Introduction agricultural activities worldwide in an attempt

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.

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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).

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 and we also look at the gaps which exist in the studies of PGPR.

Mechanisms of growth
promotion

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).

 

Studied
crops

Research exploring
the potential of PGPR for increased crop production and productivity has beed
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).

 

Gaps
in knowledge

It
is apparent that numerous studies have been done on isolation of PGPR and how
they affect growth and yield of many crops worldwide. 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.

 

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.

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