AbstractPlantgrowth promoting rhizobacteria (PGPR) are bacteria found in the rhizosphere ofplants that stimulate the growth of plants in numerous ways which can bedirectly or indirectly. For instance, they produce plant growth-promotinghormones and volatile organic compounds and may also be involved in phosphateand mineral solubilization, production of volatile organic compounds andnitrogen fixation. The potential use of such microorganisms in agriculture iscurrently feign explored worldwide as alternative ways to replace the use ofchemical fertilizers and pesticides. The understanding of the diversity of PGPRin different plant rhizospheres as well as their colonization ability andmechanisms of action will enable their rapid application in agriculture forsustainability of the environment. This article reviews the studies which havebeen done to assess the potential of PGPR for improved crop production andproductivity in Africa.

 Keywords: PGPR,Rhizosphere bacteria, Plant-microbe interactions, crop production, agriculture IntroductionOverthe past few decades, there have been increase in intensive and extensiveagricultural activities worldwide in an attempt to feed the ever-risingpopulation of people. Along with this, unanticipated environmental problemshave come up due to the continuous usage of chemical fertilizers and pesticidesto enhance crop productivity and control crop pests respectively (Alves et al.,2004; Hungria et al., 2013).  In anattempt to move towards sustainable agricultural practices and to maintain theecosystems and biodiversity, interests have been shifted towards the potentialof indigenous plant growth promoting rhizobacteria for improved and sustainablecrop production and productivity (Alves et al., 2004; Hungria et al., 2013). Several studies have been doneon the potential of these microbes even in crops.

Theterm Plant Growth Promoting Rhizobacteria (PGPR) is used to refer to soilbacteria that colonize the rhizosphere of plants, growing in or around planttissues and that stimulate plant growth by different mechanisms (Dimpka et al.,2009; Grover et al., 2011, Glick, 2012). The direct mechanisms by which PGPRpromote plant growth include biofertilization, stimulation of root growth,rhizoremediation and plant stress control, while indirect mechanisms includebio-protection by means of antibiosis, induction of systemic resistance, andcompetition against plant pathogens for nutrients and niches (Lugtenberg andKamilova, 2009).

 Common PGPR genera thathave 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). Itis apparent that numerous studies have been done on isolation of PGPR and howthey affect growth and yield of many crops worldwide. In this article, wereview the different mechanisms of plant growth promotion, we look at examplesof crops whose rhizobacteria have been studied for growth promotion whilehighlighting some of the knowledge gaps that still exist with regard to PGPR.Mechanismsof growth promotionPlantgrowth promotion mechanisms of PGPR differ from one bacteria to another.

Bio-protectionNumerousstudies have reported plant growth promotion potential of PGPR as a result ofcontrolling plant pests. Recently, Son et al., (2014) found that among selected PGPR isolates,four significantly decreased gray leaf spot disease severity with PGPR Brevibacteriumiodinum KUDC1716 providing the highest disease suppression in pepper (Capsicumannuum).

It was also found that P. polymyxa increased plantgrowth of pepper (C. annuum) by decreasing the severity of Xanthomonasaxonopodis pv. Vesicatoria (Quyet-Tien et al., 2010).NitrogenfixationSomePGPR species are capable of reducing atmospheric nitrogen (N2) into ammonia(NH3) (Franche et al.

, 2009). Such bacteria contain the nitrogenase enzyme thatenable them to perform this function (Dixon and Kahn, 2004). For instance,Rhizobia bacteria can effectively carry out biological nitrogen fixation in theroot nodules of most leguminous plants (Willems, 2007; Shridhar, 2012). Suchspecies can effectively be used to facilitate plant growth without the need fornitrogenous fertilizers.

InBrazil, Bradyrhizobium japonicum and B. elkanii are used for biologicalnitrogen fixation in soybean (Glycine max L.) prdcution (Torres et al., 2012). Biologicalnitrogen 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 developmentof the plant (Spaepen et al., 2008; Baudoin et al., 2010).

Withregards to nitrogen fixing ability of some rhizobacteria, there is still needto explore the possibility of nitrogen fixation by endophytic rhizobia speciesin 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 ofproducing indolic substances such as the IAA (Spaepen et al., 2007) and Souzaet al., (2013) were able to demonstrate that about 80% of bacteria in ricerhizospheres produce these compounds. Other studies which have observed theproduction of indolic compounds among rhizobacteria include those done byKhalid et al., (2004) and Costa et al., (2014).

The genera which have been implicatedin production of indolic compounds include Enterobacter, Escherichia, Klebsiella,Pantoea and Grimontella (Costa et al., 2013).SiderophoreproductionSiderophores are lowmolecular mass molecules (<1000 Da) that possess great specificity andaffinity for binding Fe3+ (Krewulak and Vogel, 2008) and are very important in agricultureespecially in flooded soils where excessive iron uptake by plants may lead toiron toxicity (Stein et al., 2009). This unique property has been observed insome rhizobacteria especially those associated with rice (Sauza et al, 2013).

 Siderophoreproduction by rhizobacteria associated with other plants should be exploredfurther. According to Loaces et al., (2011), the ability of endophytic bacteriato produce siderophores has been rarely studied, yet it confers competitive advantagesto plants through the exclusion of other microorganisms as well as by improvingnutrition.NutrientsolubilizationTheability of certain rhizobacteria to solubilize nutrients which are available insoil in insoluble forms is very important and helpful for plant growth becauseof improved nutrient uptake (Khan et al.

, 2009). Several phosphate solubilizingbacteria have been isolated from the rhizosphere of different plants (Souza etal., 2014; Granada et al., 2013). From rice plants, rhizobacteria which have beenassociated with phosphate solubilization include species belonging to Burkholderia,Cedecea, Cronobacter, Enterobacter, Pantoea and Pseudomonas (Chanet al., 2006; Souza et al.

, 2013). Phytateswhich are organic forms of phosphorus exist in several plants and can be goodsources of phosphorus to plants (Richardson and Simpson, 2011; Rodriguez etal., 2006).

However, these also require solubilization by bacteria that containthe phytase activity. The production of phytase has been observed in severalrhizospheric bacteria including Bacillus sp., Cellulosimicrobiumsp., Acetobacter sp., Klebsiella terrigena, Pseudomonas sp., Paenibacillussp., and Enterobacter sp.

(Idriss et al., 2002; Jorquera et al., 2011;Kumar et al., 2013, Singh et al., 2014). Such bacteria have been isolated fromthe rhizospheres of different crops such as wheat, oat (Avena sative L) andwhite clover (trifolium repens L)However,several knowledge gaps still remain to be filled. For example, very little hasbeen done but little is known concerning potassium solubilization whilepotassium is the third major macronutrient for plant growth.StudiedcropsResearch exploringthe potential of PGPR for increased crop production and productivity has beendone by several researchers.

The common knowledge now is that all plants harbora diverse community of indigenous bacteria in their rhizosphere which helpstimulate their growth naturally. Studies have shown that PGPR had positiveeffects on cereals (Shararoona et al.,2006), fruits (Kavino et al., 2010), vegetables (Kurabachew and Wydra, 2013),flowers (An et al, 2010) and spiceslike black pepper (Diby and Sarma, 2006).  ConclusionsThemost urgent need of the world today is to increase the output and yield ofcrops by means of soil fertilization and control of pests. The application anduse of PGPR can help achieve these two necessities while maintaining theecosystems at the same time.Studiesinvolving innoculation with consortia of several bacteril strains could be anlaternative to inoculation with individual strains and could lead to evenbetter results at promoting plant growth and increasing yield and productivityas has been observed in some studies (Domenech et al., 2006; Hungria et al.,2013).Theidentification of plant growth promoting characteristics in different rhizobacteriaassociated with different plants as well as assys for efficacy in vitro and invivo all contribute toi the search for alternative ways of improving cropproduction and productivity while sustaining the environemt.