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

Agrobacterium tumefaciens (AT) is the causal agent of crown gall, a major proble m in the family Rosaceae and particularly for Prunus spp. Crown gall symptoms re sult from the bacterial infection of the cells damaged mechanically at the colla r or by root parasitic nematodes. Myrobalan plum (P. cerasifera) 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. incog nita and M. javanica. The four above mentioned RKN and Myrobalan progenies segre gating for Ma were used in experiments aimed at obtaining a better knowledge of the interaction between AT and RKN in relation to the RKN resistance genes. Prun us rooted cuttings, naturally infected with the bacterium were repotted, grown a nd inoculated individually with RKN. In a first experiment, Prunus plants were ( i) either inoculated with 10,000 juveniles (J2s) of M. arenaria to provide a sho rt inoculum pressure (SIP) or (ii) inoculated by association with one M. arenari a-galled tomato root system that produced a high and durable inoculum pressure o f 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 more n umerous and more homogenous under DIP than under SIP. Nevertheless, for both ino culum regimes, AT galls were present in the RKN-susceptible clones (= carrying n one of the Ma genes) and absent in the RKN-resistant clones. Subsequent experime nts, conducted under DIP with M. arenaria, M. incognita, M. javanica and M. hapl a, also showed, for the three first species, the presence of AT galls only in RK N-susceptible clones whereas Prunus plants inoculated with M. hapla and nematode -free controls were free of AT galls. Consequently RKN act as a wound agent in t he AT infection process of Myrobalan plum only when the plant develops a compati ble reaction (i.e. when it lacks the Ma resistance genes). Considering that J2s do penetrate the roots of resistant plants, the absence of crown gall symptoms o n this material even under durable inoculum pressure strengthens the hypothesis that this nematode stage has a very weak effect on plant cells during the infect ion process. This is the first evidence of the protective effect of a RKN resist ance gene against the expression of root crown gall consecutive to RKN infection . The protective effect of Ma and presumably of other RKN resistance genes again st AT is a strong argument for their introgression into Prunus and other Rosacea e or bacterium-susceptible crops.