Endophytes : As Potential Biocontrol Agent — Review and Future Prospects

Endophytes are the microbes residing internally in the host tissues without causing visible disease symptoms. They have found involved in a balanced interaction with the plants and providing benefits such as, growth enhancement and disease resistance. In this review we hypothesize that endophytes can be employed as a potential biocontrol agent, as biocontrol is becoming most suitable disease management strategy due to its health and environment conservational benefits. This aspect of endophytes should be consider, there are several investigations that have revealed and proved the role of endophytes as best biocontrol agent. Mutualistic interaction of endophytes involve different mechanisms, as it may trigger certain genes involved in induced systemic resistance (ISR) that may initiate defense mechanism against attack of pathogens or by formulating secondary metabolites and other chemical compounds that are directly toxic to the pathogens. There is a need to explore the endophytic interaction and its mechanism of causing disease resistance more precisely.


Endophytes
Endophyte was defined as "endophytes colonize internal tissues of host without causing symptoms, but chances are there that endophytes may cause disease after completing latency period" (Petrini, 1991).The word endophytes literally means "within plants" (In Greek; endon-within and phyton-plants).Endophytes is a vast term with respect to its literal meaning, host plants and inhabitants, such as fungi (Stone, Bacon and White, 2000), bacteria (Kobayashi & Palumbo, 2000), insects (Feller, 1995) and algae (Peters, 1991).Endophyte colonizes plant tissues internally (Carroll, 1986), without causing visible disease symptoms.They live in symbiotic interaction with plants.And they also show variation in symbiotic interaction, which ranges from facultative saprobe, to parasite, to mutualistic.However, like all endophytic interactions provides nutritional benefits and protection against environmental and microbial stresses (Schulz & Boyle, 2005).different regions such as temperate, tropical regions and in boreal forests (Zhang et al., 2006).Arbuscular mycorrhiza fungi are present extensively throughout the terrestrial ecosystem, and fossil records and molecular analyses shows their association with plants from their origin millions of years ago (Redecker, Kodner, & Graham, 2000).Mutualistic bacteria has been identified in both monocots and dicots, that ranges from higher plants such as oak and pear, to lower plants like sugar beet and maize (Ryan et al., 2007).
Endophytic bacteria found within the plant system are dynamic, varied, and diverse (Sturz et al., 1997).For such plant-endophyte relationship to be stable and successful, some form of synchronization must be present.Bacterial endophytes lives, adapts and survives within the suitable environment provided by the host plants.And the host plants also get benefits from this partnership, such as growth promotion and protection (Shishido et al., 1995).

Endophytic Diversity
Endophytes show more diversity and abundance than plant pathogens within the plant systems (Ganley et al., 2004).These symbionts are very diverse, only small number of them has been characterized (Rodriguez et al., 2009).Endophytes mostly belong to phylums Basidiomycota and Ascomycota and they may be from orders Hypocreales and Xylariales of class Sordariomycetes or Loculoascomycetes (Unterseher et al., 2011).
Grasses mostly involve endophytic fungi belonging to family Clavicipitaceae, tribe Balansiae.There are five genera and about 30 species in the tribe (Luttrell & Bacon, 1977).The genera Atkinsonella and Myriogenospora contain only one specie while the genera Balansia, Balansiopsis and Epichloe' contain more than one species.Balansia is the most diverse of all having 15 species (Diehl, 1950).These genera are classified on the basis of conidia formation (Clay, 1986).These fungi are termed endophytes, found in host meristem, young leaves and inflorescence (Leuchtmann & Clay, 1988).However, most species invade vegetatively running parallel to the long axis of host leaf and stem tissue cells (Clay, 1989).
Arbuscular mycorrhiza fungi is a part of mutualistic rhizosphere, these are micro symbionts that are involved in improvement of plant nutrient uptake and provides protection against different stresses (Smith & Read, 1997).AMF involves biotrophic Glomeromycota associated with different species of plants ( Van der Heijden et al., 2015).
Review of previous studies on bacterial endophytes have characterized some of the bacterial types isolated from within the plant tissues after surface cleaning of plant tissues by using disinfectant such as sodium hypochlorite (Miche & Balandreau, 2001).The diversity of endophytes extracted from the poplar trees have been explained in a study (Porteous-Moore et al., 2006).Five taxa of endophytic bacteria were identified as Microbacterium, Pseudomonas, Clavibacter, Curtobacterium, Cellulomonas by molecular techniques such as gene sequencing and by fatty acid analyses (Zinniel et al., 2002).Number of bacterial endophytes has been extracted from the vascular tissues of citrus varieties such as E. aerogenes, Acinetobacter baumanii, Bacillus spp., Burkholderia cepacia, Citrobacter freundii, Corynebacterium spp., Arthrobacter spp., Enterobacter cloacae, and Pseudomonas aeruginosa, Acromobacter spp., Acinetobacter iwoffii, Alcaligenes-Moraxella.Some studies have concluded bacterial endophytes as polyphyletic belonging to vast range of taxa, such as Actinobacteria, α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, and Fermicutes (Miliute et al., 2015).
Rhizobacteria is also included in bacterial endophytes, playing vital role in host plants survival (Dobereiner, 1993).

Mode of Action
Number of studies has been done but how endophytes effect the plant disease severity is still unknown (Busby, Ridout, & Newcombe, 2016).Induction of host defense mechanism is consider to happen first, as Bacteria (Sequeira et al., 1977), nematodes (Kosaka et al., 2001), Viruses (Ross, 1961) and fungi (Pozo et al., 2002) induces plant defense mechanism, such as Systemic acquired resistance (SAR) and Induced systemic resistance (ISR) (Van Wees et al., 2000).For example, a fungi Colletotrichum tropicale has stimulated hundreds of genes and their expression caused greater plant immunity in Theobroma cocao (Mejia et al., 2014).
Endophytes can also minimize the defense mechanism of plant allowing other pathogens to cause disease (Houterman et al., 2008).Many studies have been done and their results have shown suppressing effects of endophytes due to competition or endophytic metabolites (Martin et al., 2015).For example, Ampelomyces spp.Suppress the powdery mildew sporulation (Kiss, 2003).
Induced systemic resistance (ISR) is a unique way by which endophytes enhances the plant defenses against number of pathogens.Various root-inhabiting mutualists, such as Trichoderma, Bacillus, mycorrhiza species and Pseudomonas triggers the immune system of plant for enhanced defenses against pathogens (Pieterse et al., 2014).
"Induced resistance" is a term used for the resistance stimulated by chemical or biological agents, which helps the plants to fight against the pathogen attacks in the future (Kuc, 1982).ISR is only initiated when endophytes colonizes the root system of host plants (Lugtenberg & Kamilova, 2009).Biofilm formation is important for the root establishment of B. subtilis, polysaccharides of host cell wall stimulates the matrix production by triggering the bacterial genes (Beauregard et al., 2013).
The endophyte adapts new lifestyle for the sake of survival, in the dynamic medium of the host cells by host-specific metabolic cues (Lahrmann et al., 2013).Trichoderma spp.establishes around the plant roots, where it forms a structure like appressorium which is an important characteristic of pathogenic fungus (Mukherjee et al., 2013).Pseudomonas, Bacillus, and Trichoderma strains for establishing themselves around plant roots uses auxin as a triggering agent for the formation of large number of lateral roots, which helps in better nutrient uptake and defense against pathogens (Contreras-Cornejo et al., 2009).
Endophytes are found responsible of producing bioactive compounds that contributes to their biocontrol activity (Akinsanya et al., 2015).An endophytic fungus Phomopsis spp. is found responsible of producing number of secondary metabolites including antimicrobial and antifungal compounds (Erbert et al., 2012).The biologically active Xanthones were found in the fermentation products of Phomopsis spp.(Yang et al., 2013).

Dependency
Review of literature shows the dependency of endophytes on biotic and abiotic factors, host and pathogen (Busby et al., 2016).Environmental factors such as humidity, pH and temperature effects the endophytic interaction of fungi (Cook & Baker, 1983).For example, Trichoderma activity is influenced by soil moisture (Jones & Bienkowski, 2015), and Candida activity is effected atmospheric conditions add strength against apple pathogen (Usall et al., 2000).In a trial, variations were observed in the endophytic activity against Dutch elm disease, indicating that there may be some abiotic factors involved that influences endophytic activity (Martin et al., 2015).
Nonconductive or poor soil conditions are thought to affect the biological control activity of endophytes against plant pathogens (Handelsman & Stabb, 1996).
Disease triangle consists of three components, host, pathogen and the environment, each component should be present for the occurrence of the disease, besides environment the other two components also influences the activity of endophytes.In case of rust disease the influence of these components has been observed (Nischwitz et al., 2005;Kiss, 2003).
But in an experiment, it has been proved that pathogen matters most in disease occurring activity.i.e., Colletotrichum gloeosporioides and Pestalotia psidii were tested against fifteen endophytic species.Fourteen of them showed defense against C. gloeosporioides, while nine were defensive against P. psidii (Pandey et al., 1993).In another finding, nine endophytic species were tested against following wheat pathogens: Drechslera tritici-repenti, Alternaria triticimaculans, Zymoseptoria tritici, and Bipolaris sorokiniana.Nine of them showed full defense against Zymoseptoria tritici and Drechslera tritici-repentis, eight showed defense against Bipolaris sorokiniana, and four against Alternaria triticimaculans (Perello et al., 2002).

Claims
In this review we have claimed that "Endophytes are potential biocontrol agent" and our claim is based on the present research literature that has been reviewed in this regard.
Endophytes are believed to have biocontrol potential against plant pathogens (Sapak et al., 2008).Presence of endophytes in plant systems provides beneficial effects (Ting et al., 2010).Many studies have concluded the role of endophytes as potential biocontrol agent mainly against pathogens of vegetable and fruit crops, as in case of Chinese cabbage (Narisawa et al., 1998).Endophytes shows biocontrol agent properties against pathogens of tomato (Hallman & Sikora, 1995), banana (Ting et al., 2008), barley (Boyle et al., 2001).Biocontrol properties are also shown by endophytes by controlling Ganoderma boninense in oil palm (Sapak et al., 2008).
Number of investigations has revealed that endophytic fungi can be used as a biocontrol tool (Sikora et al., 2008).The endophytic fungi play antagonistic role and minimizes the threat of nematode attack (Sikora, 1992).Endophytic Fusarium oxysporum decreased the number of nematodes on banana (Sikora et al., 2008).
Many studies have also concluded that some bacteria, along with endophytic bacteria (EB), enhances the symbiotic activity of AMF in the host, and can be used as biocontrol agent against plant pathogens (AzcÓnAguilar et al., 1998).So, it has been recommended to use AMF and EB together as biocontrol partners and it has been proved (Gianinazzi et al., 2010).
Endophytes consist of mutualistic symbionts that can be used as potential biocontrol agent of plant pathogens.The potential of endophytes during symbiont/host interaction has been revealed in number of studies (Sturz et al., 2000).
Endophytic bacteria live in the same environmental conditions as the plant pathogens, such as vascular wilt pathogens.This is a positive aspect for endophytes to serve as biocontrol agents.Excessive research on biocontrol properties of microbes has revealed that endophytic symbionts extracted from plant tissues shows potential as biocontrol agent against pathogens (Duijff et al., 1997), nematodes (Hallmann et al., 1998) and insects (Azevedo et al., 2000).
Biological control of plant pathogens has been observed and proved on grasses having symbiotic association with endophytes.In vitro and field demonstration has been performed and suppression of diseases have noted in case of grasses associated with endophytes (Siegel & Latch, 1991).In past, potential use of endophytes as biocontrol agent has been revealed by many investigators (Schardl, 2001;Sturz et al., 2000).

Why Biocontrol?
Because, there are growing concerns with the detrimental environmental effects of chemicals used to control plant diseases.They cause soil, water and air pollution, and are often made from expensive and non-renewable petrochemicals that have many adverse effects on natural environment.Moreover, repeated chemical treatments are required for efficient control with increases the initial economic cost (Clay, 1989).And also, use of chemicals to control diseases favors the addition of resistant mutants to pest population.As a result, biocontrol has become an important integrated management strategy (Waage & Greathead, 1988).
The chemical compounds used for the management of plant diseases are not safe for the healthy environment.Consequently, we need to devise integrated managemental strategies.So, biocontrol is employed as a reasonable strategy for disease management (Mejia et al., 2008).
As human pathogens are becoming resistant to antibiotics similarly the plant pathogens have become resistant to many chemicals used for their control.With the excessive use of chemicals pathogens have develop resistant strains.For example, Ustilago, Pythium, Phytophthora, Penicillium, Mycosphaerella, Sphaerotheca, Verticillium, Botrytis, Cercospora, Colletotrichum, Fusarium, Aspergillus and Alternaria are the fungal pathogens that have developed resistant strains against fungicides used against them (Agrios, 2005).Resistant strains of Erwinia amylovora to antibiotic streptomycin, causal agent of fire blight, had been known since the late 1950s (McManus & Jones, 1994).

Evidences
Endophytes are potential biocontrol agent is supported by number of evidences from the literature reviewed.Some members of Acremonium sp. can colonize roots or shoots and can decrease nematodes population; including Acremonium coenphialum (Pedersen et al., 1988), Acremonium lolii (Stewart, 1993) and Acremonium strictum (Goswami et al., 2008).Acremonium implicatum is a fungus that negatively affect Meloidogyne incognita causing root galls.This fungus was isolated from these galls caused due to Meloidogyne incognita (Lin et al., 2013) also from eggs of Meloidogyne hapla (Figure 1).
Greenhouse demonstration has revealed the effective role of F. oxysporum isolates in controlling R. similis in Uganda (Niere et al., 1998).Studies have proved the effectiveness of biological control of R. similis using Fusarium oxysporum in banana cultivars (Figure 2) (Pocasangre et al., 2000).Evidences have proved that   Despite of all the progress briefly described here, endophytes still need attention of researchers.As it can become an ultimate tool to handle plant diseases more effectively.Especially, the complex decline diseases that are major threat to the perennial plants and difficult to manage, e.g., destructive mango wilt disease.
At the end, by considering the great potential of endophytes the road map for the future research can be designed.
The understanding of the plant-microbe interaction should be given primary importance because by knowing this interaction better, it could one day leads to develop crop plants that can interact with endophytes/beneficial microbes more efficiently.Eventually, we can move towards gaining our goals of sustainable agriculture.