18 Mutualistic endophytic fungi have been shown to biologically control root
knot nematodes of tomatoes IITA 1998. The root knot nematodes attack tomato plants through the roots, therefore biological enhancement of the tomato plant
using mutualistic fungal endophytes will increase plant resistance to infection Sikora Pocasangre 2004. Endophytes are well adapted to the life inside the
plant and share the same ecological niche with endoparasitic nematodes, thus they are
effective at
the exact
site of
the pest
or disease
attack Sikora Pocasangre 2004.
2.11 Plant Tissue Colonization Process by Endophytic Fungi
The process of colonization of plant tissues by endophytic fungi are complex and include host recognition, spore germination, penetration and
colonization. Endophytes penetrate their host plants through natural openings or wounds
or actively
using hydrolytic
cellulases and
pectinases Hallmann et al. 1997b, forming inconspicuous infection within healthy plant
tissues for all or part of their life cycles. Plant wounding induced by biotic factors such as plant-parasitic nematodes also constitute a major factor for the entry of the
endophytic microorganisms Hallmann et al. 1998. For many years endophytic microorganisms colonizing plant tissues have
been thought to be weekly virulent pathogens Sinclair Cerkauskas 1996. The distinction between endophytic infection and latent infection is that in latent
infections, the host plant does not show any symptoms, with the infection persisting latently until symptoms are prompted to appear by environmental or
nutritional stress conditions. The state of host plant and the pathogen may also provide signals for symptom expression. Since the production of disease
symptoms is dependent upon the interaction between the host, parasite and the environment over time endophytic colonization is considered not to cause any
disease Sinclair Cerkauskas 1996. To detect endophyte colonization of plants, several methods for in situ
detection of fungal endophytes in plant tissues have been developed. A simple method involves microscopic examination of differentially stained samples of
endophyte infected plants Saha et al. 1988. This method is however time
19 consuming and less reliable since histological staining is not endophyte specific
Hahn et al. 2003. Other methods for in situ detection of endophytes include the use of monoclonal antibodies Hiat et al. 1997; Hiat et al. 1999 tissue printing
immunoblotting Gwinn et al. 1991
tissue print
immunoassay Hahn et al. 2003, electron microscopy Sardi et al. 1992 and autoradiography
You et al. 1995.
Figure 3. Light micrographs of stained endophytic mycelium inside plant tissue showing intercellular colonization by endophytic fungi. A, B.
Myceliumarrow running along the host vascular bundle VB x1000. PM: palisade mesophyll, SM: spongy mesophyll, T: tracheids Bars =
10 μm. Source; Review Iberoam Micology 2007
Majority of endophytic fungi isolated from healthy tomato tissues belong to the genus
Fusarium , followed by Acremonium, others include soil fungi
belonging to the genera Penicillium, Aspergillus and Gongronella also Trichoderma
which has biological potential is usually isolated Niere et al. 2002. The most dominant species is Fusarium oxysporum, which has been
reported as an endophyte of many crop plants including banana, tomato, rice and maize
and is an effective colonizer of plant roots Niere et al. 2002. However, Fusarium
sp are also notorious as causal agent of Fusarium wilt of many crops these are distinguished as specialised forms and physiological races, but majority
of isolates of F. oxysporum are non-pathogenic Niere et al. 2002. Two fungal endophytes F. oxysporum and Fusarium solani when added to tissue culture
plants were found to be highly effective in immobilizing root knot nematodes IITA 1998.
20
2.12 Interaction between Endophytic Fungi and Plant Parasitic Nematodes
