Abstract include biofertilization, stimulation of root growth, rhizoremediation

Abstract

Plant
growth promoting rhizobacteria (PGPR) are bacteria found in the rhizosphere of
plants that stimulate the growth of plants in numerous ways which can be
directly or indirectly. For instance, they produce plant growth-promoting
hormones and volatile organic compounds and may also be involved in phosphate
and mineral solubilization, production of volatile organic compounds and
nitrogen fixation. The potential use of such microorganisms in agriculture is
currently feign explored worldwide as alternative ways to replace the use of
chemical fertilizers and pesticides. The understanding of the diversity of PGPR
in different plant rhizospheres as well as their colonization ability and
mechanisms of action will enable their rapid application in agriculture for
sustainability of the environment. This article reviews the studies which have
been done to assess the potential of PGPR for improved crop production and
productivity in Africa.

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Keywords: PGPR,
Rhizosphere bacteria, Plant-microbe interactions, crop production, agriculture

 

Introduction

Over
the past few decades, there have been increase in intensive and extensive
agricultural activities worldwide in an attempt to feed the ever-rising
population of people. Along with this, unanticipated environmental problems
have come up due to the continuous usage of chemical fertilizers and pesticides
to enhance crop productivity and control crop pests respectively (Alves et al.,
2004; Hungria et al., 2013).  In an
attempt to move towards sustainable agricultural practices and to maintain the
ecosystems and biodiversity, interests have been shifted towards the potential
of indigenous plant growth promoting rhizobacteria for improved and sustainable
crop production and productivity (Alves et al., 2004; Hungria et al., 2013). Several studies have been done
on the potential of these microbes even in crops.

The
term Plant Growth Promoting Rhizobacteria (PGPR) is used to refer to soil
bacteria that colonize the rhizosphere of plants, growing in or around plant
tissues and that stimulate plant growth by different mechanisms (Dimpka et al.,
2009; Grover et al., 2011, Glick, 2012). The direct mechanisms by which PGPR
promote plant growth include biofertilization, stimulation of root growth,
rhizoremediation and plant stress control, while indirect mechanisms include
bio-protection by means of antibiosis, induction of systemic resistance, and
competition against plant pathogens for nutrients and niches (Lugtenberg and
Kamilova, 2009).  Common PGPR genera that
have been found to be commonly associated with different crops include Acinetobacter,
Alcaligenes, Arthrobacter, Azospirillum, Azotobacter,
Bacillus, Beijerinckia, Burkholderia, Enterobacter,
Erwinia, Flavobacterium, Rhizobium and Serratia
(Anandarai and Dinesh, 2008).

It
is apparent that numerous studies have been done on isolation of PGPR and how
they affect growth and yield of many crops worldwide. In this article, we
review the different mechanisms of plant growth promotion, we look at examples
of crops whose rhizobacteria have been studied for growth promotion while
highlighting some of the knowledge gaps that still exist with regard to PGPR.

Mechanisms
of growth promotion

Plant
growth promotion mechanisms of PGPR differ from one bacteria to another.

Bio-protection

Numerous
studies have reported plant growth promotion potential of PGPR as a result of
controlling plant pests. Recently, Son et al., (2014) found that among selected PGPR isolates,
four significantly decreased gray leaf spot disease severity with PGPR Brevibacterium
iodinum KUDC1716 providing the highest disease suppression in pepper (Capsicum
annuum). It was also found that P. polymyxa increased plant
growth of pepper (C. annuum) by decreasing the severity of Xanthomonas
axonopodis pv. Vesicatoria (Quyet-Tien et al., 2010).

Nitrogen
fixation

Some
PGPR species are capable of reducing atmospheric nitrogen (N2) into ammonia
(NH3) (Franche et al., 2009). Such bacteria contain the nitrogenase enzyme that
enable them to perform this function (Dixon and Kahn, 2004). For instance,
Rhizobia bacteria can effectively carry out biological nitrogen fixation in the
root nodules of most leguminous plants (Willems, 2007; Shridhar, 2012). Such
species can effectively be used to facilitate plant growth without the need for
nitrogenous fertilizers.

In
Brazil, Bradyrhizobium japonicum and B. elkanii are used for biological
nitrogen fixation in soybean (Glycine max L.) prdcution (Torres et al., 2012). Biological
nitrogen fixation by endophytic bacteria have also been exploited in sugarcane
(Saccharum officinarum L (Thaweenut et al.,2011)
and in wheat, IAA-producing Azospirillum has been shown to promote development
of the plant (Spaepen et al., 2008; Baudoin et al., 2010).

With
regards to nitrogen fixing ability of some rhizobacteria, there is still need
to explore the possibility of nitrogen fixation by endophytic rhizobia species
in plants other than legumes, for example in the roots of potato plants
(Terakado-Tonooka et al., 2008).

 

Production of indole-acetic acid (IAA)

Most plant-associated rhizobacteria are capable of
producing indolic substances such as the IAA (Spaepen et al., 2007) and Souza
et al., (2013) were able to demonstrate that about 80% of bacteria in rice
rhizospheres produce these compounds. Other studies which have observed the
production of indolic compounds among rhizobacteria include those done by
Khalid et al., (2004) and Costa et al., (2014). The genera which have been implicated
in production of indolic compounds include Enterobacter, Escherichia, Klebsiella,
Pantoea and Grimontella (Costa et al., 2013).

Siderophore
production

Siderophores are low
molecular mass molecules (

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