Volume 5 Supplement 7
Induced resistance and associated defence gene responses in Pinus patula
© Fitza et al; licensee BioMed Central Ltd. 2011
Published: 13 September 2011
Plants are able to incite a type of broad spectrum resistance against pathogens upon pre-treatment with biological or chemical inducers. Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR) are two types of induced resistance which lead to the accumulation of specific pathogenesis-related (PR) proteins. Non-pathogenic rhizobacteria are inducing agents for ISR and increased levels of ethylene (ET) and jasmonate (JA) are associated with this pathway, whereas SAR is associated with an increase in salicylic acid (SA) levels .
Pinus patula and P. radiata are commercially planted conifer species in South Africa, but are both highly susceptible to the causal agent of pitch canker, Fusarium circinatum. Annually, the forestry sector suffers substantial economic losses due to this disease which affects 20-30% of the planting stock. Bonello et al. (2001) showed that repeated mechanical inoculation of P. radiata with F. circinatum activated induced resistance, enhancing the protection of the tree against subsequent pathogen challenge .
Detailed knowledge of the molecular mechanisms underlying induced resistance may be useful to develop strategies to control diseases of pine trees. This study aimed to compare the efficiency of ten biological and chemical inducers in inciting resistance against F. circinatum. Additionally the molecular basis of this induced resistance was investigated by analyzing the response of selected putative defence response genes.
Ten activators of induced resistance (Bion®, Messenger®, Chitin, MeJA, Fusarium oxysporum, Pseudomonas fluorescens, SA, Kannar, Ralstonia solanacearum and potassium phosphate monobasic) were compared. A set of 80 P. patula seedlings were used per treatment. Inducers were applied at four and six months of age and F. circinatum spores (1x104) were used to challenge the seedlings a week after the booster application (six months). Disease severity was assessed six weeks after inoculation by comparing the size of the lesions on treated plants to water control plants. Three inducers that curbed symptoms most successfully were selected for further analysis. A set of 116 plants per treatment were screened weekly for eight weeks. Aerial parts of the six month old plants were harvested for RNA isolation at 24 hrs after the second application. For each treatment, three replicates, with 12 plants per replicate, were harvested. Subsequently, RNA was extracted for the reverse transcriptase quantitative PCR (RT-qPCR) analysis, where four putative defence genes were profiled using the Roche LightCycler® 480 instrument.
Results and discussion
Disease progression represented as percentage live stem in 6 month old P. patula seedlings during an eight week period post inoculation with F. circinatum. The significance levels are relative to the relevant control.
Log2 expression levels of putative defence response genes in P. patula 24 hrs after booster treatment with 10 mg/ml chitosan. Three biological replicates were used to calculate significance using the t-test.
Phenylalanine ammonia lyase
The potential of priming P. patula to defend itself against pathogen attack was explored. We tested the application of ten different inducers to enhance tolerance to F. circinatum. The application of chitosan reduced pitch canker symptoms. Reduced lesion length was observed for a period of six weeks, indicating the activation of induced resistance. Further molecular analysis suggests that the treatment may activate the phenylpropanoid pathway, which is involved in the production of secondary metabolites that have antifungal properties . Entire defence response pathways influenced by chitosan application in P. patula will be investigated in subsequent expression profiling assays.
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