Abstract
Two wild-type field populations of root-knot nematodes (Mi-Vfield, Mj-TunC2field), and two isolates selected for virulence in laboratory on resistant tomato cultivars (SM2V, SM11C2), were used to induce a resistance reaction in tomato to the soil-borne parasites. Epigenetic and metabolic mechanisms of resistance were detected and compared with those occurring in partially or fully successful infections. The activated epigenetic mechanisms in plant resistance, as opposed to those activated in infected plants, were detected by analyzing the methylated status of total DNA, by ELISA methods, and the expression level of key genes involved in the methylation pathway, by qRT-PCR. DNA hypo-methylation and down-regulation of two methyl-transferase genes (CMT2, DRM5), characterized the only true resistant reaction obtained by inoculating the Mi-1.2-carrying resistant tomato cv Rossol with the avirulent field population Mi-Vfield. On the contrary, in the roots into which nematodes were allowed to develop and reproduce, total DNA was generally found to be hyper-methylated and methyl-transferase genes up-loaded. DNA hypo-methylation was considered to be the upstream mechanism that triggers the general gene over-expression observed in plant resistance. Gene silencing induced by nematodes may be obtained through DNA hyper-methylation and methyl-transferase gene activation. Plant resistance is also characterized by an inhibition of the anti-oxidant enzyme system and activation of the defense enzyme chitinase, as opposed to the activation of such a system and inhibition of the defense enzyme glucanase in roots infested by nematodes.
Highlights
Genetic natural resistance to root-knot nematodes (RKNs) is conferred in many plant species by a single dominant resistance gene (R gene) that recognizes the proper avirulence (Avr) gene in the nematode
This “natural” selection can be mimicked in controlled greenhouse conditions by, at first, collecting the few egg masses developed on Mi-1-carrying tomato by wild-type field populations; repeated inoculation of this selected progeny leads to the generation of stable virulent isolates [27]
Epigenetic changes in total DNA methylation are being recognized of paramount importance in determining the outcome of nematode-plant interactions
Summary
Genetic natural resistance to root-knot nematodes (RKNs) is conferred in many plant species by a single dominant resistance gene (R gene) that recognizes the proper avirulence (Avr) gene in the nematode. This ‘gene-for-gene’ recognition triggers the initiation of a cascade of defense responses, which lead to the halt of nematode development. A hypersensitive reaction (HR) is observed as an early expression of tomato resistance to RKNs; it consists of a prolonged oxidative burst caused by enhanced generation and cellular concentration of reactive oxygen species (ROS), which lead to a rapid and localized cell death and tissue necrosis. Immune reactions are always characterized by a high production of pathogenesis related- (PR-) proteins, which are the executioners of plant immunity [10]
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