Symptomless infection of tomato plants by tomatinase producing Fusarium oxysporum formae speciales nonpathogenic on tomato plants

Abstract

Fusarium oxysporum f. sp. lycopersici produces extracellular enzymes known as tomatinase, which degrade the tomato phytoanticipin α-tomatine to less toxic metabolites. Tomatinase gene ( FoToml) has been found also in certain strains of F. oxysporum belonging to formae speciales nonpathogenic on tomato plants. In the current study, four FoToml-positive strains (#2, #3, #11, and #17) of F. oxysporum belonging to formae speciales different from f. sp. lycopersici and f. sp. radicis- lycopersici were examined for their ability to infect tomato plants. All of the four strains colonized not only the roots and basal stems but also upper stems of tomato plants. One of the four strains, #3 (f. sp. batatas), was transformed with the β-glucuronidase (GUS) reporter gene. A GUS-marked #3 (#3-GUS) colonized vascular vessels of all tomato cultivars tested differing in the extents of colonization in the stems among the cultivars. Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that each FoToml of the four FoToml-positive strains was expressed in the roots and stems of tomato plants, suggesting that FoToml is important for the four strains to survive in tomato plants. To see systemic induction of defense genes in tomato plants inoculated with either #2, #3, #11, or #17, accumulations of transcripts of β-1,3-glucanase and chitinase genes were analyzed. Tomato plants inoculated with #2 or #3 showed enhanced expression of acidic chitinase gene ( Chi3). Prior inoculation of tomato plants with either #2 or #3 suppressed the vascular wilt of tomato caused by subsequent challenge with F. oxysporum f. sp. lycopersici.