The Ve-mediated resistance response of the tomato to Verticillium dahliae involves H2O2, peroxidase and lignins and drives PAL gene expression

<p>Abstract</p> <p>Background</p> <p><it>Verticillium dahliae </it>is a fungal pathogen that infects a wide range of hosts. The only known genes for resistance to <it>Verticillium </it>in the Solanaceae are found in the tomato (<it>Solanum lycopersicum</it>) <it>Ve </it>locus, formed by two linked genes, <it>Ve1 </it>and <it>Ve2</it>. To characterize the resistance response mediated by the tomato <it>Ve </it>gene, we inoculated two nearly isogenic tomato lines, LA3030 (<it>ve</it>/<it>ve</it>) and LA3038 (<it>Ve</it>/<it>Ve</it>), with <it>V. dahliae</it>.</p> <p>Results</p> <p>We found induction of H₂O₂production in roots of inoculated plants, followed by an increase in peroxidase activity only in roots of inoculated resistant plants. Phenylalanine-ammonia lyase (PAL) activity was also increased in resistant roots 2 hours after inoculation, while induction of PAL activity in susceptible roots was not seen until 48 hours after inoculation. Phenylpropanoid metabolism was also affected, with increases in ferulic acid, <it>p</it>-coumaric acid, vanillin and <it>p</it>-hydroxybenzaldehyde contents in resistant roots after inoculation. Six tomato <it>PAL </it>cDNA sequences (<it>PAL1 </it>- <it>PAL6</it>) were found in the SolGenes tomato EST database. RT-PCR analysis showed that these genes were expressed in all organs of the plant, albeit at different levels. Real-time RT-PCR indicated distinct patterns of expression of the different <it>PAL </it>genes in <it>V. dahliae</it>-inoculated roots. Phylogenetic analysis of 48 partial <it>PAL </it>cDNAs corresponding to 19 plant species grouped angiosperm <it>PAL </it>sequences into four clusters, suggesting functional differences among the six tomato genes, with <it>PAL2 </it>and <it>PAL6 </it>presumably involved in lignification, and the remaining <it>PAL </it>genes implicated in other biological processes.</p> <p>An increase in the synthesis of lignins was found 16 and 28 days after inoculation in both lines; this increase was greater and faster to develop in the resistant line. In both resistant and susceptible inoculated plants, an increase in the ratio of guaiacyl/syringyl units was detected 16 days after inoculation, resulting from the lowered amount of syringyl units in the lignins of inoculated plants.</p> <p>Conclusions</p> <p>The interaction between the tomato and <it>V. dahliae </it>triggered a number of short- and long-term defensive mechanisms. Differences were found between compatible and incompatible interactions, including onset of H₂O₂production and activities of peroxidase and PAL, and phenylpropanoid metabolism and synthesis of lignins.</p