Mireille Chabaud

A diffusible factor from arbuscular mycorrhizal fungi induces symbiosis-specific MtENOD11 expression in roots of Medicago truncatula.

Using dual cultures of arbuscular mycorrhizal (AM) fungi andMedicago truncatula separated by a physical barrier, we demonstrate that hyphae from germinating spores produce a diffusible factor that is perceived by roots in the absence of direct physical contact. This AM factor elicits expression of the Nod factor-inducible gene MtENOD11, visualized using a pMtENOD11-gusA reporter. Transgene induction occurs primarily in the root cortex, with expression stretching from the zone of root hair emergence to the region of mature root hairs. All AM fungi tested (Gigaspora rosea,Gigaspora gigantea, Gigaspora margarita, and Glomus intraradices) elicit a similar response, whereas pathogenic fungi such as Phythophthora medicaginis, Phoma medicaginis var pinodella andFusarium solani f.sp. phaseoli do not, suggesting that the observed root response is specific to AM fungi. Finally, pMtENOD11-gusA induction in response to the diffusible AM fungal factor is also observed with all threeM. truncatulaNod−/Myc− mutants (dmi1,dmi2, and dmi3), whereas the same mutants are blocked in their response to Nod factor. This positive response of the Nod−/Myc− mutants to the diffusible AM fungal factor and the different cellular localization of pMtENOD11-gusA expression in response to Nod factor versus AM factor suggest that signal transduction occurs via different pathways and that expression of MtENOD11 is differently regulated by the two diffusible factors.

Large‐scale insertional mutagenesis using the Tnt1 retrotransposon in the model legume Medicago truncatula

Medicago truncatula is a fast-emerging model for the study of legume functional biology. We used the tobacco retrotransposon Tnt1 to tag the Medicago genome and generated over 7600 independent lines representing an estimated 190,000 insertion events. Tnt1 inserted on average at 25 different locations per genome during tissue culture, and insertions were stable during subsequent generations in soil. Analysis of 2461 Tnt1 flanking sequence tags (FSTs) revealed that Tnt1 appears to prefer gene-rich regions. The proportion of Tnt1 insertion in coding sequences was 34.1%, compared to the expected 15.9% if random insertions were to occur. However, Tnt1 showed neither unique target site specificity nor strong insertion hot spots, although some genes were more frequently tagged than others. Forward-genetic screening of 3237 R(1) lines resulted in identification of visible mutant phenotypes in approximately 30% of the regenerated lines. Tagging efficiency appears to be high, as all of the 20 mutants examined so far were found to be tagged. Taking the properties of Tnt1 into account and assuming 1.7 kb for the average M. truncatula gene size, we estimate that approximately 14,000-16,000 lines would be sufficient for 90% gene tagging coverage in M. truncatula. This is in contrast to more than 500,000 lines required to achieve the same saturation level using T-DNA tagging. Our data demonstrate that Tnt1 is an efficient insertional mutagen in M. truncatula, and could be a primary choice for other plant species with large genomes.

Short‐chain chitin oligomers from arbuscular mycorrhizal fungi trigger nuclear Ca2+ spiking in Medicago truncatula roots and their production is enhanced by strigolactone

The primary objective of this study was to identify the molecular signals present in arbuscular mycorrhizal (AM) germinated spore exudates (GSEs) responsible for activating nuclear Ca(2+) spiking in the Medicago truncatula root epidermis. Medicago truncatula root organ cultures (ROCs) expressing a nuclear-localized cameleon reporter were used as a bioassay to detect AM-associated Ca(2+) spiking responses and LC-MS to characterize targeted molecules in GSEs. This approach has revealed that short-chain chitin oligomers (COs) can mimic AM GSE-elicited Ca(2+) spiking, with maximum activity observed for CO4 and CO5. This spiking response is dependent on genes of the common SYM signalling pathway (DMI1/DMI2) but not on NFP, the putative Sinorhizobium meliloti Nod factor receptor. A major increase in the CO4/5 concentration in fungal exudates is observed when Rhizophagus irregularis spores are germinated in the presence of the synthetic strigolactone analogue GR24. By comparison with COs, both sulphated and nonsulphated Myc lipochito-oligosaccharides (LCOs) are less efficient elicitors of Ca(2+) spiking in M. truncatula ROCs. We propose that short-chain COs secreted by AM fungi are part of a molecular exchange with the host plant and that their perception in the epidermis leads to the activation of a SYM-dependent signalling pathway involved in the initial stages of fungal root colonization.

Prepenetration Apparatus Assembly Precedes and Predicts the Colonization Patterns of Arbuscular Mycorrhizal Fungi within the Root Cortex of Both Medicago truncatula and Daucus carota

Arbuscular mycorrhizas (AM) are widespread, ancient endosymbiotic associations that contribute significantly to soil nutrient uptake in plants. We have previously shown that initial fungal penetration of the host root is mediated via a specialized cytoplasmic assembly called the prepenetration apparatus (PPA), which directs AM hyphae through the epidermis (Genre et al., 2005). In vivo confocal microscopy studies performed on Medicago truncatula and Daucus carota, host plants with different patterns of AM colonization, now reveal that subsequent intracellular growth across the root outer cortex is also PPA dependent. On the other hand, inner root cortical colonization leading to arbuscule development involves more varied and complex PPA-related mechanisms. In particular, a striking alignment of polarized PPAs can be observed in adjacent inner cortical cells of D. carota, correlating with the intracellular root colonization strategy of this plant. Ultrastructural analysis of these PPA-containing cells reveals intense membrane trafficking coupled with nuclear enlargement and remodeling, typical features of arbusculated cells. Taken together, these findings imply that prepenetration responses are both conserved and modulated throughout the AM symbiosis as a function of the different stages of fungal accommodation and the host-specific pattern of root colonization. We propose a model for intracellular AM fungal accommodation integrating peri-arbuscular interface formation and the regulation of functional arbuscule development.

A Nuclear-Targeted Cameleon Demonstrates Intranuclear Ca2+ Spiking in Medicago truncatula Root Hairs in Response to Rhizobial Nodulation Factors

Lipochitooligosaccharide nodulation factors (NFs) secreted by endosymbiotic nitrogen-fixing rhizobia trigger Ca2+ spiking in the cytoplasmic perinuclear region of host legume root hairs. To determine whether NFs also elicit Ca2+ responses within the plant cell nucleus we have made use of a nucleoplasmin-tagged cameleon (NupYC2.1). Confocal microscopy using this nuclear-specific calcium reporter has revealed sustained and regular Ca2+ spiking within the nuclear compartment of Medicago truncatula root hairs treated with Sinorhizobium meliloti NFs. Since the activation of Ca2+ oscillations is blocked in M. truncatula nfp, dmi1, and dmi2 mutants, and unaltered in a dmi3 background, it is likely that intranuclear spiking lies on the established NF-dependent signal transduction pathway, leading to cytoplasmic calcium spiking. A semiautomated mathematical procedure has been developed to identify and analyze nuclear Ca2+ spiking profiles, and has revealed high cell-to-cell variability in terms of both periodicity and spike duration. Time-lapse imaging of the cameleon Förster resonance energy transfer-based ratio has allowed us to visualize the nuclear spiking variability in situ and to demonstrate the absence of spiking synchrony between adjacent growing root hairs. Finally, spatio-temporal analysis of the asymmetric nuclear spike suggests that the initial rapid increase in Ca2+ concentration occurs principally in the vicinity of the nuclear envelope. The discovery that rhizobial NF perception leads to the activation of cell-autonomous Ca2+ oscillations on both sides of the nuclear envelope raises major questions about the respective roles of the cytoplasmic and nuclear compartments in transducing this key endosymbiotic signal.

Arbuscular mycorrhizal hyphopodia and germinated spore exudates trigger Ca2+ spiking in the legume and nonlegume root epidermis

• The aim of this study was to investigate Ca(2+) responses to endosymbiotic arbuscular mycorrhizal (AM) fungi in the host root epidermis following pre-infection hyphopodium formation in both legumes and nonlegumes, and to determine to what extent these responses could be mimicked by germinated fungal spore exudate. • Root organ cultures of both Medicago truncatula and Daucus carota, expressing the nuclear-localized cameleon reporter NupYC2.1, were used to monitor AM-elicited Ca(2+) responses in host root tissues. • Ca(2+) spiking was observed in cells contacted by AM hyphopodia for both hosts, with highest frequencies correlating with the epidermal nucleus positioned facing the fungal contact site. Treatment with AM spore exudate also elicited Ca(2+) spiking within the AM-responsive zone of the root and, in both cases, spiking was dependent on the M. truncatula common SYM genes DMI1/2, but not on the rhizobial Nod factor perception gene NFP. • These findings support the conclusion that AM fungal root penetration is preceded by a SYM pathway-dependent oscillatory Ca(2+) response, whose evolutionary origin predates the divergence between asterid and rosid clades. Our results further show that fungal symbiotic signals are already generated during spore germination, and that cameleon-expressing root organ cultures represent a novel AM-specific bio-assay for such signals.

Mechanism of Infection Thread Elongation in Root Hairs of Medicago truncatula and Dynamic Interplay with Associated Rhizobial Colonization

In temperate legumes, endosymbiotic nitrogen-fixing rhizobia gain access to inner root tissues via a specialized transcellular apoplastic compartment known as the infection thread (IT). To study IT development in living root hairs, a protocol has been established for Medicago truncatula that allows confocal microscopic observations of the intracellular dynamics associated with IT growth. Fluorescent labeling of both the IT envelope (AtPIP2;1-green fluorescent protein) and the host endoplasmic reticulum (green fluorescent protein-HDEL) has revealed that IT growth is a fundamentally discontinuous process and that the variable rate of root hair invagination is reflected in changes in the host cell cytoarchitecture. The concomitant use of fluorescently labeled Sinorhizobium meliloti has further revealed that a bacteria-free zone is frequently present at the growing tip of the IT, thus indicating that bacterial contact is not essential for thread progression. Finally, these in vivo studies have shown that gaps within the bacterial file are a common feature during the early stages of IT development, and that segments of the file are able to slide collectively down the thread. Taken together, these observations lead us to propose that (1) IT growth involves a host-driven cellular mechanism analogous to that described for intracellular infection by arbuscular mycorrhizal fungi; (2) the non-regular growth of the thread is a consequence of the rate-limiting colonization by the infecting rhizobia; and (3) bacterial colonization involves a combination of bacterial cell division and sliding movement within the extracellular matrix of the apoplastic compartment.

Pharmacological Evidence That Multiple Phospholipid Signaling Pathways Link Rhizobium Nodulation Factor Perception in Medicago truncatula Root Hairs to Intracellular Responses, Including Ca2+ Spiking and Specific ENOD Gene Expression

Rhizobium nodulation (Nod) factors are specific lipochito-oligosaccharide signals essential for initiating in root hairs of the host legume developmental responses that are required for controlled entry of the microsymbiont. In this article, we focus on the Nod factor signal transduction pathway leading to specific and cell autonomous gene activation in Medicago truncatula cv Jemalong in a study making use of the Nod factor-inducible MtENOD11 gene. First, we show that pharmacological antagonists that interfere with intracellular ion channel and Ca2+ pump activities are efficient blockers of Nod factor-elicited pMtENOD11-β-glucuronidase (GUS) expression in root hairs of transgenic M. truncatula. These results indicate that intracellular Ca2+ release and recycling activities, essential for Ca2+ spiking, are also required for specific gene activation. Second, pharmacological effectors that inhibit phospholipase D and phosphoinositide-dependent phospholipase C activities are also able to block pMtENOD11-GUS activation, thus underlining a central role for multiple phospholipid signaling pathways in Nod factor signal transduction. Finally, pMtENOD11-GUS was introduced into all three Nod−/Myc− dmi M. truncatula mutant backgrounds, and gene expression was evaluated in response to the mastoparan peptide agonist Mas7. We found that Mas7 elicits root hair MtENOD11 expression in dmi1 and dmi2 mutants, but not in the dmi3 mutant, suggesting that the agonist acts downstream of DMI1/DMI2 and upstream of DMI3. In light of these results and the recently discovered identities of the DMI gene products, we propose an integrated cellular model for Nod factor signaling in legume root hairs in which phospholipids play a key role in linking the Nod factor perception apparatus to downstream components such as Ca2+ spiking and ENOD gene expression.

Transformation of barrel medic (Medicago truncatula Gaertn.) by Agrobacterium tumefaciens and regeneration via somatic embryogenesis of transgenic plants with the MtENOD12 nodulin promoter fused to the gus reporter gene.

SummaryFertile and stable transgenic plants of the model legume Medicago truncatula Gaertn. were obtained through transformation of leaf tissue with the disarmed Agrobacterium tumefaciens strain LBA4404 and in vitro regeneration via somatic embryogenesis. An optimised transformation/regeneration protocol has been established for two genotypes of the cultivar Jemalong, including a previously described highly embryogenic line (Nolan et al. 1989, Plant Cell Rep. 8: 278–281). Using this protocol, transgenic plantlets were obtained within 4–10 months following cocultivation with Agrobacterium. We have introduced into M. truncatula a chimeric fusion between the early nodulin MtENOD12 promoter and the gus (β-glucuronidase) reporter gene, and shown that symbiosis-specific gene expression can be elicited in the roots of such transgenic plants following the addition of purified Rhizobium nodulation factors.

A switch in Ca2+ spiking signature is concomitant with endosymbiotic microbe entry into cortical root cells of Medicago truncatula.

Ca(2+) spiking is a central component of a common signaling pathway that is activated in the host epidermis during initial recognition of endosymbiotic microbes. However, it is not known to what extent Ca(2+) signaling also plays a role during subsequent root colonization involving apoplastic transcellular infection. Live-tissue imaging using calcium cameleon reporters expressed in Medicago truncatula roots has revealed that distinct Ca(2+) oscillatory profiles correlate with specific stages of transcellular cortical infection by both rhizobia and arbuscular mycorrhizal fungi. Outer cortical cells exhibit low-frequency Ca(2+) spiking during the extensive intracellular remodeling that precedes infection. This appears to be a prerequisite for the formation of either pre-infection threads or the pre-penetration apparatus, both of which are fully reversible processes. A transition from low- to high-frequency spiking is concomitant with the initial stages of apoplastic cell entry by both microbes. This high-frequency spiking is of limited duration in the case of rhizobial infection and is completely switched off by the time transcellular infection by both microsymbionts is completed. The Ca(2+) spiking profiles associated with both rhizobial and arbuscular mycorrhizal cell entry are remarkably similar in terms of periodicity, suggesting that microbe specificity is unlikely to be encoded by the Ca(2+) signature during this particular stage of host infection in the outer cortex. Together, these findings lead to the proposal that tightly regulated Ca(2+) -mediated signal transduction is a key player in reprogramming root cell development at the critical stage of commitment to endosymbiotic infection.