Effects of UV and Phototoxins on Selected Fungal Pathogens of Citrus

Photons in the UV region of the spectrum are important for organisms since they are energy-rich and strongly absorbed by biological molecules having the potential to react with membranes, enzymes, and nucleic acids. These wavelengths can also be absorbed by specific molecules that undergo conversion to a more reactive state (light activation) which can then cause damage to molecules of critical physiological function (phototoxicity). The importance of pigments in two genera of the Citrus pathogens, Fusarium and Penicillium, was assessed for ability to protect against inactivation by UV-A, B, and C and two phototoxins activated by UV-A. Pigment-deficient mutants of both genera were isolated following UV-C mutagenesis. Direct exposure of fungal spores in suspension of wild type and pigment-deficient mutants was carried out under the appropriate light source. The UV-A activated phototoxins investigated were: α-terthienyl (α-T), which produces predominantly singlet oxygen $({}^{1}{\rm O}{}_{2})$, an excited state of oxygen, which causes chiefly membrane damage; and 8-methoxypsoralen (8-MOP), which induces cycloadduct formation in DNA. For both genera, UV-A and UV-B alone were ineffective in causing inactivation of conidia at the fluences tested. Using appropriate Escherichia coli tester strains, it was demonstrated that the UV-B source was capable of inducing DNA lesions leading to lethality, presumably cyclobutane dimers in large measure. The carotenoids in one of the Fusarium species did not appreciably protect against lethal damage induced by UV-C, but the pigments of both Penicillium species were presumably able to screen UV-C and offer protection. It is assumed that the carotenoids in the wild type Fusarium species protected against UV-A activated α-T damage by quenching singlet oxygen. The blue-green pigment(s) in P. italicum prevent DNA damage caused by 8-MOP most probably by screening the UV-A wavelengths necessary to activate the phototoxin.