The Hypersensitivity Reaction of Tomatoes Resistant to Meloidogyne incognita: Reversal by Cytokinins.

Initiation of larval growth, induction of cell necrosis, and gall formation in the host were measured as criteria of resistance or susceptibility of tomato seedlings to the root-knot nematode, Meloidogyne incognita (Kofoid &White) Chitwood. Seedlings grown at 27 C on water agar containing additions were scored 3 or 4 days after infection.In the absence of exogenous plant growth regulatory substances, approximately 73% of larvae that entered roots of susceptible plants showed growth, none induced necrosis and nearly all induced gall formation. In roots of a resistant variety, only 4% of the larvae grew, 88% induced necrosis of host cells, and only 29% induced galls. Exogenously supplied cytokinins shifted the response of the resistant plants toward the susceptible reaction. Exogenous kinetin at 0.4 and 0.8 micromolar allowed 55 and 57% of the nematodes to grow, reduced the incidence of necrosis to 32 and 31%, and increased gall formation to 73 and 65%. Three additional cytokinins, Zeatin, 6-(gamma,gamma-dimethylallylamino)purine, and 6-benzylaminopurine produced effects similar to kinetin. Exogenous indoleacetic acid, gibberellic acid, adenine, guanine, uracil, thymine, cytidine, and 6-methylaminopurine neither increased the percentage of larvae which grew nor decreased the extent of host cell necrosis.