- A tobacco leaf extract containing at least 5 % by weight, relative to the total weight of dry extract, of proteins of a molecular weight higher than 10 kDa and substantially free of molecules of a molecular weight lower than 10 kDa, said proteins preferably being selected from the group formed by the following protein families: lignin-forming anionic peroxidase, glucan endo-1,3-beta-glucosidase, endochitinase, pathogenesis-related protein, osmotin and proteinase inhibitor, and mixtures thereof; said tobacco leaf extract being characterized in that: - the content of proteins which molecular weight is higher than 500 kDa, preferably which molecular weight is higher than 400 kDa, more preferably which molecular weight is higher than 300 kDa, even more preferably which molecular weight is higher than 200 kDa, further preferably which molecular weight is higher than 150 kDa, still further preferably which molecular weight is higher than 100 kDa, and better still which molecular weight is higher than 50 kDa, is less than 15 % by weight relative to the total protein weight of the extract, preferably less than 10 % by weight relative to the total protein weight of the extract, more preferably less than 7.5 % by weight relative to the total protein weight of the extract, further preferably less than 5 % by weight relative to the total protein weight of the extract, still further preferably less than 2.5 % by weight relative to the total protein weight of the extract, still further preferably less than 1 % by weight relative to the total protein weight of the extract, and better still less than 0.5 % by weight relative to the total protein weight of the extract; and - the content of molecules of a molecular weight lower than 10 kDa is less than 5 % by weight relative to the total weight of the extract, preferably less than 2.5 % by weight and better still less than 1 % by weight. - the content of proteins which molecular weight is higher than 500 kDa, preferably which molecular weight is higher than 400 kDa, more preferably which molecular weight is higher than 300 kDa, even more preferably which molecular weight is higher than 200 kDa, further preferably which molecular weight is higher than 150 kDa, still further preferably which molecular weight is higher than 100 kDa, and better still which molecular weight is higher than 50 kDa, is less than 15 % by weight relative to the total protein weight of the extract, preferably less than 10 % by weight relative to the total protein weight of the extract, more preferably less than 7.5 % by weight relative to the total protein weight of the extract, further preferably less than 5 % by weight relative to the total protein weight of the extract, still further preferably less than 2.5 % by weight relative to the total protein weight of the extract, still further preferably less than 1 % by weight relative to the total protein weight of the extract, and better still less than 0.5 % by weight relative to the total protein weight of the extract; and - the content of molecules of a molecular weight lower than 10 kDa is less than 5 % by weight relative to the total weight of the extract, preferably less than 2.5 % by weight and better still less than 1 % by weight.