Forbs frustrate parasite penetration
Plant-on-plant parasitism often involves very specific
host–parasite interactions. However, other parasites are generalists, invading
a range of hosts with varying degrees of ‘success’. A
wide host range prompts the question posed by Cameron et al., at Aberdeen (pp. 1289–1299): what prevents the parasite invading species outside
its host range? The authors worked with Rhinanthus minor, a root parasite of grasses and legumes but not of
non-leguminous perennial dicots (‘forbs’), studying its interaction with two
grasses (Cynosurus cristatus, Phleum bertolonii), one legume (Vicia cracca) and two forbs (Leucanthemum vulgare, Plantago
lanceolata). Seedlings of target
plants and parasite were grown together and interactions were monitored. When R. minor formed a
parasitic relationship with either of the grasses or with V. cracca, there was a significant increase in parasite biomass
and in the number of flowers formed, compared with ‘free-living’ individuals.
In the abortive relationships with forbs, parasite performance was greatly
impaired, even compared with free-living individuals. Cytological and
histological examination of the host–parasite interface showed that, in
successful invasions, haustoria encompassed the host root; host tissues were
damaged mechanically and possibly also enzymically. Formation of a vascularized
penetration peg led to invasion of the host’s vascular system, sometimes with
the formation of specific conduits (oscula) between host and parasite. None of this
happened in the unsuccessful attempts to invade the two forb species: the
parasite was stopped before it could encompass the root and invade the vascular
system. There was also some evidence for a hypersensitive reaction, indicated
by host cell death at the host–parasite interface. Examination by
Fourier-transform infra-red microspectroscopy of the invasion site in host and
non-host species indicated that the latter synthesized lignin. This was visible
in light micrographs as a darkly staining layer. Thus it is containment of the
parasite, possibly accompanied by a hypersensitive reaction,
that prevents the parasitism of these two forbs by R. minor.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
