## 3 Discussion
### A Role for VAP Genes during Infection of Resistant Plants?  
Mg-VAP1 and Mg-VAP2 are specifically induced when _M. graminicola_ infects resistant rice TOG5681 and silencing of the _Mg-VAP1_ gene affects _M. graminicola_ parasitism. VAPs belong to a structurally conserved group of secreted proteins abundantly secreted during several stages of parasitism by plant- and animal-parasitic nematodes [54]. Their name originated from the allergenic protein \"antigen 5\" identified in the white-faced hornet venom [55]. Importantly, VAPs are key players in the modulation of plant immunity. Knocking down the expression of _VAP_ genes can severely affect nematode virulence [45; 56; 57; 58]. In particular, expression of _VAP_ genes are required for the onset of parasitism at the time of root invasion and migration through host tissues [58; 59]. Conversely, ectopic expression of _G. rostochiensis_ or _Heterodera schachtii VAP_ genes in potato or _Arabidopsis_ significantly increases plant susceptibility to nematode infection [46]. In addition, _A. thaliana_ lines overexpressing nematode _VAP_ genes loose basal immunity to unrelated pathogens, including fungi, bacteria, and oomycetes [46]. Modulation of plant immunity involves interaction of nematode VAPs with host proteases [45; 57]. In tomato, _Gr-VAP1_ specifically interacts with and inhibits the apoplastic papain-like cysteine protease (PLPC) Rcr3 pim, increasing the susceptibility of tomato plants to _G. rostochiensis_[45]. Additionally, [PERSON] et al. [57] showed that ectopic expression in tobacco cells of the protein HaVAP2 (minus its signal peptide) from the cereal cyst nematode _H. avenue_ allowed its transfer into the host cell nucleus where it can interact with the _Hordeum vulgare_ HvCLP peptidase. VAPs also contain a SCP/CAP domain able to bind lipids, such as leukotrienes and sterols, but little is known about endogenous ligands that are bound during parasitism [54]. Regarding the importance of VAPs in the first steps of plant-nematode interactions, especially through the modulation of plant immunity, one can easily imagine that _Mg-VAP1_ and _Mg-VAP2_ have an important role to play in _M. graminicola_ parasitism. Accordingly, we showed that silencing the _Mg-VAP1_ gene had a negative effect on the nematode ability to colonize rice roots. However, we did not observe any significant effect of _Mg-VAP_ genes in cell death suppression assays conducted in tobacco (data not shown) as observed for some _VAP_ genes [46]. Up-regulation of _Mg-VAP1_ and _Mg-VAP2_ genes when the nematode infects resistant rice plants may reflect a special requirement of their function to counteract host immunity, even if it is finally not enough to overcome rice resistance. What their role is and what their targets are in rice-compatible interactions remain to be elucidated.  
To conclude, this study gives for the first time a global view of _M. graminicola_ genes expressed in the first steps of interaction with resistant rice plants and provides with novel nematode gene sequences. This work highlighted that _M. graminicola_ adapts its gene expression depending on the plant genotype. In a hostile host environment, the nematode still (desperately) tries to infect the plant as in a susceptible host, but may also express specific genes, including those encoding _VAPs_. It will be interesting to investigate how these genes help the nematode to cope with the plant resistance responses and whether they could participate to break rice resistance to _M. graminicola_.