The xylem of rice (Oryza sativa) is colonized by Azorhizobium caulinodans

Following inoculation with Azorhizobium caulinodans ORS571 (pXLGD4), lateral root development of rice and colonization of lateral root cracks by bacteria were shown to be stimulated by the flavonoid naringenin. Rice seedlings growing aseptically in the presence of naringenin were inoculated with ORS571 (pXLGD4), carrying the lacZ reporter gene. By microscopic analysis of sections of inoculated rice roots, it has been demonstrated that the xylem of rice roots can be colonized by Azorhizobium caulinodans. We discuss whether this colonization of the xylem of rice roots by azorhizobia could provide a suitable niche for endophytic nitrogen fixation.     

Colonization of Maize and Rice Plants by Strain Bacillus megaterium C4

Bacillus megaterium C4, a nitrogen-fixing bacterium, was marked with the gfp gene. Maize and rice seedlings were inoculated with the, GFP-labeled B. megaterium C4 and then grown in gnotobiotic condition. Observation by confocal laser scanning microscope showed that the GFP-labeled bacterial cells infected the maize roots through the cracks formed at the lateral root junctions and then penetrated into cortex, xylem, and pith, and that the bacteria migrated slowly from roots to stems and leaves. The bacteria were mainly located in the intercellular spaces, although a few bacterial cells were also present within the xylem vessels, root hair cells, epidermis, cortical parenchyma, and pith cells. In addition, microscopic observation also revealed clearly that the root tip in the zone of elongation and differentiation and the junction between the primary and the lateral roots were the two sites for the bacteria entry into rice root. Therefore, we conclude that this Gram-positive nitrogen-fixer has a colonization pattern similar to those of many Gram-negative diazotrophs, such as Azospirillun brasilense Yu62 and Azoarcus sp. As far as we know, this is the first detailed report of the colonization pattern for Gram-positive diazotrophic Bacillus.     

Azorhizobium caulinodans ORS571 colonizes the xylem of Arabidopsis thaliana

Improved conditions were used for the aseptic growth of Arabidopsis thaliana to investigate whether xylem colonization of A. thaliana by Azorhizobium caulinodans ORS571 might occur. When seedlings were inoculated with ORS571 (pXLGD4) tagged with the lacZ reporter gene, nearly all of the plants showed blue regions of ORS571 colonization at lateral root cracks (LRC). The flavonoids naringenin and liquiritigenin significantly stimulated colonization of LRC by ORS571. Blue bands of ORS571 (pXLGD4) bacteria were observed histochemically in the xylem of intact roots of inoculated plants. Detailed microscopic analysis of sections of primary and lateral roots from inoculated A. thaliana confirmed xylem colonization. Xylem colonization also occurred with an ORS571 nodC mutant deficient in nodulation factors. There was no significant difference in the percentage of plants with xylem colonization or in the mean length of xylem colonized per plant between plants inoculated with either ORS571 (pXLGD4) or ORS571::nodC (pXLGD4), with or without naringenin.     

Effects of Glucosinolates and Flavonoids on Colonization of the Roots of Brassica napus by Azorhizobium caulinodans ORS571

Plants of Brassica napus were assessed quantitatively for their susceptibility to lateral root crack colonization by Azorhizobium caulinodans ORS571(pXLGD4) (a rhizobial strain carrying the lacZ reporter gene) and for the concentration of glucosinolates in their roots by high-pressure liquid chromatography (HPLC). High- and low-glucosinolate-seed (HGS and LGS) varieties exhibited a relatively low and high percentage of colonized lateral roots, respectively. HPLC showed that roots of HGS plants contained a higher concentration of glucosinolates than roots of LGS plants. One LGS variety showing fewer colonized lateral roots than other LGS varieties contained a higher concentration of glucosinolates than other LGS plants. Inoculated HGS plants treated with the flavonoid naringenin showed significantly more colonization than untreated HGS plants. This increase was not mediated by a naringenin-induced lowering of the glucosinolate content of HGS plant roots, nor did naringenin induce bacterial resistance to glucosinolates or increase the growth of bacteria. The erucic acid content of seed did not appear to influence colonization by azorhizobia. Frequently, leaf assays are used to study glucosinolates and plant defense; this study provides data on glucosinolates and bacterial colonization in roots and describes a bacterial reporter gene assay tailored easily to the study of ecologically important phytochemicals that influence bacterial colonization. These data also form a basis for future assessments of the benefits to oilseed rape plants of interaction with plant growth-promoting bacteria, especially diazotrophic bacteria potentially able to extend the benefits of nitrogen fixation to nonlegumes.     

Herbaspirillum-plant interactions: microscopical, histological and molecular aspects

Diazotrophic species in the genus Herbaspirillum (e.g. H. frisingense, H. rubrisubalbicans and H. seropedicae) associate with several economically important crops in the family Poaceae, such as maize (Zea mays), Miscanthus, rice (Oryza sativa), sorghum (Sorghum bicolor) and sugarcane (Saccharum sp.), and can increase their growth and productivity by a number of mechanisms, including nitrogen fixation. Hence, the improvement and use of these plant growth-promoting bacteria could provide economic and environmental benefits. We review the colonization processes of host plants by Herbaspirillum spp., including histological aspects and molecular mechanisms involved in these interactions, which may be epiphytic, endophytic, and even occasionally pathogenic. Herbaspirillum can recognize plant signals that modulate the expression of colonization traits and plant growth-promoting factors. Although a large proportion of herbaspirilla remain rhizospheric and epiphytic, plant-associated species in this genus are noted for their ability to colonize the plant internal tissues. Endophytic colonization by herbaspirilla begins with the attachment of the bacteria to root surfaces, followed by colonization at the emergence points of lateral roots and penetration through discontinuities of the epidermis; this appears to involve bacterial envelope structures, such as lipopolysaccharide (LPS), exopolysaccharide (EPS), adhesins and the type three secretion system (T3SS), but not necessarily the involvement of cell wall-degrading enzymes. Intercellular spaces are then rapidly occupied, proceeding to colonization of xylem and the aerial parts of the host plants. The response of the host plant includes both the recognition of the bacteria as non-pathogenic and the induction of systemic resistance to pathogens. Phytohormone signaling cascades are also activated, regulating the plant development. This complex molecular communication between some Herbaspirillum spp. and plant hosts can result in plant growth-promotion.     

The Down-Regulation of Mt4-Like Genes by Phosphate Fertilization Occurs Systemically and Involves Phosphate Translocation to the Shoots

Mt4 is a cDNA representing a phosphate-starvation-inducible gene from Medicago truncatula that is down-regulated in roots in response to inorganic phosphate (Pi) fertilization and colonization by arbuscular mycorrhizal fungi. Split-root experiments revealed that the expression of the Mt4 gene in M. truncatula roots is down-regulated systemically by both Pi fertilization and colonization by arbuscular mycorrhizal fungi. A comparison of Pi levels in these tissues suggested that this systemic down-regulation is not caused by Pi accumulation. Using a 30-bp region of the Mt4 gene as a probe, Pi-starvation-inducible Mt4-like genes were detected in Arabidopsis and soybean (Glycine max L.), but not in corn (Zea mays L.). Analysis of the expression of the Mt4-like Arabidopsis gene, At4, in wild-type Arabidopsis and pho1, a mutant unable to load Pi into the xylem, suggests that Pi must first be translocated to the shoot for down-regulation to occur. The data from the pho1 and split-root studies are consistent with the presence of a translocatable shoot factor responsible for mediating the systemic down-regulation of Mt4-like genes in roots.     

Gibberellin disturbs the balance of endogenesis hormones and inhibits adventitious root development of Pseudostellaria heterophylla through regulating gene expression related to hormone synthesis

The adventitious roots of some plants will develop into tuberous roots which are widely used in many traditional Chinese medicines, including Pseudostellaria heterophylla. If adventitious root development is inhibited, the yield of Chinese medicinal materials will be reduced. Gibberellic acid is an important phytohormone that promotes plant growth and increases the resistance to drought, flood or disease. However, the effects of gibberellic acid on adventitious roots of Pseudostellaria heterophylla are not clear. Here, we reports GA3 suppressed adventitious root development of Pseudostellaria heterophylla by disturbing the balance of endogenesis hormones. By detecting the contents of various endogenous hormones, we found that the development of adventitious roots negatively correlated with the content of CA3 in tuberous roots. Exogenous GA3 treatment decreased the diameter of adventitious roots, but increased the length of adventitious roots of Pseudostellaria heterophylla. In contrast, blocking the biosynthesis of GA3 suppressed stem growth and promoted the xylem of tuberous roots development. Moreover, exogenous GA3 treatment resulted in imbalance of endogenesis hormones by regulating their synthesis-related genes expression in xylem of tuberous roots. These results suggest GA3 broke the established distribution of hormones by regulating synthesis, transport and biological activation of hormones to activate the apical meristem and suppress lateral meristem. Regulating GA3 signaling during adventitious roots development would be one of the possible ways to increase the yield of P. heterophylla.     

Xylem loading process is a critical factor in determining Cd accumulation in the shoots of Solanum melongena and Solanum torvum

Cadmium (Cd) concentration in eggplant (Solanum melongena) fruits can be drastically reduced by grafting them with Solanum torvum rootstock. We thus examined the characteristics of Cd absorption in roots and Cd translocation from roots to shoots between S. melongena and S. torvum over 7 days using a hydroponic culture. Although there is no significant difference in Cd concentration in the roots of S. melongena and S. torvum, Cd concentration in the shoots and xylem sap was higher in S. melongena than in S. torvum. By evaluating symplastic Cd absorption in roots, using enriched isotopes ¹¹³Cd and ¹¹⁴Cd, and measuring the kinetics in xylem loading, we characterized Cd absorption and translocation for S. torvum (low Cd translocation) and S. melongena (high Cd translocation). A concentration-dependent study in roots indicated that K_m values were almost the same for species, but the V_max value was 1.5-fold higher in S. melongena than in S. torvum. A concentration-dependent study in xylem loading indicated that V_max was almost the same, but K_m values were approximately 7-fold higher in S. torvum compared to S. melongena. These results, together, suggest that the affinity for Cd in the xylem loading process is a critical factor for determining the different Cd concentrations in the shoots between both plants under low Cd concentration conditions. In addition, a metabolic inhibitor, carbonyl cyanide-m-chloro-phenyl-hydrazone (CCCP) inhibited Cd absorption and translocation from roots to shoots in both plants. This suggests that Cd absorption in roots and Cd translocation from roots to shoots via the xylem loading process, under low Cd concentration conditions, are partly mediated by an active energy-dependent process in both plants.     

Nitrate Assimilation Contributes to Ralstonia solanacearum Root Attachment,Stem Colonization,and Virulence

Ralstonia solanacearum, an economically important plant pathogen, must attach, grow, and produce virulence factors to colonize plant xylem vessels and cause disease. Little is known about the bacterial metabolism that drives these processes. Nitrate is present in both tomato xylem fluid and agricultural soils, and the bacterium''s gene expression profile suggests that it assimilates nitrate during pathogenesis. A nasA mutant, which lacks the gene encoding the catalytic subunit of R. solanacearum''s sole assimilatory nitrate reductase, did not grow on nitrate as a sole nitrogen source. This nasA mutant exhibited reduced virulence and delayed stem colonization after soil soak inoculation of tomato plants. The nasA virulence defect was more severe following a period of soil survival between hosts. Unexpectedly, once bacteria reached xylem tissue, nitrate assimilation was dispensable for growth, virulence, and competitive fitness. However, nasA-dependent nitrate assimilation was required for normal production of extracellular polysaccharide (EPS), a major virulence factor. Quantitative analyses revealed that EPS production was significantly influenced by nitrate assimilation when nitrate was not required for growth. The plant colonization delay of the nasA mutant was externally complemented by coinoculation with wild-type bacteria but not by coinoculation with an EPS-deficient epsB mutant. The nasA mutant and epsB mutant did not attach to tomato roots as well as wild-type strain UW551. However, adding either wild-type cells or cell-free EPS improved the root attachment of these mutants. These data collectively suggest that nitrate assimilation promotes R. solanacearum virulence by enhancing root attachment, the initial stage of infection, possibly by modulating EPS production.     

Real Time Live Imaging of Phytopathogenic Bacteria Xanthomonas campestris pv. campestris MAFF106712 in ‘Plant Sweet Home’

Xanthomonas is one of the most widespread phytobacteria, causing diseases on a variety of agricultural plants. To develop novel control techniques, knowledge of bacterial behavior inside plant cells is essential. Xanthomonas campestris pv. campestris, a vascular pathogen, is the causal agent of black rot on leaves of Brassicaceae, including Arabidopsis thaliana. Among the X. campestris pv. campestris stocks in the MAFF collection, we selected XccMAFF106712 as a model compatible pathogen for the A. thaliana reference ecotype Columbia (Col-0). Using modified green fluorescent protein (AcGFP) as a reporter, we observed real time XccMAFF106712 colonization in planta with confocal microscopy. AcGFP-expressing bacteria colonized the inside of epidermal cells and the apoplast, as well as the xylem vessels of the vasculature. In the case of the type III mutant, bacteria colonization was never detected in the xylem vessel or apoplast, though they freely enter the xylem vessel through the wound. After 9 days post inoculation with XccMAFF106712, the xylem vessel became filled with bacterial aggregates. This suggests that Xcc colonization can be divided into main four steps, (1) movement in the xylem vessel, (2) movement to the next cell, (3) adhesion to the host plant cells, and (4) formation of bacterial aggregates. The type III mutant abolished at least steps (1) and (2). Better understanding of Xcc colonization is essential for development of novel control techniques for black rot.     

Deciphering the route of Ralstonia solanacearum colonization in Arabidopsis thaliana roots during a compatible interaction: focus at the plant cell wall

The compatible interaction between the model plant, Arabidopsis thaliana, and the GMI1000 strain of the phytopathogenic bacterium, Ralstonia solanacearum, was investigated in an in vitro pathosystem. We describe the progression of the bacteria in the root from penetration at the root surface to the xylem vessels and the cell type-specific, cell wall-associated modifications that accompanies bacterial colonization. Within 6?days post inoculation, R. solanacearum provoked a rapid plasmolysis of the epidermal, cortical, and endodermal cells, including those not directly in contact with the bacteria. Plasmolysis was accompanied by a global degradation of pectic homogalacturonanes as shown by the loss of JIM7 and JIM5 antibody signal in the cell wall of these cell types. As indicated by immunolabeling with Rsol-I antibodies that specifically recognize R. solanacearum, the bacteria progresses through the root in a highly directed, centripetal manner to the xylem poles, without extensive multiplication in the intercellular spaces along its path. Entry into the vascular cylinder was facilitated by cell collapse of the two pericycle cells located at the xylem poles. Once the bacteria reached the xylem vessels, they multiplied abundantly and moved from vessel to vessel by digesting the pit membrane between adjacent vessels. The degradation of the secondary walls of xylem vessels was not a prerequisite for vessel colonization as LM10 antibodies strongly labeled xylem cell walls, even at very late stages in disease development. Finally, the capacity of R. solanacearum to specifically degrade certain cell wall components and not others could be correlated with the arsenal of cell wall hydrolytic enzymes identified in the bacterial genome.     

Functionalized microchannels as xylem-mimicking environment: Quantifying X. fastidiosa cell adhesion

Microchannels can be used to simulate xylem vessels and investigate phytopathogen colonization under controlled conditions. In this work, we explore surface functionalization strategies for polydimethylsiloxane and glass microchannels to study microenvironment colonization by Xylella fastidiosa subsp. pauca cells. We closely monitored cell initial adhesion, growth, and motility inside microfluidic channels as a function of chemical environments that mimic those found in xylem vessels. Carboxymethylcellulose (CMC), a synthetic cellulose, and an adhesin that is overexpressed during early stages of X. fastidiosa biofilm formation, XadA1 protein, were immobilized on the device’s internal surfaces. This latter protocol increased bacterial density as compared with CMC. We quantitatively evaluated the different X. fastidiosa attachment affinities to each type of microchannel surface using a mathematical model and experimental observations acquired under constant flow of culture medium. We thus estimate that bacterial cells present ～4 and 82% better adhesion rates in CMC- and XadA1-functionalized channels, respectively. Furthermore, variable flow experiments show that bacterial adhesion forces against shear stresses approximately doubled in value for the XadA1-functionalized microchannel as compared with the polydimethylsiloxane and glass pristine channels. These results show the viability of functionalized microchannels to mimic xylem vessels and corroborate the important role of chemical environments, and particularly XadA1 adhesin, for early stages of X. fastidiosa biofilm formation, as well as adhesivity modulation along the pathogen life cycle.     

Effects on plant growth and root-knot nematode infection of an endophytic GFP transformant of the nematophagous fungus Pochonia chlamydosporia

Pochonia chlamydosporia (Pc123) is a fungal parasite of nematode eggs which can colonize endophytically barley and tomato roots. In this paper we use culturing as well as quantitative PCR (qPCR) methods and a stable GFP transformant (Pc123gfp) to analyze the endophytic behavior of the fungus in tomato roots. We found no differences between virulence/root colonization of Pc123 and Pc123gfp on root-knot nematode Meloidogyne javanica eggs and tomato seedlings respectively. Confocal microscopy of Pc123gfp infecting M. javanica eggs revealed details of the process such as penetration hyphae in the egg shell or appressoria and associated post infection hyphae previously unseen. Pc123gfp colonization of tomato roots was low close to the root cap, but increased with the distance to form a patchy hyphal network. Pc123gfp colonized epidermal and cortex tomato root cells and induced plant defenses (papillae). qPCR unlike culturing revealed reduction in fungus root colonization (total and endophytic) with plant development. Pc123gfp was found by qPCR less rhizosphere competent than Pc123. Endophytic colonization by Pc123gfp promoted growth of both roots and shoots of tomato plants vs. uninoculated (control) plants. Tomato roots endophytically colonized by Pc123gfp and inoculated with M. javanica juveniles developed galls and egg masses which were colonized by the fungus. Our results suggest that endophytic colonization of tomato roots by P. chlamydosporia may be relevant for promoting plant growth and perhaps affect managing of root-knot nematode infestations.     

Maize Root Lectins Mediate the Interaction with Herbaspirillum seropedicae via N-Acetyl Glucosamine Residues of Lipopolysaccharides

Herbaspirillum seropedicae is a plant growth-promoting diazotrophic betaproteobacterium which associates with important crops, such as maize, wheat, rice and sugar-cane. We have previously reported that intact lipopolysaccharide (LPS) is required for H. seropedicae attachment and endophytic colonization of maize roots. In this study, we present evidence that the LPS biosynthesis gene waaL (codes for the O-antigen ligase) is induced during rhizosphere colonization by H. seropedicae. Furthermore a waaL mutant strain lacking the O-antigen portion of the LPS is severely impaired in colonization. Since N-acetyl glucosamine inhibits H. seropedicae attachment to maize roots, lectin-like proteins from maize roots (MRLs) were isolated and mass spectrometry (MS) analysis showed that MRL-1 and MRL-2 correspond to maize proteins with a jacalin-like lectin domain, while MRL-3 contains a B-chain lectin domain. These proteins showed agglutination activity against wild type H. seropedicae, but failed to agglutinate the waaL mutant strain. The agglutination reaction was severely diminished in the presence of N-acetyl glucosamine. Moreover addition of the MRL proteins as competitors in H. seropedicae attachment assays decreased 80-fold the adhesion of the wild type to maize roots. The results suggest that N-acetyl glucosamine residues of the LPS O-antigen bind to maize root lectins, an essential step for efficient bacterial attachment and colonization.     

The incidence of arbuscular mycorrhiza in two submerged Isoëtes species

We investigated the occurrence of arbuscular mycorrhizal fungi in the roots of Isoëtes lacustris and I. echinospora. These submerged lycopsids are the only macrophyte species inhabiting the bottom of two acidified glacial lakes in the Czech Republic. Arbuscular mycorrhizal (AM) fungi were detected in the roots of both species but the percentage of root colonization was both low and variable. Nevertheless, planting Littorella uniflora in the sediments from Isoëtes rhizosphere revealed high levels of viable AM propagules in both lakes. While AM colonization of Isoëtes roots did not exceed 25%, the average colonization of Littorella roots amounted to more than 80%. Although colonization of quillwort roots by AM fungi is evident, the taxonomic identity and role of these AM fungi in plant growth remain unclear. In addition to AM fungi, root-colonizing dark septate endophytic fungi were observed in both Isoëtes species.     

Quantitative and Microscopic Assessment of Compatible and Incompatible Interactions between Chickpea Cultivars and Fusarium oxysporum f. sp. ciceris Races

Background

Fusarium wilt caused by Fusarium oxysporum f. sp. ciceris, a main threat to global chickpea production, is managed mainly by resistant cultivars whose efficiency is curtailed by Fusarium oxysporum f. sp. ciceris races.

Methodology

We characterized compatible and incompatible interactions by assessing the spatial-temporal pattern of infection and colonization of chickpea cvs. P-2245, JG-62 and WR-315 by Fusarium oxysporum f. sp. ciceris races 0 and 5 labeled with ZsGreen fluorescent protein using confocal laser scanning microscopy.

Findings

The two races colonized the host root surface in both interactions with preferential colonization of the root apex and subapical root zone. In compatible interactions, the pathogen grew intercellularly in the root cortex, reached the xylem, and progressed upwards in the stem xylem, being the rate and intensity of stem colonization directly related with the degree of compatibility among Fusarium oxysporum f. sp. ciceris races and chickpea cultivars. In incompatible interactions, race 0 invaded and colonized ‘JG-62’ xylem vessels of root and stem but in ‘WR-315’, it remained in the intercellular spaces of the root cortex failing to reach the xylem, whereas race 5 progressed up to the hypocotyl. However, all incompatible interactions were asymptomatic.

Conclusions

The differential patterns of colonization of chickpea cultivars by Fusarium oxysporum f. sp. ciceris races may be related to the operation of multiple resistance mechanisms.     

Cadmium distribution in the root tissues of solanaceous plants with contrasting root-to-shoot Cd translocation efficiencies

Root-to-shoot cadmium (Cd) translocation in Solanum torvum is lower than that of the eggplant Solanum melongena; therefore, grafting S. melongena onto S. torvum rootstock can effectively reduce the Cd concentration in eggplant fruits. We hypothesized that Cd transport in S. torvum roots is restricted in the path between the epidermis and xylem vessel; hence, we investigated the Cd distribution in the roots at the micron-scale. Elemental maps of Cd, Zn and Fe accumulation in S. melongena and S. torvum root sections were obtained by synchrotron micro X-ray fluorescence spectrometry. The Cd was localized in both the stele and the epidermis of the S. melongena root cross sections regardless of the distance from the root apex. In S. torvum root sections taken at 30 and 40 mm above the root apex, a higher abundance of Cd was found within the cells of the endodermis and pericycle. The results suggested that the symplastic uptake and xylem loading of Cd in S. torvum roots were restricted, and thereby, the Cd that was unable to be loaded into the xylem accumulated in the endodermis and in the pericycle. Because symplastic uptake differs only slightly between the two species, the difference in xylem loading would explain the comparatively lower Cd concentration in S. torvum shoots.     

Sites, pathways, and mechanism of absorption of Cu-EDDS complex in primary roots of maize (Zea Mays L.): anatomical, chemical and histochemical analysis

To study the mechanism of chelant-metal complexes to be absorbed into plant roots in the presence of different concentration chelating agents, the sites, pathways, and mechanism of absorption of Cu-EDDS complex ([S, S’]-ethylene diamine disuccinic acid) in maize (Zea mays L.) primary roots were systematically studied. The results showed that, at low concentrations of the Cu-EDDS complex (<200 μmol L^?1) in hydroponic culture, the complex was passively absorbed mainly from the apoplastic spaces where lateral roots penetrate the endodermis and the cortical region into the root xylem, the lateral root zone were the main absorption sites. At higher concentrations (<3,000 μmol L^?1), under hydroponic culture and soil culture conditions, the passage cells, which form a physiological barrier controlling ion absorption, were either injured or killed, and the complex could enter the root xylem. Injury to the physiological barrier was a key factor in the complex being absorbed by roots in substantially larger quantities. In addition, the histochemical analysis of rubeanic acid can also be used for other researches involving Cu, and the negative–pressure measuring device provides a new research tool for studying the apoplastic absorption of other metal–chelating complexes, molecules, and ions.     

Sucrose triggers a novel signaling cascade promoting Bacillus subtilis rhizosphere colonization

Beneficial rhizobacteria promote plant growth and protect plants against phytopathogens. Effective colonization on plant roots is critical for the rhizobacteria to exert beneficial activities. How bacteria migrate swiftly in the soil of semisolid or solid nature remains unclear. Here we report that sucrose, a disaccharide ubiquitously deployed by photosynthetic plants for fixed carbon transport and storage, and abundantly secreted from plant roots, promotes solid surface motility (SSM) and root colonization by Bacillus subtilis through a previously uncharacterized mechanism. Sucrose induces robust SSM by triggering a signaling cascade, first through extracellular synthesis of polymeric levan, which in turn stimulates strong production of surfactin and hyper-flagellation of the cells. B. subtilis poorly colonizes the roots of Arabidopsis thaliana mutants deficient in root-exudation of sucrose, while exogenously added sucrose selectively shapes the rhizomicrobiome associated with the tomato plant roots, promoting specifically bacilli and pseudomonad. We propose that sucrose activates a signaling cascade to trigger SSM and promote rhizosphere colonization by B. subtilis. Our findings also suggest a practicable approach to boost prevalence of beneficial Bacillus species in plant protection.Subject terms: Soil microbiology, Bacteriology