Effects of Abscisic Acid on the Transfer of Sucrose From Host, Pelargonium zonale (L.) Aiton, to a Phanerogamic Parasite, Cuscuta reflexa Roxb

Abstract

Using isolated Pelargonium leaves parasitised by Cuscuta, we have established by means of autoradiographs that, in vivo, abscisic acid (ABA) is exported from the host leaf petiole to the attached parasite shoot. Experiments with simplified models (parasitised host stem slices, unparasitised host stem slices, isolated haustoria and parasitised host stem after removal of haustoria) showed that, in vitro, [2-14C]ABA is rapidly accumulated in haustorium tissues, and the internal/external ratio of ABA becomes greater than 1 within 45 min. The very fast ABA uptake, strongly dependent on the pH, may involve facilitated diffusion in addition to the diffusion of the undissociated acid. The high level of ABA concentration in Cuscuta, especially in haustoria, might be explained by ABA import from host tissues. In Cuscuta stem and haustoria, the hormone enhances the uptake and decreases the efflux of sucrose. In host tissues, ABA enhances phloem unloading of sucrose and decreases sucrose uptake. Thus, ABA may have a central role in the host-parasite relationship by enhancing sucrose transfer.