Role of the white collar 1 photoreceptor in carotenogenesis, UV resistance, hydrophobicity, and virulence of Fusarium oxysporum

Abstract

Knockout mutants of Fusarium oxysporum lacking the putative photoreceptor Wc1 were impaired in aerial hyphae, surface hydrophobicity, light-induced carotenogenesis, photoreactivation after UV treatment, and upregulation of photolyase gene transcription. Infection experiments with tomato plants and immunodepressed mice revealed that Wc1 is dispensable for pathogenicity on plants but required for full virulence on mammals.

FIG. 1.

Expression analysis of phr1 and hyd1 genes by semiquantitative RT-PCR of the wild type (wt), the Δwc1 mutant, and the Δwc1+wc1 strain grown on PDA plates under white light for the indicated time periods. Increasing numbers of amplification rounds were used, and the transcript signal was compared with that of the actin gene (act1). Amplification of genomic DNA (gDNA) was used as control for transcript size.

FIG. 2.

(A) Incidence of Fusarium wilt on tomato plants (cv. Monika) caused by the wild type (squares), the Δwc1 mutant (triangles), and the Δwc1+wc1 strain (crosses). Severity of disease symptoms was recorded at different times after inoculation, using an index ranging from 1 (healthy plant) to 5 (dead plant) (16). Uninoculated plants were used as control (diamonds). Error bars indicate the standard deviations of the results from 20 plants for each treatment. The experiment was performed twice with similar results. (B) Virulence of different strains of F. oxysporum in immunodepressed mice. Groups of 10 mice were infected with 2 × 10⁷ microconidia of the wild type (squares), the Δwc1 mutant (triangles), and the Δwc1+wc1 strain (inverted triangles) as described previously (28). Percent survival was plotted for 15 days. The results for infection with mutant Δwc1 differed significantly from the results for infection with the wild-type and the Δwc1+wc1 strains (P < 0.01). The experiment was performed three times. Data shown are from one representative experiment. Conditions were approved by the Animal Welfare Committee of Universitat Rovira I Virgili.