Ectopic expression of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.) <Emphasis Type="Italic">CsTIR</Emphasis>/<Emphasis Type="Italic">AFB</Emphasis> genes enhance salt tolerance in transgenic <Emphasis Type="Italic">Arabidopsis</Emphasis>

Auxin receptors TIR1/AFBs play an essential role in a series of signaling network cascades. These F-box proteins have also been identified to participate in different stress responses via the auxin signaling pathway in Arabidopsis. Cucumber (Cucumis sativus L.) is one of the most important crops worldwide, which is also a model plant for research. In the study herein, two cucumber homologous auxin receptor F-box genes CsTIR and CsAFB were cloned and studied for the first time. The deduced amino acid sequences showed a 78% identity between CsTIR and AtTIR1 and 76% between CsAFB and AtAFB2. All these proteins share similar characteristics of an F-box domain near the N-terminus, and several Leucine-rich repeat regions in the middle. Arabidopsis plants ectopically overexpressing CsTIR or CsAFB were obtained and verified. Shorter primary roots and more lateral roots were found in these transgenic lines with auxin signaling amplified. Results showed that expression of CsTIR/AFB genes in Arabidopsis could lead to higher seeds germination rates and plant survival rates than wild-type under salt stress. The enhanced salt tolerance in transgenic plants is probably caused by maintaining root growth and controlling water loss in seedlings, and by stabilizing life-sustaining substances as well as accumulating endogenous osmoregulation substances. We proposed that CsTIR/AFB-involved auxin signal regulation might trigger auxin mediated stress adaptation response and enhance the plant salt stress resistance by osmoregulation.     

Volatile compounds from beneficial or pathogenic bacteria differentially regulate root exudation,transcription of iron transporters,and defense signaling pathways in <Emphasis Type="Italic">Sorghum bicolor</Emphasis>

Key message

Our results show that Sorghum bicolor is able to recognize bacteria through its volatile compounds and differentially respond to beneficial or pathogens via eliciting nutritional or defense adaptive traits.

Abstract

Plants establish beneficial, harmful, or neutral relationships with bacteria. Plant growth promoting rhizobacteria (PGPR) emit volatile compounds (VCs), which may act as molecular cues influencing plant development, nutrition, and/or defense. In this study, we compared the effects of VCs produced by bacteria with different lifestyles, including Arthrobacter agilis UMCV2, Bacillus methylotrophicus M4-96, Sinorhizobium meliloti 1021, the plant pathogen Pseudomonas aeruginosa PAO1, and the commensal rhizobacterium Bacillus sp. L2-64, on S. bicolor. We show that VCs from all tested bacteria, except Bacillus sp. L2-64, increased biomass and chlorophyll content, and improved root architecture, but notheworthy A. agilis induced the release of attractant molecules, whereas P. aeruginosa activated the exudation of growth inhibitory compounds by roots. An analysis of the expression of iron-transporters SbIRT1, SbIRT2, SbYS1, and SbYS2 and genes related to plant defense pathways COI1 and PR-1 indicated that beneficial, pathogenic, and commensal bacteria could up-regulate iron transporters, whereas only beneficial and pathogenic species could induce a defense response. These results show how S. bicolor could recognize bacteria through their volatiles profiles and highlight that PGPR or pathogens can elicit nutritional or defensive traits in plants.

     

Iron homeostasis in tropical <Emphasis Type="Italic">indica</Emphasis> rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars having contrasting grain iron concentration

Iron homeostasis was studied in two tropical indica rice cultivars viz. Sharbati (high Fe) and Lalat (low Fe) having contrasting grain Fe concentration. Plants were hydroponically grown with 5 concentrations of Fe (0.05, 2, 5, 15, 50 mg L^?1) till maturity. The effect of incremental Fe treatment on the plant was followed by analyzing accumulation of ferritin protein, activities of aconitase enzyme, enzymes of anti-oxidative defense and accumulation of hydrogen peroxide and ascorbic acid. Plant growth was adversely affected beyond 15 mg L^?1 of Fe supplementation and effects of Fe stress (both deficiency and excess) were more apparent on the high Fe containing cultivar Sharbati than the low Fe containing Lalat. Level of ferritin protein and aconitase activity increased up to 5 mg L^?1 of Fe concentration. Lalat continued to synthesize ferritin protein at much higher Fe level than Sharbati and the cultivar also had higher activities of peroxidase, superoxide dismutase and glutathione reductase. It was concluded that the tolerance of Lalat to Fe stress was because of its higher intrinsic ability to scavenge free radicals of oxidative stress for possessing higher activity of antioxidative enzymes. This, together with its capacity to sequester the excess Fe in ferritin protein over a wider range of Fe concentrations made it more tolerant to Fe stress.     

Impact of high temperature on sucrose translocation,sugar content and inulin yield in <Emphasis Type="Italic">Cichorium intybus</Emphasis> L. var. <Emphasis Type="Italic">sativum</Emphasis>

Aims

Dianthus caryophyllus is a commercially important ornamental flower. Plant growth promoting rhizobacteria are increasingly applied as bio-fertilisers and bio-fortifiers. We studied the effect of a rhizospheric isolate Klebsiella SGM 81 strain to promote D. caryophyllus growth under sterile and non-sterile conditions, to colonise its root system endophytically and its impact on the cultivatable microbial community. We identified the auxin indole-3-acetic acid (IAA) production of Klebsiella SGM 81 as major bacterial trait most likely to enhance growth of D. caryophyllus.

Methods

ipdC dependent IAA production of SGM 81 was quantified using LC-MS/MS and localised proximal to D. caryophyllus roots and correlated to root growth promotion and characteristic morphological changes. SGM 81 cells were localised on and within the plant root using 3D rendering confocal microscopy of gfp expressing SGM 81. Using Salkowski reagent IAA production was quantified and localised proximal to roots in situ. The effect of different bacterial titres on rhizosphere bacterial population was CFU enumerated on nutrient agar. The genome sequence of Klebsiella SGM 81 (accession number PRJEB21197) was determined to validate PGP traits and phylogenic relationships.

Results

Inoculation of D. caryophyllus roots with Klebsiella SGM 81 drastically promoted plant growth when grown in agar and soil, concomitant with a burst in root hair formation, suggesting an increase in root auxin activity. We sequenced the Klebsiella SGM 81 genome, identified the presence of a canonical ipdC gene in Klebsiella SGM 81, confirmed bacterial production and secretion of IAA in batch culture using LC-MS/MS and localised plant dependent IAA production by SGM 81 proximal to roots. We found Klebsiella SGM 81 to be a rhizoplane and endophytic coloniser of D. caryophyllus roots in a dose dependent manner. We found no adverse effects of SGM 81 on the overall rhizospheric microbial population unless supplied to soil in very high titres.

Conclusion

Klebsiella SGM 81 effectively improves root traits of D. caryophyllus in a dose dependent manner, likely through tryptophan dependent IAA production in the rhizoplane and potentially within the intercellular spaces of root tissue. Under optimal plant growth promoting conditions in non-sterile soil, the high total microbial titre in the rhizosphere supports a mutualistic relationship between Klebsiella SGM 81 and carnation that potentially extends to the wider rhizosphere microbiota.

     

<Emphasis Type="Italic">Piriformospora indica</Emphasis>-mediated salinity tolerance in <Emphasis Type="Italic">Aloe vera</Emphasis> plantlets

Piriformospora indica, a root endophytic fungus, has been reported to promote growth of many plants under normal condition and allow the plants to survive under stress conditions. However, its impact on an important medicinal plant Aloe vera L. has not been well studied. Therefore, this study was undertaken to investigate the effect of P. indica on salinity stress tolerance of A. vera plant. P. indica inoculated and non-inoculated A. vera plantlets were subjected to four levels of salinity treatment- 0, 100, 200 and 300 mM NaCl. The salinity stress decreased the ability of the fungus to colonize roots of A. vera but the interaction of A. vera with P. indica resulted in an overall increase in plant biomass and greater shoot and root length as well as number of shoots and roots. The photosynthetic pigment (Chl a, Chl b and total Chl) and gel content were significantly higher for the fungus inoculated A. vera plantlets, at respective salinity concentrations. Furthermore, the inoculated plantlets had higher phenol, flavonoid, flavonol, aloin contents and radical scavenging activity at all salinity concentrations. The higher phenolic and flavonoid content may help the plants ameliorate oxidative stress resulting from high salinity.     

<Emphasis Type="Italic">Bacillus methylotrophicus</Emphasis> M4-96 isolated from maize (<Emphasis Type="Italic">Zea mays</Emphasis>) rhizoplane increases growth and auxin content in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> via emission of volatiles

Plant growth-promoting rhizobacteria stimulate plant growth and development via different mechanisms. In this study, we characterized the effect of volatiles from Bacillus methylotrophicus M4-96 isolated from the maize rhizosphere on root and shoot development, and auxin homeostasis in Arabidopsis thaliana. Phytostimulation occurred after 4 days of interaction between M4-96 and Arabidopsis grown on opposite sides of divided Petri plates, as revealed by enhanced primary root growth, root branching, leaf formation, and shoot biomass accumulation. Analysis of indole-3-acetic acid content revealed two- and threefold higher accumulation in the shoot and root of bacterized seedlings, respectively, compared to uninoculated plants, which was correlated with increased expression of the auxin response marker DR5::GUS. The auxin transport inhibitor 1-naphthylphthalamic acid inhibited primary root growth and lateral root formation in axenically grown seedlings and antagonized the plant growth-promoting effects of M4-96. Analysis of bacterial volatile compounds revealed the presence of four classes of compounds, including ten ketones, eight alcohols, one aldehyde, and two hydrocarbons. However, the abundance of ketones and alcohols represented 88.73 and 8.05%, respectively, of all airborne signals detected, with acetoin being the main compound produced. Application of acetoin had a different effect from application of volatiles, suggesting that either the entire pool or acetoin acting in concert with another unidentified compound underlies the strong phytostimulatory response. Taken together, our results show that B. methylotrophicus M4-96 generates bioactive volatiles that increase the active auxin pool of plants, stimulate the growth and formation of new organs, and reprogram root morphogenesis.     

Mechanisms of thaxtomin A-induced root toxicity revealed by a thaxtomin A sensitive <Emphasis Type="Italic">Arabidopsis</Emphasis> mutant (<Emphasis Type="Italic">ucu2</Emphasis>-<Emphasis Type="Italic">2/gi</Emphasis>-<Emphasis Type="Italic">2</Emphasis>)

Key message

The Arabidopsis mutant ( ucu2 - 2/gi - 2 ) is thaxtomin A, isoxaben and NPA-sensitive indicated by root growth and ion flux responses providing new insights into these compounds mode of action and interactions.

Abstract

Thaxtomin A (TA) is a cellulose biosynthetic inhibitor (CBI) that promotes plant cell hypertrophy and cell death. Electrophysiological analysis of steady-state K⁺ and Ca²⁺ fluxes in Arabidopsis thaliana roots pretreated with TA for 24 h indicated a disturbance in the regulation of ion movement across the plant cell membrane. The observed inability to control solute movement, recorded in rapidly growing meristematic and elongation root zones, may partly explain typical root toxicity responses to TA treatment. Of note, the TA-sensitive mutant (ucu2-2/gi-2) was more susceptible with K⁺ and Ca²⁺ fluxes altered between 1.3 and eightfold compared to the wild-type control where fluxes altered between 1.2 and threefold. Root growth inhibition assays showed that the ucu2-2/gi-2 mutant had an increased sensitivity to the auxin 2,4-D, but not IAA or NAA; it also had increased sensitivity to the auxin efflux transport inhibitor, 1-naphthylphthalamic acid (NPA), but not 2,3,5- Triiodobenzoic acid (TIBA), when compared to the WT. The NPA sensitivity data were supported by electrophysiological analysis of H⁺ fluxes in the mature (but not elongation) root zone. Increased sensitivity to the CBI, isoxaben (IXB), but not dichlobenil was recorded. Increased sensitivity to both TA and IXB corresponded with higher levels of accumulation of these toxins in the root tissue, compared to the WT. Further root growth inhibition assays showed no altered sensitivity of ucu2-2/gi-2 to two other plant pathogen toxins, alternariol and fusaric acid. Identification of a TA-sensitive Arabidopsis mutant provides further insight into how this CBI toxin interacts with plant cells.

     

Effect of microalgae hydrolysate foliar application (<Emphasis Type="Italic">Arthrospira platensis</Emphasis> and <Emphasis Type="Italic">Scenedesmus</Emphasis> sp.) on <Emphasis Type="Italic">Petunia x hybrida</Emphasis> growth

In horticultural practice accelerated plant development and particularly earlier flowering, has been reported with microalgae applications. Therefore, the objective of this work was to study the effects of foliar spraying with Scenedesmus sp. and Arthrospira platensis hydrolysates on Petunia x hybrida plant development and leaf nutrient status. Three treatments were tested: T1 (foliar application with water, the control), T2 (foliar application with Arthrospira), and T3 (foliar application with Scenedesmus). Foliar spraying was applied five times (0, 14, 28, 35, and 42 days after transplanting). The concentration of both microalgae was 10 g L^?1. At the end of the trial biometric parameters and nutrient concentration in photosynthetic organs (the leaves) were measured. The results of this assay show that foliar application of Scenedesmus accelerated plant development in terms of higher rates of root growth, leaf and shoot development, and earliness of flowering. Arthrospira enhanced the root dry matter, the number of flowers per plant, and the water content. Nevertheless, a reduction was found in the conductive tissue (stem + petiole) dry weight with Arthrospira compared with Scenedesmus and the control. The results also show that microalgae hydrolysate supply can improve the plant nutrient status. Based on these results, it is advisable to use Scenedesmus hydrolysates in foliar applications to increase the blooming of Petunia x hybrida.     

Endophytic <Emphasis Type="Italic">Alcaligenes</Emphasis> Isolated from Horticultural and Medicinal Crops Promotes Growth in Okra (<Emphasis Type="Italic">Abelmoschus</Emphasis><Emphasis Type="Italic">esculentus</Emphasis>)

The potential of endophytic bacteria to act as biofertilizers and bioprotectants has been demonstrated, and considerable progress has been made in explaining their role in plant protection. In the present study, three endophytic bacterial strains (BHU 12, BHU 16 isolated from the leaves of Abelmoschus esculentus, and BHU M7 isolated from the leaves of Andrographis paniculata) were used which displayed high sequence similarity to Alcaligenes faecalis. The biofilm formation ability of these endophytic strains in the presence of okra root exudates confirms their chemotactic ability, an initial step for successful endophytic colonization. Further, reinoculation of spontaneous rifampicin-tagged mutants into okra seedlings revealed a CFU count above 10⁵ cells g^?1 of all three endophytic strains in root samples during the first 15 days of plant growth. The CFU count increased up to 10¹³ by 30 days of plant growth, followed by a gradual decline to approximately 10¹⁰ cells g^?1 at 45 days of plant growth. Systemic endophytic colonization was further supported by 2, 3, 5-triphenyl tetrazolium chloride staining and fluorescence imaging of ds-RED expressing conjugants of the endophytic strains. The strains were further assessed for their plausible in vivo and in vitro plant growth-promoting and antagonistic abilities. Our results demonstrated that the endophytic strains BHU 12, BHU 16, and BHU M7 augmented plant biomass by greater than 40 %. Root and shoot lengths of okra plants when primed by BHU 12, BHU 16, and BHU M7 increased up to 34 and 14.5 %, respectively. The endophytic isolates also exhibited significant in vitro antagonistic potential against the collar rot pathogen Sclerotium rolfsii. In summary, our results demonstrate excellent potential of the three endophytic bacterial strains as biofertilizers and biocontrol agents, indicating the possibility for use in sustainable agriculture.     

Promotion of Peanut Growth by Co-inoculation with Selected Strains of <Emphasis Type="Italic">Bradyrhizobium</Emphasis> and <Emphasis Type="Italic">Azospirillum</Emphasis>

The ability of inoculated rhizobial strains to increase root nodulation of host legumes often depends on their competitiveness with existing native soil strains. Results of studies to date on rhizobial inoculation for improvement of peanut (Arachis hypogaea L.) production in Argentina have been inconsistent and controversial. In many cases, nodulation and yield of peanut crops have been increased by inoculation of specific rhizobial strains. Native peanut-nodulating strains are generally present in soils of agricultural areas, but their growth-promoting effect is often lower than that of inoculated strains. Many species of the genus Bradyrhizobium interact in a host-specific manner with legume species and form nitrogen-fixing root nodules. Other free-living rhizobacteria such as species of the genus Azospirillum are facultatively capable of interacting with legume roots and promoting plant growth. We evaluated and compared the effects of various single inoculation and co-inoculation treatments on peanut growth parameters in greenhouse and field experiments. In the greenhouse studies, co-inoculation with various Bradyrhizobium strains (native 15A and PC34, and recommended peanut inoculant C145), and Azospirillum brasilense strain Az39 generally resulted in increases in the measured parameters. The growth-promoting effect of 15A was similar to or higher than that of C145. In the field studies, 15A-Az39 co-inoculation had a greater promoting effect on measured growth parameters than did C145-Az39 co-inoculation. Our findings indicate that careful selection of native rhizobacterial strains adapted to peanut soils is useful in strategies for growth promotion, and that 15A in particular is a promising candidate for future inoculant formulation.     

<Emphasis Type="Italic">Trichoderma</Emphasis> Modulates Stomatal Aperture and Leaf Transpiration Through an Abscisic Acid-Dependent Mechanism in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Trichoderma species are widespread phytostimulant fungi that act through biocontrol of root pathogens, modulation of root architecture, and improving plant adaptation to biotic and abiotic stress. With the major challenge to better understand the contribution of Trichoderma symbionts to plant adaptation to climate changes and confer stress tolerance, we investigated the potential of Trichoderma virens and Trichoderma atroviride in modulating stomatal aperture and plant transpiration. Arabidopsis wild-type (WT) seedlings and ABA-insensitive mutants, abi1-1 and abi2-1, were co-cultivated with either T. virens or T. atroviride, and stomatal aperture and water loss were determined in leaves. Arabidopsis WT seedlings inoculated with these fungal species showed both decreased stomatal aperture and reduced water loss when compared with uninoculated seedlings. This effect was absent in abi1-1 and abi2-1 mutants. T. virens and T. atroviride induced the abscisic acid (ABA) inducible marker abi4:uidA and produced ABA under standard or saline growth conditions. These results show a novel facet of Trichoderma-produced metabolites in stomatic aperture and water-use efficiency of plants.     

Halophilic rhizobacteria from <Emphasis Type="Italic">Distichlis spicata</Emphasis> promote growth and improve salt tolerance in heterologous plant hosts

Rhizobacteria are central components of the plant microbiome and influence root development and function. Desciphering how rhizobacteria contribute to plant performance under adverse environments is a major research challenge. The aims of the present study were to isolate and characterize rhizobacteria from the halophilic grass Distichlis spicata and to test their possible growth promoting and salt protective properties in Arabidopsis thaliana, Cucumis sativus, and Citrullus lanatus. To determine their possible plant growth promoting properties, 38 rhizobacterial isolates were co-cultivated with Arabidopsis seedlings in vitro. Out of these, two halophilic bacteria, LBEndo1 and KBEcto4, were selected following their strong shoot and root biostimulation. 16S rRNA sequencing identified LBEndo1 as Bacillus sp. and KBEcto4 as Pseudomonas lini. Both strains improved growth under standard and saline conditions, which correlated with IAA and siderophore production, as well as phosphate solubilization. Additionally, the KBEcto4 strain expresses the ACC deaminase enzyme (acdS gene), and slightly increases auxin redistribution within Arabidopsis roots expressing an auxin-inducible gene construct. These data reveal the potential of saltgrass (Distichlis spicata) rhizobacteria to promote growth and confer salt tolerance to Arabidopsis and crop plants.     

The crucial role of roots in increased cadmium-tolerance and Cd-accumulation in the pea mutant <Emphasis Type="Italic">SGECd</Emphasis><Superscript><Emphasis Type="Italic">t</Emphasis></Superscript>

Elucidation of mechanisms underlying plant tolerance to cadmium, a widespread toxic soil pollutant, and accumulation of Cd in plants are urgent tasks. For this purposes, the pea (Pisum sativum L.) mutant SGECd^t (obtained by treatment of the laboratory pea line SGE with ethylmethane sulfonate) was reciprocally grafted with the parental line SGE, and four scion/rootstock combinations were obtained: SGE/SGE, SGECd^t/SGECd^t, SGE/SGECd^t, and SGECd^t/SGE. They were grown in hydroponics in the presence of 1 μM CdCl₂ for 30 d. The SGE and SGECd^t scions on the SGECd^t rootstock had a higher root and shoot biomass and an elevated root and shoot Cd content compared with the grafts having SGE rootstock. Only the grafts with the SGE rootstock showed chlorosis and roots demonstrating symptoms of Cd toxicity. The content of nutrient elements in roots (Fe, K, Mg, Mn, Na, P, and Zn) was higher in the grafts having the SGECd^t rootstock, and three elements, namely Ca, Fe, and Mn, were efficiently transported by the SGECd^t root to the shoot of these grafts. The content of other measured elements (K, Mg, Na, P, and Zn) was similar in the root and shoot in all the grafts. Then, the non-grafted plants were grown in the presence of Cd and subjected to deficit or excess concentrations of Ca, Fe, or Mn. Exclusion of these elements from the nutrient solution retained or increased differences between SGE and SGECd^t in growth response to Cd toxicity, whereas excess of Ca, Fe, or Mn decreased or eliminated such differences. The obtained results assign a principal role of roots to realizing the increased Cd-tolerance and Cdaccumulation in the SGECd^t mutant. Efficient translocation of Ca, Fe, and Mn from roots to shoots appeared to counteract Cd toxicity, although Cd was actively taken up by roots and accumulated in shoots.     

Overexpression of a <Emphasis Type="Italic">Miscanthus sacchariflorus</Emphasis> yellow stripe-like transporter <Emphasis Type="Italic">MsYSL1</Emphasis> enhances resistance of <Emphasis Type="Italic">Arabidopsis</Emphasis> to cadmium by mediating metal ion reallocation

The yellow stripe-like (YSL) family of transporters mediates the uptake, translocation, and distribution of various mineral elements in vivo by transferring metal ions chelated with phytosiderophore or nicotianamine (NA). However, little is known about the roles of the YSL genes against cadmium in planta. In this study, we first cloned and characterized a vital member of the YSL gene family, MsYSL1, from the bioenergy plant Miscanthus sacchariflorus. MsYSL1 localized in the plasma membrane and was widely expressed throughout the whole seedling with the highest expression level in the stem. In addition, its expression in the root was stimulated by excess manganese (Mn), cadmium (Cd), and lead, and a shortage of iron (Fe), zinc (Zn), and copper. Functional complementation in yeast indicated that MsYSL1 showed transport activity for Fe(II)–NA and Zn–NA, but not for Cd–NA. Although they exhibited no significant differences versus the wild type under normal cultivation conditions, MsYSL1-overexpressing Arabidopsis lines displayed a higher resistance to Cd accompanied by longer root lengths, lower Cd, Zn, and Mn levels in roots, and higher Cd, Fe, and Mn translocation ratios under Cd stress. Moreover, genes related to NA synthesis, metal translocation, long-distance transport, and Cd exclusion were highly induced in transgenic lines under Cd stress. Thus, MsYSL1 may be an essential transporter for diverse metal–NAs to participate in the Cd detoxification by mediating the reallocation of other metal ions.     

An ATP-Binding Cassette Transporter Gene from <Emphasis Type="Italic">Cucumis sativus</Emphasis> L., <Emphasis Type="Italic">CsABC19</Emphasis>, Is Involved in Propamocarb Stress in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

A propamocarb-responsive gene named CsABC19 was isolated from a cucumber cultivar ‘D0351’ using a homologous cloning strategy. The full-length cDNA of CsABC19 was 921 bp with a complete ORF encoding 306 amino acids. Quantitative real-time PCR analysis revealed that CsABC19 was induced in the root, stem, leaf, and fruit by propamocarb and the expression levels of CsABC19 seemed to be different in different tissues. Further functional analysis showed that CsABC19 transgenic Arabidopsis plants appeared better growth performance under propamocarb stress and lower propamocarb residues. Our findings suggest that CsABC19 plays a crucial role in plant responses to propamocarb stress and also provide new clues for the mechanism regulation of the responses to propamocarb stress in cucumber.     

Endophytic microbes <Emphasis Type="Italic">Bacillus</Emphasis> sp. LZR216-regulated root development is dependent on polar auxin transport in <Emphasis Type="Italic">Arabidopsis</Emphasis> seedlings

Key message

Endophytic microbes Bacillus sp. LZR216 isolated from Arabidopsis root promoted Arabidopsis seedlings growth. It may be achieved by promoting the lateral root growth and inhibiting the primary root elongation.

Abstract

Plant roots are colonized by an immense number of microbes, including epiphytic and endophytic microbes. It was found that they have the ability to promote plant growth and protect roots from biotic and abiotic stresses. But little is known about the mechanism of the endophytic microbes-regulated root development. We isolated and identified a Bacillus sp., named as LZR216, of endophytic bacteria from Arabidopsis root. By employing a sterile experimental system, we found that LZR216 promoted the Arabidopsis seedlings growth, which may be achieved by promoting the lateral root growth and inhibiting the primary root elongation. By testing the cell type-specific developmental markers, we demonstrated that Bacillus sp. LZR216 increases the DR5::GUS and DR5::GFP expression but decreases the CYCB1;1::GUS expression in Arabidopsis root tips. Further studies indicated that LZR216 is able to inhibit the meristematic length and decrease the cell division capability but has little effect on the quiescent center function of the root meristem. Subsequently, it was also shown that LZR216 has no significant effects on the primary root length of the pin2 and aux1-7 mutants. Furthermore, LZR216 down-regulates the levels of PIN1-GFP, PIN2-GFP, PIN3-GFP, and AUX1-YFP. In addition, the wild-type Arabidopsis seedlings in the present of 1 or 5 µM NPA (an auxin transport inhibitor) were insensitive to LZR216-inhibited primary root elongation. Collectively, LZR216 regulates the development of root system architecture depending on polar auxin transport. This study shows a new insight on the ability of beneficial endophytic bacteria in regulating postembryonic root development.