Some fungi grow within plant tissue but do not cause lesions or other disease symptoms and are referred to as endophytes. These organisms can be mutualistic if they defend the plants against herbivores or pathogens and parasites. Acremonium spp. may secrete general toxins that influence on grazing mammals and herbivorous insects, and induce plant root modification which decrease nematode feeding and reproduction . A number of endophytic fungi are nonpathogenic isolates of ordinary plant pathogens like Fusarium oxysporum that during in vitro tests secreted metabolites which were toxic to Radopholus similis, Meloidogyne incognita and Pratylenchus zeae [274, 275]. Even though this fungus can decrease the numbers of nematodes developing in roots, its mode of action is not clear. This can be due to toxin production, competition for space in the roots, alteration of the physiological state of root tissue, or colonization of feeding cells to the detriment of nematodes [2, 276]. Neotyphodium spp. in the leaves of grasses may also rely on a toxic secretion mechanism to lessen nematode infestations in roots .
Arbuscular mycorrhizal (AM) fungi are the well known plant root associated endophytes. These fungi are obligate symbiotic parasites of plants that have been widely reported to enhance the growth of nematode-infected plants and, in some cases, to decrease nematode infestations [277-279]. Plant growth enhancement is happened by improving plant access to nutrients, particularly phosphorus, and especially under conditions of poor nutrient availability. These fungi also assist access to and uptake of water and alleviate heavy metal toxicity .
Roots were shared as a resource for food and space between plant parasitic nematodes and AM fungi. According to proximity in tissue, more reciprocal effects were expected between AM fungi and endoparasitic nematodes. Migratory endoparasitic nematodes were the only group whose numbers were greater on AMF-infected plants .
Role of AM fungi in suppressing nematode damage to plant and in decreasing nematode densities in the soil has been illustrated in many cases, although most of them include Meloidogyne species. Nematode multiplication rate can be reduced if plant roots are colonized by AM fungi before nematode invasion. The greatest decreases in nematode infestations usually occurring in roots extensively occupied by the fungus before the nematodes invade . Production and secretion of root diffusates may be interfered or some nematotoxic compounds can be produced by AM fungi, however their exact mode(s) of action is not well understood . The efficacy of endophytes depends on the plant colonized and the species of troublesome nematode .
Endophytic fungi, especially AM fungi are produced commercially as crop-growth enhancers. They have the advantage that they can be applied as seed treatments, and then they will multiply rapidly and colonize the rhizosphere and plant roots. This can result in protecting the plants from nematode invasion . The level of nematode management can be satisfactory, although the effect of different isolates of the same species can differ distinctly in suppressing nematode damage . Active isolates of F. oxysporum against R. similis are being developed and undergoing field trial on banana plantations in Central America and East Africa . However it is speculated that induced resistance has an important role in the interaction, but the modes of action are poorly understood .
Some nematophagous fungi have the ability to colonize plant roots as a probable survival strategy . It is demonstrated that P chlamydosporia and P. rubescens endophytically colonize barley roots . The plant defense reactions were probably induced by nematophagous fungi, but these never prevented root colonization. The nematode-trapping and toxin-producing fungi cause necrotic areas on roots at their initial stages of colonization, but were never later observed, even when the fungi proliferated in epidermal and cortical cells. It seemed that monocotyledon plants extensively colonized by nematophagous fungi resulted in producing abundant mycelia, conidia and chlamydospores . The egg parasite fungi, like Pochonia spp., that grow as endophytic fungi may have higher chance to parasitize eggs of economically important endoparasitic nematodes (like cyst and root-knot species) inside the roots and to decrease succeeding spread and roots infection by the second generation of juveniles. Some structures similar to trapping devices were seen in epidermal cells colonized by A. oligospora, which can use to entrap newly hatched juveniles escaping the roots. The ability to colonize plant roots by nematophagous fungi is a novel area of research that deserves in-depth investigations .
The endophytic root colonization potential of different groups of nematophagous species was investigated recently. The egg-parasite P. chlamydosporia and the toxin-producing Pleurotus djamor had the ability to endophytic colonization of barley roots. The nematode-trapping species A. oligospora, D. dactyloides, and N. robustus were all also capable of similar root colonization. Only the endo-parasitic fungi H. rhossiliensis and Nematoctonus pachysporus were not capable of endophytic root colonization [66, 282, 283]. The fungi penetrated into plant cell walls of epidermis and cortex cells by means of appressoria, and developed inter- and intracellularly. It was the first time that appressoria formation was seen in A. oligospora [66, 283].
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