et al. 2011 ). There are a number of reports describing various bioactive metabolites
produced by fungal endophytes which help the plant to increase the resistance
against biotic stress (Guo et al. 2008 ; Suryanarayanan et al. 2009 ). For example,
production of two macrocyclic alkaloids, pyrrocidines A and B with antibiotic
activity, by the endophytic fungusAcremonium zeaehas been implicated in the
protection of its host, maize, against pathogenic and mycotoxin producing fungi
(He et al. 2002 ). In grasses and herbaceous plants, the endophytes are known to
produce toxic alkaloids that prevent or poison invertebrate and vertebrate herbi-
vores (Rodriguez et al. 2009 ). Accordingly in symbiotically conferred stress tol-
erance, endophytes act as a biological trigger to activate host defense system more
rapidly and strongly (Rodriguez and Redman 2008 ). At the same time some
endophytes are capable of enhancing the hosts’allelopathic effects on other species
growing close by, being an opponent for the space and nutrients (Newcombe et al.
2009 ). Apart from the above benefits, many endophytes are reported to enhance
uptake of phosphorus, and other important elements for plant growth, capable of
fixing nitrogen and producing plant hormones such as auxin, indole acetic acid,
which are essential for regulation of plant growth and development (Guo et al.
2008 ).
9.4 Biological Rationale in Plant Selection
Due to the vast number of plant species in the world, creative and imaginative
strategies are necessary to quickly narrow down the search for bioactive endo-
phytes. This provides the best opportunities to isolate endophytes prone to produce
novel bioactive products. Plants from distinct environmental settings and/or with an
unconventional biology are considered to be a promising source for isolating novel
endophytes bearing new secondary metabolites (Strobel 2003 ). Strobel and Daisy
( 2003 ) reported several reasonable hypotheses governing the plant selection for
isolating bioactive endophytes. Selection of plants from a unique environment,
having unusual biology, using novel approaches for survival is one such strategy.
Mangrove environments are an example for hosting such plants. A second tactic is
the selection of plants that have a historic background, which have been exploited
as a source of traditional medicine. Third, plants that are endemic, having an
unusual longevity or that occupy a certain ancient land mass, have the prospect of
lodging such endophytes. Finally, plants growing in areas of high biodiversity, such
as rainforest ecosystems, are potential sources housing novel and bioactive endo-
phytic fungi (Strobel and Daisy 2003 ).
194 P.B. Ratnaweera and E.D. de Silva