I. V. Maksimov

Plant growth promoting rhizobacteria as alternative to chemical crop protectors from pathogens (review)

The review analyses data on physiological and biochemical influence of rhizospheric and endophytic plant growth promoting rhizobacteria (PGPR) on induced the mechanisms of resistance of plants and the possibility of their using in agricultural for to protect crop from pathogens and phytophages. Resistance of plants promoted by PGPR due to their endosymbiotic interrelationships is directly achieved by producing peptide antibiotics and hydrolases of chitin and glucan and also because plants form their own system of induced resistance, accompanied by changes in the balance of defensive proteins, phytohormones, and pro-/antioxidant status.

Effects of salicylic and jasmonic acids on the components of pro/antioxidant system in potato plants infected with late blight

The effects of salicylic acid (SA) and jasmonic acid (JA) on plant defense responses were studied with aseptic potato (Solanum tuberosum L.) plantlets infected with Phytophthora infestans (Mont.) de Bary. Plant treatment with 10−6 M SA or 10−7 M JA induced plant resistance; the mixture of these acids was most efficient. After treatment with these compounds, phenolic compounds were accumulated and peroxidase was activated in the sites of pathogen localization, and this might be the reason of resistance enhancement. In addition, more H2O2 was accumulated in infected plants treated with JA or its mixture with SA but not in plants treated with SA alone. It might occur because of observed inhibition of catalase and activation of isoperoxidase with the isoelectric point (pI) of ∼9.3, which manifests an affinity for the pathogen cell wall. The data obtained allow us to recommend the application of these compounds for potato plant protection against late blight.

The Control of Wheat Defense Responses during Infection with Bipolaris sorokiniana by Chitooligosaccharides

We studied the effects of low-molecular-weight water-soluble derivatives of chitin, chitooligsaccharides (ChOS) with a mol wt of 5–10 kD and the degree of acetylation of 65% on expression and molecular heterogeneity of peroxidase and the content of phytohormones in wheat plants differing in their resistance to the causal agents of root rots. Plant infection or their treatment with ChOS induced expression of the gene for anionic peroxidase and enhanced enzyme activity, and these processes depended on the degree of wheat resistance. They were more intense in the resistant cultivar. Treatment of susceptible plants with ChOS prevented a pathogen-induced drop in the cytokinin level, thus simulating defensive responses, which are characteristic of the resistant plants.

Chitin-specific peroxidases in plants.

The activity of various plant peroxidases and the ability of their individual isoforms to bind chitin was studied. Some increase in peroxidase activity was observed in crude extracts in the presence of chitin. Activated peroxidases of some species fell in the fraction not sorbed on chitin and those of other species can bind chitin. Only anionic isoperoxidases from oat (Avena sativa), rice (Oryza sativa), horseradish (Armoracia rusticana), garden radish (Raphanus sativus var. radicula), peanut (Arachis hypogaea), and tobacco (Nicotiana tabacum Link et Otto) were sorbed on chitin. Both anionic and cationic isoforms from pea (Pisum sativum), galega (Galega orientalis), cucumber (Cucumis sativus), and zucchini (Cucurbita pepo L.) were sorbed on chitin. Peroxidase activation under the influence of chitin was correlated to the processes that occur during hypersensitive reaction and lignification of sites, in which pathogenic fungus penetrates into a plant. The role of chitin-specific isoperoxidases in inhibition of fungal growth and connection of this phenomenon with structural characteristics of isoperoxidases are also discussed.

Changes in the Phytohormone Levels in Wheat Calli as Affected by Salicylic Acid and Infection with Tilletia caries, a Bunt Pathogenic Agent

The role of salicylic acid (SA) in growth regulation and the change in the levels of phytohormones (IAA, ABA, and cytokinins) were studied in the wheat calli co-cultured with bunt pathogen Tilletia caries. Calli infection with T. caries resulted in the hypertrophied callus growth and simultaneous increase in phytohormone level. The addition of SA to the nutrient media decreased the callus growth induced by the pathogen, whereas the level of investigated phytohormones was not affected. In the SA-treated infected calli, the formation of necrotic lesions was observed in the zones of contact of the fungal mycelium with callus cells that limited pathogen growth. The authors suggest that the stabilization of the hormonal balance of plant cells at pathogenesis is one of the possible mechanisms of the SA protective action in vitro and in vivo. Hence, co-culturing wheat calli and T. caries fungus appeared to be a convenient model for assessing SA protective action.

Salicylic acid increases the defense reaction against bunt and smut pathogens in wheat calli

The role of salicylic acid (SA) in regulating wheat calli growth and peroxidase activity in the co-culture with bunt (Tilletia caries (D.C.) Tul.) and smut (Ustilago tritici Pers.) pathogens was studied in this work. We found that the influence of SA increased the number of globular high-density sites with meristema-like cells and reduced the number of low-structured sites with parenchyma-like cells in wheat calli. SA inhibited fungi spores germination, mycelium growth, and formation of its new spores during the co-cultivation of wheat calli with bunt or smut agent. Interaction of fungi mycelium with wheat calli treated by SA led to formation of zones with hypersensitive reaction in calli high-structured sites with meristema-like cells. The obtained data about the influence of SA on activation of wheat calli growth and the peroxidases with pI ~3.5 and ~9.8, which can bind to the mycelium of T. caries or U. tritici are discussed in this paper.

Abscisic acid in the plants-pathogen interaction

Information available concerning the role of ABA in the interaction between plants and pathogenic microorganisms allows a conclusion that this phytohormone is required for plant defense. For the development of plant resistance, short-term increases in the ABA level are of importance during the early stages of plant interaction with pathogens, which trigger anti-stress programs in plants, primarily related to the synthesis of callose. At the same time, high ABA concentrations maintained for a long time reduce efficiency of defense systems controlled by salicylic and jasmonic acids and ethylene. ABA was shown to suppress expression of some genes of defense proteins, including those involved in the synthesis and metabolism of phenolic compounds and lignin. ABA is evidently involved in plant defense mechanisms against pathogens as a regulatory element.

The effect of chitooligosaccharides on hydrogen peroxide production and anionic peroxidase activity in wheat coleoptiles

We studied the effects of chitooligosaccharides (ChOS) with a mol wt of 5 kD, the degree of acetylation of 65%, and the concentrations from 0.01 to 100 mg/l on the content of hydrogen peroxide in incubation medium and the activity of anionic peroxidase (pI ∼ 3.5) in the segments of wheat (Triticum aestivum) coleoptiles. H2O2 production and peroxidase activity were found to be dependent on the ChOS concentration. After 3 h of incubation, the highest H2O2 level in medium was observed at 0.01 mg/l ChOS, whereas after 6h, at 1 mg/l. After 3 h of incubation, ChOS suppressed peroxidase activity. After 6 h of incubation, high ChOS concentrations enhanced peroxidase activity. IAA favored H2O2 accumulation in medium and suppressed anionic peroxidase. The involvement of ChOS in the control of the level of reactive oxygen species and anionic peroxidase activity in plant cells is suggested.

Changes in the Levels of IAA, ABA, and Cytokinins in Wheat Seedlings Infected with Tilletia caries

The seedling growth and the content of endogenous phytohormones in wheat seedlings were estimated 3, 6, and 9 days after infection with the bunt pathogen (Tilletia caries) (DC.)TUL. The infection of a pathogen-susceptible species Triticum aestivum L. and a resistant species T. timopheevii Zhuk. resulted, respectively, in an increase and a decrease in the seedling growth and the IAA content as compared to the control. The cytokinin content increased in both species, and the increase in T. timopheevii was more rapid. The pathogen-induced increase in auxin content is suggested to enhance fungal invasion of plants. In the susceptible species, a high ABA concentration was retained for a longer period of time and could act as a factor of virulence. At the same time, in the resistant species, an increase in ABA content was transient and seems to trigger plant defense mechanisms.

Molecular peculiarities of the chitin-binding peroxidases of plants

The chitin-binding ability of isoperoxidases isolated from 23 plants of different species was studied. The activation of peroxidases in a protein extract in the presence of this polysaccharide was found for 14 of the studied plants. Anionic isoperoxidases were shown to be sorbed on chitin and eluted from them with 1M NaCl for 16 of the plant species. Cationic isoforms of the peroxidases of some species of the Fabaceae and Cucurbitaceae plant families also bound to chitin. An immunochemical similarity was found between the chitin-binding isoperoxidases of taxonomically distant plant species (the Pomaceous, Fabaceae, and Cucurbitaceae). Moreover, a high homology of the molecular structures of the polysaccharide-binding sites was revealed for the anionic peroxidases of rice, wheat, oat, zucchini, cucumber, and radish. We propose the existence of a special class of plant peroxidases that bind with polysaccharides (chitin) and participate in the protective reactions of plants against pathogens.