Biotically induced resistance

Table I. Some Reports of Biotically Induced Resistance In Plants...

Initial reaction. In all known cases of effective biotic sensitization of plants reported to date, a critical factor appears to be the necrosis of host cells in the zone of initial infection. However, while non-necrotic infections are ineffective inducers, necrosis per se is not effective in inducing resistance. Injury by abiotic agents such as heat, chemicals, dry ice, or various extracts from plants and microbes does not protect cucumbers against lagenarium (8-10). Infection of tobacco by a wide variety of Peronosporales fungi other than P. tabacina frequently causes severe necrosis, but does not induce systemic resistance against blue mold (Tiizun and Kuc, unpublished). 

During our research on BABA-induced resistance we noticed that the priming effect of this chemical was not restricted to the plant s reaction to biotic stresses. BABA-treated plants were also sensitized to react faster and more effectively to abiotic stresses. BABA-treated Arabidopsis, for example showed a 75% survival rate following a treatment of two days at -5 °C, whereas control plants were all killed [83]. We also observed faster reactions at the molecular and phenotypical level to high salt, heat and drought treatment [8,69]. All these observations point to an important involvement of BABA in the expression of general priming mechanisms of stress tolerance. 

There are two kinds of SAR activator biotic and abiotic. Biotic activators include extracts from plants and microbes. For example, excellent control of powdery mildews was observed by application of extract of Rheynoutria sachalinensis the extracts from Bacillus subtilis have been shown to induce resistance in barley, especially against powdery mildew. It was reported that some plant growth-promoting rhizobacteria may be able to protect plants from foliar diseases when used as a seed treatment or by seed soaking. A strain of Pseudomonas was found to be able to protect cucumber against broad spectrum of diseases. Chitosan and laminarin are two typical resistance-related biotic molecules. Biotic plant activators harpin and ComCat have been commercialized. 

In addition to this local resistance, biotic agents may induce systemic resistance. Infection of the first true leaf of cucumber plants with Colletotrichum lagenarium immunized other aerial plant parts against infection by lagenarium, Cladosporium cucumerinum and Pseudomonas lachrymans (16). Bean plants appeared systemically protected against Colletotrichum lindemuthlanum when the first leaf had been inoculated with a non-pathogenic race of this fungus or with... 

We have already described that a number of rhizobial LPS mutants are sym-biotically defective because they likely induce an increased defense response by the host and/or are more sensitive to the host defense response. One structural feature of rhizobial LPS that appears to be important is the presence of OPS since its absence appears to result in a more robust plant defense response. We have also suggested (above) that the lack of OPS exposes the anionic COS on the bacterial surface which may make the rhizobial cell more sensitive to antimicrobial cationic peptides. Recent work in our laboratory (Brown, unpublished) has shown that a mutation of R. leguminosarum biovar viciae 3841 which specifically results in the loss of GalA residues from the core increases resistance to cationic peptides. It has also recently been shown that R. etli CE3 mutants in IpxE and IpxF, which are unable to remove the 1 and 4 -phosphates and, therefore have LA with increased anionic character, show increased sensitivity to cationic peptides (Ingram et al., 2010). 

General Occurrence of Resistance Induced by Biotic Agents... 

Spatial and Temporal relationships. A common characteristic of biotic sensitization of plants is a latent or lag period between initiation of the inducer infection and manifestation of disease resistance. In the cucumber/TNV or C lagenarium system, enhanced resistance against pathogens is first manifest about 48-72 hr after initial infection and maximal resistance is achieved by 120-144 hr... 

While biotic sensitization of plants against pathogens may act systemically, not all organs or tissues are necessarily protected equally. Recent work in our laboratory has shown that the extent of resistance against anthracnose or angular leaf spot of leaves in different positions on immunized cucumber plants varies in a complex manner not necessarily directly proportional to proximity of the inducer lesions (73). Removal of epidermal layers from immunized cucumber leaves reduces resistance to anthracnose (74). 

In V. vinifera, stilbenoids can either be constitutively expressed or induced through an abiotic or biotic stress. While there are exceptions, the stilbenoids are primarily constitutively expressed in the lignified organs (roots and stems) and are mainly induced substances, serving as phytoalexins, in leaves and berry skins [22, 23]. Within the roots and stems, stilbenoids are found as major constituents. In leaves, stilbenoids have been shown to be highly inducible, and a number of stilbenoids have been identified in infected and treated leaves. The stilbenoids in roots, stems, and leaves have been widely studied due to their importance in disease resistance. While these plant parts are not commonly utilized in the diet, they may be sources of novel stilbenoids with potent biological activities. 

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