Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5968_Библиотеки_им_академика_М_И_Перельмана
.pdf
via various plant growth-promoting substances. They not only help in enhanced
plant nutrition but also increase plant vigor and stress tolerance (Vacheron et al.
2013a). The concept of rhizosphere was introduced by Hiltner (Hiltner 1904
), while
the term PGPR was coined b y Joseph W. Kloepper in the late 1970s which described
it as a narrow stretch of soil adjoining the roots where microbial populations are
rejuvenated by root activities. The use of PGPR has tremendously increased during
the past few years all around the world, in the sector of agriculture. Several reports
have demonstrated the contribution of PGPR to the benefit of agronomically impor-
tant crops and have shown a significant increase in growt
h and yield when inoculated
together. Functions of PGPR in promoting plant vigor, increasing yields, providing
systemic resistance, and controlling root diseases in plants like Arabidopsis thaliana,
tomato, pepper, carrot, etc. have shown the necessity of applying PGPR into
commercial substrates. Therefore, peat-based substrates are now being used with
PGPR in various plant improvement programs. Applicability of PGPR can be seen in
various areas where the effect
s lie from stimulation of plant growth by plant
hormone production, soil bioremediation to improve soil structure, and enhanced
nutrient absorption by increasing the root volume and availability of nutrients within
the rhizosphere. PGPR application also inhibits the activity of plant pathogens. They
also produce several phytohormones, siderophores, and fungicides and are used as
biofertilizers.
244 K. Kiran and G. Sharma
The simultaneous direct a
nd indirect processes conducted by PGPR include the
solubilization of chemical nutrients, the promotion of alternative pathways of nutri-
ent uptake, and the production of phytohormones. Also, PGPR synthesize various
antimicrobial compounds and induces systemic resistance in plants which indirectly
prevents or mitigates the effects of various phytopathogenic organisms (Oleńska
et al. 2020). Based on capabilities, over the last two decades, several potential strains
of PGPR have been characterized and evaluated in green house, field studies, and
laboratories (Martínez-Viveros et al. 2010). But then due to the lack of precise
information of the effect that PGPR have on the plant growth and crop production,
the implementation of this technology is still limited in the agriculture industry
(Vejan et al. 2019).
Loss in crop production and reduced cultivable land area due to climate change
has significantly rose the temperature globally (Mohanty et al.
2021; Shah et al.
2021a). Industrial revolution and human interventions to compensate the crop
production with massive usage of chemical inputs have constantly unestablished
the environment (Delitte et al. 2021). Over the evolution, eventually the stress
conditions originating from either of the biotic, abiotic, chemical, or soil have
shown the adaptability of plant roots for a mutualistic mechanism with the microbes
to protect themselves and develop further to survive. Recent computational and
bioinformatics developments made to identify bacterial strains with PGPR properties
from huge genomic data have simplified the process of identification and selection of
such strains. These advancements have further enhanced the understanding of the
regulatory mecha nisms of the rhizosphere zone for better growth and development
of plants under the most adverse conditions. This chapter aims to cover an overall
https://t.me/medicina_free
(continued)
15 Regulation of PGPR-Related Genes in Medicinal Plants in Adverse Conditions 245
Table 15.1 List of PGPR isolated from various medicinal plants and their properties
Name of PGPR Host medicinal plant Property Reference
Pseudomonas
aeruginosa
Crocus sativus Biocontrol and plant
growth enhancement
Ahmed et al.
(
2019)
Bacillus megaterium,
Azospirillum
amazonense and
Azotobacter sp.
Curcuma longa Higher flavonoids and
curcumin content,
antioxidant activity
Dutta and Neog
(
2016)
Pseudomonas and
Azotobacter spp.
Cymbopogon citratus Increase in essential
oil content and yield
Mirzaei et al.
(
2020)
Rhizobium sp.,
Pseudomonas
plecoglossicida,
Bacillus toyonensis
Datura stramonium Changes in alkaloids
and amino acid profile
Rahmoune et al.
(
2019)
Pantoea dispersa and
Pseudomonas
aeruginosa
Eichhornia crassipes Salinity tolerant and
increase in
phosphorus
(P) solubilization
Kuramae et al.
(
2020)
Lysobacter sp. Glycyrrhiza uralensis Glycyrrhizic acid
biosynthesis
Zhong et al. (2022)
Pseudomonas
fl
uorescence
Hibiscus sabdariffa Plant growth
promotion and water
stress tolerance
Sanayei et al.
(
2021)
Azospirillum,
Pseudomonas and
Bacillus spp.
Hyssopus officinalis Increase in essential
oil and chlorophyll
content and yield
Sharifi (2017)
Microbacterium
aurantiacum
Nicotiana
benthamiana
Production
of VOCs Prabhukarthikeyan
et al. (
2020)
Pseudomonas putida,
Azotobacter
chroococcum, and
Azospirillum
lipoferum spp.
Ocimum basilicum Increase in growth
and N, P, K content
and essential oils
Ordookhani et al.
(
2011)
Azospirillum
baldaniorum
Ocimum basilicum Drought stress
tolerance
Mariotti et al.
(2021)
Azospirillum,
Alcaligenes,
Arthrobacter,
Acinetobacter,
Bacillus,
Burkholderia,
Enterobacter,
Erwinia,
Flavobacterium,
Pseudomonas,
Rhizobium, and
Serratia spp.
Ocimum basilicum,
Marrubium vulgare,
Melissa officinalis,
Origanum syriacum,
Quisqualis indica,
Solidago virgaurea,
Melilotus officinalis,
Cymbopogon citratus,
Matricaria
chamomilla, Thymus
vulgaris, and
Majorana hortensis
PGPR traits such as
production of IAA,
HCN, siderophore,
phosphate and
potassium
solubilization,
chitinase activity
Jansson and
Hofmockel (
2018)
Pseudomonas
fluorescence
Rosmarinus officinalis Increase in essential
oil content in salinity
conditions
Ordookhani et al.
(2011)
https://t.me/medicina_free
aspect of PGPR and the related association with plants with medicinal properties
(Table 15.1).
246 K. Kiran and G. Sharma
Table 15.1 (continued)
Name of PGPR Host medicinal plant Property Reference
Pseudomonas
fluorescens
Rosmarinus officinalis Increase in dry matter,
yield, nutrient uptake,
photosynthetic
pigments, and content
of flavonoid and
essential oil
Kasmaei et al.
(
2019)
Bacillus subtilis,
Pseudomonas
aeruginosa, and
Cedecea lapagei
Rosmarinus officinalis Increase in dry matter,
yield, nutrient uptake,
photosynthetic
pigments
Sharma et al.
(
2021)
Priestia endophytica Trigonella foenum-
graecum
Increase in nitrogen
assimilation
and
secondary metabolite
production
Rizvi et al. (2022)
Azospirillum,
Azotobacter,
Pseudomonas, and
Bacillus spp.
Withania somnifera Plant growth and
increase in alkaloid
content
Rajasekar and
Elango (
2011)
15.2 PGPR and Plants: Mechanism of Actions
PGPR rhizosphere zone, through direct and indirect mechanisms, can potentially
enhance the plant growth, yield, and development. The most vital mechanisms
include sequestration of metals and degradation of xenobioti c compounds thereby
helping in bioremediation of heavy metal (Glick 2010), 1-Aminocyclopropane-1-
carboxylate (ACC) deaminase (acdS) production to reduce ethylene-induced stress
(Glick
2004), production of phytohormones like IAA, CK, GA, and ABA, supplying
nitrogen (nitrogen fixation) and iron chelation by producing siderophore, enacting as
a biocontrol agent, solubilization of minerals, and mainly enhancing resistance
against any non-biological resistance (Compant et al. 2005; Glick 2014). The
released compounds affect the plant metabolic process either directly by improving
its growth and development, or indirectly to work on the enhancement of the nutrient
absorption of plants by boosting their adaptive capacity (Grobelak et al.
2015). The
phytohormones produced help in processes of cell division, root extension, and cell
enlargement (Kudoyarova et al.
2015). Rhizobacterial microorganisms have a major
role in the management of abiotic stresses in a precise manner. PGPR like Strepto-
myces, Burkholderia, Bacillus, Azospirillum, Pseudomonas, Brady rhizobium,
Aeromonas, and Acetobacter have distinct roles in improving crop yield in salt-
affected soils, while few others are reported to provide resistance under drought
conditions (Kaushal and Wani 2016). PGPR-induced drought resistance
https://t.me/medicina_free
mechanisms against different stresses are known to be associated with physiological
and chemical changes (Kaushal and Wani 2016; Zhang et al. 2021). Some known
fungal species from Trichoderma and Coniothyrium genera promote plant growth by
suppressing the plant pathogens as biocontrol agents (Chen et al.
2016; De Vrije
et al. 2001).
15 Regulation of PGPR-Related Genes in Medicinal Plants in Adverse Conditions 247
15.2.1 Direct Mechanisms
15.2.1.1 Auxins
Auxin (IAA) is reported to be synthesized and released as a secondary metabolite by
about 80% of the microbes present in the rhizosphere that help in plant growth
(Patten and Glick 1996). Indole-3-acetic acid (IAA) is the most pertinent among the
other naturally occurring auxins (Spaepen et al.
2007). There are some inactive
forms of auxins too, for example, the 4-chloroindole-3-acetic acid (Korasick et al.
2013). The multiple functional areas of auxins lie within the geotropic and phototro-
pic responses, differentiation of vascular tissue, apical dominance, root initiation,
and/or elongation and cell division. IAA produced by rhizobacteria overall changes
the auxin sources by increasing the surface area and length of the roots; the level of
root exudates therefore increases and helps in better uptake of nutrients (Ali et al.
2010). The level of auxin concentrations preferred for promoting plant growth is
varied among plants, and shoots require far more auxin concentrations than roots for
effective promotion of plant growth (Cheng et al.
2013; Glick 2012).
Plant-synthesized auxin could be optimal to suboptimal depending on which the
bacterial-synthesized auxin is produced (Glick
2012). The bacterial synthesized
auxin by plant growth-promoting bacteria (P GPB) can promote root growth/devel-
opment to optimize the plant auxin concentrations or may suppress root develop-
ment where the plant auxin concentration is already optimal (Spaepen et al. 2007).
Tryptophan is the main amino acid out of which auxin is produced at a considerably
low concentrations, by primarily three biosynthetic pathways. These pathways,
namely, the indole pyruvic acid (IPA) pathway, the indole acetamide (IAM) path-
way, the indole acetaldoxime (IAOx)/indoleacetonitrile (IAN) pathway (Duca et al.
2014), the indole acetaldehyde (IAH) pathway, and the tryptamine pathway. PGPB-
producing auxins may possess all the three biosynthetic pathways indicating a vital
role of IAA synthesis in the lifecycle and the proper functioning of the bacterium.
Among the three pathways, IPA pathway is related to epiphytic and rhizosphere
fitness. IAA actively interacts with the involved biosynthetic genes and governs the
process of activation, inactivation, overexpression, or underexpression of the genes.
For example, the regulation of the ipdC gene involved in IAA synthesis in PGPB is
controlled by the amount of IAA produced (Spaepen and Vanderleyden 2011). This
was first reported in Ancylostoma braziliense Sp245 as PGPB with such observation
(Broek et al. 2005).
https://t.me/medicina_free
248 K. Kiran and G. Sharma
15.2.1.2 Gibberellins
Gibberellins like other phytohormones are efficient energy converters of the elicitor
signals. They are made of a large group of tetracyclic diterpenoid carboxylic acids
containing either C20 or C19 carbon skeletons (Dodd et al.
2010; Hedden and
Thomas
2012).
Out of the several gibberellins (GA1–GA136) structures known to date (Hedden
and Thomas
2012), only four have been identified in bacteria, namely, GA1, GA3,
GA4, and GA20 (Gupta et al. 2016), and GA1 and GA4 are the most active among
the four (Nelson and Steber
2016). Functionally gibberellins play important roles in
plant growth stimulation and exhilaration of important growth processes like stem
elongation, seed germination, flowering, and fruit setting (Zaidi et al.
2015).
The prominent PGPB production of GAs is known from species Achromobacter
xylosoxidans, Gluconobacter diazotrophicus, Acinetobacte r calcoaceticus,
Rhizobia, Azotobacter spp., Bacillus spp., Herbaspirillum seropedicae, and
Azospirillum spp. (Dodd et al. 2010; Deka et al. 2015 ). The gibberellin biosynthetic
pathways in bacteria are not that well known as they are in the case of plants and
fungi. Nevertheless, depending on the action mechanism of the gibberellin signaling
system, gibberellins are reported to both induce and inhibit shoot development and
root growth, respectively. For example, the DELLA repressor is responsible to
activate various gibberellin-inducing genes (Nelson and Steber 2016; Martínez
et al. 2016; Minguet et al. 2014; Wang et al. 2015).
15.2.1.3 Cytokinin
Little is known about the roles of bacterial cytokinins. They are synthesized in the
root tips and transported through the xylem to the shoot by translocation. The widely
known cytokinin form plant is zeatin, first isolated from Zea mays (Schäfer et al.
2015) while kinetin was the first to be discovered. Cytokinin synthesis primarily
takes place in roots and then is distributed throughout the plant. The two groups of
cytokinins are the adenine type including kinetin and zeatin and the phenyl urea type
which is diphenyl urea and thidiazuron. The functional area of cytokinins is to
regulate apical dominance, cell division, root elongation, seed germination, xylem
and chloroplast differentiation, flower and fruit development, nutritional signaling,
leaf senescence, and plant-pathogen interactions (Sakakibara 2006). Cytokinin gene
expression within PGPB has been reported. Inoculation of lettuce with Bacillus
subtilis not only increased the cytokinin content but also enhanced the plant growth
(Arkhipova et al. 2005).
Sinorhizobium meliloti, a genetically modified bacterial strain producing approx-
imately five times more cytokinin as compared to the wild type, was reported to
enormously increase the size of alfalfa plants under extreme drought stress unlike the
wild-type strain (Xu et al. 2012). This overproduction of the cytokinin was helpful to
protect the alfalfa plants from senescence due to drought conditions.
15.2.1.4 ACC Deaminase
The effect of ethylene can be seen in almost all tissues and stages of plant develop-
ment. Ethylene production depends on various conditions a plant is exposed to
https://t.me/medicina_free
externally as well as on the concentrations of other plant hormones present within
(Gamalero and Glick 2015). A rise in ethylene production is observed as a response
to several stresses like accumulation of metals, deposition of organic and inorganic
chemicals, extreme temperatures conditions, flood or water deficit, ultraviolet light
exposures, damage caused by insects, harmful microbial pathogen infections, and
external mechanical wounding (Ali et al.
2014
; Barnawal et al. 2012). However,
overproduction of ethylene by the action of ACC oxidase within plant tissues
elevates ethylene stress thereby affecting the root and shoot development. This
situation is well handled by PGPB that are able to produce ACC deaminase leading
to normal plant development.
15 Regulation of PGPR-Related Genes in Medicinal Plants in Adverse Conditions 249
The higher concentration of
plant ethylene dramatically worsens the effect of the
initial stress that had triggered the ethylene response. Therefore, any remedy done
resulting in downregulation of the ethylene production should also reduce the
amount of initial stress that the plant was facing. PGPR expressing ACC deaminase
lower plant ACC levels eventually resulting in low plant ethylene levels and thus
decreasing the harmful effect on plants from different stresses (Glick
2014).
The ACC is converted to α-ketobutyrate and ammonia by ACC deaminase in the
PGPB. It is also reported that application of ACC deaminase-containing PGPB in
advance to plants generally lowers the concentration of ethylene production in
response to the stress caused by pathogen attack and mitigates harmful effects that
the plants experience from the pathogen attack (Glick
2012; Toklikishvili et al.
2010). To date, this approach has been shown to be quite effective with a variety of
plants and with numerous pathogens. However, these results have so far been limited
to controlled conditions within the greenhouses or growth chamber experiments.
Proteins encoding the gene that regulates ethylene biosynthesis are classified as the
ETR proteins. The first member of these proteins was identified in mutant
Arabidopsis in the reports published by Bleecker et al. (Bleecker et al. 1988). Plants
secrete tryptophan, which is taken up by the PGPB bound to the roots of the plant to
be converted to IAA. This IAA is then secreted by bacterium and used by plants
(Glick 2014). Also, the ACC synthase-activated transcription due to the increased
IAA in turn results in the increase in ACC levels thus promoting higher level of
ethylene. Overall, PGPB-produced IAA could either enhance plant growth or inhibit
plant growth through the action mechanism of ethylene produced. Beneficially, the
action mechanism of PGPB-produced ACC deaminase reduces the level of ACC that
was newly formed in the plant. This enables bacterial IAA to enhance plant growth
and also decreases the level of ethylene production within the plant.
15.2.1.5 Nitrogen Fixation
Nitrogen is essential for the growth of plants and bacteria. Nitrogen deficiency in soil
is fixed by using various nitrogenous fertilizers to make up for the necess ary plant
requirements (Zhang et al. 2015). About 78% of gaseous nitrogen [N
2
] is not utilized
by plants until it is converted to ammonia (Baas et al. 2014). This reaction requires a
significant amount of energy, which is general ly provided by the use of fossil fuels or
through biological nitrogen fixation process in the presence of ATP (Fig.
15.1), and
also majorly through exogenous supply. Several organisms capable of fixing
https://t.me/medicina_free
250 K. Kiran and G. Sharma
Fig. 15.1 Graphical abstract demonstrating the roles of various PGPR species toward plant growth and development. (Image background created using https://
www.canva.com/)
https://t.me/medicina_free
nitrogen symbiotically associated with specific plants have been identified. For
example Rhizobium, Sinorhizobium, Azorhizobium, Allorhizobium, Mesorhizobium,
Bradyrhizobium, Frankia, Azoarcus, Achromobacter, Burkholderia, and
Herbaspirillum are few leguminous bacter ia (Babalola
2010; Pé rez-Montaño et al.
2014; Turan et al. 2016). The gram-negative bacteria, rhizobia, form specific
symbiotic relationships mainly with selected legumes. Some cereal crops have also
been observed to be colonized by rhizobia (Geddes et al.
2015;
Oldroyd and Dixon
2014). The complex series of developmental changes take place within the plant
after the colonization of bacteria which leads to root nodule formation (Gage
2004).
I
nside the root nodule, this cell-wall-less bacteroid fixes atmospheric nitrogen by
using nitrogenase enzyme and producing ammonia. This mutualistic relationship
involves the supply of fixed carbon from the plants to the bacterium for its growth,
while the bacteria continue to fix nitrogen within the nodules. However the benefi-
cial role of nitrogenase enzyme production during the process nitrogen fixation is
countered with a loss leading to reduction of H
+
to H
2
, which is lost to the
atmosphere. This drastically reduces the nitrogen-fixing proces s by approximately
30%. There are a few strains of rhizobia having the capability of retrieving the lost
H
2
from the atmosphere and converting it back into H
+
to produce ATP. They do this
with the help of internal hydrogenase enzyme present in the bacteria (Adams et al.
1981).
15 Regulation of PGPR-Related Genes in Medicinal Plants in Adverse Conditions 251
15.2.1.6 Siderophores
Siderophores are more commonly known to be efficient iron chelators. Their struc-
ture consists of small peptide molecules containing side chains and functional groups
to which ferric ions can bind (Goswami et al.
2016). Quite many species-specific
siderophores used have been screened from microbes (Sandy and Butler 2009).
PGPB synthesize siderophores that can prevent the spreading of phytopathogens.
Siderophores bind tightly to most of the Fe
3+
present in the rhizosphere of the host
plant and provide it either to the PGPB or the host plant. This does not allow the
fungal/bacterial pathogens in the rhizosphere, where the biocontrol PGPB are bound
from receiving the required iron for their growth. Thus, preventing the pathogens to
proliferate due to lack of iron, PGPB siderophores are reported to have a much
higher affinity for iron as compared to the fungal siderophores (Kloepper et al.
1980). Several reports have demonstrated the role of siderophores as iron chelators
for example from Chryseobacterium spp. Supply of iron to tomato plants was seen
by C138 (Radzki et al.
2013); similarly supplementation of Pseudomonas strains
showed enhanced plant growth and increase in germination rates (Sharma and Johri
2003).
15.2.2 Indirect Mechanisms
The major mechanism used by PGPB to reverse the harmful effects of
phytopathogens is through synthesis of various antibiotics (Haas and Keel 2003;
Raaijmakers and Mazzola 2012). However, the method of action of any antibiotic-
https://t.me/medicina_free
synthesizing PGPB varies accordingly depending on the field conditions. Also,
growth and expression of the biocontrol PGPB can be altered in in vitro conditions
before applying them to redirect their activity (Glick 2015). Many antibiotics have
been derived from bacteria of the genera Bacillus and Pseudomonas. They produce a
variety of metabolites acting against multiple microbes (Goswami et al.
2016).
252 K. Kiran and G. Sharma
Plants respond by synth
esizing different fungal cell wall degrading enzymes
against infections caused by fungal pathogens. The enzymes include chitinase,
protease, and lipase. Some biocontrol PGPB synthesize a similar group of cell
wall degrading enzymes (Chernin et al.
1995). Bacillus sp. JS has been demonstrated
to cause an upregulation of the PR-2 and PR-3 genes that encode β-1,3-glucanase
and chitinase, respectively (Kim et al. 2015). Additionally some biocontrol PGPB
overmatch the phytopathogens, say for nutrients or binding sites on the plant roots
(Barahona et al. 2011; Innerebner et al. 2011). Generally, PGPB competitiven ess
works in synergy with other biocontrol mechanisms to overcome the effectiveness of
phytopathogens. For example Porcel et al. (
2014) showed the competition ability of
Bacillus megaterium in improved growth of tomato plant.
Yet several other biocontrol PGPB synthesize chemicals like hydrogen cyanide
(HCN) but in so low concentrations that in case, if these bacteria were used as the
only biocontrol mechanism, then in most instances, the low level of HCN would not
be effective at mitigating the expansion of most fungal phytopathogens. However, it
is mostly observed that biocontrol PGPB that produce HCN can also synthesize few
antibiotics or cell wall degrading enzymes (Ramette et al.
2006). Moreover, it has
been observed that the low level of HCN synthesized by the bacterium ensures that
the fungal pathogen does not develop resistance to the particular antifungal agent in
specific. Thus PGPB synthesizing HCN acts synergistically with other procedures of
biocontrol followed by the same bacterium. Studies have shown that cytochrome c
oxidase and other important metalloenzymes are inhibited by HCN toxicity ability
(Nandi et al.
2017). Likewise, many bacterial species from genus Alcaligenes,
Rhizobium, Bacillus, Pseudomonas, and Aeromonas have been shown to be HCN
producers (Ahmad et al. 2008; Das et al. 2017; Zachow et al. 2017). Studies have
reported suppression of tomato root-knot disease caused by Melo idogyne javanica
due to the effect of HCN (Siddiqui et al. 2006) and also as a control of Odontotermes
obesus, which is a crop pest found in India (Kumar et al. 2015).
The process of induced systemic resistance (ISR) where nonpathogenic microbes
like PGPB reduce the detrimental effects of plant pathogens by activating a resis-
tance mechanism in the plants was initially demonstrated by Van Peer and Schippers
(1992). ISR is known to express specifically at the induction site and prepare the
plant against a wide range of pathogens than targeting any specific pathogen.
Various PGPB are also known to produce salicylic acid (SA) that as a signal
molecule turns on a mechanism similar to ISR in plants, termed as “systemic
acquired resistance” or SAR (Chen et al. 1999). While this mechanism is extremely
rare in PBPB as compared to ISR, both the mechanisms are regulated by different
signaling pathways and their effectiveness is also different (Ton et al. 2002).
Naturally, bacterial cells use the mechanism of quorum sensing to identify the
presence of similar and/or different types of bacteria. On an optimal threshold cell
https://t.me/medicina_free
density, the bacteria through chemical signals begin to alter their metabolism by
turning on different sets of genes (Cornforth et al. 2014), such that similar bacteria
that are in vicinity to each other may start acting in a synchronized manner.
15 Regulation of PGPR-Related Genes in Medicinal Plants in Adverse Conditions 253
In several cases certa
in bacteria synthesize autoinducers, secreted outsi de of the
bacterial cells. Above an optimum level autoinducers bind to bacterial cellular
receptors and trigger a signal transduction chain reaction causing large-scale (popu-
lation) alterations in bacterial gene expression with the actions of a specified group
of cells. One way to quench the process of quorum sensing is to utilize a PGPB that
can produce enzymes like lactonase that degrade the pathogen-produced autoinducer
following pretreating plant seedlings with such PGPB. These experiments are still
restricted to labs and not in the field.
Certain bacterial phytopathogens are known to be lysed by specific
bacteriophages or bacterial viruses (Frampton et al. 2012). It is possibly done
through isolating and characterizing bacteriophages that may lyse the target patho-
gen but have no effect on any other bacterial strains. A mixture of bacteriophage
strains as a spray reduces the possibility of development of any bacteriophage-
resistant mutants of the target pathogen. Since the binding sites of the bacteriophages
are different on the surface of the pathogen. Bacteriophages generally are quite
sensitive to UV light (Buttimer et al. 2017) and therefore are sprayed onto plants at
evening hours when the UV light intensity is quite low. Currently, a few
bacteriophage-based biocontrol agents have been licensed for use, for example, the
bacterial pathogen Xanthom onas campestris pv. vesicatoria, causing bacterial spot
of tomatoes and peppers, Pseudomonas syringae pv. actinidiae, causing canker
disease in kiwifruit, and Pseudomonas syringae pv. tomato, causing bacterial
speck on tomatoes (Peitl et al. 2017; Yu et al. 2016). Depending on their life
cycle, they can be lytic phages or lysogenic phages. The treatment of bacterial
infections with phages is termed as phage therapy (Álvarez and Biosca
2017). It
has been successfully used in treating plant phytopathogenic diseases (Fujiwara et al.
2011; Balogh et al. 2010; Bae et al. 2012; Frampton et al. 2014). Few detailed
studies on bacteriophages as biocontrol agents are published by Buttimer et al.
(
2017), Abedon et al. (2017), Jones et al. (2007), and O’Brien (2017) etc.
15.3 Status of Medicinal Plants in the World
Medicinal plants constitute around 10% of all vascular plants; these are around
350,000 estimated in number with more than 5 lakh known species (Fonnegra 2007;
Joppa et al. 2011; Pimm et al. 2014). The use of medicinal plants eventually has
refined over generations, and majority of them are known for their beneficial
properties used in several traditional medicines. Therapeutic properties of medicinal
plants and their method of treatment on humans were well known since the eigh-
teenth century, but the active compounds responsible were not explored which
remained unknown (Faridi et al. 2010). Present-day practices have replaced the
use of medicinal plants directly. Also, the world medicine needs required produc-
tion, through pharmaceutical industry in large quantities which are majorly based on
https://t.me/medicina_free
Соседние файлы в папке Библиотека им академика М.И. Перельмана