Striga gesnerioides parasitising cowpea: Development of infection structures and mechanisms of penetration

Abstract

Striga gesnerioides (Scrophulariaceae) is an obligate hemiparasitic angiosperm, that infects the roots of cowpea (Vigna unguiculata). The development of Striga seedlings was characterized after transfer to the surface of cowpea roots using cryo-scanning electron microscopy. Contact with the surface of the root caused distinct morphological changes in the Striga radicles. Radicle elongation ceased 8 h after contact with host roots and radicle hairs developed. These hairs adhered to the surface of the host root. Early stages of root penetration, i.e. prior to connection to the host vascular system were examined by light microscopy and transmission electron microscopy to determine the mechanisms by which Striga invades root tissue. Cowpea roots were penetrated by S. gesnerioides radicles after 48 h. Host cells immediately surrounding the invading Striga were compressed, but other host cells were not affected. Immunocytochemistry, using monoclonal antibodies JIM 5 and JIM 7, showed that Striga advanced intercellularly through the root tissues without extensive removal of pectins at the site of penetration.