At The Intersection of Phenylpropanoid Metabolism in Petunia
Plants are sessile organisms which in comparison with animals, had to develop a different set of mechanisms to adapt to environmental queues, such as abiotic and biotic stresses. An even more complex strategy must have developed through evolution to ensure reproductive success. Plant secondary metabolites, are represented by far more than the approximately 100,000 low molecular weight compounds described so far. Volatile organic compounds one group of secondary metabolites is important for several ecological functions. They act as attractants for pollinators and seed disperser to enhance reproduction and genetic variation, as well as representing a defense mechanism against herbivores as well as pathogens, and they allow for plant-plant communication. This work has focused on three critical steps in phenylpropanoids/benzenoid metabolism, which represent the second largest class of volatile organic compounds (Knudsen and Gershenzon, 2006). Arabidopsis thaliana predominantly produces terpenoid compounds, however, closer investigation of the Arabidopsis genome revealed the presence of two genes designated as aromatic L-amino acid decarboxylases (AADCs) with high homology to the recently identified Petunia hybrida phenylacetaldehyde synthase involved in phenylacetaldehyde production. This work includes the biochemical characterization and functional analysis in planta of an aromatic aldehyde synthase (AtAAS), which catalyzes the conversion of phenylalanine and 3,4-dihydroxy-L-phenylalanine to phenylacetaldehyde and dopaldehyde, respectively. Petunia (Petunia hybrida cv 'Mitchell Diploid') in contrast to Arabidopsis produces a unique blend of volatile phenylpropanoid/benzenoid compounds. Benzoic acid (BA) and its derivatives can be generated via two different routes the β-oxidative and non-β-oxidative pathway. The first reaction in the β-oxidative route is the activation of cinnamic acid (CA) to its CoA thioester, likely catalyzed by a 4-coumarate:CoA ligase (4CL) type enzyme. Investigation of this particular step lead to isolation of a cinnamoyl-CoA ligase (CNL) and 4CL enzyme, both of which were biochemically characterized and analyzed for their gene expression profiles. An RNAi strategy was applied to enable evaluation of CNL function in planta. 4CL was further analyzed for its impact on the production of the phenylpropenes eugenol and isoeugenol. Isoeugenol is one of the major volatile compounds released by Petunia hybrida cv 'Mitchell Diploid'. These phenylpropenes are derived from phenylalanine (Phe) and share the initial biosynthetic steps with lignin formation. Important intermediates in this biochemical pathway are hydroxycinnamic CoA esters, formed by activation of hydroxycinnamic acid derivatives, likely catalyzed by 4CL. This study includes the in planta analysis of 4CL function.
Dudareva, Purdue University.
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