas syringae pv. syringae, P. syringae pv. tabaci and Peronospora tabacina infections. While N. glutinosa is very susceptible to Cercospora nicotianae, Chalara elegans and Phytophthora parasitica var. nicotianae, the hybrid is as resistant as N. debneyi to these fungi. The resistance of the hybrid is linked to high levels of chitinase, β-1,3-glucanase, peroxidase (PO) and polyphenoloxidase (PPO). The parental species contain much lower levels of these enzymes unless they are locally infected with necrotizing viruses. In those cases, chitinase, β-1,3-glucanase and PO, but not PPO, increase in the whole plant to levels comparable with those found in the hybrid, and resistance to secondary infection develops (systemic acquired resistance). All the acidic isoenzymes of PO present in both parental species after infection are constitutively expressed in the hybrid. The activity of phenylalanine ammonia-lyase was similar in all plants. These results reinforce the hypothesis that hydrolases and PO are involved in resistance against various pathogens.