Inducible plant responses triggered by phytochemical and bacterial elicitors
Farag, Mohamed Ali Ali
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Plants release elevated levels of volatile organic compounds (VOCs) in response to insect or mechanical damage with some volatile components serving as chemical signals for attracting or repelling other organisms. Plants exposed to biogenic VOCs present in the environment can also modulate plant growth and development, although the nature of VOC-triggered plant responses has yet to be characterized for most plant species. The overall goal of this research has been to examine how certain environmental VOCs can trigger primary and secondary responses in plants. Specifically we examined (1) metabolic changes triggered by Ce alcohols and aldehydes ubiquitously released from damaged plants focusing on the model systems tomato and maize, as well as (2) C4 alcohols emitted from plant growth promoting rhizobacteria in the classic model plant Arabidopsis. Metabolic changes were monitored at genomic levels by reverse transcriptase-PCR (RT-PCR), slot blot analysis and GUS fusion assays. Changes in metabolic levels were monitored by GC, HPLC and Western analyses. The Ce-volatile (£)-2-hexenal triggered the release of local and systemic mono-and sesquiterpenes in tomato which was shown to be mediated through the jasmonic acid (JA) signaling pathway. Release of VOCs triggered with Ce-volatile treatment did not affect the accumulation of proteinase inhibitor enzymes (PI) or stored phytochemicals. In maize, Ce-com volafile (Z)-3-hexenoI resulted in an increase in transcript level for a series of defense genes including pal (phenyl alanine ammonium lyase), lox (lipoxygenase), igl (indole-3-glycerol phosphate lyase) and mpi (maize-proteinase inhibitor). The induction of transcripts were compared with metabolites generated from the respective pathways. Structure activity relationships has established that a series of Cft- alcohols were more active than tested aldehydes in triggering VOC emissions in both model systems. Biochemical and plant assays showed that the bacterial volatile elicitor 2,3- butanediol triggered growth promotion and induced disease resistance in Arabidopsis. Using transgenic and mutant lines of Arabidopsis, the signal pathway activated by bacterial volatiles was found to be dependent on cytokinin activation for growth promotion and dependent on ethylene signaling for induced pathogen resistance. These data provide new insight into the role of short chain alcohols and aldehydes as signaling molecules mediating plant-plant or plant-microbial interactions.