Interaction of root-knot nematodes (RKN) and the bacterium Agrobacterium tumefaciens in roots of Prunus cerasifera: evidence of the protective effectof the Ma RKN resistance genes against expression of crown gall symptoms

Agrobacterium tumefaciens (AT) is the causal agent of crown gall, a major p roblem in the family Rosaceae and particularly for Prunus spp. Crown gall s ymptoms result from the bacterial infection of the cells damaged mechanical ly at the collar or by root parasitic nematodes. Myrobalan plum (P. cerasif era) is susceptible to AT and is not a host for the root-knot nematode (RKN ), M. hapla. Some clones of this plum carry single Ma resistance genes that control M. arenaria, M. incognita and M. javanica. The four above mentione d RKN and Myrobalan progenies segregating for Ma were used in experiments a imed at obtaining a better knowledge of the interaction between AT and RKN in relation to the RKN resistance genes. Prunus rooted cuttings, naturally infected with the bacterium were repotted, grown and inoculated individuall y with RKN. In a first experiment, Prunus plants were (i) either inoculated with 10,000 juveniles (J2s) of M. arenaria to provide a short inoculum pre ssure (SIP) or (ii) inoculated by association with one M. arenaria-galled t omato root system that produced a high and durable inoculum pressure of the same nematode species. Four months after RKN inoculation, plants were rate d for nematode and bacterial root galling symptoms. RKN and AT galls were m ore numerous and more homogenous under DIP than under SIP. Nevertheless, fo r both inoculum regimes, AT galls were present in the RKN-susceptible clone s (= carrying none of the Ma genes) and absent in the RKN-resistant clones. Subsequent experiments, conducted under DIP with M. arenaria, M. incognita , M. javanica and M. hapla, also showed, for the three first species, the p resence of AT galls only in RKN-susceptible clones whereas Prunus plants in oculated with M. hapla and nematode-free controls were free of AT galls. Co nsequently RKN act as a wound agent in the AT infection process of Myrobala n plum only when the plant develops a compatible reaction (i.e. when it lac ks the Ma resistance genes). Considering that J2s do penetrate the roots of resistant plants, the absence of crown gall symptoms on this material even under durable inoculum pressure strengthens the hypothesis that this nemat ode stage has a very weak effect on plant cells during the infection proces s. This is the first evidence of the protective effect of a RKN resistance gene against the expression of root crown gall consecutive to RKN infection . The protective effect of Ma and presumably of other RKN resistance genes against AT is a strong argument for their introgression into Prunus and oth er Rosaceae or bacterium-susceptible crops.