Laboratoire de Biologie cellulaire et Moléculaire, Institut National de la Recherche Agronomique and Uni-versité Victor Segalen Bordeaux 2, 33883 Villenave d'Ornon cedex, France
Two hundred and fifty-seven transposon Tn4001 mutants of Spiroplasma citri strain GII3 were used for transmission assays by the leafhopper vector Circulifer haematoceps into periwinkle (Catharanthus roseus) plants. Multiplication of the mutants in the two hosts, the leafhopper and the plant, as well as the symptom expression in the plant were studied. Two mutants, GMT 470 and GMT 553, caused no symptoms on plants. Tn4001 is inserted as a single copy in the genome of these mutants. Mutant GMT 470 did not multiply, or multiplied only poorly, in the leaf-hopper and was not transmitted by the insect to the plant, nor to culture medium through Parafilm membrane. The growth rate of GMT 470 in SP4 medium was twice as slow as that of wild-type strain GII3. Mutant GMT 553 multiplied in the leafhopper as well as the wild-type spiro-plasma, and was transmitted by the leafhoppers into the plants, where it reached the same titers as the wild-type strain but in approximately twice as much time. The plants containing high titers of mutant GMT 553 remained symptomless for several weeks. However, symptoms began to develop at a time when revertants that had lost the transposon were detected.