Infection and colonization of peanut pods by Aspergillus parasiticus and the expression of the aflatoxin biosynthetic gene, nor-1, in infection hyphae

Abstract

The aflatoxigenic fungi, Aspergillus flavus and A. parasiticus infect a wide variety of crops, all of which produce oil-rich seed. A histological study of the host–pathogen interaction between peanut,Arachis hyphogea , and A. parasiticus was performed in a system where peanuts remained attached to the plant and were inoculated without wounding. For infection studies, a genetically-tagged strain of A. parasiticus, G5, was engineered to harbor the β-glucuronidase (GUS) reporter gene under control of the nor-1 promoter from the aflatoxin biosynthetic pathway. There was a similar temporal pattern of aflatoxin B1 production and appearance of GUS activity in cultures ofA. parasiticus G5. This strain was used to follow infection and aflatoxin production during colonization of undamaged, drought-stressed peanuts. The fungus colonized all tissues of the peanut pod and appeared to gain ingress through the corky layer of the pericarp. Both intra- and inter-cellular colonization were observed. Fungal colonization of the cotyledons resulted in visible depletion of storage bodies within cells. Two morphologically distinct types of hyphae, wider hyphae and narrower hyphae, were seen throughout the pod tissues. Statistical analysis revealed that the narrower hyphae were significantly more likely to produce GUS activity than wider ones. GUS activity was found in hyphae infecting the pericarp, embryo and cotyledons indicating expression of aflatoxin biosynthetic genes in these tissues. Interestingly, GUS activity was not observed in the hyphae colonizing the testa.