Abstract include biofertilization, stimulation of root growth, rhizoremediation


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



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.

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).

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.

of growth promotion

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


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).


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.

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).

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).


Siderophores are low
molecular mass molecules (<1000 Da) that possess great specificity and affinity for binding Fe3+ (Krewulak and Vogel, 2008) and are very important in agriculture especially in flooded soils where excessive iron uptake by plants may lead to iron toxicity (Stein et al., 2009). This unique property has been observed in some rhizobacteria especially those associated with rice (Sauza et al, 2013).   Siderophore production by rhizobacteria associated with other plants should be explored further. According to Loaces et al., (2011), the ability of endophytic bacteria to produce siderophores has been rarely studied, yet it confers competitive advantages to plants through the exclusion of other microorganisms as well as by improving nutrition. Nutrient solubilization The ability of certain rhizobacteria to solubilize nutrients which are available in soil in insoluble forms is very important and helpful for plant growth because of improved nutrient uptake (Khan et al., 2009). Several phosphate solubilizing bacteria have been isolated from the rhizosphere of different plants (Souza et al., 2014; Granada et al., 2013). From rice plants, rhizobacteria which have been associated with phosphate solubilization include species belonging to Burkholderia, Cedecea, Cronobacter, Enterobacter, Pantoea and Pseudomonas (Chan et al., 2006; Souza et al., 2013). Phytates which are organic forms of phosphorus exist in several plants and can be good sources of phosphorus to plants (Richardson and Simpson, 2011; Rodriguez et al., 2006). However, these also require solubilization by bacteria that contain the phytase activity. The production of phytase has been observed in several rhizospheric bacteria including Bacillus sp., Cellulosimicrobium sp., Acetobacter sp., Klebsiella terrigena, Pseudomonas sp., Paenibacillus sp., and Enterobacter sp. (Idriss et al., 2002; Jorquera et al., 2011; Kumar et al., 2013, Singh et al., 2014). Such bacteria have been isolated from the rhizospheres of different crops such as wheat, oat (Avena sative L) and white clover (trifolium repens L) However, several knowledge gaps still remain to be filled. For example, very little has been done but little is known concerning potassium solubilization while potassium is the third major macronutrient for plant growth. Studied crops Research exploring the potential of PGPR for increased crop production and productivity has been done by several researchers. The common knowledge now is that all plants harbor a diverse community of indigenous bacteria in their rhizosphere which help stimulate their growth naturally. Studies have shown that PGPR had positive effects on cereals (Shararoona et al., 2006), fruits (Kavino et al., 2010), vegetables (Kurabachew and Wydra, 2013), flowers (An et al, 2010) and spices like black pepper (Diby and Sarma, 2006).     Conclusions The most urgent need of the world today is to increase the output and yield of crops by means of soil fertilization and control of pests. The application and use of PGPR can help achieve these two necessities while maintaining the ecosystems at the same time. Studies involving innoculation with consortia of several bacteril strains could be an laternative to inoculation with individual strains and could lead to even better results at promoting plant growth and increasing yield and productivity as has been observed in some studies (Domenech et al., 2006; Hungria et al., 2013). The identification of plant growth promoting characteristics in different rhizobacteria associated with different plants as well as assys for efficacy in vitro and in vivo all contribute toi the search for alternative ways of improving crop production and productivity while sustaining the environemt.


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