Association of epidermal lignification with nonhost resistance of cucurbits to fungi

Abstract

Etiolated seedlings of cucumber, muskmelon, watermelon, pumpkin, and squash were inoculated with Colletotrichum lagenarium (Pass.) Ell. & Halst. race 1, Colletotrichum atramentarium (Berk. & Br.) Taub., or Helminthosporium carbonum Ull. The fungi germinated and produced appressoria equally well on all hosts. Penetrations into host cells, however, were routinely observed only in the interactions of C. lagenarium with cucumber, muskmelon, and watermelon. Histochemical staining of the other host–pathogen interactions (nonhost interactions) revealed that the lack of penetration by fungi into nonhost tissue was associated with the deposition of lignin in the upper and lateral host epidermal cell walls around appressoria. Treatment of the lignified tissues with 0.5 M NaOH or hot ethanol did not alter staining reactions. Treatment with hot aminoethanol, a delignifying agent, did eliminate stainings. Cupric oxide oxidation of lignified epidermal cell walls from each host revealed that each host produced a p-coumaryl-rich, guaiacyl-poor ligninlike material. These results suggest that lignification may be a general nonhost resistance response in the Cucurbitaceae.