The Interplay Between Salicylic and Jasmonic Acid During Phytopathogenesis - Salicylic Acid: Plant Growth and Development - page 280

The smallest necrotic lesions were observed after treatment with the mixture of
these acids. Thus, a mixture of SA and JA formed out to the most efficient inducer
of resistance in potato plants to late blight. It is of importance that treatment with
SA and JA did not exert any visual effect on the morphology of healthy, non-
infected potato leaves. As potato plants are characterized by the constitutively high
content of SA (Panina et al. 2005 ). Both SA and JA are involved in the regulation
of potato resistance to late blight (Vasyukova et al. 2008 ) by increasing the level of
H 2 O 2 in infected tissues (Hung et al. 2006 ; Liu et al. 2008 ).
24 h after inoculation, peroxidase was activated locally at the site of contact
between plant leaf epidermal cells and pathogen mycelium (Fig. 2 a-d). It was
stronger after plant pretreatment with SA and/or JA. In the last, color reaction was
most intense in the zone of stomata in infected plants (Fig. 2 d, e). The accumu-
lation of phenolic compounds in cell walls of leaf epidermal cells in the zone of
infection is known to be an important component of plant defense responses
against pathogens. In our experiments, plant treatment with SA and JA induced
this process (Fig. 2 g-j). Phenolic compounds accumulated mostly after treatment
with SA, which is evidently an important inducer of these compounds in potato
plants. Similar changes have also been observed earlier after treatment with SA
analog, benzothiodiazole (Hukkanen et al. 2007 ). In our experiments, accumula-
tion of phenolic compounds in treated plants (Fig. 2 f-j) was correlated with local
peroxidase activation in the zone of infection (Fig. 2 c, d).
Enzyme activity was detected not only in the space between stomatal guard
cells but also on the pathogen cell walls (Fig. 2 f). DAB did not stain P. infestans
mycelium, grown on nutrient medium. Therefore, it may be suggested that local
reaction in infected tissues is determined by the host extracellular peroxidases and
interacting with the surface of P. infestans mycelium.
5 Affinity of Potato Isoperoxidases to P. infestans Cell
Walls and Chitin
Some potato peroxidases are known to manifest ion-exchange affinity for chitin,
the component of fungal cell walls. However, the cell walls of the late blight causal
agent contains predominantly b-1,3-glucans (Hardham and Shan 2009 ). In our
experiments, it was found that potato isoperoxidase with pI *9.3 interacted
mainly with P. infestans cell walls. As evident from Fig. 3 (lane g), this isoper-
oxidase was activated by JA treatment and infection.
In addition, such an increase in enzymatic activity could result from changes in
the conformation of the enzymatic molecules due to the high electrostatic activity
of polysaccharides (Dunand et al. 2002 ). It can be proposed that PO sorption on
chitin could not be considered to be a classic ion exchange process because both the
anionic and cationic isoforms of the plant POs interact with chitin (Maksimov et al.
2005 ). Additionally, it contains 3 high anionic POs (3.5, 3.7, 4.0) but only 2 of them
(3.5 and 3.7) are adsorbed on chitin (Fig. 3 a).