The involvement of organic acids in soil fertility, plant health and environment sustainability.

Increasing demand for safe food by an ever-growing human population emphasizes the urgency for increasing crop yields and reducing the losses caused by abiotic and biotic stresses; a partial solution to this problem is to develop a better understanding of plant-microbe interactions. Plant roots continuously release a wide range of compounds including organic acids in root exudates. These root exudates stimulate growth of specific microbial communities in the rhizosphere, which affect complex biological and physico-chemical interactions occurring between plant roots and the surrounding soil environment. In addition, organic acids are also released by different microbes and during decomposition of organic matter and plant residues in the soil. Interestingly, the available organic acids in the rhizosphere play crucial roles in various physio-chemical processes including the chemoattraction of microbes (both beneficial and pathogenic), mineralization and solubilization of complex minerals (P, K and Zn), biocontrol of phytopathogens, induction of systemic resistance, biogas formation, mitigation of abiotic stresses and, detoxification of metals and residual pesticides. Thus, organic acids play a significant role in the sustainable management of the soil ecosystem and in environmental sustainability. This review discusses the role of organic acids in the stimulation or enrichment of specific root-associated microbial communities and their effect on plant-microbe interactions at the root surface. In addition, the potential for root microbiome modification to enhance nutrient cycling and nutrient acquisition, and in amelioration of environmental stresses for increasing food production is discussed.

Cloning and characterization of Aiia, an acylhomoserine lactonase from Bacillus cereus RC1 to control soft rot causing pathogen Lelliottia amnigena RCE.

Bacterial pathogenesis-associated characteristics such as biofilm formation, synthesis of hydrolyzing enzymes, and toxins are regulated by Acyl Homoserine Lactones (AHLs), small peptides and diffusing signal factors (DSF). Lelliottia amnigena is gram negative bacteria and its pathogenicity is regulated by the luxR and luxI class of quorum sensing. The signaling molecules and their concentrations are essential for the virulence of the pathogenic bacterium. To suppresses the pathogenicity; the concentration of signalling molecules must be controlled or degraded. The lactonase have the ability to hydrolyze lactones of different chain length. The present study deals with a newer approach to control the pathogenesis of Lelliottia amnigena through isolation and characterization of Aiia lactonase from Bacillus cereus RC1. Aiia lactonase specific primers were used to amplify the gene, and the sequence thus obtained was submitted to the Genbank database under accession # OK643884.1. The gene was cloned in pBE-S shuttle vector and transformed in the recombinant host. The expressed and purified protein had a molecular weight of 28.00 KDa and exhibited its optimum activity at 37℃ by inhibiting the violacein pigment of the monitor strain Chromobacterium violaceum MTCC 2656. The proteinaceous nature of the purified molecule was confirmed by incubating it in the presence of proteinase K for 1 h. The activity of the pathogenesis-related protein, polygalacturonase was drastically reduced in the presence of the purified Aiia protein. The purified protein also showed a zone of inhibition when plated together with Lelliottia amnigena RCE (MZ712952.1). Searches of the Conserved Domain Database suggested that this protein belonged to the Metallo-beta-lactamase superfamily and is closely related to Aiia from B. thuringiensis serovar kurstaki. Modeling of the protein structure was done using I-TASSER; a C-score of 0.55 suggested that the model was of good quality. To be used commercially, this recombinant protein needs to be purified at an industrial scale; it can then be used to repress the growth of soft rot causing bacteria in horticultural crops during their storage period.

Mitigation of lead (Pb) toxicity in rice cultivated with either ground water or wastewater by application of acidified carbon.

Toxicity induced by a high concentration of lead (Pb) can significantly decrease plant's growth, gas exchange, and yield attributes. It can also causes cancer in humans. The use of organic amendments, especially biochar, can alleviate Pb toxicity in different crops. The application of biochar can decrease the uptake of Pb by plant roots. However, the high pH of thermo-pyrolyzed biochar makes it an unfit amendment for high pH soils. As Pb is an acute toxin and its uptake in rice is a major issue, the current experiment was conducted to explore the efficacy of chemically produced acidified carbon (AC) to mitigate Pb toxicity in rice. Lead was introduced in concentrations of 0, 15, and 30 mg kg-1 soil in combination with 0, 0.5, and 1% AC, underground water (GW) and wastewater (WW) in rice plants. The addition of 1% AC significantly improved the plant height (52 and 7%), spike length (66 and 50%), 1000 grains weight (144 and 71%) compared to 0% AC under GW and WW irrigation, respectively at 30 mg Pb kg-1 soil (30 Pb) toxicity. Similar improvements in the photosynthetic rate, transpiration rate and stomatal conductance also validated the effectiveness of 1% AC over 0% AC. A significant decrease in electrolyte leakage and plant Pb concentration by application of 0.5 and 1% AC validates the effectiveness of these treatments for mitigating 30 Pb toxicity in rice compared to 0% AC under GW or WW irrigation. In conclusion, 1% AC is an effective amendment in alleviating Pb toxicity in rice irrigated with GW or WW at 30 Pb.

The extreme plant-growth-promoting properties of Pantoea phytobeneficialis MSR2 revealed by functional and genomic analysis.

Numerous Pantoea strains are important because of the benefit they provide in the facilitation of plant growth. However, Pantoea have a high level of genotypic diversity and not much is understood regarding their ability to function in a plant beneficial manner. In the work reported here, the plant growth promotion activities and the genomic properties of the unusual Pantoea phytobeneficialis MSR2 are elaborated, emphasizing the genetic mechanisms involved in plant colonization and growth promotion. Detailed analysis revealed that strain MSR2 belongs to a rare group of Pantoea strains possessing an astonishing number of plant growth promotion genes, including those involved in nitrogen fixation, phosphate solubilization, 1-aminocyclopropane-1-carboxylic acid deaminase activity, indoleacetic acid and cytokinin biosynthesis, and jasmonic acid metabolism. Moreover, the genome of this bacterium also contains genes involved in the metabolism of lignin and other plant cell wall compounds, quorum-sensing mechanisms, metabolism of plant root exudates, bacterial attachment to plant surfaces and resistance to plant defences. Importantly, the analysis revealed that most of these genes are present on accessory plasmids that are found within a small subset of Pantoea genomes, reinforcing the idea that Pantoea evolution is largely mediated by plasmids, providing new insights into the evolution of beneficial plant-associated Pantoea.

Indole-3-acetic acid biosynthesis and its regulation in plant-associated bacteria.

Numerous studies have reported the stimulation of plant growth following inoculation with an IAA-producing PGPB. However, the specific mode of IAA production by the PGPB is rarely elucidated. In part, this is due to the overwhelming complexity of IAA biosynthesis and regulation. The promiscuity of the enzymes implicated in IAA biosynthesis adds another element of complexity when attempting to decipher their role in IAA biosynthesis. To date, the majority of research on IAA biosynthesis describes three separate pathways classified in terms of their intermediates-indole acetonitrile (IAN), indole acetamide (IAM), and indole pyruvic acid (IPA). Each of these pathways is mediated by a set of enzymes, many of which are traditionally assumed to exist for that specific catalytic role. This lends the possibility of missing other, novel, enzymes that may also incidentally serve that function. Some of these pathways are constitutively expressed, while others are inducible. Some enzymes involved in IAA biosynthesis are known to be regulated by IAA or by IAA precursors, as well as by a multitude of environmental cues. This review aims to provide an update to our current understanding of the biosynthesis and regulation of IAA in bacteria. KEY POINTS: • IAA produced by PGPB improves bacterial stress tolerance and promotes plant growth. • Bacterial IAA biosynthesis is convoluted; multiple interdependent pathways. • Biosynthesis of IAA is regulated by IAA, IAA-precursors, and environmental factors.

Role of textile effluent fertilization with biosurfactant to sustain soil quality and nutrient availability.

The textile industry is one of the world's most pollution generating industries, and management of its toxic effluent has become a global issue. However, the use of textile effluent as source of nutrients can be a viable option due to the presence of some essential minerals, although the presence of several toxic elements can deteriorate soil health. Therefore, experiments were conducted to identify the potential of textile effluent fertilization together with biosurfactant amendment to increase the soil health and nutrient status for agricultural sustainability. In this study, soil fertilized with different concentrations of textile effluent treatments (i.e. T1A, T2A and T3A) was treated with two different concentrations of biosurfactants to accurately appraise the suitability of a set of soil quality parameters including, physical, chemical, biochemical, and biological activities. A thorough analysis of all soil health parameters was undertaken before sowing the seeds and after cultivation of either Triticum aestivum (cv. LOK-1 and GW-496) or Capsicum annum (cv. GVC-101 and GVC-121). The results indicated that lower concentrations of textile effluent are a good source of nutrients while biosurfactants served as good solubilizers of metals and made it more available at the higher effluent concentration (T2C). Under these conditions, not only did fertilizers significantly increased, but also the microbial population as well as the enzymatic activity of soil was enhanced. Lower concentrations of micronutrients in the soil after crop cultivation is attributed to the higher level of their availability to the plants during their growth and development. This study provides new insight into the biosurfactants application to enhance soil micronutrients availability and increase crop productivity using textile effluents as a source of nutrients.

Plant-archaea relationships: a potential means to improve crop production in arid and semi-arid regions.

Crop production in arid and semi-arid regions of the world is limited by several abiotic factors, including water stress, temperature extremes, low soil fertility, high soil pH, low soil water-holding capacity, and low soil organic matter. Moreover, arid and semi-arid areas experience low levels of rainfall with high spatial and temporal variability. Also, the indiscriminate use of chemicals, a practice that characterizes current agricultural practice, promotes crop and soil pollution potentially resulting in serious human health and environmental hazards. A reliable and sustainable alternative to current farming practice is, therefore, a necessity. One such option includes the use of plant growth-promoting microbes that can help to ameliorate some of the adverse effects of these multiple stresses. In this regard, archaea, functional components of the plant microbiome that are found both in the rhizosphere and the endosphere may contribute to the promotion of plant growth. Archaea can survive in extreme habitats such as areas with high temperatures and hypersaline water. No cases of archaea pathogenicity towards plants have been reported. Archaea appear to have the potential to promote plant growth, improve nutrient supply and protect plants against various abiotic stresses. A better understanding of recent developments in archaea functional diversity, plant colonizing ability, and modes of action could facilitate their eventual usage as reliable components of sustainable agricultural systems. The research discussed herein, therefore, addresses the potential role of archaea to improve sustainable crop production in arid and semi-arid areas.

ACC deaminase plays a major role in Pseudomonas fluorescens YsS6 ability to promote the nodulation of Alpha- and Betaproteobacteria rhizobial strains.

Ethylene acts as a major regulator of the nodulation process of leguminous plants. Several rhizobial strains possess the ability to modulate plant ethylene levels through the expression of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase; however, rhizobia present low enzymatic activities. One possible alternative to this problem resides on the use of free-living bacteria, such as Pseudomonas, presenting high levels of ACC deaminase activity that may be used as adjuvants in the nodulation process by decreasing inhibitory ethylene levels. Nevertheless, not much is understood about the specific role of ACC deaminase in the possible role of free-living bacteria as nodulation adjuvants. Therefore, this work aims to study the effect of ACC deaminase in the plant growth-promoting bacterium, Pseudomonas fluorescens YsS6, ability to facilitate alpha- and beta-rhizobia nodulation. The ACC deaminase-producing P. fluorescens YsS6 and its ACC deaminase mutant were used in co-inoculation assays to evaluate their impact in the nodulation process of alpha- (Rhizobium tropici CIAT899) and beta-rhizobia (Cupriavidus taiwanensis STM894) representatives, in Phaseolus vulgaris and Mimosa pudica plants, respectively. The results obtained indicate that the wild-type P. fluorescens YsS6, but not its mutant defective in ACC deaminase production, increase the nodulation abilities of both alpha- and beta-rhizobia, resulting in an increased leguminous plant growth. Moreover, this is the first report of the positive effect of free-living bacteria in the nodulation process of beta-rhizobia. The modulation of inhibitory ethylene levels by free-living ACC deaminase-producing bacteria plays an important role in facilitating the nodulation process of alpha- and beta-rhizobia.

Plant health: feedback effect of root exudates-rhizobiome interactions.

The well-being of the microbial community that densely populates the rhizosphere is aided by a plant's root exudates. Maintaining a plant's health is a key factor in its continued existence. As minute as rhizospheric microbes are, their importance in plant growth cannot be overemphasized. They depend on plants for nutrients and other necessary requirements. The relationship between the rhizosphere-microbiome (rhizobiome) and plant hosts can be beneficial, non-effectual, or pathogenic depending on the microbes and the plant involved. This relationship, to a large extent, determines the fate of the host plant's survival. Modern molecular techniques have been used to unravel rhizobiome species' composition, but the interplay between the rhizobiome root exudates and other factors in the maintenance of a healthy plant have not as yet been thoroughly investigated. Many functional proteins are activated in plants upon contact with external factors. These proteins may elicit growth promoting or growth suppressing responses from the plants. To optimize the growth and productivity of host plants, rhizobiome microbial diversity and modulatory techniques need to be clearly understood for improved plant health.

Co-occurrence patterns of microbial communities affected by inoculants of plant growth-promoting bacteria during phytoremediation of heavy metal-contaminated soils.

Phytoremediation assisted by plant growth-promoting bacteria (PGPB) is an alternative method of cleaning up toxic metals from soil. However, the interactions among indigenous soil microorganisms following PGPB inoculation are far from fully understood, although these interactions are conducive to evaluate the effectiveness of PGPB. Here, we used Illumina Miseq sequencing and network analysis to decipher the co-occurrence patterns of bacterial communities following PGPB inoculation during phytoremediation of heavy metal contaminated soil. Miseq sequencing revealed that PGPB inoculation changed the bacterial community composition one day after inoculation, with minor changes continuing to be observed ten days after inoculation. This suggested that PGPB inoculants did not proliferate extensively in a new environment. Network analysis showed that PGPB inoculation altered the co-occurrence patterns, dominant modules and topological roles of individual OTUs. In the presence of PGPB inoculants the bacterial community had more complex and compact associations. Moreover, PGPB inoculation increased the percentage of connectors, indicating that PGPB may contribute to more intensified interactions among OTUs from different modules; consequently, the microbial community would be more ordered and efficient. The enhanced co-occurrence associations in the PGPB-inoculated bacterial network may contribute to the plant growth-promoting effects of PGPB during phytoremediation of heavy metal-contaminated soil.

Indole acetic acid overproduction transformants of the rhizobacterium Pseudomonas sp. UW4.

The plant growth-promoting rhizobacterium Pseudomonas sp. UW4 was transformed to increase the biosynthesis of the auxin, indole-3-acetic acid (IAA). Four native IAA biosynthesis genes from strain UW4 were individually cloned into an expression vector and introduced back into the wild-type strain. Quantitative real-time polymerase chain reaction analysis revealed that the introduced genes ami, nit, nthAB and phe were all overexpressed in these transformants. A significant increase in the production of IAA was observed for all modified strains. Canola plants inoculated with the modified strains showed enhanced root elongation under gnotobiotic conditions. The growth rate and 1-aminocyclopropane-1-carboxylate deaminase activity of transformant strains was lower compared to the wild-type. The indoleacetic acid biosynthesis pathways and the role of this phytohormone in the mechanism of plant growth stimulation by Pseudomonas sp. UW4 is discussed.

Improvement of Cupriavidus taiwanensis Nodulation and Plant Growth Promoting Abilities by the Expression of an Exogenous ACC Deaminase Gene.

Several rhizobial strains possess the ability to modulate leguminous plants ethylene levels by producing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase. While the effect of ACC deaminase has been studied in several rhizobia belonging to the Alphaproteobacteria class, not much is understood about its impact in the nodulation abilities of rhizobia belonging to the Betaproteobacteria class, which are common symbionts of Mimosa species. In this work, we report the impact of ACC deaminase production by the Betaproteobacterium, Cupriavidus taiwanensis STM894, and its role in the nodulation of Mimosa pudica. C. taiwanensis STM894 was studied following its transformation with the plasmid pRKACC, containing an ACC deaminase gene. The expression of the exogenous ACC deaminase led to increased nodulation and M. pudica growth promotion by C. taiwanensis STM894. These results indicate that ACC deaminase plays an important role in modulating ethylene levels that inhibit the nodulation process induced by both rhizobia belonging to the Alpha and Betaproteobacteria class.

Tomato ethylene sensitivity determines interaction with plant growth-promoting bacteria.

Background and Aims: Plant growth-promoting bacteria (PGPB) are soil micro-organisms able to interact with plants and stimulate their growth, positively affecting plant physiology and development. Although ethylene plays a key role in plant growth, little is known about the involvement of ethylene sensitivity in bacterial inoculation effects on plant physiology. Thus, the present study was pursued to establish whether ethylene perception is critical for plant-bacteria interaction and growth induction by two different PGPB strains, and to assess the physiological effects of these strains in juvenile and mature tomato ( Solanum lycopersicum ) plants. Methods: An experiment was performed with the ethylene-insensitive tomato never ripe and its isogenic wild-type line in which these two strains were inoculated with either Bacillus megaterium or Enterobacter sp. C7. Plants were grown until juvenile and mature stages, when biomass, stomatal conductance, photosynthesis as well as nutritional, hormonal and metabolic statuses were analysed. Key Results: Bacillus megaterium promoted growth only in mature wild type plants. However, Enterobacter C7 PGPB activity affected both wild-type and never ripe plants. Furthermore, PGPB inoculation affected physiological parameters and root metabolite levels in juvenile plants; meanwhile plant nutrition was highly dependent on ethylene sensitivity and was altered at the mature stage. Bacillus megaterium inoculation improved carbon assimilation in wild-type plants. However, insensitivity to ethylene compromised B. megaterium PGPB activity, affecting photosynthetic efficiency, plant nutrition and the root sugar content. Nevertheless, Enterobacter C7 inoculation modified the root amino acid content in addition to stomatal conductance and plant nutrition. Conclusions: Insensitivity to ethylene severely impaired B. megaterium interaction with tomato plants, resulting in physiological modifications and loss of PGPB activity. In contrast, Enterobacter C7 inoculation stimulated growth independently of ethylene perception and improved nitrogen assimilation in ethylene-insensitive plants. Thus, ethylene sensitivity is a determinant for B. megaterium , but is not involved in Enterobacter C7 PGPB activity.

Survey of Plant Growth-Promoting Mechanisms in Native Portuguese Chickpea Mesorhizobium Isolates.

Rhizobia may possess other plant growth-promoting mechanisms besides nitrogen fixation. These mechanisms and the tolerance to different environmental factors, such as metals, may contribute to the use of rhizobia inocula to establish a successful legume-rhizobia symbiosis. Our goal was to characterize a collection of native Portuguese chickpea Mesorhizobium isolates in terms of plant growth-promoting (PGP) traits and tolerance to different metals as well as to investigate whether these characteristics are related to the biogeography of the isolates. The occurrence of six PGP mechanisms and tolerance to five metals were evaluated in 61 chickpea Mesorhizobium isolates previously obtained from distinct provinces in Portugal and assigned to different species clusters. Chickpea microsymbionts show high diversity in terms of PGP traits as well as in their ability to tolerate different metals. All isolates synthesized indoleacetic acid, 50 isolates produced siderophores, 19 isolates solubilized phosphate, 12 isolates displayed acid phosphatase activity, and 22 exhibited cytokinin activity. Most isolates tolerated Zn or Pb but not Ni, Co, or Cu. Several associations between specific PGP mechanisms and the province of origin and species clusters of the isolates were found. Our data suggests that the isolate's tolerance to metals and ability to solubilize inorganic phosphate and to produce IAA may be responsible for the persistence and distribution of the native Portuguese chickpea Mesorhizobium species. Furthermore, this study revealed several chickpea microsymbionts with potential as PGP rhizobacteria as well as for utilization in phytoremediation strategies.

Mechanisms of action of plant growth promoting bacteria.

The idea of eliminating the use of fertilizers which are sometimes environmentally unsafe is slowly becoming a reality because of the emergence of microorganisms that can serve the same purpose or even do better. Depletion of soil nutrients through leaching into the waterways and causing contamination are some of the negative effects of these chemical fertilizers that prompted the need for suitable alternatives. This brings us to the idea of using microbes that can be developed for use as biological fertilizers (biofertilizers). They are environmentally friendly as they are natural living organisms. They increase crop yield and production and, in addition, in developing countries, they are less expensive compared to chemical fertilizers. These biofertilizers are typically called plant growth-promoting bacteria (PGPB). In addition to PGPB, some fungi have also been demonstrated to promote plant growth. Apart from improving crop yields, some biofertilizers also control various plant pathogens. The objective of worldwide sustainable agriculture is much more likely to be achieved through the widespread use of biofertilizers rather than chemically synthesized fertilizers. However, to realize this objective it is essential that the many mechanisms employed by PGPB first be thoroughly understood thereby allowing workers to fully harness the potentials of these microbes. The present state of our knowledge regarding the fundamental mechanisms employed by PGPB is discussed herein.

Bacterial Modulation of Plant Ethylene Levels.

A focus on the mechanisms by which ACC deaminase-containing bacteria facilitate plant growth.Bacteria that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, when present either on the surface of plant roots (rhizospheric) or within plant tissues (endophytic), play an active role in modulating ethylene levels in plants. This enzyme activity facilitates plant growth especially in the presence of various environmental stresses. Thus, plant growth-promoting bacteria that express ACC deaminase activity protect plants from growth inhibition by flooding and anoxia, drought, high salt, the presence of fungal and bacterial pathogens, nematodes, and the presence of metals and organic contaminants. Bacteria that express ACC deaminase activity also decrease the rate of flower wilting, promote the rooting of cuttings, and facilitate the nodulation of legumes. Here, the mechanisms behind bacterial ACC deaminase facilitation of plant growth and development are discussed, and numerous examples of the use of bacteria with this activity are summarized.

Isolation and characterization of endophytic plant growth-promoting bacteria from date palm tree (Phoenix dactylifera L.) and their potential role in salinity tolerance.

Endophytic bacteria were isolated from date palm (Phoenix dactylifera L.) seedling roots, characterized and tested for their ability to help plants grow under saline conditions. Molecular characterization showed that the majority of these strains belonged to the genera Bacillus and Enterobacter and had different degrees of resistance to various antibiotics. Some of these strains were able to produce the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and the plant growth regulatory hormone indole-3-acetic acid (IAA). Some strains were also able to chelate ferric iron (Fe(3+)) and solubilize potassium (K(+)), phosphorus (PO 4 (3-) ) and zinc (Zn(2+)), and produce ammonia. The results also showed that ACC deaminase activity and IAA production was slightly increased in some strains in response to an increase in NaCl concentration in the growth media. Consistent with these results, selected strains such as PD-R6 (Paenibacillus xylanexedens) and PD-P6 (Enterobacter cloacae) were able to enhance canola root elongation when grown under normal and saline conditions as demonstrated by a gnotobiotic root elongation assay. These results suggest that the isolated and characterized endophytic bacteria can alter ethylene and IAA levels and also facilitate nutrient uptake in roots and therefore have the potential role to promote the growth and development of date palm trees growing under salinity stress.

Characterization of a nitrilase and a nitrile hydratase from Pseudomonas sp. strain UW4 that converts indole-3-acetonitrile to indole-3-acetic acid.

Indole-3-acetic acid (IAA) is a fundamental phytohormone with the ability to control many aspects of plant growth and development. Pseudomonas sp. strain UW4 is a rhizospheric plant growth-promoting bacterium that produces and secretes IAA. While several putative IAA biosynthetic genes have been reported in this bacterium, the pathways leading to the production of IAA in strain UW4 are unclear. Here, the presence of the indole-3-acetamide (IAM) and indole-3-acetaldoxime/indole-3-acetonitrile (IAOx/IAN) pathways of IAA biosynthesis is described, and the specific role of two of the enzymes (nitrilase and nitrile hydratase) that mediate these pathways is assessed. The genes encoding these two enzymes were expressed in Escherichia coli, and the enzymes were isolated and characterized. Substrate-feeding assays indicate that the nitrilase produces both IAM and IAA from the IAN substrate, while the nitrile hydratase only produces IAM. The two nitrile-hydrolyzing enzymes have very different temperature and pH optimums. Nitrilase prefers a temperature of 50°C and a pH of 6, while nitrile hydratase prefers 4°C and a pH of 7.5. Based on multiple sequence alignments and motif analyses, physicochemical properties and enzyme assays, it is concluded that the UW4 nitrilase has an aromatic substrate specificity. The nitrile hydratase is identified as an iron-type metalloenzyme that does not require the help of a P47K activator protein to be active. These data are interpreted in terms of a preliminary model for the biosynthesis of IAA in this bacterium.

A bioinformatics approach to the determination of genes involved in endophytic behavior in Burkholderia spp.

The vast majority of plants harbor endophytic bacteria that colonize a portion of the plant's interior tissues without harming the plant. Like plant pathogens, endophytes gain entry into their plants hosts through various mechanisms. Bacterial endophytes display a broad range of symbiotic interactions with their host plants. The molecular bases of these plant-endophyte interactions are currently not fully understood. In the present study, a set of genes possibly responsible for endophytic behavior for genus Burkholderia was predicted and then compared and contrasted with a number (nine endophytes from different genera) of endophytes by comparative genome analysis. The nine endophytes included Burkholderia phytofirmans PsJN, Burkholderia spp. strain JK006, Azospirillum lipoferum 4B, Enterobacter cloacae ENHKU01, Klebsiella pneumoniae 342, Pseudomonas putida W619, Enterobacter spp. 638, Azoarcus spp. BH72, and Serratia proteamaculans 568. From the genomes of the analyzed bacterial strains, a set of bacterial genes orthologs was identified that are predicted to be involved in determining the endophytic behavior of Burkholderia spp. The genes and their possible functions were then investigated to establish a potential connection between their presence and the role they play in bacterial endophytic behavior. Nearly all of the genes identified by this bioinformatics procedure encode function previously suggested in other studies to be involved in endophytic behavior.

Indole-3-acetic acid in plant-microbe interactions.

Indole-3-acetic acid (IAA) is an important phytohormone with the capacity to control plant development in both beneficial and deleterious ways. The ability to synthesize IAA is an attribute that many bacteria including both plant growth-promoters and phytopathogens possess. There are three main pathways through which IAA is synthesized; the indole-3-pyruvic acid, indole-3-acetamide and indole-3-acetonitrile pathways. This chapter reviews the factors that effect the production of this phytohormone, the role of IAA in bacterial physiology and in plant-microbe interactions including phytostimulation and phytopathogenesis.