Root Signal Research Articles

Silicon mitigates potassium deficiency by enhanced remobilization and modulated potassium transporter regulation

Silicon (Si) improves plant growth and development especially under stress conditions. Although Si alleviates growth suppression and deficiency symptoms of several nutrient disorders, underlying regulatory processes have remained indefinite. Here, we analyzed the impact of Si on nutritional, metabolic and transcriptional responses in roots and shoots of barley subjected to potassium (K) deficiency. On the long run, Si nutrition improved root and shoot growth as well as metabolite homeostasis and partly reverted the K deficiency signature of the transcriptome back to adequate only in fully expanded leaves but hardly in roots. In the short term, Si supply to K-starved roots enhanced first the expression of the vacuolar K exporter KCO1 together with root-to-shoot translocation rates of rubidium (Rb) that was used as tracer for K. The typical K deficiency response, marked by upregulation of the K importer genes HAK1 and AKT1, set in a few days later but then at several-fold higher levels. Lower root K and higher shoot K contents in Si-supplied plants indicated more efficient remobilization of root K pools and higher root-to-shoot translocation, which on the long run restored K-dependent metabolic processes in shoots for the sake of continued assimilate provision to roots. These results provide a sequence of physiological processes by which Si partially recovers plants from K deficiency and indicate that Si interferes with systemic K deficiency signaling in roots to alter the regulation of K transporters. • · Mitigation of K deficiency by Si supply relies on an improved metabolic status of the leaves for sustained assimilate provision of roots • Under K starvation, Si enables stronger depletion of the root K pool and enhances K remobilization for root-to-shoot translocation • In K-starved plants, Si activates vacuolar K exporter genes in roots and subsequently stimulates the K deficiency response

Time Series Transcriptome Analysis in Medicago truncatula Shoot and Root Tissue During Early Nodulation.

In response to colonization by rhizobia bacteria, legumes are able to form nitrogen-fixing nodules in their roots, allowing the plants to grow efficiently in nitrogen-depleted environments. Legumes utilize a complex, long-distance signaling pathway to regulate nodulation that involves signals in both roots and shoots. We measured the transcriptional response to treatment with rhizobia in both the shoots and roots of Medicago truncatula over a 72-h time course. To detect temporal shifts in gene expression, we developed GeneShift, a novel computational statistics and machine learning workflow that addresses the time series replicate the averaging issue for detecting gene expression pattern shifts under different conditions. We identified both known and novel genes that are regulated dynamically in both tissues during early nodulation including leginsulin, defensins, root transporters, nodulin-related, and circadian clock genes. We validated over 70% of the expression patterns that GeneShift discovered using an independent M. truncatula RNA-Seq study. GeneShift facilitated the discovery of condition-specific temporally differentially expressed genes in the symbiotic nodulation biological system. In principle, GeneShift should work for time-series gene expression profiling studies from other systems.

Protoplast Dissociation and Transcriptome Analysis Provides Insights to Salt Stress Response in Cotton.

As one of the pioneer crops widely planted in saline-alkaline areas, Gossypium provides daily necessities, including natural fiber, vegetable proteins, and edible oils. However, cotton fiber yield and quality are highly influenced by salt stress. Therefore, elucidating the molecular mechanisms of cotton in response to salinity stress is importance to breed new cultivars with high tolerance. In this study, we first developed a method for single-cell RNA-seq based on isolating protoplast from cotton root tips; then, we studied the impact of salinity stress on gene expression profiling and their dynamic changes using the developed high-efficiency method for protoplast dissociation suitable for single-cell RNA-seq. A total of 3391 and 2826 differentially expressed genes (DEGs) were identified in salt-treated samples before and after protoplast dissociation, respectively, which were enriched into several molecular components, including response to stimulus, response to stress, and cellular macromolecule metabolic process by gene ontology (GO) analysis. Plant hormone signal transduction, phenylpropanoid biosynthesis, and MAPK signaling pathway were found to be enriched via Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Twenty-two and nine salinity-responsive DEGs participated in plant hormone signaling and MAPK signaling in roots, before and after protoplast dissociation, respectively; six upregulated DEGs were involved in ABA signaling transduction, namely, Ga04G2111, Ga07G0142, Ga09G2061, Ga10G0262, Ga01G0063, and Ga08G1915 which indicates their potential functions on plants adapting to salt stress. Additionally, 384 and 257 transcription factors (TFs) were differentially expressed in salt-stress roots before and after protoplast dissociation, respectively, of which significantly up-regulated TFs mainly belonged to the AP2/ERF-ERF family, which implied their potential roles responding to salt stress. These results not only provide novel insights to reveal the regulatory networks in plant’s root response to salt stress, but also lay the solid foundation for further exploration on cellular heterogeneity by single-cell transcriptome sequencing.

Quercetin and 1-methyl-2-oxindole mimic root signaling that promotes spore germination and mycelial growth of Gigaspora margarita.

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs, and the difficulty of growing them in asymbiotic or monoxenic (AMF + root) conditions limits research and their large-scale production as biofertilizer. We hypothesized that a combination of flavanols and strigolactones can mimic complex root signaling during the presymbiotic stages of AMF. We evaluated the germination, mycelial growth, branching, and auxiliary cell clusters formation by Gigaspora margarita during the presymbiotic stage in the presence (or absence) of transformed Cichorium intybus roots in basal culture medium enriched with glucose, a flavonol (quercetin or biochanin A) and a strigolactone analogue (1-Methyl-2-oxindole or indole propionic acid). With quercetin (5µM), methyl oxindole (2.5nM), and glucose (8.2g/L) in the absence of roots, the presymbiotic mycelium of G. margarita grew without cytoplasmic retraction and produced auxiliary cells over 71days similar to presymbiotic mycelium in the presence of roots but without glucose, strigolactones, and flavonols. Our results indicate that glucose and a specific combination of certain concentrations of a flavonol and a strigolactone might be used in asymbiotic or monoxenic liquid or semisolid cultures to stimulate AMF inoculant bioprocesses.

Iron in leaves: chemical forms, signalling, and in-cell distribution.

Iron (Fe) is an essential transition metal. Based on its redox-active nature under biological conditions, various Fe compounds serve as cofactors in redox enzymes. In plants, the photosynthetic machinery has the highest demand for Fe. In consequence, the delivery and incorporation of Fe into cofactors of the photosynthetic apparatus is the focus of Fe metabolism in leaves. Disturbance of foliar Fe homeostasis leads to impaired biosynthesis of chlorophylls and composition of the photosynthetic machinery. Nevertheless, mitochondrial function also has a significant demand for Fe. The proper incorporation of Fe into proteins and cofactors as well as a balanced intracellular Fe status in leaf cells require the ability to sense Fe, but may also rely on indirect signals that report on the physiological processes connected to Fe homeostasis. Although multiple pieces of information have been gained on Fe signalling in roots, the regulation of Fe status in leaves has not yet been clarified in detail. In this review, we give an overview on current knowledge of foliar Fe homeostasis, from the chemical forms to the allocation and sensing of Fe in leaves.

Exposure of Brassica to Red Light Antagonizes Low Production of Indole-3-Acetic Acid in Leaf Through Root Signaling Under Stress Conditions.

Plant leaf is highly sensitive to various growth-promoting and growth-restraining components. This sensitivity is normally caused by the alteration of different phytohormones (predominately by indole-3-acetic acid (IAA)), when the plants exposed to certain environmental conditions. We exposed the hydroponically grown Brassica campestris seedlings (7 days old) to red and green light in order to observe its effect on IAA secretion at leaf. The evaluated data showed that red light antagonized the low production of IAA in leaf by initiating the root signaling through the flavonoid production and high redox activity. The study also explored the link between the differential phytohormonal response and biotic or abiotic stress elimination in leaf through root signaling under green or red light. The results exhibited that the biotic (Pseudomonas syringae or Frankia alni) or abiotic stresses (100 mm AgNO3 or 100 mm tert-butyl alcohol) inhibited flavonoids at the roots and resisted the restoration of IAA at the leaf. However, under green light where IAA was not inhibited, the stresses could not produce flavonoid at the root and further passing the signals to leaf. The results concluded that the growth and photosynthetic rates of the seedlings were improved under red light exposure through flavonoid inducing stresses.

Mechanisms of increasing salt resistance of vegetables by grafting

Soil salinity is a serious threat to horticultural production. Improving salt tolerance of vegetables by breeding is a difficult task as salt tolerance is a quantitative trait, regulated by a large number of genes. As a traditional agronomic method, grafting, which is widely practiced in vegetables, especially among members of Cucurbitaceae and Solanaceae, is a useful tool for reducing yield loss caused by salinity. However, the underlying mechanisms of this phenomenon remain largely unknown. Numerous studies have been conducted to uncover these mechanisms by which grafting improves salt tolerance in vegetables. This review summarizes the studies that have been conducted on this topic. In this review, the effects of salt stress on vegetable crops were discussed, and the four main mechanisms by which grafting increases the salt resistance of vegetables, namely, restricting the transport of toxic ions, enhancing the antioxidant system, enhancing the stability of the photosynthetic system, and sending root signals, were discussed. This review concludes by identifying several prospects for future research on increasing the adoption of grafting in vegetables under salinity stress.

Flow-signal correlation in seal whisker array sensing

Phocid seals detect and track artificial or biogenic hydrodynamic trails based on mechanical signals of their whisker arrays. In this paper, we investigated the correlations between flow structures and whisker array signals using a simplified numerical model of fluid–structure interaction (FSI). Three-dimensional (3D) wakes of moving paddles in three different shapes (rectangular plate, undulated plate, and circular cylinder) were simulated using an in-house immersed-boundary-method-based computational fluid dynamics solver. One-way FSI was then simulated to obtain the dynamic behavior and root signal of each whisker in the two whisker arrays on a seal head in each wake. The position, geometry, and material properties of each whisker were modeled based on the measurements reported in literatures. The correlations between the wake structures and whisker array signals were analyzed. It was found that the patterns of the signals on the whisker arrays can reflect the strength, timing, and moving trajectories of the jets induced by the vortices in the wakes. Specifically, the rectangular plate generates the strongest starting vortex ring as well as the strongest jets, while the undulated plate generates the weakest ones. These flow features are fully reflected by the largest whisker signal magnitude in the rectangular plate sensing and the smallest one in the undulated plate sensing. Moreover, the timing of the signal initiation and the maximum signal agree well with the timing of the jet reaching the arrays and the maximum flow speed, respectively. The correlation coefficient between the moving trajectories of the jet and the movement of the high signal level region in the array was found to be higher than 0.9 in the rectangular plate case. The results provide a physical insight into the mechanisms of seal whisker flow sensing.

Temporal Control of Morphogenic Factor Expression Determines Efficacy in Enhancing Regeneration.

Background: Regeneration of fertile plants from tissue culture is a critical bottleneck in the application of new plant breeding technologies. Ectopic overexpression of morphogenic factors is a promising workaround for this hurdle. Methods: Conditional overexpression of WUS and ARF5Δ was used to study the effect of timing the overexpression of these morphogenic factors during shoot regeneration from root explants in Arabidopsis. In addition, their effect on auxin-signaling activation was examined by visualization and cytometric quantification of the DR5:GFP auxin-signaling reporter in roots and protoplasts, respectively. Results: The induced expression of both WUS and ARF5Δ led to an activation of auxin signaling in roots. Activation of auxin signaling by WUS and ARF5Δ was further quantified by transient transformation of protoplasts. Ectopic overexpression of both WUS and ARF5Δ enhanced regeneration efficiency, but only during the shoot-induction stage of regeneration and not during the callus-induction stage. Conclusions: The overexpression of WUS and ARF5Δ both lead to activation of auxin signaling. Expression during the shoot-induction stage is critical for the enhancement of shoot regeneration by WUS and ARF5Δ.

Early signalling processes in roots play a crucial role in the differential salt tolerance in contrasting Chenopodium quinoa accessions.

Significant variation in epidermal bladder cell (EBC) density and salt tolerance (ST) exists amongst quinoa accessions, suggesting that salt sequestration in EBCs is not the only mechanism conferring ST in this halophyte. In order to reveal other traits that may operate in tandem with salt sequestration in EBCs and whether these additional tolerance mechanisms acted mainly at the root or shoot level, two quinoa (Chenopodium quinoa) accessions with contrasting ST and EBC densities (Q30, low ST with high EBC density versus Q68, with high ST and low EBC density) were studied. The results indicate that responses in roots, rather than in shoots, contributed to the greater ST in the accession with low EBC density. In particular, the tolerant accession had improved root plasma membrane integrity and K+ retention in the mature root zone in response to salt. Furthermore, superior ST in the tolerant Q68 was associated with faster and root-specific H2O2 accumulation and reactive oxygen species-induced K+ and Ca2+ fluxes in the root apex within 30 min after NaCl application. This was found to be associated with the constitutive up-regulation of the membrane-localized receptor kinases regulatory protein FERONIA in the tolerant accession. Taken together, this study shows that differential root signalling events upon salt exposure are essential for the halophytic quinoa; the failure to do this limits quinoa adaptation to salinity, independently of salt sequestration in EBCs.

Arbuscular Mycorrhizal Symbiosis Triggers Major Changes in Primary Metabolism Together With Modification of Defense Responses and Signaling in Both Roots and Leaves of Vitis vinifera.

Grapevine (Vitis vinifera L.) is one of the most important crops worldwide but is subjected to multiple biotic and abiotic stresses, especially related to climate change. In this context, the grapevine culture could take advantage of symbiosis through association with arbuscular mycorrhizal fungi (AMF), which are able to establish symbiosis with most terrestrial plants. Indeed, it is well established that mycorrhization improves grapevine nutrition and resistance to stresses, especially water stress and resistance to root pathogens. Thus, it appears essential to understand the effect of mycorrhization on grapevine metabolism and defense responses. In this study, we combined a non-targeted metabolomic approach and a targeted transcriptomic study to analyze changes induced in both the roots and leaves of V. vinifera cv. Gewurztraminer by colonization with Rhizophagus irregularis (Ri). We showed that colonization of grapevine with AMF triggers major reprogramming of primary metabolism in the roots, especially sugar and fatty acid metabolism. On the other hand, mycorrhizal roots had decreased contents of most sugars and sugar acids. A significant increase in several fatty acids (C16:1, linoleic and linolenic acids and the C20 arachidonic and eicosapentaenoic acids) was also detected. However, a downregulation of the JA biosynthesis pathway was evidenced. We also found strong induction of the expression of PR proteins from the proteinase inhibitor (PR6) and subtilase (PR7) families in roots, suggesting that these proteins are involved in the mycorrhiza development but could also confer higher resistance to root pathogens. Metabolic changes induced by mycorrhization were less marked in leaves but involved higher levels of linoleic and linolenic acids and decreased sucrose, quinic, and shikimic acid contents. In addition, Ri colonization resulted in enhanced JA and SA levels in leaves. Overall, this study provides a detailed picture of metabolic changes induced by AMF colonization in a woody, economically important species. Moreover, stimulation of fatty acid biosynthesis and PR protein expression in roots and enhanced defense hormone contents in leaves establish first insight in favor of better resistance of grapevine to various pathogens provided by AMF colonization.

Nitric oxide signalling in roots is required for MYB72-dependent systemic resistance induced by Trichoderma volatile compounds in Arabidopsis.

Volatile compounds (VCs) of Trichoderma fungi trigger induced systemic resistance (ISR) in Arabidopsis that is effective against a broad spectrum of pathogens. The root-specific transcription factor MYB72 is an early regulator of ISR and also controls the activation of iron-deficiency responses. Nitric oxide (NO) is involved in the regulation of MYB72-dependent iron-deficiency responses in Arabidopsis roots, but the role of NO in the regulation of MYB72 and ISR by Trichoderma VCs remains unexplored. Using in vitro bioassays, we applied Trichoderma VCs to Arabidopsis seedlings. Plant perception of Trichoderma VCs triggered a burst of NO in Arabidopsis roots. By suppressing this burst using an NO scavenger, we show the involvement of NO in Trichoderma VCs-mediated regulation of MYB72 expression. Using an NO scavenger and the Arabidopsis lines myb72 and nia1nia2 in in planta bioassays, we demonstrate that NO signalling is required in the roots for activation of Trichoderma VCs-mediated ISR against the leaf pathogen Botrytis cinerea. Analysis of the defence-related genes PR1 and PDF1.2 points to the involvement of root NO in priming leaves for enhanced defence. Our results support a key role of root NO signalling in the regulation of MYB72 expression during the activation of ISR by Trichoderma VCs.

The CaM1-associated CCaMK-MKK1/6 cascade positively affects lateral root growth via auxin signaling under salt stress in rice.

Ca2+/calmodulin (CaM)-dependent protein kinases (CCaMKs) and mitogen-activated protein kinase kinases (MAPKKs) are two types of kinases that regulate salt stress response in plants. It remains unclear, however, how they cooperatively affect lateral root growth under salt stress. Here, two conserved phosphorylation sites (S102 and T118) of OsCaM1 were identified, and found to affect the ability to bind to Ca2+in vitro and the kinase activity of OsCCaMK in vivo. OsCCaMK specifically interacted with OsMKK1/6 in a Ca2+/CaM-dependent manner. In vitro kinase and in vivo dual-luciferase assays revealed that OsCCaMK phosphorylated OsMKK6 while OsMKK1 phosphorylated OsCCaMK. Overexpression and antisense-RNA repression expression of OsCaM1-1, and CRISPR/Cas9-mediated gene editing mutations of OsMKK1, OsMKK6, and OsMKK1/6 proved that OsCaM1-1, OsMKK1, and OsMKK6 enhanced the auxin content in roots and lateral root growth under salt stress. Consistently, OsCaM1-1, OsMKK1, and OsMKK6 regulated the transcript levels of the genes of this cascade, and salt stress-related and lateral root growth-related auxin signaling under salt stress in rice roots. These findings demonstrate that the OsCaM1-associated OsCCaMK-OsMKK1/6 cascade plays a critical role in recruiting auxin signaling in rice roots. These results also provide new insight into the regulatory mechanism of the CaM-mediated phosphorylation relay cascade to auxin signaling in lateral root growth under salt stress in plants.

N-Nitrosomelatonin, an efficient nitric oxide donor and transporter in Arabidopsis seedlings

Nitric oxide (NO) produced in plant cells has the unique ability to interact with various other biomolecules, thereby facilitating its own as well as their signaling and associated actions at their sites of biosynthesis and at other sites via transcellular long distance transport of the molecular complexes. Melatonin (Mel) is one such biomolecule produced in plant cells which has fascinated plant biologists with regard to its molecular crosstalk with other molecules to serve its roles as a growth regulator. Present work reports the synthesis of N-nitrosomelatonin (NOMela) and its preferential uptake by Arabidopsis seedlings roots and long distance transport to the leaves through vascular strands. Equimolar (250 μM) concentrations of NOMela and S-nitrosoglutathione (GSNO) in aqueous solutions bring about 52.8% more release of NO from NOMela than from GSNO. Following confocal laser scanning microscopic (CLSM) imaging, Pearson's correlation coefficient analysis of the Scatter gram of endogenously taken up NOMela demonstrates significant NO signal in roots emanating from mitochondria. NOMela (250 μM) taken up by Arabidopsis seedling roots also proved more efficient as a NO transporter from primary root to leaves than 250 μM of GSNO. These novel observations on NOMela thus hold promise to decipher its crucial role as a NO carrier and reservoir in plant cells, and also as a facilitator of melatonin action in plant development.

Modeling Auxin Signaling in Roots: Auxin Computations.

Auxin signaling and patterning is an inherently complex process, involving polarized auxin transport, metabolism, and signaling, its effect on developmental zones, as well as growth rates, and the feedback between all these different aspects. This complexity has led to an important role for computational modeling in unraveling the multifactorial roles of auxin in plant developmental and adaptive processes. Here we discuss the basic ingredients of auxin signaling and patterning models for root development as well as a series of key modeling studies in this area. These modeling studies have helped elucidate how plants use auxin signaling to compute the size of their root meristem, the direction in which to grow, and when and where to form lateral roots. Importantly, these models highlight how auxin, through patterning of and collaborating with other factors, can fulfill all these roles simultaneously.

Variation of Glucosinolate Contents in Clubroot-Resistant and -Susceptible Brassica napus Cultivars in Response to Virulence of Plasmodiophora brassicae.

The present study investigated the changes in total and individual glucosinolates (GSLs) in roots and leaves of different clubroot-resistant and -susceptible oilseed rape cultivars following artificial inoculation with Plasmodiophora brassicae isolates with different virulence. The results showed significant differences in clubroot incidence and severity as well as in the amount of total and individual glucosinolates between oilseed rape cultivars in response to virulence of the pathogen. Single among with total aliphatic and total indolic glucosinolate contents were significantly lower in leaves of susceptible cultivars compared to resistant ones due to the infection. Similarly, single and total aliphatic as well as indolic glucosinolate contents in roots were lower in susceptible cultivars compared to resistant cultivars analyzed. The different isolates of P. brassicae seem to differ in their ability to reduce gluconasturtiin contents in the host. The more aggressive isolate P1 (+) might be able to suppress gluconasturtiin synthesis of the host in a more pronounced manner compared to the isolate P1. A possible interaction of breakdown products of glucobrassicin with the auxin receptor transport inhibitor response 1 (TIR1) is hypothesized and its possible effects on auxin signaling in roots and leaves of resistant and susceptible cultivars is discussed. A potential interplay between aliphatic and indolic glucosinolates that might be involved in water homeostasis in resistant cultivars is explained.

Circadian Variation of Root Water Status in Three Herbaceous Species Assessed by Portable NMR.

Roots are at the core of plant water dynamics. Nonetheless, root morphology and functioning are not easily assessable without destructive approaches. Nuclear Magnetic Resonance (NMR), and particularly low-field NMR (LF-NMR), is an interesting noninvasive method to study water in plants, as measurements can be performed outdoors and independent of sample size. However, as far as we know, there are no reported studies dealing with the water dynamics in plant roots using LF-NMR. Thus, the aim of this study is to assess the feasibility of using LF-NMR to characterize root water status and water dynamics non-invasively. To achieve this goal, a proof-of-concept study was designed using well-controlled environmental conditions. NMR and ecophysiological measurements were performed continuously over one week on three herbaceous species grown in rhizotrons. The NMR parameters measured were either the total signal or the transverse relaxation time T2. We observed circadian variations of the total NMR signal in roots and in soil and of the root slow relaxing T2 value. These results were consistent with ecophysiological measurements, especially with the variation of fluxes between daytime and nighttime. This study assessed the feasibility of using LF-NMR to evaluate root water status in herbaceous species.

Association mapping reveals novel genomic regions controlling some root and stolon traits in tetraploid potato (Solanum tuberosum L.).

The online version contains supplementary material available at 10.1007/s13205-021-02727-6.

JASMONATE RESISTANT 1 negatively regulates root growth under boron deficiency in Arabidopsis.

Boron (B) is an essential micronutrient for plant growth and development. Jasmonic acid (JA) plays pivotal roles in plant growth, but the underlying molecular mechanism of JA involvement in B-deficiency-induced root growth inhibition is yet to be explored. In this study, we investigated the response of JA to B deficiency and the mechanism of JAR1-dependent JA signaling in root growth inhibition under B deficiency in Arabidopsis. B deficiency enhanced JA signaling in roots, and root growth inhibition was partially restored by JA biosynthesis inhibition. The jar1-1 (jasmonate-resistant 1, JAR1) mutant, and mutants of coronatine-insensitive 1 (coi1-2) and myc2 defective in JA signaling showed insensitivity to B deficiency. The ethylene-overproduction mutant eto1 and ethylene-insensitive mutant etr1 showed sensitivity and insensitivity to B deficiency, respectively, suggesting that ethylene is involved in the inhibition of primary root growth under B deficiency. Furthermore, after a decline in levels of EIN3, which may contribute to root growth, ethylene signaling was weakened in the jar1-1 mutant root under B deficiency. Under B deficiency, B concentrations were increased in the roots and shoots of the jar1-1 mutant, owing to the large root system and its activity. Therefore, our findings revealed that JA, which is involved in the inhibition of root growth under B deficiency, is regulated by JAR1-activated JA and ethylene signaling pathways.

Dynamics of miRNA mediated regulation of legume symbiosis.

Symbiotic nitrogen fixation in legume nodules is important in soils with low nitrogen availability. The initiation and sustainability of symbiosis require cellular reprogramming that involves the miRNA-mediated inhibition or activation of specific nodulation genes. The high-throughput sequencing of small RNA libraries has identified miRNAs and their targets, which are the major players in the post-transcriptional gene regulation (PTGS) of the different stages of legume-rhizobia symbiosis ranging from bacterial colonization and organogenesis to symbiotic nitrogen fixation. Here, we present an overview of information obtained from the miRNA libraries from nodulating tissues that have been sequenced to date. The functional analysis of miRNAs has revealed roles in phytohormone homeostasis and spatio-temporal regulation, as well as the mobility of miRNAs and their functions in shoot to root signalling that affects diverse functions, including bacterial entry, meristem division and differentiation, nitrogen fixation and senescence. Furthermore, small RNA fragments of rhizobial origin repress complementary plant mRNAs. We also consider the roles of miRNAs in determinate or indeterminate nodules. Taken together, this overview confirms that miRNAs are master regulators of the legume-rhizobia symbiosis.