Chitosan (b 1–4 linked N-glucosamin

Chitosan (b 1–4 linked N-glucosamine) has been shown to induce
defense responses [21,22,31–33] and resistance against
pathogens [15,31,34,35] in different plant species. In this study,
chitosan compounds of different molecular weights and degrees
of acetylation caused a differential response in the protein accumulation
and enzymatic activity of particular defenses in tobacco
leaves of 25-days-old plants; this response was dependent on the
elicitor concentration and the method of application. Regarding
protein accumulation, the time delay between treatment and protein
determination was critical for both the levels of protein accumulation
as well as increases in protein concentration in the
chitosan treatments relative to the controls (Fig. 1).
The highest protein levels were detected when protein levels
were determined 72 h after treatment, regardless of whether the
treatment was by foliar spray or as drench to the rhizosphere. This
behavior could be related to the transient character of the defense
responses mobilized after plant treatment with elicitors because
plants, depending on elicitor concentrations, can usually only activate
their defense signaling responses for a limited period of time
due to the energy cost needed to keep up its basal resistance [36–
38]. Thus, protein levels induced by chitosan treatments at 72 h,
representing proteins and enzymes related to the process of induced
resistance, might be reduced to basal levels as time passed.
This could be the reason why the protein levels in the leaves of 25-
day-old tobacco plants treated with chitosan derivatives at seeding
are lower than in the same plants treated 72 h before protein
determination.
All three enzymes evaluated in this work were activated differentially
in tobacco leaves when treated with the chitosan derivatives
tested. b-1,3-glucanase is essential for tobacco protection
against P. nicotianae [39] because this enzyme degrades b-1–3 glucan
polymers, an important structure of the oomycete cell wall.