The Ve-mediated resistance response of the tomato to Verticillium dahliae involves H2O2, peroxidase and lignins and drives PAL gene expression.
BACKGROUND
Verticillium dahliae is a fungal pathogen that infects a wide range of hosts. The only known genes for resistance to Verticillium in the Solanaceae are found in the tomato (Solanum lycopersicum) Ve locus, formed by two linked genes, Ve1 and Ve2. To characterize the resistance response mediated by the tomato Ve gene, we inoculated two nearly isogenic tomato lines, LA3030 (ve/ve) and LA3038 (Ve/Ve), with V. dahliae.


RESULTS
We found induction of H2O2 production 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, p-coumaric acid, vanillin and p-hydroxybenzaldehyde contents in resistant roots after inoculation. Six tomato PAL cDNA sequences (PAL1 - PAL6) 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 PAL genes in V. dahliae-inoculated roots. Phylogenetic analysis of 48 partial PAL cDNAs corresponding to 19 plant species grouped angiosperm PAL sequences into four clusters, suggesting functional differences among the six tomato genes, with PAL2 and PAL6 presumably involved in lignification, and the remaining PAL genes implicated in other biological processes. 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.


CONCLUSIONS
The interaction between the tomato and V. dahliae triggered a number of short- and long-term defensive mechanisms. Differences were found between compatible and incompatible interactions, including onset of H2O2 production and activities of peroxidase and PAL, and phenylpropanoid metabolism and synthesis of lignins.
Departamento de Biología Animal, Biología Vegetal y Ecología, Universidad de La Coruña, 15071 La Coruña, Spain.
Carmen
C
Gayoso
Federico
F
Pomar
Esther
E
Novo-Uzal
Fuencisla
F
Merino
Oskar Martínez
OM
de Ilárduya
D001483:Base Sequence; D020869:Gene Expression Profiling; D015971:Gene Expression Regulation, Enzymologic; D018506:Gene Expression Regulation, Plant; D054884:Host-Pathogen Interactions; D006861:Hydrogen Peroxide / Q000378:metabolism*; D007113:Immunity, Innate / Q000235:genetics; D007527:Isoenzymes / Q000235:genetics / Q000378:metabolism; D008031:Lignin / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000235:genetics / Q000378:metabolism* / Q000382:microbiology; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D009195:Peroxidase / Q000378:metabolism*; D019800:Phenol / Q000378:metabolism; D010650:Phenylalanine Ammonia-Lyase / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D010935:Plant Diseases / Q000235:genetics / Q000382:microbiology; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000235:genetics / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012689:Sequence Homology, Nucleic Acid; D017550:Spectroscopy, Fourier Transform Infrared; D020172:Verticillium / Q000502:physiology
D007527:Isoenzymes; D008562:Membrane Glycoproteins; D010940:Plant Proteins; D011956:Receptors, Cell Surface; C430462:Ve1 protein, Lycopersicon esculentum; C430463:Ve2 protein, Lycopersicon esculentum; D019800:Phenol; D008031:Lignin; D006861:Hydrogen Peroxide; D009195:Peroxidase; D010650:Phenylalanine Ammonia-Lyase