INVITRO SELECTION FOR RESISTANCE TO VERTICILLIUM WILT IN ALFALFA (MEDICAGO-SATIVA L) USING A FUNGAL CULTURE FILTRATE

A filtrate from mycelial cultures of Verticillium albo-atrum was found to inhibit the cell growth and reduce the viability of alfalfa (Medicago sativa) suspension cultures. The toxicity of the filtrate was enhanced at pH 7·5 relative to pH 5·5. At least a portion of the filtrate toxicity was lost during dialysis at > 3500 kD exclusion. A similar filtrate preparation from Verticillium dahliae, which does not cause disease symptoms in alfalfa, was also toxic to the alfalfa suspension culture cells. Cells resistant to the V. albo-atrum filtrate were selected during culture in the presence of the toxic filtrate as evidenced by the successive increases in packed cell volume and cell viability that occurred in alfalfa cultures over four subculture cycles into fresh filtrate-containing medium. Plants regenerated from filtrate-containing alfalfa cultures initiated from two genotypes had significantly lower average disease severity indices than plants regenerated from control cultures and the donor plants. Also, the average disease severity indices of plants regenerated from filtrate-treated cultures of one genotype decreased from cycle to cycle, but remained the same for plants regenerated from control cultures. In general, the study showed that V. albo-atrum fungal filtrate and alfalfa suspension culture cells can be used to examine non-specific host-pathogen interactions and to obtain plants with increased resistance to verticillium wilt. © 1991 Academic Press Limited.