Co-evolutionary relationship between plants and …IJRAR1BHP176 International Journal of Research and Analytical Reviews (IJRAR) 1133 Co-evolutionary relationship between plants and

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IJRAR1BHP176 International Journal of Research and Analytical Reviews (IJRAR) www.ijrar.org 1133

Co-evolutionary relationship between plants and

phytopathogens Sheetal Chopra1,2, Jastin Samuel1,2,*

1Waste Valorization Research lab, Lovely Professional University, Phagwara – 144411, Punjab.

2School of Bioengineering and Biosciences, Lovely Professional University, Phagwara – 144411, Punjab.

Abstract: Plant- microbe interactions are a vital aspect to work on and have been studied for several years and after lots of

research what is currently understood is that bacteria have optimistic force on plant growth and development. Plants can also

choose their microbiome by secreting particular metabolites in the soil microenvironment. Plant pathogens are posing serious

peril to economically important crops worldwide. Plant pathogens are the main culprit for the huge range devastation of

agricultural crops around the globe, and can affect productivity of crops vulnerable to diseases both in the field (pre- harvest)

and post-harvest. Various different kinds of plant pathogens are found such as from small protein structures i.e. Virus to

bacteria, fungi, nematodes and parasitic plants. The present review will discuss about the belowground plant pathogenic

interactions and the mechanism involved in the interactions and other biologically control mechanism to prevent plant

pathogenic infection in agricultural crops.

Keywords: Rhizosphere; plant pathogens; defense system, chemotaxis and secondary metabolites

1. Introduction:

Soil ecosystem is key player in the agriculture, so its continued functioning is vital for agricultural system. The activities

governed by physical and biological environment associated with plant root system have an important role in productivity and

quality of crops. The roots are inhabited by microorganisms which affect plant growth by biological processes, thus,

contributing in raising crop productivity (Korir et al 2017). Soil is resident of enormously diverse population of micro and

macro-organisms. The knowledge of the soil aspects such as bacterial diversity, functions and nutrient availability plays an

important role in improving agricultural practices and conservation methods. The change in the comparative range and

functionality of soil microbial population influence nutrient acquisition and organization and efficiency of native plant

population. The microbial population present in soil system influence mineral solubilisation and mobilization processes. The

soil region where the root system is influenced by processes facilitated by microorganisms are called rhizosphere. Rhizosphere

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term was pioneer by Hiltner in 1904. Its origin is from Greek word- ‘rhiza’ meaning root and ‘sphere’ meaning the field of

influence. Rhizosphere is area around roots in the soil where maximum level of bacterial activity is found. It spreads from a

few millimeters of the root surface and often provides an important environment for some beneficial associations and plant

pathogen interactions (Chen et al 2014). The environment of rhizosphere is highly competitive for different microorganisms to

occupy spaces and nutrients. Therefore, those organisms, either beneficial or pathogenic that compete effectively with others

in colonizing plant tissues and for nutrients will proliferate in the microenvironment as compared to weakened ones.

Traditionally, the root system provides anchorage to plant and helps in uptake of nutrients and water, but it is produces

chemicals that arbitrate several underground interactions. Plant- microbe interactions is a vital aspect to work on and have

been studied for several years and after lots of research what is currently understood is that bacteria have optimistic force on

plant growth and improvement. Plants can also choose their microbiome by secreting particular metabolites in the soil

microenvironment. Plant- microbe interaction has been associated to root exudation. Plants release high amount of carbon

compounds in the soil. These compounds are known as chemo-attractants for microbes and help in directing the microbe’s

movement in the stream of chemical gradients. This movement is commonly referred to as chemotaxis and plays an essential

part in communication between root of host plant and microbes. The chemicals secreted by roots of host plant have positive

interaction, negative interaction and neutral interaction with microorganism in the rhizosphere (Morgan et al 2005).

Pseudomonas, Rhizobium and Agrobacterium species are known to increase their root colonization efficiency in the

rhizosphere due to chemo-tactic response (Brencic and Winans 2005; Neal et al 2012; Haichar et al 2014). Chemotaxis

provides competitive benefit to bacteria in colonization of plant root surfaces, necessary for the organization of beneficial

interactions. Motile soil bacteria can sense and act in response to different chemical signals released by plant roots.

The root hair zones, root tips and the points of emergence of secondary roots are main sites from where colonization is

initiated as these areas contain plentiful amount of root exudates (Scharf et al 2016). Various plant species have dissimilar

composition of root exudates due to difference in stages of development or environmental conditions (Lakshmanan et al

2014). In plant, root system has a key role in plant development and uptake of nutrients and water. Many root rot diseases

unenthusiastically control root function, cause root decay which ultimately leads to plant death, causing striking decrease in

plant yield and quality. It is evident that diseases in plants caused by a particular species of pathogenic microbes or even one

specific strain.

Plant pathogens are posing serious peril to economically important crops worldwide. Viruses, bacteria and fungi are main

causative agents among major pathogens. Reduction in many different crops viz. 78% in crops comprising fruits, 54% in




vegetable crops and 32% loss in cereal crops are recorded due to spread of diseases by harmful pathogens. Plant pathogens are

the main culprit for the huge range devastation of agricultural crops around the globe, and can affect productivity of crops

vulnerable to diseases both in the field (pre- harvest) and post-harvest. Various different kinds of plant pathogens are found

such as from small protein structures i.e. Virus to bacteria, fungi, nematodes and parasitic plants.

The present review will discuss about the belowground plant pathogenic interactions and the mechanism involved in the

interactions and other biologically control mechanism to prevent plant pathogenic infection in agricultural crops.

2. Plant-Pathogenic interaction in rhizospheric ecosystem:

2.1 Types of pathogens:

Pathogens infect plants by various modes of nutrient uptake. On the basis of their mode of infection pathogens are divided into

three groups: necrotrophs, biotrophs and hemibiotrophs. The recognition of plant pathogen lifestyle is very essential to combat

its affect, if it is not recognized early enough then it may aggravate damage in plant. The different kinds of lifestyle of

pathogens inside plant are described as follows:

2.1.1 Biotrophic pathogens

Biotrophic pathogens shares a close relationship with living plant cells as they grow up, imitate and obtain nutrients from alive

plant tissue. Some examples of biotrophic pathogens are Cladosporium fulvum it is causative agent of tomato leaf mold and

Ustilago maydis which is causative agent of corn smut. These pathogens have specific host range i.e. restricted to particular

types of plants. The haustoria is the specialized nutrient absorbing structures that penetrates host plant through the plasma

membrane and receive the nutrient in the liquid form and liberate some chemicals known as effector molecules. During

invasion of phytopathogen, if host cell dies then pathogen is incapable of colonizing that host plant as it needs live host cell for

survival. There are many efficient defense reaction reported against biotrophs such as ROS production, HR, Programmed cell

death and SAR and Salicylic acid signaling is also studied in particular host such as Arabidopsis.

2.1.2 Necrotrophic pathogens

These pathogens kills host plant cells before invasion, as they rely on dead plant tissues for their feed. They usually enter the

host through dead cells or injuries and release toxins and enzymes such as cell-wall degrading enzymes to wipe out dead




tissues of host plant. Necrotrophs not only survive inside the host but they are also capable of surviving outside the host and

also have ability to grow on synthetic media. Sometimes toxins released by pathogens are incapable to destroy or kill host cell

due to genotyopic variation, the different composition of toxins or the difference in the secretion time or fluctuation in actual

concentration i.e. in low amount. Cochliobolus, Alternaria and Botrytis are some species of fungal nectrotrophs. To recognize

the necrotrophs at early stages is very essential for plant host to combat the HR, Programmed cell death (PCD) and oxidative

burst.

2.1.3 Hemibiotrophic pathogens

Hemibiotrophs have intermediate lifestyle, in the beginning they have biotrophic mode of life with the host, therefore kill the

host cells and change their mode of life to necrotrophs. The bacterial species P. syringae and fungus species Phytophthora,

Phythium and Fusarium have this kind of lifestyle and some species in the genera Colletotrichum and Venturia. Most of the

agronomically significant phytopathogens belong to this group of pathogens.

2.2 Rhizospheric Pathogens:

Plant pathogens present in soil are the main cause in the reduction of productivity of food, feed, fiber and fuel crops. These

plant pathogens depend upon host for reproduction, development and for establishment in the rhizosphere. Pathogens can

survive successfully in soil by formation of protective structures. The harmful microorganisms present in soil include fungi

such as Fusarium oxysporum, Verticillium sp. and Rhizoctonia solani and oomycetes include Phythuim sp. and Phytophthora

sp. and nematodes. (Coninck et al 2014). These pathogens cause damping off in seedlings which restrict root development,

stunted growth, and wilting and plant death. The phytopathogenic bacteria in temperate climates are agronomically less

significant than fungi, oomycetes and also nematodes. Aphanomyces euteiches is a root pathogenic oomycete which infects

many legumes and cause major loss of economy in temperate regions (Marra et al 2006; Mathesius 2009). It causes browning

of roots along with decrease in root weight. Oospores appear from infected roots which afterwards re-infect other plants.

Another group of destructive oomycete which infect many plants is Phytophthora sp (Tyler et al 2006). The bacterial genera

such as Pectoacterium, Ralstonia cause substantial economic damage in some crops. Agrobacterium tumefaciens, Ralstonia

solanacearum, Dickeya dadanthi and Dickeya solani and Pectobacterium carotovorum and Pectobacterium atrosepticum, all

attack plant via roots (Mansfield et al 2012). Agrobacterium sp. and protist Plasmodiophora brassicae is root infecting

microbes that induce gall formation in roots (Coninck et al 2014). Viruses also infect plants with the help of vectors like




nematodes or zoosporic fungi for invasion in roots (Campbell 1996; Macfarlane 2003). Various crops and infecting plant

pathogens in rhizosphere are listed in Table I.

Table I : Plant pathogens present in the rhizosphere

Plant species Pathogens References

Rice (Orzya sativa

L.)

Xanthomonas oryzae pv. oryzicola,

Burkholderia glumae, Xanthomonas

oryzae pv. oryzae, Phythium sp.,

Magnaporthe oryzae, Rhizoctonia

solani, Ustilaginoidia virens,

Cochliobolus miyabeanus,

Fusarium fujikuroi,

Coninck et al 2014; Liu

and Wang 2016.

Wheat (Triticum

aestivum)

Pseudomonas syringae subsp

Syringae, Gaumannomyces

graminis var tritici,

Xanthomonas campestris pv.

Translucens,Erwinia

rhapontici,Claviceps

purpurea,Ustaligo tritici, Phythium

sp, Magnaporthe grisea

Pechanova and Pechan

2015.

Maize (Zea mays)

Colletotrichum graminicola,

Aspergillus flavus, Fusarium

verticillioides, F. graminearum,

Curvularia lunata

Coninck et al2014;

Pechanova and Pechan

2015.

Potato (Solanum

tuberosum)

Streptomyces scabies,

Pectobacterium carotovorum,

Berendsen et al2012.





Dickeyasolani, Clavibacter

michiganensis, Ralstonia

solanacearum, Alternaria solani,

Phytophtora infestans, Sclerotinia

sclerotiorum

Cotton (Gossypium

hirsutum)

Xanthomonas axonopodis,

Rhizoctonia solani, Fusarium sp,

Pythium sp, Verticilium

dahlia,Colletotrichum gossypii

Zhang et al 2008.

Tomato (Solanum

lycopersicum)

Phytophthora, Fusarium oxysporum

f.sp. lycopersici (Fol)

Coninck et al2014.

Peas (Pisum sativum

L. cv. Duel)

Aphanomyces euteiches,Ascochyta

sp., Phytophthora clandestine,

Rhizoctonia solani, Phoma

mediaginis

Tinivella et al 2009,

Barbetti et al 2007.

Bean (Phaseolus

vulgaris)

Colletotrichum lindemuthianum,

Aphanomyces euteiches,

Phytophthora clandestine, Phoma

mediaginis

Tinivella et al 2009,

Barbetti et al 2007.

Citrus

Pseudomonas syringae, Xylella

fastidiosa, Xanthomonas

campestris pv. citri, Candidatus

liberibacter asisticus, Botrytis

cinerea, Phytophthora palmivora,

Berendsen et al 2012.


https://en.wikipedia.org/wiki/Ralstonia_solanacearum

https://en.wikipedia.org/wiki/Ralstonia_solanacearum

https://en.wikipedia.org/wiki/Xanthomonas_citri

https://en.wikipedia.org/wiki/Rhizoctonia_solani

https://en.wikipedia.org/wiki/Fusarium

https://en.wikipedia.org/wiki/Pythium

https://en.wikipedia.org/wiki/Colletotrichum_gossypii

https://en.wikipedia.org/wiki/Xylella_fastidiosa

https://en.wikipedia.org/wiki/Xylella_fastidiosa

https://en.wikipedia.org/wiki/Xanthomonas_campestris

https://en.wikipedia.org/wiki/Xanthomonas_campestris




Ashbya gossypii, Colletotrichum

gloeosporioides, Trichoderma

viride, Penicillium digitatum,

Alternaria citri

3. Interaction mechanism of plant pathogens:

Plants and their pathogens shared co-evolutionary relationship in nature. Plant pathogens have parasitic mode of colonization

and they release some molecules (proteins) together known as effectors to various cellular compartments of the host plant to

ensure their colonization inside plant. it is seen that when pathogen colonized inside plant then it alters plant defense system to

some extent (van der Hoorn and Kamoun 2008; Abramovitch et al 2006). Better understanding of these interactions and its

mechanism involving molecular aspect of the initiation of pathogen colonization process inside plant host and pathogenicity is

a critical task. The term “effector” is used for degradative enzymes, toxins, PAMPs, and MAMPs in the field of plant-microbe

interactions.

To deliver those effector molecules inside host plant a special secretary system is needed, for example, the secretion system of

type III (T3SS) in Gram negative microbes (Zhou and Chai 2008; Abramovitch et al 2006). For transfer of pathogenicity

inside the host cell, biotrophic fungi and oomycetes have developed a special structure called haustoria (Dodds et al 2009).

Some host selective toxin such as ToxA of Pyrenophora tritici-repentis (necrotrophic fungus) can enter plant cells without the

pathogen (Manning and Ciuffetti 2005). Plant surface receptor, target plant machinery such as photosynthetic machinery

facilitates its entry into plant host, boost in the amount of reactive oxygen species (ROS) and eradicate the host (Manning et al

2008). Cladosporium fulvum (pathogen of tomato) produces some effectors in the extracellular interface of plant-pathogen and

Plant pathogens have T3SS effectors whose core role is to repress the plant innate immune system (Pelgrom and Ackerveken

2016). Effector molecules are capable of altering morphological characters and behaviour of host plant, as seen in

Xanthomonas citri. The pthA expression in cells of citrus plants results in the canker growth, and the hyperplastic lesion

symptom become visible as macroscopic. These cankers cause expansion of bacterial colonies inside the host plant from

infected tissue to the other regions. In plant- microbe interactions, many plant beneficial organisms lead to formation of

block apoplastic plant defense system (Misas- Villamil and van der Hoorn 2008).


https://en.wikipedia.org/wiki/Ashbya_gossypii



protective niches in the form of special structures for their enhancing their dispersal, for example root nodules developed in

rhizobia (Oldroyd and Downie 2008) and crown galls in Agrobacterium spp. (Chalupowicz et al 2006). Several plant

pathogens generate hormones such as auxins and cytokinins which shows phenomenon of phytohormone mimicry. Cytokinins

are reported to be produced by Rhodococcus fascians and Streptomyces turgidiscabies (Hogenhout and Loria 2008).

Gibberellins are also produced by some fungi viz Gibberrella fujikuroi which cause foolish seedling disease in rice (Kawaide

2006). Plant pathogens are major cause in the destruction of crops and it is very important to understand the interaction

mechanism with host plant to overcome this threat.

4. Role of different components in plant pathogen interactions:

Various studies have revealed that numerous number of plant compounds hold ecological and chemical defensive role, which

give rise to the new arena of research called as ecological biochemistry. In this section we will discuss about some of the plant

components which play role in these interactions:

I) Secondary Metabolites:

Secondary metabolites are synthesized by plant to protect the plant against various plant pathogens. Different metabolites have

different roles such as some aid plant for contacting various other organisms and some protect plants from abiotic stresses e.g.

UV-B radiations. Therefore, they are important components which help in plant improvement and growth. In plant, for

containing specific compounds secondary metabolites are divided into three categories: Phenolics, Terpenes and

Nitrogen/Sulfur. Terpenes contain 5-C isoterpenoid as their prime unit which are toxic to herbivores and kill them. Amino

acids are the major contributor of nitrogen nd sulfur containing secondary metabolites. Phenolics are produced by Shikimic

acid pathway which gives defensive ability to plants. It is considered that more than 100,000 metabolites are present in plant

defense system which is result of plant pathogen interactions in million of years.

In plant-pathogenic interactions, secondary metabolites are key players. In most of the previous studies it is evident that

secondary metabolites have a key position in defense response against pathogenic interactions. Their production and activation

is assisted by microbial detection by means of defense proteins or MAMPs recognition by pattern recognition patterns. Due to

their diversity in the structure and biosynthetic pathways, various criteria has been introduced foe their classification in plant

immunity. These criteria’s are based on common precursors, mechanism of action and core structure. The most commonly

used criteria for their classification is the way of synthesis and increase in defense related phytochemicals. De novo production




of metabolites due to an infection named as phytoalexins, while production and storage of defense related metabolites in plant

tissues is termed as Phytoanticipins.

II) Plant hormones:

Plant hormones are produced by plants and are involved in the overall development of plant viz. plays role in cell division, cell

elongation, tissue segregation, nutrient acquisition, abscission, ripening and apical dominance. Plant hormones are produced

by microorganisms colonizing the root to encourage plant development by enhancing the length and density of root hairs. Root

surface area id increased by enlargement in root hairs which helps in enhancing the potential of plant to uptake water and

mineral nutrients (Volkmar and Bremer 1998). Plant hormones such as auxins, gibberellins, cytokinins and abscisic acid are

produced by microbes residing in plant root. Some defense hormones such as salicylic acid, jasmonic acid and ethylene are

also produced by plants which help in plants in acquiring resistance against phytopathogens. These plant hormones protect

plant in opposition to both biotic and abiotic stresses (Fujita et al 2006).

AUXINS: It is reported that auxins a plant hormone is produced by plant beneficial rhizobacteria (Patten and Glick 1996).

Auxins are secondary metabolites or signaling molecules which alters the function of gene of some microbes which that create

hindrance in plant growth and improvement. As in seen in plants, microorganism also exhibit Multiple pathways are involved

in production of auxins. Production of root exudates is facilitated by the tryptophan present in them. IAA is commonly

produced by various plant growth promoting bacterial genus viz. Pseudomonas, Bradyrhizobium, Azospirillum, Alcaligenes,

Rhizobium Enterobacter, Aeromonas, and Comamonas (Tailor and Joshi 2014). It helps fungus to invade and colonize the

plants which in turn modifies the plant’s basal defense mechanisms and plant growth stimulation. In Zea mays and

Arabidopsis thaliana, Trichoderma inoculation affects root system architecture which helps in the formation of root (lateral)

and as well as its hair growth thus enhancing the plant yield (Contreras-Cornejo et al 2009).

CYTOKININS: Rhizospheric microorganism also produces cytokinins. The role of cytokinins produced by microorganisms is

still not clear and complete in previous studies (Olanrewaju et al 2017). The compostion of plant hormones varies when plant

growth promoting bacteria is inoculated in the plant, as these bacteria contain cytokinin gene. It is evident from the lettuce

plant investigation, in which the inoculation of Bacillus subtilis into the plants by injection yielded cytokinin that was

observed in the plants (Arkhipova et al 2005).




GIBBERELLINS: Gibberellins hormones have a vital role in germination of seed, elongation of the stem, flowering, ad fruit

setting. This is mainly attributed to the elevated rate of photosynthesis and the chlorophyll content affected by the hormone

(Zaidi et al 2015). Deficiency of this hormone results in the lessening in number and length of the lateral roots. First time, this

hormone was found in Azospirillium brasilense and Rhizobium (Tewari and Arora 2013; Tien et al 1979). Many PGPB are

known for producing gibberellins viz. Agrobacterium, Rhizobium, diazotrophicus, Gluconobacter, Bacillus, Azospirillum,

Clostridium, Azotobacter, Flavobacterium, Pseudomonas, Micrococcus, Xanthomonas, Arthrobacter, and Herbaspirillum

seropedicae, Achromobacter xylosoxidans, Burkholderia and Acinetobacter calcoaceticus (Deka et al 2015; Dodd et al 2010).

ABSCISIC ACID: ABA negatively affects disease resistance in plant pathogen interactions. It is seen that some of the

pathogens produce ABA by themselves or induce ABA accumulation. On the contrary, activation of plant defense system

results in inhibition of ABA accumulation and signalling. For prioritizing immune responses mutual antagonism is probably

needed or ABA-regulated abiotic stress reaction in changing environments. It is advantageous for plants to alleviate immune

responses under low humidity viz. drought where ABA signalling is activated, because pathogen virulence prefers high

humidity (Aung, Jiang, & He 2018). Conversely, plants enhance disease resistance during immune activation by actively

reducing the inhibitory effect of ABA. Depending upon the stages and mode of pathogen infection ABA contributes in

suppressing plant immunity (Ton et al 2009).

5. Plant response towards these interactions:

Interaction between the induced strain and host plant trigger successful induction of plant defense (Van Wees et al 2008).

Different rhizospheric bacteria are involved in initiation of Induced Systemic Resistance (ISR) in variety of plants species

when invading microbes are either beneficial or pathogenic (Bakker et al 2003, 2007). SAR and ISR are two different types of

defense signalling pathway. Plant hormones as Salicylic acid (SA) and Jasmonic acid (JA) play role in both beneficial and

pathogenic communication. SA is a signal molecule which has important role in regulation of pathogen-induced Systemic

Acquired Resistance (SAR) (Van Wees et al 2008). SA dependent defense is effective against biotropic pathogens which

inhabit live plant (Glazebrook 2005). JA /ET (Ethylene) are signal molecules which play important role in Induced Systemic

Resistance (ISR) which give resistance against necrotrophic pathogens which kills plant cell. Alternaria brassicicola is a

necrotrophic fungus which is resisted by ISR (Induced Systemic Resistance) defense signaling but not by SAR (Systemic

Acquired Resistance) signaling pathway. Turnip crinkle virus is biotropic and is resisted by SAR and not by ISR (Ton et al

2002). It has been revealed that SA levels rapidly increase in wild-type rice roots and pea at initial stages of Arbruscular

Myccorhiza (AM) interaction due to formation of hyphopodia on the root surface (Blilou et al 1999, 2000). The increase in SA




seen at initial level of AM colonization in Arabdopsis is due to presence of fungal strains in roots which trigger defense-gene

(Liu et al 2003). Similar results were observed in wild type pea roots and alfa alfa roots when they were inoculated with

Rhizobium strain. Inoculation with mutant strain malfunctioned in Nod factor production abolished symbiont detection by this

plant (Martinez-Abarca et al 1998). The documented literature states that when symbiont invades the roots then the plant

defense system involving SA is triggered. Later, production of SA ceases to facilitate colonization. However, the defense

system remains active if the symbiont is perceived as a pathogenic at initial stages of interaction (Gutjahr et al 2009). JA has

been reported to be secreted by roots, as in soyabean and wheat in the rhizosphere (Creelman and Mullet 1995), showing that

microorganism present in soil near roots might be exposed to JA molecules. The direct effect of JA on AM fungi is not known.

Rhizobia induced expression of Nod genes and production of Nod factors has been observed upon application of JA

exogenously and methyl jasmonate (MeJA). The combination of two plant hormones such as JA and flavonoids showed

enhanced Nod factor production (Mabood et al 2006).

6. How to prevent these interactions?

Several mechanisms are involved to prevent these interactions which are carried out by special agents called biocontrol agents.

These agents are produced by rhizospheric microbes for reducing the risk of infection inside plants by the attack of

phytopathogen (Lugtenberg and Kamilova, 2009; Glick, 2012). Plant growth promoting bacteria have primary approach of

biocontrol activity such as ruling out the niche, activating the systemic resistance, competitive approach in pathogens for

nutrients, different kinds of volatile compounds are also present such as hydrogen cyanide, siderophores known as iron

chelating compounds and production of compounds (antibiotics) used in antibiosis. Among the plant pathogens most harmful

are fungus followed by viruses and bacteria. In plants, plant receptors are present which receive elicitor signals from microbes

for combating the infection inside plant parts. After receiving the signals, plant restrain the attack by phytopathogens. Bacillus,

Escherichia, Pseudomonas, Rhizobium and Agrobacterium are the genera which produces such elicitors.

ANTIBIOSIS: Plant growth promoting bacteria exhibit capacity to inhibit entry or colonization of pathogenic bacteria inside

plant by secreting different kinds of antibiotics (Raaijmakers and Mazzola 2012). Antibiotics produced by microbes have low

molecular weight and even in low concentration they are capable of inhibiting growth or metabolic activities of plant

pathogens. Agrobacterium radiobacter strainK84 produced a well-known biocontrol agent (Agrocin 84), first of its kind which

is capable of controlling pathogenic effect of Agrobacterium tumefaciens. Pseudomonas produces various different kinds of

antibiotics viz. cyclic lipopeptides (cLPs), phenazines (PHZ), pyrrolnitrin (PRN), phloroglucinol, pyoluteorin (PLT), 2,4-

diacetylphloroglucinol (DAPG), and HCN.




LYTIC ENZYME: Rhizospheric microorganism produces lytic enzyme which helps in degrading cell wall of pathogenc fungi

causing infection inside plant. the production of lytic enzymes by rhizospheric microbes is an ecologically naturally evolved

activity to biocontrol and defend the plants from phyto-pathogens. There are various kinds of lytic enzymes viz. Proteases (cell

wall proteins degrading enzyme), β-1, 3- glucanases (lyse β-1, 3 glucan), Chitinases (Chitin degrading enzyme), Cellulases

(cellulose lysis); Xylanase (xylans); Pectinases (pectin degrading enzyme) (Vaddepalli et al 2017); etc. These enzymes help in

inhibiting progression of pathogen and block the activities of pathogens by several mechanisms.

HYDROGEN CYANIDE: Various plant growth promoting bacteria such as Rhizobium, Chromobacterium, Aeromonas,

Bacillus, Pseudomonas, Burkholderia, Cyanobacteria and Alcaligenes produces volatile secondary metabolites known as

HCN that function as biocontrol agent against phytopathogens (Ahemad and Kibret 2014; Blumer and Haas 2000) and

Meloidogyne javanica Thielaviopsis basicola (causative agent of tomato root knot and tobacco black root rot respectively) has

been concealed by HCN (Siddiqui et al 2006). HCN works by different modes and one of the most important modes is by

inhibiting the cytochrome c oxidase of electron transport chain of pathogenic microbes. The inhibition of ETC in pathogenic

microorganism results in energy liberation and finally causing death of harmful microbe (Nandi et al 2017).

COMPETITION: In rhizosphere, PGPRs shows competition with pathogens for inadequate nutrients availablility. Due to

presence of biocontrol activity in PGPR, they inhibit the colonization of other competing microbes around the plant root

surface and dominates rhizosphere. PGPR competes with population of phytopathogen by reducing the nutrient availability.

This competition limits the binding of plant pathogen to the plant and makes it very difficult for them to proliferate. PGPR

produces low molecular weight compounds known as siderophores which chelate irons and creates starvation in the

phytopathogens for iron nutrient which ultimately leads to suppression of the pathogen.

Conclusion and future perspectives:

Sustainable development is need of the hour for reducing problems faced by current agriculture. The increment in the crop

productivity is very important as population is increasing drastically. Plant pathogenic interactions give us great approach to

understand the mechanism developed by plant naturally to combat the infection caused by phytopathogen. More knowledge is

required to understand the virulence ability of pathogens and their attacking mechanism which will help plant pathologists to

develop new techniques to prevent our crops from the dreadful pathogens. When the scenario is clear about the mechanism

involved in the infection process and the response given by plant defense system then we can adopt new breeding techniques




and genetic manipulation to produce crop varieties with effective resistance against phytopathogens. Various tools like

microarray analysis give us a picture of the total number of genes (upregulation or downregulation) affected during infection

process. Bioinformatical analysis tools should be used to derive as much as information as possible from the recently cloned

genomes of plant and pathogens alilke.

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