BACKGROUND
microbial formulations are used as additives in agriculture promising stimulation of root production, thus enhanced uptake of water and nutrients, resistance to pathogens and increased resilience to environmental stresses such as drought, salinity and heavy metal contamination  <cit> .
soil microbes can also play a more direct role in plant nutrient acquisition, especially for those nutrients that are inherently less available in soils, such as phosphorus   <cit> .
microbial metabolism is dependent on a source of labile carbon  and the rhizosphere is far richer in microorganisms than the surrounding bulk soil due to the substantial exudation of c by plant roots.
bacteria colonise only a small proportion of the root surface, largely the junctions between epidermal cells and the regions surrounding emerging lateral roots where c is secreted  <cit> .
a number of these rhizobacteria species increase root production through effects on plant hormonal signalling processes: either by production of hormones in the bacteria themselves  <cit>  or by perturbation of endogenous concentrations  <cit>  or transport  <cit>  within the plant.
however, evidence to support positive yield benefits from the use of individual strains, or commercial mixtures, of rhizobacteria in field soils is mixed, suggesting an incomplete understanding of the mechanisms and interactions involved.
this study focuses specifically on the auxin-producing bacterium bacillus amyloliquefaciens fzb <dig>  and the nature of the resulting plant-microbe interactions involved in plant p uptake.
auxin is a plant hormone which regulates a large number of root biological processes including the regulation of cell division and differentiation in processes as diverse as root hair production, meristem maintenance, root gravitrophism and lateral root production.
a large proportion of rhizosphere bacteria synthesise auxin  <cit> , and it is proposed that this is responsible for the promotion of root growth by plant-associated azospirillum, bacillus, pseudomonas and rhizobium species  <cit> .
this auxin production is hypothesised to be a component of a bacterial colonisation mechanism whereby the auxin-induced stimulation of root growth and branching leads to an increase in the area available for bacterial colonisation and so increased c supply  <cit> .
the process of auxin production has been shown to be similar in bacteria and plants  <cit> , and is often sensitive to environmental tryptophan  levels  <cit> .
one of the major objectives of the increase in root production stimulated by microbial inoculation is the increased acquisition of phosphorus .
the concentration of freely available inorganic p  in soil solution is typically very low, due to its propensity to bind strongly to soil surfaces or form insoluble complexes with cations  <cit> .
this means that pi availability is often a limiting factor in plant growth and development, and so increasing a plant’s ability to forage for pi is desirable for crop production.
a number of physiological factors determine pi uptake efficiency in cereals including: lateral root branching and elongation  <cit> ; root hair density  <cit> ; exudation of organic acid anions and phosphatases into the rhizosphere  <cit> ; and formation of symbioses with mycorrhizal fungi  <cit> .
the technical difficulties involved in using bioengineering to exploit these traits  <cit> , alongside some nations’ reluctance to embrace such technologies, has contributed to the use of bacterial and fungal inoculants to improve crop pi uptake ability and therefore increase yields  <cit> .
b. amyloliquefaciens fzb <dig> is a plant growth-promoting bacterium that has been demonstrated to promote the growth of roots it has colonised  <cit> .
this strain has been shown to produce large quantities of auxin, with increases in production following the addition of tryptophan to the culture media  <cit> .
this study aims to assess the nature of the biological interaction between b. amyloliquefaciens fzb <dig> and the triticum aestivum root system: with the hypothesis that any effects of b. amyloliquefaciens fzb <dig> colonisation on root pi uptake rates are dependent upon the soil pi concentration.
as the primary mode of b. amyloliquefaciens fzb42-root interaction observed so far is auxin production  <cit> , this interaction was investigated further by analysing the effects of exogenous auxin application upon both root pi-related gene expression, and root exudation of organic c.
RESULTS
bacillus amyloliquefaciens fzb <dig> effect on root production
in the experimental conditions described above, the application of b. amyloliquefaciens fzb <dig> as a seed dressing stimulated t. aestivum root production in live soil of both low and high pi concentration .
treatment resulted in significant increases in the length of the seminal root  and first order lateral root  per plant under b. amyloliquefaciens fzb <dig> treatment in low pi soil , and length of the seminal root per plant  in high pi soil .
bacillus amyloliquefaciens fzb <dig> impacts pi uptake from low pi environments
the average total pi uptake per plant after three weeks growth was not significantly different in the b. amyloliquefaciens fzb <dig> treatment from the un-inoculated controls  in both low- and high-pi soils.
when expressed on a per unit root surface area basis, however, b. amyloliquefaciens fzb <dig> treated root systems grown in low pi soils were significantly less effective at acquiring pi than un-inoculated controls .
there was, however, no significant difference in endpoint pi uptake per unit root surface area between un-inoculated controls and b. amyloliquefaciens fzb <dig> treatments in high pi soils .
one week old, soil-grown, t. aestivum root systems were grown in soil with either a high or low exogenous pi supply: it was found that b. amyloliquefaciens fzb <dig> seed-treatment resulted in a reduced pi uptake rate under low external pi conditions compared with the un-inoculated controls .
plants had a significantly lower rate of pi uptake per root surface area than the controls , with the average pi uptake being  <dig>  nmol plant- <dig>  and  <dig>  nmol plant- <dig> .
however, under a higher exogenous pi supply  there was no significant difference between the treatments , with the larger root system of the b. amyloliquefaciens fzb <dig> treated plants acquiring an average of  <dig>  nmol plant- <dig> pi compared with the  <dig>  nmol plant- <dig> pi of the controls.
impact of bacillus amyloliquefaciens fzb <dig> on p transporter expression
the relative expression of genes encoding individual cellular pi transporters was also assayed.
of the six pi transporters tested, a significant reduction was found in the relative expression of tapht <dig>  and tapht <dig> , in both the b. amyloliquefaciens fzb <dig> and iaa treatments relative to the control .
of the five pi-associated genes tested, only pap <dig> showed any significant difference in expression under b. amyloliquefaciens fzb <dig> treatment relative to the control, with its expression increasing  <dig> % over two fold over untreated controls .
the expression of none of these genes was significantly affected by the iaa treatment .
impact of iaa on root exudation
the exogenous application of iaa to 1-week-old t. aestivum root systems was assessed using 3-o-m-glucose as a marker of sugar exudation.
the application of 1 mm iaa produced a significant increase in 3-o-m-glucose release when expressed as a function of root system surface area .
discussion
auxin mediated bio-stimulation of t. aestivum root system growth comes at a cost in pi uptake rate
this study presents new experimental data demonstrating the complex nature of plant-microbial interactions in the rhizosphere.
b. amyloliquefaciens fzb <dig>  like a large proportion of rhizosphere microbes, secretes auxin as a component of its interaction with the plant  <cit> .
the results of the present study demonstrate that exogenous auxin application to the t. aestivum root system, whilst capable of stimulating an increase in root production, also reduced root pi uptake rate per unit root surface area from low pi soils .
root pi uptake is mediated by phosphate transporter  proteins which are phosphate : h+ symporters that use electrochemical gradients to drive pi and proton symport into the plant  <cit> .
in t. aestivum the tapht gene family is proposed to encode a group of closely related high affinity phosphate transporter proteins whose expression in root tissue has been shown to be increased  or unchanged  in response to decreased root pi supply  <cit> .
tapht <dig>  &  <dig> ,  <dig>  and  <dig>  expression has conversely been shown to be reduced in response to low p supply under field conditions, but this has been attributed to the effects of high levels of mycorrhizal colonisation  <cit> .
given that tapht <dig>  and tapht <dig>  have previously been identified as having significantly up-regulated expression in roots growing in low pi environments  <cit> , the lower pi uptake rates displayed by b. amyloliquefaciens fzb <dig> treated plants in low pi conditions in the present study  could be explained by the b. amyloliquefaciens fzb <dig> treatment reducing tapht <dig>  and tapht <dig>  expression .
the b. amyloliquefaciens fzb <dig> treatment effect upon tapht <dig>  and tapht <dig>  expression was also mimicked by the exogenous iaa application, which strongly implies that this is at least in part an auxin mediated process, and so an intrinsic component of the b. amyloliquefaciens fzb42-root interaction .
we therefore propose that b. amyloliquefaciens fzb <dig> auxin production lowered root tapht <dig>  and tapht <dig>  expression, directly resulting in the depressed plant pi uptake levels observed in the pot experiments .
it is possible that this effect enables auxin producing rhizobacteria to better compete for localised p in low p soils: auxin application has previously been shown to perturb the expression pattern of pht genes in arabidopsis <cit> , and microbial re-modelling of pht expression has also been demonstrated using arbuscular mycorrhizal fungi  <cit> .
our results have implications for the use of auxins in future biotechnological applications.
specifically, the use of auxins, or auxin-producing micro-organisms, to stimulate root production should be carefully mapped to environmental p conditions to ensure optimal pi uptake, plant growth and yield.
internal p-mobilisation gene expression
treatment with b. amyloliquefaciens fzb <dig> resulted in the up-regulation of pap <dig> expression, whereas the levels of pap <dig>  ips <dig>  and rns remained unchanged .
this increase in pap <dig> expression is potentially a homeostatic adjustment by the plant to recycle more p, counteracting the reduced pi uptake caused by the auxin released by b. amyloliquefaciens fzb <dig> treatment.
the lack of an ips <dig>  expression response in the b. amyloliquefaciens fzb <dig> and iaa treated roots indicates that the effects of auxin upon plant pi gene expression is nuanced rather than a wholesale shift in perceived pi status.
bacillus amyloliquefaciens fzb <dig> seed treatment in higher pi status soils
in contrast to low pi soil, there was no significant negative effect of b. amyloliquefaciens fzb <dig> treatment on pi uptake in soil with a high pi concentration .
this could be due to the unchanged expression levels of tapht <dig> , tapht <dig>  &  <dig>   which are still expressed in high pi environments, or the presence of tapht <dig>  which is expressed in roots independently of environmental pi concentration  <cit> .
tapht <dig>  expression was found to be significantly reduced under b. amyloliquefaciens fzb <dig> treatment , but is also highly homologous to tapht <dig>  and tapht <dig>  &  <dig>  <cit> .
this implies that, despite their proposed functional similarity, their expression is controlled by different factors.
b. amyloliquefaciens fzb <dig> treatment did, however, induce a beneficial effect on plant root production in high pi soils  meaning that, under these conditions, the plants were capable of realising the many benefits of a large root system, such as increased uptake of water and nutrients, without the pitfall of suppressed pi uptake per root surface area.
it is interesting to note that the size of this increase was far smaller than that in low pi soils, probably due to this effect being partly swamped by the increases in root elongation and root branching that are normally observed after elevating environmental pi  <cit> .
therefore the use of b. amyloliquefaciens fzb <dig> treatment within high pi soils, or alongside conventional pi fertilisation, is potentially beneficial to agricultural systems: the auxin produced providing enhanced root production with none of the negative effects upon pi uptake observed in low pi environments in this study .
stimulation of root organic c exudation provides an insight into rhizobacteria auxin production
as the rate of microbial metabolism in most agricultural soils is limited by the availability of labile c compounds  <cit> , the rate of organic c exudation from root systems has a great influence on the microbial community.
this results in an elevated bacterial population in the rhizosphere compared with the bulk soil  <cit> .
it has been estimated that as many as 80% of bacterial species populating the rhizosphere produce auxin, where they are hypothesised to be the beneficiaries of an increase in colonisable root area through auxin stimulated root production  <cit> .
the results of the present study shown in figure  <dig> indicate that the resulting elevated concentrations of auxin in the rhizosphere can stimulate increased net exudation of sugars from plant roots.
as the exudation of sugars from cereal roots is largely a passive process  <cit> , it would imply that auxin has either  down-regulated the plasma membrane h+-atpase driven hexose h+-co-transport which recaptures sugars lost from the root, or  increased membrane permeability facilitating faster efflux.
the latter hypothesis is supported by studies in t. aestivum callus cultures and maize membrane vesicles showing higher membrane permeability and ion leakage from cells treated with iaa and minimal direct effect on h+-atpase functioning  <cit> .
that this increased sugar loss was measurable after only one hour in the present study implies that it is likely to be a direct plant root response to auxin rather than an indirect effect.
this is a previously un-described effect of bacterial auxin production, and a priority for future research is to assess the response relationship to the dose of auxin applied, with reference to known rates of production from rhizosphere bacterial communities.
the results of the present study shown in figure  <dig> also have wider implications for the mechanisms by which the rate of organic molecule exudation by plant roots is controlled.
organic acids have been shown to be exuded by the roots of many species in response to pi deficiency, displacing pi from inorganic precipitates found in the soil  <cit> .
the root tip and root hairs have been shown to be the site of the bulk of this exudation  <cit> , and the root tip is also the location of significant auxin response maxima, the pattern of which is tightly controlled  <cit> .
alterations in the auxin fluxes within the root tip are therefore a potential mechanism for controlling c exudation by roots.
this process could synergistically promote root uptake of pi in low pi environments, with greater concentrations of exuded organic acid anions both: chemically displacing pi from insoluble soil complexes  <cit> , and increasing the labile c source available to rhizosphere microbes which may accelerate biological mobilisation of soil organic p  <cit> .
longevity of bacillus amyloliquefaciens fzb <dig> treatment effects
in the direct 33pi uptake experiments of the present study the b. amyloliquefaciens fzb <dig> treatment caused a proportionally greater reduction in pi uptake in the low pi status soil after one week  than was observed at the end point of the three week pot trial .
it is possible, therefore, that after the first week of growth, the b. amyloliquefaciens fzb <dig> has been increasingly outcompeted at the root surface by endogenous soil microbes  <cit> .
this assumes that b. amyloliquefaciens fzb <dig> colonisation of the seed coat has enabled root surface colonisation as seen in bacillus treatment of zea mays seeds  <cit> .
however, even if this was the case, the greater size of root system established during the crucial first three weeks of root development in t. aestivum is likely to persist.
this would therefore provide potential longer-term benefits for increased crop yield.
CONCLUSIONS
one consequence of the demand for increased agricultural food production is the growth in the use of bio-stimulatory products that are marketed on the basis of increasing crop nutrient uptake for a much lower cost than that of extra nutrient application in fertiliser.
b. amyloliquefaciens fzb <dig>  like many rhizosphere microbes, produces auxin which is likely to be a major component of its interaction with the root system of crop plants such as t. aestivum.
the study demonstrates that b. amyloliquefaciens fzb <dig> auxin production can drive a reduction in the expression of the high-affinity pi transporters tapht <dig>  and tapht <dig> , alongside a reduction of the root’s capacity to take up pi from soils with a low pi concentration.
at the same time b. amyloliquefaciens fzb <dig> stimulates a greater rate of early growth of both seminal and lateral roots in t. aestivum.
the results presented here provide new insight into the complex biological interactions between the t. aestivum root system and auxin producing bacteria in the rhizosphere, and the potential trade-off of these different effects for crop p nutrition.
the reduced root pi uptake caused by the addition of b. amyloliquefaciens fzb <dig>  and its interaction with soil pi concentration, highlights the importance of acquiring a detailed understanding of the interaction of any new microbial strain with crop root systems across a range of environmental conditions, before their widespread use in agriculture is advocated.
this should reduce the risk of unanticipated adverse effects, identify how the efficacy of different products varies with environmental conditions and provide farmers with the evidence to make an informed cost-benefit analysis of the value of such products compared with alternatives.

 microbial formulations are used as additives in agriculture promising stimulation of root production, thus enhanced uptake of water and nutrients, resistance to pathogens and increased resilience to environmental stresses such as drought, salinity and heavy metal contamination. soil microbes can also play a more direct role in plant nutrient acquisition, especially for those nutrients that are inherently less available in soils, such as phosphorus. microbial metabolism is dependent on a source of labile carbon and the rhizosphere is far richer in microorganisms than the surrounding bulk soil due to the substantial exudation of c by plant roots. bacteria colonise only a small proportion of the root surface, largely the junctions between epidermal cells and the regions surrounding emerging lateral roots where c is secreted. a number of these rhizobacteria species increase root production through effects on plant hormonal signalling processes: either by production of hormones in the bacteria themselves or by perturbation of endogenous concentrations or transport within the plant. however, evidence to support positive yield benefits from the use of individual strains, or commercial mixtures, of rhizobacteria in field soils is mixed, suggesting an incomplete understanding of the mechanisms and interactions involved. this study focuses specifically on the auxin-producing bacterium bacillus amyloliquefaciens fzb and the nature of the resulting plant-microbe interactions involved in plant p uptake. auxin is a plant hormone which regulates a large number of root biological processes including the regulation of cell division and differentiation in processes as diverse as root hair production, meristem maintenance, root gravitrophism and lateral root production. a large proportion of rhizosphere bacteria synthesise auxin, and it is proposed that this is responsible for the promotion of root growth by plant-associated azospirillum, bacillus, pseudomonas and rhizobium species. this auxin production is hypothesised to be a component of a bacterial colonisation mechanism whereby the auxin-induced stimulation of root growth and branching leads to an increase in the area available for bacterial colonisation and so increased c supply. the process of auxin production has been shown to be similar in bacteria and plants, and is often sensitive to environmental tryptophan levels. one of the major objectives of the increase in root production stimulated by microbial inoculation is the increased acquisition of phosphorus. the concentration of freely available inorganic p in soil solution is typically very low, due to its propensity to bind strongly to soil surfaces or form insoluble complexes with cations. this means that pi availability is often a limiting factor in plant growth and development, and so increasing a plant’s ability to forage for pi is desirable for crop production. a number of physiological factors determine pi uptake efficiency in cereals including: lateral root branching and elongation; root hair density; exudation of organic acid anions and phosphatases into the rhizosphere; and formation of symbioses with mycorrhizal fungi. the technical difficulties involved in using bioengineering to exploit these traits, alongside some nations’ reluctance to embrace such technologies, has contributed to the use of bacterial and fungal inoculants to improve crop pi uptake ability and therefore increase yields. b. amyloliquefaciens fzb is a plant growth-promoting bacterium that has been demonstrated to promote the growth of roots it has colonised. this strain has been shown to produce large quantities of auxin, with increases in production following the addition of tryptophan to the culture media. this study aims to assess the nature of the biological interaction between b. amyloliquefaciens fzb and the triticum aestivum root system: with the hypothesis that any effects of b. amyloliquefaciens fzb colonisation on root pi uptake rates are dependent upon the soil pi concentration. as the primary mode of b. amyloliquefaciens fzb42-root interaction observed so far is auxin production, this interaction was investigated further by analysing the effects of exogenous auxin application upon both root pi-related gene expression, and root exudation of organic c.  bacillus amyloliquefaciens fzb effect on root production in the experimental conditions described above, the application of b. amyloliquefaciens fzb as a seed dressing stimulated t. aestivum root production in live soil of both low and high pi concentration. treatment resulted in significant increases in the length of the seminal root and first order lateral root per plant under b. amyloliquefaciens fzb treatment in low pi soil, and length of the seminal root per plant in high pi soil. bacillus amyloliquefaciens fzb impacts pi uptake from low pi environments the average total pi uptake per plant after three weeks growth was not significantly different in the b. amyloliquefaciens fzb treatment from the un-inoculated controls in both low- and high-pi soils. when expressed on a per unit root surface area basis, however, b. amyloliquefaciens fzb treated root systems grown in low pi soils were significantly less effective at acquiring pi than un-inoculated controls. there was, however, no significant difference in endpoint pi uptake per unit root surface area between un-inoculated controls and b. amyloliquefaciens fzb treatments in high pi soils. one week old, soil-grown, t. aestivum root systems were grown in soil with either a high or low exogenous pi supply: it was found that b. amyloliquefaciens fzb seed-treatment resulted in a reduced pi uptake rate under low external pi conditions compared with the un-inoculated controls. plants had a significantly lower rate of pi uptake per root surface area than the controls, with the average pi uptake being  nmol plant- and  nmol plant-. however, under a higher exogenous pi supply there was no significant difference between the treatments, with the larger root system of the b. amyloliquefaciens fzb treated plants acquiring an average of  nmol plant- pi compared with the  nmol plant- pi of the controls. impact of bacillus amyloliquefaciens fzb on p transporter expression the relative expression of genes encoding individual cellular pi transporters was also assayed. of the six pi transporters tested, a significant reduction was found in the relative expression of tapht and tapht, in both the b. amyloliquefaciens fzb and iaa treatments relative to the control. of the five pi-associated genes tested, only pap showed any significant difference in expression under b. amyloliquefaciens fzb treatment relative to the control, with its expression increasing % over two fold over untreated controls. the expression of none of these genes was significantly affected by the iaa treatment. impact of iaa on root exudation the exogenous application of iaa to 1-week-old t. aestivum root systems was assessed using 3-o-m-glucose as a marker of sugar exudation. the application of 1 mm iaa produced a significant increase in 3-o-m-glucose release when expressed as a function of root system surface area. discussion auxin mediated bio-stimulation of t. aestivum root system growth comes at a cost in pi uptake rate this study presents new experimental data demonstrating the complex nature of plant-microbial interactions in the rhizosphere. b. amyloliquefaciens fzb like a large proportion of rhizosphere microbes, secretes auxin as a component of its interaction with the plant. the results of the present study demonstrate that exogenous auxin application to the t. aestivum root system, whilst capable of stimulating an increase in root production, also reduced root pi uptake rate per unit root surface area from low pi soils. root pi uptake is mediated by phosphate transporter proteins which are phosphate: h+ symporters that use electrochemical gradients to drive pi and proton symport into the plant. in t. aestivum the tapht gene family is proposed to encode a group of closely related high affinity phosphate transporter proteins whose expression in root tissue has been shown to be increased or unchanged in response to decreased root pi supply. tapht &, and expression has conversely been shown to be reduced in response to low p supply under field conditions, but this has been attributed to the effects of high levels of mycorrhizal colonisation. given that tapht and tapht have previously been identified as having significantly up-regulated expression in roots growing in low pi environments, the lower pi uptake rates displayed by b. amyloliquefaciens fzb treated plants in low pi conditions in the present study could be explained by the b. amyloliquefaciens fzb treatment reducing tapht and tapht expression. the b. amyloliquefaciens fzb treatment effect upon tapht and tapht expression was also mimicked by the exogenous iaa application, which strongly implies that this is at least in part an auxin mediated process, and so an intrinsic component of the b. amyloliquefaciens fzb42-root interaction. we therefore propose that b. amyloliquefaciens fzb auxin production lowered root tapht and tapht expression, directly resulting in the depressed plant pi uptake levels observed in the pot experiments. it is possible that this effect enables auxin producing rhizobacteria to better compete for localised p in low p soils: auxin application has previously been shown to perturb the expression pattern of pht genes in arabidopsis, and microbial re-modelling of pht expression has also been demonstrated using arbuscular mycorrhizal fungi. our results have implications for the use of auxins in future biotechnological applications. specifically, the use of auxins, or auxin-producing micro-organisms, to stimulate root production should be carefully mapped to environmental p conditions to ensure optimal pi uptake, plant growth and yield. internal p-mobilisation gene expression treatment with b. amyloliquefaciens fzb resulted in the up-regulation of pap expression, whereas the levels of pap ips and rns remained unchanged. this increase in pap expression is potentially a homeostatic adjustment by the plant to recycle more p, counteracting the reduced pi uptake caused by the auxin released by b. amyloliquefaciens fzb treatment. the lack of an ips expression response in the b. amyloliquefaciens fzb and iaa treated roots indicates that the effects of auxin upon plant pi gene expression is nuanced rather than a wholesale shift in perceived pi status. bacillus amyloliquefaciens fzb seed treatment in higher pi status soils in contrast to low pi soil, there was no significant negative effect of b. amyloliquefaciens fzb treatment on pi uptake in soil with a high pi concentration. this could be due to the unchanged expression levels of tapht, tapht & which are still expressed in high pi environments, or the presence of tapht which is expressed in roots independently of environmental pi concentration. tapht expression was found to be significantly reduced under b. amyloliquefaciens fzb treatment, but is also highly homologous to tapht and tapht &. this implies that, despite their proposed functional similarity, their expression is controlled by different factors. b. amyloliquefaciens fzb treatment did, however, induce a beneficial effect on plant root production in high pi soils meaning that, under these conditions, the plants were capable of realising the many benefits of a large root system, such as increased uptake of water and nutrients, without the pitfall of suppressed pi uptake per root surface area. it is interesting to note that the size of this increase was far smaller than that in low pi soils, probably due to this effect being partly swamped by the increases in root elongation and root branching that are normally observed after elevating environmental pi. therefore the use of b. amyloliquefaciens fzb treatment within high pi soils, or alongside conventional pi fertilisation, is potentially beneficial to agricultural systems: the auxin produced providing enhanced root production with none of the negative effects upon pi uptake observed in low pi environments in this study. stimulation of root organic c exudation provides an insight into rhizobacteria auxin production as the rate of microbial metabolism in most agricultural soils is limited by the availability of labile c compounds, the rate of organic c exudation from root systems has a great influence on the microbial community. this results in an elevated bacterial population in the rhizosphere compared with the bulk soil. it has been estimated that as many as 80% of bacterial species populating the rhizosphere produce auxin, where they are hypothesised to be the beneficiaries of an increase in colonisable root area through auxin stimulated root production. the results of the present study shown in figure  indicate that the resulting elevated concentrations of auxin in the rhizosphere can stimulate increased net exudation of sugars from plant roots. as the exudation of sugars from cereal roots is largely a passive process, it would imply that auxin has either down-regulated the plasma membrane h+-atpase driven hexose h+-co-transport which recaptures sugars lost from the root, or increased membrane permeability facilitating faster efflux. the latter hypothesis is supported by studies in t. aestivum callus cultures and maize membrane vesicles showing higher membrane permeability and ion leakage from cells treated with iaa and minimal direct effect on h+-atpase functioning. that this increased sugar loss was measurable after only one hour in the present study implies that it is likely to be a direct plant root response to auxin rather than an indirect effect. this is a previously un-described effect of bacterial auxin production, and a priority for future research is to assess the response relationship to the dose of auxin applied, with reference to known rates of production from rhizosphere bacterial communities. the results of the present study shown in figure  also have wider implications for the mechanisms by which the rate of organic molecule exudation by plant roots is controlled. organic acids have been shown to be exuded by the roots of many species in response to pi deficiency, displacing pi from inorganic precipitates found in the soil. the root tip and root hairs have been shown to be the site of the bulk of this exudation, and the root tip is also the location of significant auxin response maxima, the pattern of which is tightly controlled. alterations in the auxin fluxes within the root tip are therefore a potential mechanism for controlling c exudation by roots. this process could synergistically promote root uptake of pi in low pi environments, with greater concentrations of exuded organic acid anions both: chemically displacing pi from insoluble soil complexes, and increasing the labile c source available to rhizosphere microbes which may accelerate biological mobilisation of soil organic p. longevity of bacillus amyloliquefaciens fzb treatment effects in the direct 33pi uptake experiments of the present study the b. amyloliquefaciens fzb treatment caused a proportionally greater reduction in pi uptake in the low pi status soil after one week than was observed at the end point of the three week pot trial. it is possible, therefore, that after the first week of growth, the b. amyloliquefaciens fzb has been increasingly outcompeted at the root surface by endogenous soil microbes. this assumes that b. amyloliquefaciens fzb colonisation of the seed coat has enabled root surface colonisation as seen in bacillus treatment of zea mays seeds. however, even if this was the case, the greater size of root system established during the crucial first three weeks of root development in t. aestivum is likely to persist. this would therefore provide potential longer-term benefits for increased crop yield.  one consequence of the demand for increased agricultural food production is the growth in the use of bio-stimulatory products that are marketed on the basis of increasing crop nutrient uptake for a much lower cost than that of extra nutrient application in fertiliser. b. amyloliquefaciens fzb like many rhizosphere microbes, produces auxin which is likely to be a major component of its interaction with the root system of crop plants such as t. aestivum. the study demonstrates that b. amyloliquefaciens fzb auxin production can drive a reduction in the expression of the high-affinity pi transporters tapht and tapht, alongside a reduction of the root’s capacity to take up pi from soils with a low pi concentration. at the same time b. amyloliquefaciens fzb stimulates a greater rate of early growth of both seminal and lateral roots in t. aestivum. the results presented here provide new insight into the complex biological interactions between the t. aestivum root system and auxin producing bacteria in the rhizosphere, and the potential trade-off of these different effects for crop p nutrition. the reduced root pi uptake caused by the addition of b. amyloliquefaciens fzb and its interaction with soil pi concentration, highlights the importance of acquiring a detailed understanding of the interaction of any new microbial strain with crop root systems across a range of environmental conditions, before their widespread use in agriculture is advocated. this should reduce the risk of unanticipated adverse effects, identify how the efficacy of different products varies with environmental conditions and provide farmers with the evidence to make an informed cost-benefit analysis of the value of such products compared with alternatives. 
it then examines the effects on root production, pi uptake, pi-related gene expression and organic carbon exudation. seed treatment with b. amyloliquefaciens fzb increased root production at low environmental pi concentrations, but significantly repressed root pi uptake. at high external pi concentrations, root production was promoted by b. amyloliquefaciens fzb but pi uptake was unaffected. whilst such bacteria promote root production in vitro, the nature of the bacteria-plant interaction in live soil, particularly concerning any effects on nutrient uptake, are not known. the use of auxin-producing rhizosphere bacteria as agricultural products promises increased root production and therefore greater phosphate uptake. we conclude that, alongside promoting root production, auxin biosynthesis by b. amyloliquefaciens fzb both re-models pi transporter expression and elevates organic c exudation.
 the use of auxin-producing rhizosphere bacteria as agricultural products promises increased root production and therefore greater phosphate uptake. whilst such bacteria promote root production in vitro, the nature of the bacteria-plant interaction in live soil, particularly concerning any effects on nutrient uptake, are not known. this study uses bacillus amyloliquefaciens fzb an auxin-producing rhizobacterium, as a dressing on triticum aestivum seeds. it then examines the effects on root production, pi uptake, pi-related gene expression and organic carbon exudation.  seed treatment with b. amyloliquefaciens fzb increased root production at low environmental pi concentrations, but significantly repressed root pi uptake. this coincided with an auxin-mediated reduction in expression of the pi transporters tapht and tapht. applied exogenous auxin also triggered an increase in root c exudation. at high external pi concentrations, root production was promoted by b. amyloliquefaciens fzb but pi uptake was unaffected.  we conclude that, alongside promoting root production, auxin biosynthesis by b. amyloliquefaciens fzb both re-models pi transporter expression and elevates organic c exudation. this shows the potential importance of rhizobacterial-derived auxin following colonisation of root surfaces, and the nature of this bacteria-plant interaction in soil. bacillus amyloliquefaciens fzb42seed treatmentwheatauxinphosphaterootexudation 