Abstract
The Avr9 peptide elicitor from the fungus Cladosporium fulvum, the bacterial pathogen Pseudomonas syringae pathovar tomato carrying the avirulence gene avrPto (Pst (avrPto)), and the organophosphorous insecticide fenitrothion induce resistance-related responses in tomato lines carrying the Cf-9, Pto, and Fen genes, respectively. These responses were associated with synthesis of p-coumaroyloctopamine and p-coumaroylnoradrenaline, a novel compound for plants. In susceptible near isogenic tomato lines (Cf-0, pto, fen) and wounded tomato leaves, the levels of these compounds were reduced or undetectable. The elevated levels of p-coumaroyloctopamine and p-coumaroylnoradrenaline were accompanied by elevated mRNA levels of genes encoding phenylalanine ammonia lyase, p-coumarate CoA ligase, and hydroxycinnamoyl-CoA:tyramine N-(hydroxycinnamoyl)transferase (THT), enzymes that are involved in the hydroxycinnamic acid amide biosynthesis. Southern hybridization indicated that THT is encoded by a multigene family in tomato. Four different THT full-length cDNAs were derived by reverse transcriptase-PCR using degenerate primers based on potato and tobacco THT sequences. Transcripts for all four homologs were present in unchallenged tomato leaves, but only tomTHT1-3 was highly expressed following challenge with Pst (avrPto). Furthermore, tomTHT1-3 showed a more substantial and rapid induction in the incompatible interaction than in the compatible interaction. The cDNAs tomTHT1-3, tomTHT7-1, and tomTHT7-8 encoded proteins with a high degree of amino acid sequence homology, although the recombinant proteins had different preferences for octopamine and noradrenaline. The fourth cDNA, tomTHT1-4, directed synthesis of a truncated enzymatically inactive protein due to the presence of a premature stop codon.
Highlights
Plant defense against microbial attack is associated with activation of plant responses that serve to restrict the growth of the pathogen and/or to eliminate it
The elevated levels of pcoumaroyloctopamine and p-coumaroylnoradrenaline were accompanied by elevated mRNA levels of genes encoding phenylalanine ammonia lyase, p-coumarate CoA ligase, and hydroxycinnamoyl-CoA:tyramine N-(hydroxycinnamoyl)transferase (THT), enzymes that are involved in the hydroxycinnamic acid amide biosynthesis
We have compared the synthesis of soluble phenolics in response to both biotic and abiotic agents using nearly isogenic tomato lines that differ in their response to these agents
Summary
Necrotic reaction that resembles the HR, whereas fen plants show only sporadic appearance of necrotic spots [10, 11]. By monitoring the profile of soluble phenolics, we have shown preferential induction of the synthesis of two phenolic compounds in those interactions that are associated with resistance This is seen with both biotic elicitation and fenitrothion treatment. Conjugates of hydroxycinnamic acids with tyramine and derivatives of tyramine, such as dopamine, methoxytyramine, and octopamine, are found in a wide range of plants Synthesis of these amides has been shown previously to be activated by treatment with fungal elicitors, by attempted infection by fungi, viruses, and bacteria, and in some cases by wounding [13,14,15,16,17,18,19]. We report that the different THT isoforms in tomato have different acceptor preferences and that the isoform that is most highly expressed upon bacterial challenge has a significantly higher affinity for noradrenaline than either the potato or tobacco enzymes
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