Growth stimulation and fruit yield improvement of greenhouse tomato plants by inoculation with Pseudomonas putida or Trichoderma atroviride: Possible role of indole acetic acid (IAA)

Five bacteria (Pseudomonas fluorescens, P. fluorescens subgroup G strain 2, P. marginalis, P. putida subgroup B strain 1 and P. syringae strain 1) and three fungi (Penicillium brevicompactum, P. solitum strain 1 and Trichoderma atroviride) were evaluated to determine their promoting effect on the growth of mature healthy tomato plants grown under hydroponic conditions. P. putida and T. atroviride were shown to improve fruit yields in rockwool and in organic medium. The production or degradation of indole acetic acid (IAA) by the two microorganisms was investigated as possible mechanisms for plant growth stimulation. Both P. putida and T. atroviride were shown to produce IAA. The production of IAA by the two microorganisms was stimulated in vitro by the addition of l-tryptophan, tryptamine and tryptophol (200 μg ml⁻¹) in the culture medium. P. putida and T. atroviride also increased the fresh weight of both the shoot and the roots of tomato seedlings grown in the presence of increasing concentrations of l-tryptophan (up to 0.75 mM). Both microorganisms showed partial degradation of IAA in vitro when grown in a minimal medium with or without sucrose. In addition, the capacity of these microorganisms to reduce the deleterious effect of exogenous IAA was investigated using tomato seedlings. The results showed that the roots of tomato seedlings grown in the presence of increasing concentrations of IAA (0-10 μg ml⁻¹) were significantly longer when seeds were previously treated with P. putida or T. atroviride. The reduction in the detrimental effect of IAA on root elongation could be associated with a reduced ethylene production resulting from a decrease of its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) by microbial degradation of IAA in the rhizosphere and/or by ACC deaminase activity present in both microorganisms.     

Improvement of Brassica napus growth under cadmium stress by cadmium-resistant rhizobacteria

This study focuses on the characterization of four bacterial isolates from heavy metal-polluted rhizosphere in order to examine their plant growth promoting (PGP) activity. The PGP activity on the canola (Brassica napus) of the strains which showed cadmium resistance and multiple PGP traits was assessed in the presence and in the absence of Cd²⁺. The strains, Pseudomonas tolaasii ACC23, Pseudomonas fluorescens ACC9, Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14 showed 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity. They also synthesized ACCD enzyme in vitro when 0.4 mM Cd²⁺ was added to the growth medium. The presence of the metal, however, reduced the ACCD activity in Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14, while it did not affect the ACCD activity of P. tolaasii ACC23 and P. fluorescens ACC9. ACC9 and ACC23 produced indole acetic acid (IAA) and siderophores, while ACC14 produced only IAA. IAA and siderophores were produced more actively under Cd-stress.Root elongation assays conducted on B. napus under gnotobiotic conditions demonstrated increases (from 34% up to 97%) in root elongation of inoculated canola seedlings compared to the control plants. Subsequently, the effect of inoculation with these strains on growth and uptake of Cd²⁺ in roots and shoots of canola was studied in pot experiments using Cd-free and Cd-treated (15 μg Cd²⁺ g^?1 dw) soil. Inoculation with P. tolaasii ACC23, P. fluorescens ACC9 and Mycobacterium sp. ACC14 promoted the growth of plants at concentrations of 0 and 15 μg Cd²⁺ g^?1 soil. The maximum growth was observed in the plants inoculated with P. tolaasii ACC23. The strains did not influence the specific accumulation of cadmium in the root and shoot systems, but all increased the plant biomass and consequently the total cadmium accumulation.The present observations showed that the bacterial strains used in this study protect the plants against the inhibitory effects of cadmium, probably due to the production of IAA, siderophores and ACCD activity.     

Biological activity of humic substances extracted from soils under different vegetation cover

Abstract

Comparisons were made between the chemical compositions of humic substances extracted from three soils covered by different vegetation and their biological activities assayed using 15‐ and 30‐day‐old seedlings of Pinus sylvestris and Picea abies. The growth, “α‐amylase and invertase activity were affected by humic fractions and by gibberellic acid (GA), indicating that humic matter had a gibberellin‐like activity. The isoenzymatic polymorphism in the electrophoretic patterns of esterase was influenced in a similar way by all humic fractions and by indoleacetic acid (IAA), which also suggested that the humic fractions exhibited an auxin‐like activity. The humic fractions extracted from the grassland, exhibiting higher amounts of phenolic and a considerable amount of carboxyl carbon, showed the best metabolic effect. The forest humic fractions, characterized by lower phenolic carbon content, appeared less effective in influencing plant metabolism, whereas the grassland‐forest humic substances proved to be even more less effective. The auxin‐ and the gibberellin‐like activities were related to a high content of phenolic and carboxylic groups. These results presented evidence that the biological activity of the humic substances was attributed to their chemical structure and to their functional groups, which could interact with hormone‐binding proteins in the membrane systems, evoking a hormone‐like response.     

低磷胁迫下熊猫豆侧根增多的生理机制研究

采用溶液培养方法,通过外源添加生长素吲哚-3-乙酸（IAA）及生长素极性运输抑制剂2,3,5-三碘苯甲酸（TIBA）,研究了低磷胁迫下熊猫豆根系构型与生长素相关代谢酶活性之间的关系,以阐明生长素在低磷胁迫下塑造植物根系构型的作用。结果表明,熊猫豆在低磷胁迫下形成特定根系构型,具体表现为侧根增多。IAA在侧根发生过程中起重要作用,外源IAA可以在一定程度上模拟低磷信号引导的侧根发生,而生长素极性运输抑制剂TIBA则显著抑制侧根发生。根系中生长素代谢相关酶生长素氧化酶（IAAO）和过氧化物酶（POD）活性在根构型形成过程中呈现规律性变化。低磷环境下熊猫豆根系中的IAAO、 POD酶活性均高于正常施磷的处理。外源IAA可以增加这两种酶的活性,而TIBA则能够逆转低磷诱导的IAAO、 POD活性的增加。     

Indole acetic acid production by Rhizobium: Effect of 2-ketoglutaric acid

I ndole acetic acid (IAA) production from tryptophan by cell suspensions of Rhizobium trifolii, R. leguminosarum, R. phaseoli and R. lupini was studied in the presence or in the absence of 2-ketoglutaric acid. In R. lupini, production of IAA was strongly enhanced by the ketoacid, but in fast growing rhizobia it was less enhanced or unaffected. On the other hand, glutamic acid inhibits IAA production by R. meliloti, but stimulates IAA production in both R. leguminosarum and R. phaseoli. A hypothesis is proposed to explain these results.     

IAA and 4‐Cl‐IAA Increases the Growth and Nitrogen Fixation in Mung Bean

A commonly occurring auxin indole‐3‐aceticacid (IAA) and a rarely occurring chlorosubstituted auxin (4‐Cl‐IAA) were compared for their impact on growth and nitrogen metabolism in mung bean for the first time. The plants were generated from healthy and Rhizobium coated seeds in earthen pots. The seedlings at 7 and/or 14 days were percolated with 0, 10^?10, 10^?8, or 10^?6 M of IAA or 4‐Cl‐IAA. The plants were sampled at 30 days after sowing (DAS) to assess the growth and various biochemical characteristics. The auxins significantly enhanced the growth (length and dry mass of shoot and root), nodule fresh mass, nitrogenase activity in fresh nodules, leaf carbonic anhydrase activity, chlorophyll content, and rate of photosynthesis. The effect of the auxins lasted up to the harvest where the seed yield, 100 seed mass, and number of pods per plant were significantly affected by the auxins. At a moderate concentration (10^?8 M), 4‐Cl‐IAA generated the best response. However, a comparable response was generated by the higher concentration (10^?6 M) of 4‐Cl‐IAA. The application of the hormone twice (at 7 and 14 DAS) was much more effective than single application (at 7 or 14 DAS). It was concluded that IAA and 4‐Cl‐IAA improved the growth and nitrogen fixation in mung bean. The 4‐Cl‐IAA proved more effective than IAA.     

Co-inoculation of Pseudomonas spp. with Rhizobium improves growth and symbiotic performance of fodder galega (Galega orientalis Lam.)

The effect of Rhizobium galegae alone and in combination with root colonising Pseudomonas strains on the growth of fodder galega (Galega orientalis Lam.) was studied under greenhouse conditions in potting soil containing low levels of nitrogen. Eight weeks after sowing combined inoculations of fodder galega with R. galegae bv. orientalis HAMBI 540 and Pseudomonas trivialis 3Re27 or Pseudomonas extremorientalis TSAU20 had increased shoot and root dry matter, as compared with inoculation with R. galegae HAMBI 540 alone. Both Pseudomonas strains produced indole-3-acetic acid (IAA) in culture but R. galegae did not. While the cellulase producing strain P. trivialis 3Re27 was able to significantly increase nodule numbers and nitrogen content of the co-inoculated plants, the cellulase-negative P. extremorientalis TSAU20 showed no significant stimulation of nodule numbers and nitrogen content in roots. We conclude that P. trivialis 3Re27 improve rhizobia–legume interactions, acting as “rhizobium helper bacteria”. The production of IAA and/or cellulase by Pseudomonas strains may contribute to such a positive effect.     

暖季型草坪草对高温胁迫的生理响应

以沟叶结缕草(Zoysia matrella)与结缕草(Z.japonica)为材料, 测定了高温胁迫下两种暖季型草坪草叶片主要抗性生理指标的变化, 结果表明: 40 ℃高温处理下, 两种结缕草叶片渗透调节物质(可溶性糖、可溶性蛋白质、脯氨酸)和ABA、GA3、IAA、MDA含量均显著增加, SOD、POD与CAT酶活性都显著增强; 两种结缕草叶片的Chla、Chlb、Car含量以及Chla/Chlb的比值则出现了不同程度的下降.高温下, 沟叶结缕草渗透调节物质的积累量和ABA、GA3、IAA含量的增加均高于结缕草, SOD和CAT活性的增强也大于结缕草, 表现出更强的耐热特性.     

Influence of clay minerals on the formation of humic substances by Epicoccum nigrum and Stachybotrys chartarum

The influence of montmorillonite, kaolinite and finely ground quartz on the formation of humic acid-type polymers by Epicoccum nigrum and Stachybotrys chartarum was studied. The fungi were grown in shake and in deep (4 cm) or shallow (1.5 cm) stationary cultures. In general, clay shortened the time required for the formation of dark colored substances and increased the amounts of humic acid-type polymers in the culture solutions or extracted from the cells with NaOH. In some tests, the time of maximum humic polymer accumulation was much earlier in the presence of clay but total amounts formed in check cultures eventually equalled that of the cultures with clay additions. The ratios of the total humic acid to the total cell substance or to the glucose consumed were also generally increased by montmorillonite. Kaolinite and quartz exerted a similar but less pronounced effect. In deep stationary cultures of S. chartarum, total growth and humic acid formation was sometimes depressed by the higher concentrations of montmorillonite but in shallow cultures biomass and humic acid formation were increased. In cultures with an initial pH of 6.0, humic acid polymers were formed in the cells before they appeared in the solutions. During autolysis, some of the cellular polymers were either released into the medium or became more readily extractable with NaOH. The clays did not appreciably alter the chemical properties of the humic polymers, namely, C and N contents, exchange capacity, COOH groups, total acidity, or phenols released upon sodium amalgam reduction. In the presence of clays, phenols were formed more quickly in the culture solutions, but the kinds and relative amounts did not appear to be altered. Clays did not significantly affect oxygen consumption during autoxidation of phenol mixtures. The observations indicate that, by affecting growth and metabolism, the clays indirectly influence phenolic polymer formation.     

Brominated phenols as auxin-like molecules

2,6-Dibromophenol (DBP) was reported as an auxin-like molecule using molecular quantum similarity measures. In this study, the auxin activity of this molecule and its chlorinated homologue is further determined using a bacterial biosensor: the auxin-inducible ipdC promoter of Azospirillum brasilense. We were able to demonstrate that DBP can induce gene expression, but to a lesser extent than the auxin indole-3-acetic acid (IAA) and that DBP is not an antagonist for the IAA signalling pathway. To investigate the role of the bromine groups, the molecule 2,6-dichlorophenol (DCP) was also tested for gene expression induction. However, no induction could be observed. In a second part, DBP and other molecules were modelled in the auxin-binding pocket of the plant auxin receptor TIR1 to evaluate theoretical binding energies. Both DBP and its chlorinated homologue DCP are not strong ligands compared to other known auxins such as IAA. The importance of a carboxylated side chain for optimal binding (and probably auxin activity) was demonstrated.     

Characterization of functional traits of two fluorescent pseudomonads isolated from basidiomes of ectomycorrhizal fungi

Some functional traits of Pseudomonas fluorescens 92 and BBc6, two strains isolated, respectively, from the basidiome of the ectomycorrhizal fungi Suillus grevillei and Laccaria laccata, were evaluated. A rifampicin-resistant mutant of P. fluorescens 92 (P. fluorescens 92R1) showed a significant in vivo plant growth promotion effect on cucumber plants. Quantitative analysis of enzymatic and physiological activities on different substrates showed that P. fluorescens 92 produced about a three times higher level of avicelase than BBc6, while equivalent amounts of β-glucosidase were produced by both strains. Satisfactory levels of neutral phosphomonoesterase and medium levels of acid phosphomonoesterase were produced by both. Only P. fluorescens BBc6 produced a very low amount of phosphodiesterase. Both strains produced high amounts of IAA and siderophores. Both strains showed on an iron deficient medium a very high antagonistic activity against the phytopathogenic fungus Heterobasidion annosum. Purification of fluorescent siderophores by copper-chelate chromatography showed that P. fluorescens 92 produced one pyoverdin (Pf92) and BBc6 two pyoverdins (PfBI and PfBII). A good inhibitory activity against mycelial growth of H. annosum was also observed when using the pyoverdines purified by affinity chromatography. Further purification by reverse phase high pressure liquid chromatography produced multiple fractionation of the three pyoverdins. Analysis of the reverse-phase purified pyoverdines by electronspray ionization mass spectrometry gave for pyoverdin Pf92 the mass value of 1213.8 and for both PfBI and PfBII the mass value of 1305.7. The presence of iron-chelating forms and sodium adducts were also evidenced.     

Screening of rhizobacteria isolated from maize (Zea mays L.) in Rio Grande do Sul State (South Brazil) and analysis of their potential to improve plant growth

Plant growth-promoting rhizobacteria (PGPR) are considered to have a beneficial effect on host plants and may facilitate plant growth by different mechanisms. In this work, the influence of different soil types on the bacterial diversity and the stimulatory effects of selected PGPR on two cultivars of maize were investigated. A set of 292 strains was isolated from the roots and rhizosphere soil of maize cultivated in five different areas of the Rio Grande do Sul State in Brazil. 16S rDNA-PCR-RFLP and 16S rDNA partial sequencing were used for identification, and the Shannon–Weaver index was used to evaluate bacterial diversity. We evaluated the ability of each isolate to produce indole acetic acid (IAA), siderophores and solubilize phosphates. On the basis of multiple PGP traits, six isolates were selected to test their potential as plant growth-promoting rhizobacteria on maize plants. In both the roots and the rhizospheric soil of maize, the dominant bacterial genera identified were Klebsiella and Burkholderia. IAA producers were distributed widely among isolates, regardless of the sampling site. Approximately 42% of the isolates exhibited at least two attributes, and 24% showed all three PGP traits. Three strains, identified as Achromobacter, Burkholderia, and Arthrobacter, were effective as PGPR in both of the cultivars evaluated.     

Characterization and partial purification of proteinases from the highly alkaline midgut of the humivorous larvae of Pachnoda ephippiata (Coleoptera: Scarabaeidae)

Proteinases in the humus-feeding larva of Pachnoda ephippiata were partially purified and characterized. Proteinase activity from the midgut was alkali-stable and its pH optimum for activity was about pH 12. Nine proteolytic bands were visible on zymogram gels containing gelatin; one band of 19 kDa was dominant. P. ephippiata mainly employed serine proteinases for digestive proteolysis. The combination of strong midgut alkalinity and midgut proteinases with pronounced alkali stability, a high proteolytic activity on model humic acids, and a large tolerance to high humic acid concentrations would enable the P. ephippiata to gain nutrients from soil by digesting the proteinaceous component of soil humic substances.     

Stress-responsive indole-3-acetic acid biosynthesis by Azospirillum brasilense SM and its ability to modulate plant growth

Plant growth promotion by Azospirillum brasilense SM has been attributed to its indole-3-acetic acid (IAA) production. Analysis of IAA biosynthesis by this strain under nutrient stresses, likely environmental fluctuations and long-term batch cultures suggested that they significantly influenced this function, with some conditions (fluctuations in temperature) triggering IAA accumulation. In long-term batch cultures (of 30 days), the bacterial population was maintained at a specific cell density and produced IAA even after a sharp decline in population size, albeit fluctuations were observed in both the parameters. Long-term bacterial cultures under nitrogen starvation showed the same trend in cell viability; however, a continuous increase in IAA accumulation was seen over time. This study has shown that A. brasilense strain SM has the potential to be a competent rhizospheric bacterium as it can beneficially influence the growth of sorghum. Further, it also has the ability to promote the growth of a number of other plants like mung bean, maize, and wheat. The benefit of this characteristic of strain SM can be directly accrued to a range of plants with which it may associate so as to improve their yield.     

Do bacterial-feeding nematodes stimulate root proliferation through hormonal effects?

We prepared soil with greater populations of bacterial-feeding nematodes either by stimulating the native populations of the soil, adding an additional mixed community of nematodes, or by adding Caenorhabditis elegans, to investigate the effects of bacterial-feeding nematodes on root morphology, soil auxin (indolyl-3-acetic acid—IAA) concentrations and microbial community structure. In the presence of enhanced bacterial-feeding nematode populations, tomato plants had a more highly branched root system with longer and thinner roots. Root system development was greater with native nematodes than C. elegans. The changes of root morphology were accompanied by an increase of soil IAA content and an altered microbial community structure. Bacterial-feeding nematodes may have affected plant growth by stimulating hormone production through grazing-induced changes to the soil microbial community.     

在受控条件下植物促生菌对扁豆的生长，结瘤和养分积累的效应

Application of plant growth-promoting rhizobacteria (PGPR) has been shown to increase legume growth and development under field and controlled environmental conditions. The present study was conducted to isolate plant growth-promoting rhizobacteria (PGPR) from the root nodules of lentil (Lens culinaris Medik.) grown in arid/semi-arid region of Punjab, Pakistan and examined their plant growth-promoting abilities. Five bacterial isolates were isolated, screened in vitro for plant growth-promoting (PGP) characteristics and their effects on the growth of lentil were assessed under in vitro, hydroponic and greenhouse (pot experiment) conditions. All the isolates were Gram negative, rod-shaped and circular in form and exhibited the plant growth-promoting attributes of phosphate solubilization and auxin (indole acetic acid, IAA) production. The IAA production capacity ranged in 0.5-11.0 μg mL-1 and P solubilization ranged in 3-16 mg L-1 . When tested for their effects on plant growth, the isolated strains had a stimulatory effect on growth, nodulation and nitrogen (N) and phosphorus (P) uptake in plants on nutrient-deficient soil. In the greenhouse pot experiment, application of PGPR significantly increased shoot length, fresh weight and dry weight by 65%, 43% and 63% and the increases in root length, fresh weight and dry weight were 74%, 54% and 92%, respectively, as compared with the uninoculated control. The relative increases in growth characteristics under in vitro and hydroponic conditions were even higher. PGPR also increased the number of pods per plant, 1 000-grain weight, dry matter yield and grain yield by 50%, 13%, 28% and 29%, respectively, over the control. The number of nodules and nodule dry mass increased by 170% and 136%, respectively. After inoculation with effective bacterial strains, the shoot, root and seed N and P contents increased, thereby increasing both N and P uptake in plants. The root elongation showed a positive correlation (R2 = 0.67) with the IAA production and seed yield exhibited a positive correlation (R2 = 0.82) with root nodulation. These indicated that the isolated PGPR rhizobial strains can be best utilized as potential agents or biofertilizers for stimulating the growth and nutrient accumulation of lentil.     

Stimulatory effect of phosphate-solubilizing fungal strains (Aspergillus awamori and Penicillium citrinum) on the yield of chickpea (Cicer arietinum L. cv. GPF2)

The effect of six phosphate-solubilizing fungi (PSF, two strains of Aspergillus awamori, and four of Penicillium citrinum) isolated from rhizosphere of various crops, was observed on the growth and seed production of chickpea plants (Cicer arietinum L. cv. GPF2) in pot experiments. The phosphate (P) solubilizing activity of PSF in liquid varied from 38 to 760 μg ml^?1 for tricalcium phosphate (TCP) and 28–248 μg ml^?1 for mussoorie rock phosphate (MRP). All PSF isolates were biocompatible and produced growth-promoting hormone, Indole acetic acid (IAA), varying in concentration from 2.5 to 9.8 μg ml^?1. Of the various pot experiments carried out in green house, maximum stimulatory effect on chickpea plants growth was observed by inoculation of two A. awamori strains. This treatment resulted in 7–12% increase in shoot height, nearly three-fold increase in seed number and two-fold increase in seeds weight as compared to the control (un-inoculated) plants. Inoculation of four strains of P. citrinum exhibited lesser stimulatory effect. It showed 7% increase in shoot height, two-fold increase in seed number and 87% increase in seeds weight as compared to the control plants. However, a consortium of all the six fungal isolates showed no stimulatory effect on chickpea plants growth.     

Protective Activity of Humic Substances in Wheat Seedlings in Water Deficit Conditions

We conducted a comparative evaluation of the protective effect of humic substances (HSs) derived from coal and peat in seedlings of soft wheat Triticum aestivum L. in water deficit conditions caused by a PEG-6000 hyperosmotic solution. The protective effect of HSs was found to increase with an increase in the content of phenolic fragments. This finding may indicate that the antioxidant activity of HS was the main protective activity mechanism of HS in water deficit conditions.     

Characterization of Heavy Metal-Resistant Endophytic Yeast Cryptococcus sp. CBSB78 from Rapes (Brassica chinensis) and Its Potential in Promoting the Growth of Brassica spp. in Metal-Contaminated Soils

A Cd-, Pb-, Zn-, Cu-resistant endophytic yeast CBSB78 was isolated from surface-sterilized rape roots. The isolate was identified as Cryptococcus sp. based on the ITS1?C5.8S?CITS2 sequence analysis. The strain was resistant to 20 mM Cd²⁺, 20 mM Pb²⁺, 10 mM Zn²⁺, and 7 mM Cu²⁺. The yeast CBSB78 was a low indole-3-acetic acid (IAA) producer and possessed low 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity. Overall, 29.4?C244 % of survival rates increased and the dry weight of Brassica alboglabra showed a 41.1 % increase when it was inoculated into the seedlings. The inoculation of CBSB78 could also increase the extraction amounts of Cd, Pb, and Zn by B. alboglabra simultaneously in the multi-metal contaminated soils, which showed the potential to improve extraction efficacy of Cd, Pb, Zn by B. alboglabra seedlings in the field.     

Arbuscular mycorrhizal fungi inoculation and exogenous indole-3-acetic acid application induce antioxidant defense response to alleviate cadmium toxicity in Broussonetia papyrifera

Cadmium (Cd) contamination in soil poses a huge threat to plants even at low concentrations; Broussonetia papyrifera has great potential in remediation of soil heavy metal contamination. However, whether exogenous indole-3-acetic acid (IAA) application and arbuscular mycorrhizal fungi (AMF) have synergistic effects on Cd tolerance of B. papyrifera remains unclear. To investigate the effects of AMF inoculation and IAA application on the tolerance of B. papyrifera to Cd stress, two experiments were conducted: the first to investigate the effect of AMF (Rhizophagus irregularis) inoculation on the tolerance of B. papyrifera to Cd stress and the second to investigate the combined effects of AMF inoculation and IAA application on the tolerance of B. papyrifera to Cd stress. Parameters including endogenous hormone concentration, antioxidant defense response, malondialdehyde (MDA) content, and gene expression related to antioxidant enzyme system and hormone were measured. The results indicated that AMF alleviated Cd toxicity of B. papyrifera by reducing MDA content and improving antioxidant enzyme activities and Cd absorption capacity. Furthermore, the combination of AMF inoculation and IAA application had a synergetic effect on the tolerance of B. papyrifera to Cd stress through upregulating BpAUX1 and BpAUX2, which might contribute to root growth and root xylem synthesis, and by upregulating BpSOD2 and BpPOD34 to enhance the antioxidant enzyme system. This work provides a new insight into the application of IAA in the remediation of soil Cd pollution by mycorrhizal plants.