Endophyte enters end-zone
Mistletoe (Viscum album) is relatively common in the
area of the UK
in which I live. Indeed, there are several specimens within a few minutes’ walk
from my office. In other parts of the world, this group is represented by
numerous species; here we focus on dwarf mistletoes in the genus Arceuthobium.
Several members of this genus provide a very unusual illustration of the highly
intimate relationship that often develops between host and parasite, as
described by David Lye (Sidney, British
Columbia, pp. 953–963). Thus, when A. americanum parasitizes Pinus
contorta, isophasic growth occurs, i.e. the growth of host and endophyte become synchronized to a large extent. This does
not occur when the same mistletoe parasitizes Picea glauca, even though
the latter parasitism is also successful. Lye has investigated the parasitism
of Pseudostuga menziesii (Douglas
fir) by A. douglasii in which isophasic growth also occurs, focusing on
facets of host and endophyte growth and especially the growth of the endophyte
in host apical buds. From his very full account, several notable features
emerge. Firstly, the endophytic strands of the parasite were distributed within
the host in predictable patterns. Secondly, and unexpectedly, the parasite was
present in the dormant buds of the host, suggesting that it had grown into them
as they developed. Thirdly, within some of the dormant buds, the endophyte had
actually grown into preformed leaves. Thus, the endophyte grows much further
into host tissues than previously thought. The author sums up succinctly: ‘Throughout
the undifferentiated areas where the host prepares for the next year’s
longitudinal growth the filaments of the parasite are already present’. It is
therefore ready to gear its growth to the new growth of the host and to
establish its own emergent shoots. It is certainly a most remarkable phenomenon
and it will be fascinating to know how it is controlled.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
