Strigolactones fine-tune the root system

Involvement of Auxin, Flavonoids and Strigolactones in the Different Rooting Ability of European Chestnut (Castanea sativa) and Hybrids (Castanea crenata × Castanea sativa)

The aim of this study was to investigate the differences between Castanea sativa Mill. and Castanea crenata Siebold & Zucc. × Castanea sativa Mill. in rooting ability in relation to endogenous levels of auxin, auxin cofactors and inhibitors that influence rooting success. Leafy cuttings of the two commercial cultivars 'Marsol' and 'Maraval' (Castanea crenata × Castanea sativa) and the native accession 'Kozjak' (Castanea sativa) were analyzed. Endogenous indole-3-acetic acid (IAA) concentration was assessed at the beginning of propagation (day 0); in addition, strigolactones, flavonoids, rooting ability and quality were assessed 120 days after. The concentration of endogenous IAA in 'Maraval' (324.34 ± 28.66 ng g-1) and 'Marsol' (251.60 ± 35.44 ng g-1) was significantly higher than in 'Kozjak' (112.87 ± 35.44 ng g-1). The best rooting result was observed with the genotypes 'Maraval' (100.00 ± 0.00%) and 'Marsol' (90.48 ± 6.15%). A significantly lower strigol concentration was observed in the roots of 'Maraval' (75.54 ± 17.93 ng g-1) compared with other genotypes. The total flavonoid concentration in 'Maraval' was significantly higher (2794.99 ± 187.13 μg g-1) than in 'Kozjak' (1057.38 ± 61.05 μg g-1). Our results indicate that the concentration of endogenous IAA has a significant influence on rooting success. The results further indicate that in the case of flavonoids and strigolactones, not only the individual compounds but also their ratio is important for rooting success. Correlation coefficients calculated between analyzed compounds and rooting success point toward specific functions of flavonoids and strigolactones in the rooting of Castanea that need to be functionally analyzed.

Modeling the effects of strigolactone levels on maize root system architecture

Maize is the most in-demand staple crop globally. Its production relies strongly on the use of fertilizers for the supply of nitrogen, phosphorus, and potassium, which the plant absorbs through its roots, together with water. The architecture of maize roots is determinant in modulating how the plant interacts with the microbiome and extracts nutrients and water from the soil. As such, attempts to use synthetic biology and modulate that architecture to make the plant more resilient to drought and parasitic plants are underway. These attempts often try to modulate the biosynthesis of hormones that determine root architecture and growth. Experiments are laborious and time-consuming, creating the need for simulation platforms that can integrate metabolic models and 3D root growth models and predict the effects of synthetic biology interventions on both, hormone levels and root system architectures. Here, we present an example of such a platform that is built using Mathematica. First, we develop a root model, and use it to simulate the growth of many unique 3D maize root system architectures (RSAs). Then, we couple this model to a metabolic model that simulates the biosynthesis of strigolactones, hormones that modulate root growth and development. The coupling allows us to simulate the effect of changing strigolactone levels on the architecture of the roots. We then integrate the two models in a simulation platform, where we also add the functionality to analyze the effect of strigolactone levels on root phenotype. Finally, using in silico experiments, we show that our models can reproduce both the phenotype of wild type maize, and the effect that varying strigolactone levels have on changing the architecture of maize roots.

Strigolactone signaling gene from soybean GmMAX2a enhances the drought and salt-alkaline resistance in Arabidopsis via regulating transcriptional profiles of stress-related genes

Strigolactone (SL) is a new plant hormone, which not only plays an important role in stimulating seed germination, plant branching, and regulating root development, but also plays an important role in the response of plants to abiotic stresses. In this study, the full-length cDNA of a soybean SL signal transduction gene (GmMAX2a) was isolated, cloned and revealed an important role in abiotic stress responses. Tissue-specific expression analysis by qRT-PCR indicated that GmMAX2a was expressed in all tissues of soybean, but highest expression was detected in seedling stems. Moreover, upregulation of GmMAX2a transcript expression under salt, alkali, and drought conditions were noted at different time points in soybean leaves compared to roots. Additionally, histochemical GUS staining studies revealed the deep staining in P_GmMAX2a: GUS transgenic lines compared to WT indicating active involvement of GmMAX2a promoter region to stress responses. To further investigate the function of GmMAX2a gene in transgenic Arabidopsis, Petri-plate experiments were performed and GmMAX2a OX lines appeared with longer roots and improved fresh biomass compared to WT plants to NaCl, NaHCO₃, and mannitol supplementation. Furthermore, the expression of several stress-related genes such as RD29B, SOS1, NXH1, AtRD22, KIN1, COR15A, RD29A, COR47, H+-APase, NADP-ME, NCED3, and P5CS were significantly high in GmMAX2a OX plants after stress treatment compared to WT plants. In conclusion, GmMAX2a improves soybean tolerance towards abiotic stresses (salt, alkali, and drought). Hence, GmMAX2a can be considered a candidate gene for transgenic breeding against various abiotic stresses in plants.

Strigolactones as a hormonal hub for the acclimation and priming to environmental stress in plants

Strigolactones are phytohormones with many attributed roles in development, and more recently in responses to environmental stress. We will review evidence of the latter in the frame of the classic distinction among the three main stress acclimation strategies (i.e., avoidance, tolerance and escape), by taking osmotic stress in its several facets as a non-exclusive case study. The picture we will sketch is that of a hormonal family playing important roles in each of the mechanisms tested so far, and influencing as well the build-up of environmental memory through priming. Thus, strigolactones appear to be backstage operators rather than frontstage players, setting the tune of acclimation responses by fitting them to the plant individual history of stress experience.

MAX2-dependent competence for callus formation and shoot regeneration from Arabidopsis thaliana root explants

Although the division of the pericycle cells initiates both lateral root development and root-derived callus formation, these developmental processes are affected differently in the strigolactone and karrikin/KARRIKIN INSENSITIVE 2 (KAI2) ligand signalling mutant more axillary growth 2 (max2). Whereas max2 produces more lateral roots than the wild type, it is defective in the regeneration of shoots from root explants. We suggest that the decreased shoot regeneration of max2 originates from delayed formation of callus primordium, yielding less callus material to regenerate shoots. Indeed, when incubated on callus-inducing medium, the pericycle cell division was reduced in max2 and the early gene expression varied when compared with the wild type, as determined by a transcriptomics analysis. Furthermore, the expression of the LATERAL ORGAN BOUNDARIES DOMAIN genes and of callus-induction genes was modified in correlation with the max2 phenotype, suggesting a role for MAX2 in the regulation of the interplay between cytokinin, auxin, and light signalling in callus initiation. Additionally, we found that the in vitro shoot regeneration phenotype of max2 might be caused by a defect in KAI2, rather than in DWARF14, signalling. Nevertheless, the shoot regeneration assays revealed that the strigolactone biosynthesis mutants max3 and max4 also play a minor role.

Role of Strigolactones: Signalling and Crosstalk with Other Phytohormones

Plant hormones play important roles in controlling how plants grow and develop. While metabolism provides the energy needed for plant survival, hormones regulate the pace of plant growth. Strigolactones (SLs) were recently defined as new phytohormones that regulate plant metabolism and, in turn, plant growth and development. This group of phytohormones is derived from carotenoids and has been implicated in a wide range of physiological functions including regulation of plant architecture (inhibition of bud outgrowth and shoot branching), photomorphogenesis, seed germination, nodulation, and physiological reactions to abiotic factors. SLs also induce hyphal branching in germinating spores of arbuscular mycorrhizal fungi (AMF), a process that is important for initiating the connection between host plant roots and AMF. This review outlines the physiological roles of SLs and discusses the significance of interactions between SLs and other phytohormones to plant metabolic responses.

Impairment in karrikin but not strigolactone sensing enhances root skewing in Arabidopsis thaliana

Roots form highly complex systems varying in growth direction and branching pattern to forage for nutrients efficiently. Here mutations in the KAI2 (KARRIKIN INSENSITIVE) α/β-fold hydrolase and the MAX2 (MORE AXILLARY GROWTH 2) F-box leucine-rich protein, which together perceive karrikins (smoke-derived butenolides), caused alteration in root skewing in Arabidopsis thaliana. This phenotype was independent of endogenous strigolactones perception by the D14 α/β-fold hydrolase and MAX2. Thus, KAI2/MAX2 effect on root growth may be through the perception of endogenous KAI2-ligands (KLs), which have yet to be identified. Upon perception of a ligand, a KAI2/MAX2 complex is formed together with additional target proteins before ubiquitination and degradation through the 26S proteasome. Using a genetic approach, we show that SMAX1 (SUPPRESSOR OF MAX2-1)/SMXL2 and SMXL6,7,8 (SUPPRESSOR OF MAX2-1-LIKE) are also likely degradation targets for the KAI2/MAX2 complex in the context of root skewing. In A. thaliana therefore, KAI2 and MAX2 act to limit root skewing, while kai2's gravitropic and mechano-sensing responses remained largely unaffected. Many proteins are involved in root skewing, and we investigated the link between MAX2 and two members of the SKS/SKU family. Though KLs are yet to be identified in plants, our data support the hypothesis that they are present and can affect root skewing.

Rooting of eucalypt cuttings as a problem-solving oriented model in plant biology

Species of Eucalyptus are some of the most planted trees in the world, providing fiber, cellulose, energy, and wood for construction and furniture in renewable fashion, with the added advantage of fixing large amounts of atmospheric carbon. The efficiency of eucalypts in forestry relies mostly on the clonal propagation of selected genotypes both as pure species and interspecific hybrids. The formation of new roots from cambium tissues at the base of cuttings, referred to as adventitious rooting (AR), is essential for accomplishing clonal propagation successfully. AR is a highly complex, multi-level regulated developmental process, affected by a number of endogenous and environmental factors. In several cases, highly desirable genotypes from an industrial point of view carry along the undesirable trait of difficulty-to-root (recalcitrance). Understanding the bases of this phenotype is needed to identify ways to overcome recalcitrance and allow efficient clonal propagation. Herein, an overview of the state-of-the-art on the basis of AR recalcitrance in eucalypts addressed at various levels of regulation (transcript, protein, metabolite and phenotype), and OMICs techniques is presented. In addition, a focus is also provided on the gaps that need to be filled in order to advance in this strategic biological problem for global forestry industry relying on eucalypts.

Strigolactones: new plant hormones in the spotlight

The development and growth of plants are regulated by interplay of a plethora of complex chemical reactions in which plant hormones play a pivotal role. In recent years, a group of new plant hormones, namely strigolactones (SLs), was discovered and identified. The first SL, strigol, was isolated in 1966, but it took almost 20 years before the details of its structure were fully elucidated. At present, two families of SLs are known, one having the stereochemistry of (+)-strigol and the other that of (-)-orobanchol, the most abundant naturally occurring SL. The most well-known bioproperty of SLs is the germination of seeds of the parasitic weeds Striga and Orobanche. It is evident that SLs are going to play a prominent role in modern molecular botany. In this review, relevant molecular and bioproperties of SLs are discussed. Items of importance are the effect of stereochemistry, structure-activity relationships, design and synthesis of analogues with a simple structure, but with retention of bioactivity, introduction of fluorescent labels into SLs, biosynthetic origin of SLs, mode of action in plants, application in agriculture for the control of parasitic weeds, stimulation of the branching of arbuscular mycorrhizal (AM) fungi, and the control of plant architecture. The future potential of SLs in molecular botany is highlighted.

CRISPR/Cas9-Mediated Mutagenesis of Carotenoid Cleavage Dioxygenase 8 (CCD8) in Tobacco Affects Shoot and Root Architecture

Strigolactones (SLs) are a class of phytohormones that regulate plant architecture. Carotenoid cleavage dioxygenase (CCD) genes are involved in the biosynthesis of SLs and are identified and characterized in many plants. However, the function of CCD genes in tobacco remains poorly understood. In this study, two closely related genes NtCCD8A and NtCCD8B were cloned from tobacco (Nicotiana tabacum L.). The two NtCCD8 genes are orthologues of the tomato (Solanum lycopersicum) carotenoid cleavage dioxygenase 8 (SlCCD8) gene. NtCCD8A and NtCCD8B were primarily expressed in tobacco roots, but low expression levels of these genes were detected in all plant tissues, and their transcript levels significantly increased in response to phosphate limitation. NtCCD8A and NtCCD8B mutations were introduced into tobacco using the CRISPR/Cas9 system and transgenic tobacco lines for both ntccd8 mutant alleles were identified. The ntccd8a and ntccd8b mutant alleles were inactivated by a deletion of three nucleotides and insertion of one nucleotide, respectively, both of which led to the production of premature stop codons. The ntccd8 mutants had increased shoot branching, reduced plant height, increased number of leaves and nodes, and reduced total plant biomass compared to wild-type plants; however, the root-to-shoot ratio was unchanged. In addition, mutant lines had shorter primary roots and more of lateral roots than wild type. These results suggest that NtCCD8 genes are important for changes in tobacco plant architecture.

Strigolactones are common regulators in induction of stomatal closure in planta

Strigolactones (SLs) have been implicated in many plant biological processes, including growth and development and the acclimation to environmental stress. We recently reported that SLs intrinsically acted as prominent regulators in induction of stomatal closure. Here we present evidence that the effect of SLs on stotamal closure is not limited to Arabidopsis, and thus SLs could serve as common regulators in the modulation of stomatal apertures of various plant species. Nevertheless, TIS108, a SL-biosynthetic inhibitor, exerted no effect on stomatal apertures. In addition, the SL receptor mutant atd14-5, similar to SL-deficient and more axillary growth 2 (max2) mutants, exhibited hypersensitivity to drought stress. Altogether, these results reinforce the role of SLs as common regulators in stress resilience.

Identification and functional analysis of the HvD14 gene involved in strigolactone signaling in Hordeum vulgare

In this study, the barley HvD14 gene encoding α/β hydrolase, which is involved in strigolactone (SL) signaling, was identified. Bioinformatics analysis revealed that the identified gene is an orthologue of the D14, AtD14 and PhDAD2 genes that have been described in rice, Arabidopsis thaliana and petunia, respectively. Using TILLING strategy, an hvd14.d mutant that carried the G725A transition, located in the second exon, was identified. This mutation led to the substitution of a highly conserved glycine-193 to glutamic acid in the conserved fragment of the α/β hydrolase domain of the HvD14 protein. The plants that carry the hvd14.d allele were semi-dwarf and produced a higher number of tillers in comparison to the wild-type (WT) parent cultivar. Additionally, the root architecture of mutant plants was affected: the total length of the seminal roots was significantly reduced, and the density of the lateral roots was higher than in the WT. Plants with the hvd14.d allele were insensitive to treatment with GR24, which is the synthetic analogue of SL. Analysis of the indole-3-acetic acid (IAA) concentration in the lateral buds showed no differences between the WT and mutant plants. By contrast, the WT seedlings treated with GR24 developed a lower number of tillers, longer primary roots with a reduced number of lateral roots and had an increased concentration of IAA in lateral buds. This paper describes the first barley SL mutant and shows the potential functions of SLs in barley growth and development.

Strigolactones: new plant hormones in action

MAIN CONCLUSION

The key step in the mode of action of strigolactones is the enzymatic detachment of the D-ring. The thus formed hydroxy butenolide induces conformational changes of the receptor pocket which trigger a cascade of reactions in the signal transduction. Strigolactones (SLs) constitute a new class of plant hormones which are of increasing importance in plant science. For the last 60 years, they have been known as germination stimulants for parasitic plants. Recently, several new bio-properties of SLs have been discovered such as the branching factor for arbuscular mycorrhizal fungi, regulation of plant architecture (inhibition of bud outgrowth and of shoot branching) and the response to abiotic factors, etc. To broaden horizons and encourage new ideas for identifying and synthesising new and structurally simple SLs, this review is focused on molecular aspects of this new class of plant hormones. Special attention has been given to structural features, the mode of action of these phytohormones in various biological actions, the design of SL analogs and their applications.

Strigolactones: new plant hormones in action

The key step in the mode of action of strigolactones is the enzymatic detachment of the D-ring. The thus formed hydroxy butenolide induces conformational changes of the receptor pocket which trigger a cascade of reactions in the signal transduction. Strigolactones (SLs) constitute a new class of plant hormones which are of increasing importance in plant science. For the last 60 years, they have been known as germination stimulants for parasitic plants. Recently, several new bio-properties of SLs have been discovered such as the branching factor for arbuscular mycorrhizal fungi, regulation of plant architecture (inhibition of bud outgrowth and of shoot branching) and the response to abiotic factors, etc. To broaden horizons and encourage new ideas for identifying and synthesising new and structurally simple SLs, this review is focused on molecular aspects of this new class of plant hormones. Special attention has been given to structural features, the mode of action of these phytohormones in various biological actions, the design of SL analogs and their applications.

Strigolactone versus gibberellin signaling: reemerging concepts?

MAIN CONCLUSION

In this review, we compare knowledge about the recently discovered strigolactone signaling pathway and the well established gibberellin signaling pathway to identify gaps of knowledge and putative research directions in strigolactone biology. Communication between and inside cells is integral for the vitality of living organisms. Hormonal signaling cascades form a large part of this communication and an understanding of both their complexity and interactive nature is only beginning to emerge. In plants, the strigolactone (SL) signaling pathway is the most recent addition to the classically acting group of hormones and, although fundamental insights have been made, knowledge about the nature and impact of SL signaling is still cursory. This narrow understanding is in spite of the fact that SLs influence a specific spectrum of processes, which includes shoot branching and root system architecture in response, partly, to environmental stimuli. This makes these hormones ideal tools for understanding the coordination of plant growth processes, mechanisms of long-distance communication and developmental plasticity. Here, we summarize current knowledge about SL signaling and employ the well-characterized gibberellin (GA) signaling pathway as a scaffold to highlight emerging features as well as gaps in our knowledge in this context. GA signaling is particularly suitable for this comparison because both signaling cascades share key features of hormone perception and of immediate downstream events. Therefore, our comparative view demonstrates the possible level of complexity and regulatory interfaces of SL signaling.

Characterization of DWARF14 Genes in Populus

Strigolactones are a new class of plant hormones regulating shoot branching and symbiotic interactions with arbuscular mycorrhizal fungi. Studies of branching mutants in herbaceous plants have identified several key genes involved in strigolactone biosynthesis or signaling. The strigolactone signal is perceived by a member of the α/β-fold hydrolase superfamily, known as DWARF14 (D14). However, little is known about D14 genes in the woody perennial plants. Here we report the identification of D14 homologs in the model woody plant Populus trichocarpa. We showed that there are two D14 homologs in P. trichocarpa, designated as PtD14a and PtD14b that are over 95% similar at the amino acid level. Expression analysis indicated that the transcript level of PtD14a is generally more abundant than that of PtD14b. However, only PtD14a was able to complement Arabidopsis d14 mutants, suggesting that PtD14a is the functional D14 ortholog. Amino acid alignment and structural modeling revealed substitutions of several highly conserved amino acids in the PtD14b protein including a phenylalanine near the catalytic triad of D14 proteins. This study lays a foundation for further characterization of strigolactone pathway and its functions in the woody perennial plants.

The Physiology of Adventitious Roots

Adventitious roots are plant roots that form from any nonroot tissue and are produced both during normal development (crown roots on cereals and nodal roots on strawberry [Fragaria spp.]) and in response to stress conditions, such as flooding, nutrient deprivation, and wounding. They are important economically (for cuttings and food production), ecologically (environmental stress response), and for human existence (food production). To improve sustainable food production under environmentally extreme conditions, it is important to understand the adventitious root development of crops both in normal and stressed conditions. Therefore, understanding the regulation and physiology of adventitious root formation is critical for breeding programs. Recent work shows that different adventitious root types are regulated differently, and here, we propose clear definitions of these classes. We use three case studies to summarize the physiology of adventitious root development in response to flooding (case study 1), nutrient deficiency (case study 2), and wounding (case study 3).

Strigolactones spatially influence lateral root development through the cytokinin signaling network

Strigolactones are important rhizosphere signals that act as phytohormones and have multiple functions, including modulation of lateral root (LR) development. Here, we show that treatment with the strigolactone analog GR24 did not affect LR initiation, but negatively influenced LR priming and emergence, the latter especially near the root-shoot junction. The cytokinin module ARABIDOPSIS HISTIDINE KINASE3 (AHK3)/ARABIDOPSIS RESPONSE REGULATOR1 (ARR1)/ARR12 was found to interact with the GR24-dependent reduction in LR development, because mutants in this pathway rendered LR development insensitive to GR24. Additionally, pharmacological analyses, mutant analyses, and gene expression analyses indicated that the affected polar auxin transport stream in mutants of the AHK3/ARR1/ARR12 module could be the underlying cause. Altogether, the data reveal that the GR24 effect on LR development depends on the hormonal landscape that results from the intimate connection with auxins and cytokinins, two main players in LR development.

Apocarotenoids: A New Carotenoid-Derived Pathway

Carotenoids are precursors of carotenoid derived molecules termed apocarotenoids, which include isoprenoids with important functions in plant-environment interactions such as the attraction of pollinators and the defense against pathogens and herbivores. Apocarotenoids also include volatile aromatic compounds that act as repellents, chemoattractants, growth simulators and inhibitors, as well as the phytohormones abscisic acid and strigolactones. In plants, apocarotenoids can be found in several types of plastids (etioplast, leucoplast and chromoplast) and among different plant tissues such as flowers and roots. The structural similarity of some flower and spice isoprenoid volatile organic compounds (β-ionone and safranal) to carotenoids has led to the recent discovery of carotenoid-specific cleavage oxygenases, including carotenoid cleavage dioxygenases and 9-cis-epoxydioxygenases, which tailor and transform carotenoids into apocarotenoids. The great diversity of apocarotenoids is a consequence of the huge amount of carotenoid precursors, the variations in specific cleavage sites and the modifications after cleavage. Lycopene, β-carotene and zeaxanthin are the precursors of the main apocarotenoids described to date, which include bixin, crocin, picrocrocin, abscisic acid, strigolactone and mycorradicin.The current chapter will give rise to an overview of the biosynthesis and function of the most important apocarotenoids in plants, as well as the current knowledge about the carotenoid cleavage oxygenase enzymes involved in these biosynthetic pathways.

Stereochemical Assignment of Strigolactone Analogues Confirms Their Selective Biological Activity

Strigolactones (SLs) are new plant hormones with various developmental functions. They are also soil signaling chemicals that are required for establishing beneficial mycorrhizal plant/fungus symbiosis. In addition, SLs play an essential role in inducing seed germination in root-parasitic weeds, which are one of the seven most serious biological threats to food security. There are around 20 natural SLs that are produced by plants in very low quantities. Therefore, most of the knowledge on SL signal transduction and associated molecular events is based on the application of synthetic analogues. Stereochemistry plays a crucial role in the structure-activity relationship of SLs, as compounds with an unnatural D-ring configuration may induce biological effects that are unrelated to SLs. We have synthesized a series of strigolactone analogues, whose absolute configuration has been elucidated and related with their biological activity, thus confirming the high specificity of the response. Analogues bearing the R-configured butenolide moiety showed enhanced biological activity, which highlights the importance of this stereochemical motif.

The plastid terminal oxidase: its elusive function points to multiple contributions to plastid physiology

Plastids have retained from their cyanobacterial ancestor a fragment of the respiratory electron chain comprising an NADPH dehydrogenase and a diiron oxidase, which sustain the so-called chlororespiration pathway. Despite its very low turnover rates compared with photosynthetic electron flow, knocking out the plastid terminal oxidase (PTOX) in plants or microalgae leads to severe phenotypes that encompass developmental and growth defects together with increased photosensitivity. On the basis of a phylogenetic and structural analysis of the enzyme, we discuss its physiological contribution to chloroplast metabolism, with an emphasis on its critical function in setting the redox poise of the chloroplast stroma in darkness. The emerging picture of PTOX is that of an enzyme at the crossroads of a variety of metabolic processes, such as, among others, the regulation of cyclic electron transfer and carotenoid biosynthesis, which have in common their dependence on the redox state of the plastoquinone pool, set largely by the activity of PTOX in darkness.

Nitric oxide mediates strigolactone signaling in auxin and ethylene-sensitive lateral root formation in sunflower seedlings

Strigolactones (SLs) play significant role in shaping root architecture whereby auxin-SL crosstalk has been observed in SL-mediated responses of primary root elongation, lateral root formation and adventitious root (AR) initiation. Whereas GR24 (a synthetic strigolactone) inhibits LR and AR formation, the effect of SL biosynthesis inhibitor (fluridone) is just the opposite (root proliferation). Naphthylphthalamic acid (NPA) leads to LR proliferation but completely inhibits AR development. The diffusive distribution of PIN1 in the provascular cells in the differentiating zone of the roots in response to GR24, fluridone or NPA treatments further indicates the involvement of localized auxin accumulation in LR development responses. Inhibition of LR formation by GR24 treatment coincides with inhibition of ACC synthase activity. Profuse LR development by fluridone and NPA treatments correlates with enhanced [Ca(2+)]cyt in the apical region and differentiating zones of LR, indicating a critical role of [Ca(2+)] in LR development in response to the coordinated action of auxins, ethylene and SLs. Significant enhancement of carotenoid cleavage dioxygenase (CCD) activity (enzyme responsible for SL biosynthesis) in tissue homogenates in presence of cPTIO (NO scavenger) indicates the role of endogenous NO as a negative modulator of CCD activity. Differences in the spatial distribution of NO in the primary and lateral roots further highlight the involvement of NO in SL-modulated root morphogenesis in sunflower seedlings. Present work provides new report on the negative modulation of SL biosynthesis through modulation of CCD activity by endogenous nitric oxide during SL-modulated LR development.

From lateral root density to nodule number, the strigolactone analogue GR24 shapes the root architecture of Medicago truncatula

In the rhizosphere, strigolactones not only act as crucial signalling molecules in the communication of plants with parasitic weeds and arbuscular mycorrhiza, but they also play a key role in regulating different aspects of the root system. Here we investigated how strigolactones influence the root architecture of Medicago truncatula. We provide evidence that addition of the synthetic strigolactone analogue GR24 has an inhibitory effect on the lateral root density. Moreover, treatment with GR24 of Sinorhizobium meliloti-inoculated M. truncatula plants affects the nodule number both positively and negatively, depending on the concentration. Plants treated with 0.1 µM GR24 had a slightly increased number of nodules, whereas concentrations of 2 and 5 µM strongly reduced it. This effect was independent of the autoregulation of nodulation mechanism that is controlled by SUPER NUMERIC NODULE. Furthermore, we demonstrate that GR24 controls the nodule number through crosstalk with SICKLE-dependent ethylene signalling. Additionally, because the expression of the nodulation marker EARLY NODULATION11 was strongly reduced in GR24-treated plants, we concluded that strigolactones influence nodulation at a very early stage of the symbiotic interaction.

Strigolactones, a novel carotenoid-derived plant hormone

Strigolactones (SLs) are carotenoid-derived plant hormones and signaling molecules. When released into the soil, SLs indicate the presence of a host to symbiotic fungi and root parasitic plants. In planta, they regulate several developmental processes that adapt plant architecture to nutrient availability. Highly branched/tillered mutants in Arabidopsis, pea, and rice have enabled the identification of four SL biosynthetic enzymes: a cis/trans-carotene isomerase, two carotenoid cleavage dioxygenases, and a cytochrome P450 (MAX1). In vitro and in vivo enzyme assays and analysis of mutants have shown that the pathway involves a combination of new reactions leading to carlactone, which is converted by a rice MAX1 homolog into an SL parent molecule with a tricyclic lactone moiety. In this review, we focus on SL biosynthesis, describe the hormonal and environmental factors that determine this process, and discuss SL transport and downstream signaling as well as the role of SLs in regulating plant development.

Melatonin: plant growth regulator and/or biostimulator during stress?

Melatonin regulates the growth of roots, shoots, and explants, to activate seed germination and rhizogenesis and to delay induced leaf senescence. The antioxidant properties of melatonin would seem to explain, at least partially, its ability to fortify plants subjected to abiotic stress. In this Review we examine recent data on the gene-regulation capacity of melatonin that point to many interesting features, such as the upregulation of anti-stress genes and recent aspects of the auxin-independent effects of melatonin as a plant growth regulator. This, together with the recent data on endogenous melatonin biosynthesis induction by environmental factors, makes melatonin an interesting candidate for use as a natural biostimulating treatment for field crops.

Characterization of MORE AXILLARY GROWTH genes in Populus

Background

Strigolactones are a new class of plant hormones that play a key role in regulating shoot branching. Studies of branching mutants in Arabidopsis, pea, rice and petunia have identified several key genes involved in strigolactone biosynthesis or signaling pathway. In the model plant Arabidopsis, MORE AXILLARY GROWTH1 (MAX1), MAX2, MAX3 and MAX4 are four founding members of strigolactone pathway genes. However, little is known about the strigolactone pathway genes in the woody perennial plants.

Methodology/Principal Finding

Here we report the identification of MAX homologues in the woody model plant Populus trichocarpa. We identified the sequence homologues for each MAX protein in P. trichocarpa. Gene expression analysis revealed that Populus MAX paralogous genes are differentially expressed across various tissues and organs. Furthermore, we showed that Populus MAX genes could complement or partially complement the shoot branching phenotypes of the corresponding Arabidopsis max mutants.

Conclusion/Significance

This study provides genetic evidence that strigolactone pathway genes are likely conserved in the woody perennial plants and lays a foundation for further characterization of strigolactone pathway and its functions in the woody perennial plants.

Strigolactones inhibit caulonema elongation and cell division in the moss Physcomitrella patens

In vascular plants, strigolactones (SLs) are known for their hormonal role and for their role as signal molecules in the rhizosphere. SLs are also produced by the moss Physcomitrella patens, in which they act as signaling factors for controlling filament extension and possibly interaction with neighboring individuals. To gain a better understanding of SL action at the cellular level, we investigated the effect of exogenously added molecules (SLs or analogs) in moss growth media. We used the previously characterized Ppccd8 mutant that is deficient in SL synthesis and showed that SLs affect moss protonema extension by reducing caulonema cell elongation and mainly cell division rate, both in light and dark conditions. Based on this effect, we set up bioassays to examine chemical structure requirements for SL activity in moss. The results suggest that compounds GR24, GR5, and 5-deoxystrigol are active in moss (as in pea), while other analogs that are highly active in the control of pea branching show little activity in moss. Interestingly, the karrikinolide KAR1, which shares molecular features with SLs, did not have any effect on filament growth, even though the moss genome contains several genes homologous to KAI2 (encoding the KAR1 receptor) and no canonical homologue to D14 (encoding the SL receptor). Further studies should investigate whether SL signaling pathways have been conserved during land plant evolution.

A 2-component system is involved in the early stages of the Pisolithus tinctorius-Pinus greggii symbiosis

Ectomycorrhizal symbiosis results in profound morphological and physiological modifications in both plant and fungus. This in turn is the product of differential gene expression in both co-symbionts, giving rise to specialized cell types capable of performing novel functions. During the precolonization stage, chemical signals from root exudates are sensed by the ectomycorrizal fungus, and vice versa, which are in principle responsible for the observed change in the developmental symbionts program. Little is known about the molecular mechanisms involved in the signaling and recognition between ectomycorrhizal fungi and their host plants. In the present work, we characterized a novel lactone, termed pinelactone, and identified a gene encoding for a histidine kinase in Pisolithus tictorius, which function is proposed to be the perception of the aforementioned metabolites. In this study, the use of closantel, a specific inhibitor of histidine kinase phosphorylation, affected the capacity for fungal colonization in the symbiosis between Pisolithus tinctorius and Pinus greggii, indicating that a 2-component system (TCS) may operate in the early events of plant-fungus interaction. Indeed, the metabolites induced the accumulation of Pisolithus tinctorius mRNA for a putative histidine kinase (termed Pthik1). Of note, Pthik1 was able to partially complement a S. cerevisiae histidine kinase mutant, demonstrating its role in the response to the presence of the aforementioned metabolites. Our results indicate a role of a 2-component pathway in the early stages of ectomycorrhizal symbiosis before colonization. Furthermore, a novel lactone from Pinus greggii root exudates may activate a signal transduction pathway that contributes to the establishment of the ectomycorrhizal symbiosis.

Adventitious roots and lateral roots: similarities and differences

In addition to its role in water and nutrient uptake, the root system is fundamentally important because it anchors a plant to its substrate. Although a wide variety of root systems exist across different species, all plants have a primary root (derived from an embryonic radicle) and different types of lateral roots. Adventitious roots, by comparison, display the same functions as lateral roots but develop from aerial tissues. In addition, they not only develop as an adaptive response to various stresses, such as wounding or flooding, but also are a key limiting component of vegetative propagation. Lateral and adventitious roots share key elements of the genetic and hormonal regulatory networks but are subject to different regulatory mechanisms. In this review, we discuss the developmental processes that give rise to lateral and adventitious roots and highlight knowledge acquired over the past few years about the mechanisms that regulate adventitious root formation.