Most AAL toxin-sensitive Nicotiana species are resistant to the tomato fungal pathogen Alternaria alternata f. sp lycopersici

The phytopathogenic fungus Alternaria alternata f. sp, lycopersici produces AAL toxins required to colonize susceptible tomato (Lycopersicon esculentu m) plants, AAL toxins and fumonisins of the unrelated fungus Fusarium monil iforme are sphinganine-analog mycotoxins (SAMs), which are toxic for some p lant species and mammalian cell lines, Insensitivity of tomato to SAMs is d etermined by the Alternaria Stem canker gene 1 (Asc-1), and sensitivity is associated with a mutated Asc-1, We show that SAM-sensitive species occur a t a low frequency in the Nicotiana genus and that candidate Asc-1 homologs are still present in those species, In Nicotiana spp,, SAM-sensitivity and insensitivity also is mediated by a single codominant locus, suggesting tha t SAM-sensitive genotypes are host for A, alternata f. sp, lycopersici, Nic otiana umbratica plants homozygous for SAM-sensitivity are indeed susceptib le to A. alternata f, sp, lycopersici, In contrast, SAM-sensitive genotypes of Nicotiana spegazinii, Nicotiana acuminata var. acuminata, Nicotiana bon ariensis, and Nicotiana langsdorffii are resistant to A. alternata f. sp, l ycopersici infection concomitant with localized cell death. Additional (non host) resistance mechanisms to A. alternata f. sp, lycopersici that are not based on an insensitivity to SAMs are proposed to be present in Nicotiana species.