Benzothiadiazole (BTH) acts as a priming agent in plant defence leading to a reduction in penetration and development of the root-knot nematode Meloidogyne incognita in susceptible tomato roots. Changes in
lignin biosynthesis in the susceptible tomato cv. Roma following
nematode infection and/or BTH treatment were investigated in comparison to the resistant cv. Rossol. Both untreated and BTH-treated susceptible infected roots (galls) showed an increased level of expression of
lignin synthesis-related genes (PAL, C4H, HCT and F5H) at early times during
infection (2-4 days post inoculation).
Peroxidase (soluble and cell-wall bound, POX)
enzyme activities increased after inoculation with M. incognita and the priming effect of BTH treatment was evident at later stages of
infection (7 days post inoculation). As expected, the induction of PAL and POXs and
lignin synthesis-related genes was faster and greater in resistant roots after
infection. Histochemical analysis revealed accumulation of higher
lignin levels at later
infection stages in BTH-treated galls compared to untreated ones. Furthermore, the monomer composition of
lignin indicated a different composition in guaiacyl (G) and syringyl (S) units in BTH-treated galls compared to untreated galls. The increase in G units made G/S ratio similar to that in the resistant genotype. Overall,
lignin played a critical role in tomato defence to M. incognita in response to BTH.