A strigolactone signal is required for adventitious root formation in rice

Background and Aims Strigolactones (SLs) and their derivatives are plant hormones that have recently been identified as regulating root development. This study examines whether SLs play a role in mediating production of adventious roots (ARs) in rice (Oryza sativa), and also investigates possible interactions between SLs and auxin.Methods Wild-type (WT), SL-deficient (d10) and SL-insensitive (d3) rice mutants were used to investigate AR development in an auxin-distribution experiment that considered DR5::GUS activity, [³H] indole-3-acetic acid (IAA) transport, and associated expression of auxin transporter genes. The effects of exogenous application of GR24 (a synthetic SL analogue), NAA (α-naphthylacetic acid, exogenous auxin) and NPA (N-1-naphthylphalamic acid, a polar auxin transport inhibitor) on rice AR development in seedlings were investigated.Key Results The rice d mutants with impaired SL biosynthesis and signalling exhibited reduced AR production compared with the WT. Application of GR24 increased the number of ARs and average AR number per tiller in d10, but not in d3. These results indicate that rice AR production is positively regulated by SLs. Higher endogenous IAA concentration, stronger expression of DR5::GUS and higher [³H] IAA activity were found in the d mutants. Exogenous GR24 application decreased the expression of DR5::GUS, probably indicating that SLs modulate AR formation by inhibiting polar auxin transport. The WT and the d10 and d3 mutants had similar expression of DR5::GUS regardless of exogenous application of NAA or NPA; however, AR number was greater in the WT than in the d mutants.Conclusions The results suggest that AR formation is positively regulated by SLs via the D3 response pathway. The positive effect of NAA application and the opposite effect of NPA application on AR number of WT plants also suggests the importance of auxin for AR formation, but the interaction between auxin and SLs is complex.     

Interaction of Phytohormones in Regulating the Axillary Bud Growth in Pea

In the present study the interactions of GR24, a synthetic analog of newly discovered plant hormones strigolactones (SLs), with cytokinin (CK), benzyladenine (BA), auxin naphthaleneacetic acid (NAA), gibberellic acid (GA₃) and abscisic acid (ABA) in the regulation of axillary bud growth in pea (Pisum sativum L.) were investigated. The hormones were applied directly to buds at node 1 and 2 and the dose-response experiments were performed on 8–10-day-old SL-deficient rms1 and rms5–1 mutants, branching SL-sensitive rms2–1 mutants and wild-type plants. In the mutant plants the treatment with 5 μM GR24 completely inhibited bud growth while BA up to 100 μM stimulated it. The combined application of GR24 and BA showed that GR24 efficiency to reduce bud outgrowth constantly declines as CK-stimulated bud growth increased, with the inhibiting effect of GR24 abolished at 100 μM BA applied. GA₃ accelerated bud outgrowth, but did not interfere with GR24 inhibitory action. NAA reduced bud growth in intact SL-sensitive rms2–1 mutant and also in SL-insensitive rms3–2 and rms4–1 mutants. The NAA effect was observed already at 0.5 μM, however, even at a response saturating concentration of 500 μM its inhibiting effect was inferior to that of 5 μM GR24. At combined treatment the effectiveness of GR24 in suppressing bud growth decreased with a decrease in NAA-inhibited bud growth, suggesting that auxin might act as a suppressor of SL action. ABA strongly inhibited the bud outgrowth and, like CK and auxin, reduced the inhibitory effectiveness of GR24. The revealed ability of CK, ABA and auxin to suppress bud response to SLs is supposed to result from phytohormone signaling crosstalks.     

Identification of Differentially Expressed Proteins and Phosphorylated Proteins in Rice Seedlings in Response to Strigolactone Treatment

Strigolactones (SLs) are recently identified plant hormones that inhibit shoot branching and control various aspects of plant growth, development and interaction with parasites. Previous studies have shown that plant D10 protein is a carotenoid cleavage dioxygenase that functions in SL biosynthesis. In this work, we used an allelic SL-deficient d10 mutant XJC of rice (Oryza sativa L. spp. indica) to investigate proteins that were responsive to SL treatment. When grown in darkness, d10 mutant seedlings exhibited elongated mesocotyl that could be rescued by exogenous application of SLs. Soluble protein extracts were prepared from d10 mutant seedlings grown in darkness in the presence of GR24, a synthetic SL analog. Soluble proteins were separated on two-dimensional gels and subjected to proteomic analysis. Proteins that were expressed differentially and phosphoproteins whose phosphorylation status changed in response to GR24 treatment were identified. Eight proteins were found to be induced or down-regulated by GR24, and a different set of 8 phosphoproteins were shown to change their phosphorylation intensities in the dark-grown d10 seedlings in response to GR24 treatment. Analysis of these proteins revealed that they are important enzymes of the carbohydrate and amino acid metabolic pathways and key components of the cellular energy generation machinery. These proteins may represent potential targets of the SL signaling pathway. This study provides new insight into the complex and negative regulatory mechanism by which SLs control shoot branching and plant development.     

Effects of strigolactone-biosynthesis inhibitor TIS108 on Arabidopsis

TIS108 is a triazole-type strigolactone (SL)-biosynthesis inhibitor that reduces the level of 2′-epi-5-deoxystrigol (epi-5DS) in rice. Here we report the effects of TIS108 on Arabidopsis. Treatment of TIS108 increased the number of branches and repressed root hair elongation as was observed in SL-deficient mutants, and co-application of GR24, a synthetic SL analog, recovered the TIS108-induced phenotype to that of wild-type. In addition, MAX3 and MAX4 genes in the SL-biosynthesis pathway were upregulated in TIS108-treated Arabidopsis, probably due to feedback regulation caused by SL deficiency. These results indicate that TIS108 is an effective tool for regulating SL production in Arabidopsis.     

Strigolactones affect lateral root formation and root-hair elongation in Arabidopsis

Strigolactones (SLs) have been proposed as a new group of plant hormones, inhibiting shoot branching, and as signaling molecules for plant interactions. Here, we present evidence for effects of SLs on root development. The analysis of mutants flawed in SLs synthesis or signaling suggested that the absence of SLs enhances lateral root formation. In accordance, roots grown in the presence of GR24, a synthetic bioactive SL, showed reduced number of lateral roots in WT and in max3-11 and max4-1 mutants, deficient in SL synthesis. The GR24-induced reduction in lateral roots was not apparent in the SL signaling mutant max2-1. Moreover, GR24 led to increased root-hair length in WT and in max3-11 and max4-1 mutants, but not in max2-1. SLs effect on lateral root formation and root-hair elongation may suggest a role for SLs in the regulation of root development; perhaps, as a response to growth conditions.     

New branching inhibitors and their potential as strigolactone mimics in rice

Strigolactones (SLs) are rhizosphere communication chemicals. Recent studies of highly branched mutants revealed that SL or its metabolites work as a phytohormone to inhibit shoot branching. When SLs are exogenously applied to the rice d10-1 mutant that has a highly branched phenotype caused by a defect in the SL biosynthesis gene (CCD8), they inhibit tiller bud outgrowth (branching in rice) of the mutant. We focused our attention on the SL function as a phytohormone and tried to find new chemicals mimicking the hormonal action of SL by screening chemicals that inhibit branching of rice d10-1 mutant. Fortunately, we found 5-(4-chlorophenoxy)-3-methylfuran-2(5H)-one (3a) as a new chemical possessing SL-like activity against the rice d10-1 mutant. Then, we prepared several derivatives of 3a (3b-3k) to examine their ability to inhibit shoot branching of rice d10-1. These derivatives were synthesized by a one-pot coupling reaction between phenols and halo butenolide to give 5-phenoxy 3-methylfuran-2(5H)-one (3) derivatives, which possess a common substructure with SLs. Some of the derivatives showed SL-like activity more potently than GR24, a typical SL derivative, in a rice assay. As SLs also show activity by inducing seed germination of root parasitic plants, the induction activity of these derivatives was also evaluated. Here we report the structure-activity relationships of these compounds.     

Strigolactone Positively Controls Crown Root Elongation in Rice

Strigolactones are recently identified plant hormones that inhibit shoot branching. Pleiotropic defects in strigolactone-deficient or -insensitive mutants indicate that strigolactones control various aspects of plant growth and development. However, our understanding of the hormonal function of strigolactones in plants is very limited. In this study we demonstrate that rice dwarf mutants that are strigolactone-deficient or -insensitive exhibit a short crown root phenotype. Exogenous application of GR24, a synthetic strigolactone analog, complemented the crown root defect in strigolactone-deficient mutants but not in strigolactone-insensitive mutants. These observations imply that strigolactones positively regulate the length of crown roots. Histological observations revealed that the meristematic zone is shorter in dwarf mutants than in wild type, suggesting that strigolactones may exert their effect on roots via the control of cell division. We also show that crown roots of wild type, but not dwarf mutants, become longer under phosphate starvation.     

Differential Activity of Striga hermonthica Seed Germination Stimulants and Gigaspora rosea Hyphal Branching Factors in Rice and Their Contribution to Underground Communication

Strigolactones (SLs) trigger germination of parasitic plant seeds and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. There is extensive structural variation in SLs and plants usually produce blends of different SLs. The structural variation among natural SLs has been shown to impact their biological activity as hyphal branching and parasitic plant seed germination stimulants. In this study, rice root exudates were fractioned by HPLC. The resulting fractions were analyzed by MRM-LC-MS to investigate the presence of SLs and tested using bioassays to assess their Striga hermonthica seed germination and Gigaspora rosea hyphal branching stimulatory activities. A substantial number of active fractions were revealed often with very different effect on seed germination and hyphal branching. Fractions containing (−)−orobanchol and ent-2''-epi-5-deoxystrigol contributed little to the induction of S. hermonthica seed germination but strongly stimulated AM fungal hyphal branching. Three SLs in one fraction, putative methoxy-5-deoxystrigol isomers, had moderate seed germination and hyphal branching inducing activity. Two fractions contained strong germination stimulants but displayed only modest hyphal branching activity. We provide evidence that these stimulants are likely SLs although no SL-representative masses could be detected using MRM-LC-MS. Our results show that seed germination and hyphal branching are induced to very different extents by the various SLs (or other stimulants) present in rice root exudates. We propose that the development of rice varieties with different SL composition is a promising strategy to reduce parasitic plant infestation while maintaining symbiosis with AM fungi.     

Exogenous strigolactones impact metabolic profiles and phosphate starvation signalling in roots

Strigolactones (SLs) are important ex-planta signalling molecules in the rhizosphere, promoting the association with beneficial microorganisms, but also affecting plant interactions with harmful organisms. They are also plant hormones in-planta, acting as modulators of plant responses under nutrient-deficient conditions, mainly phosphate (Pi) starvation. In the present work, we investigate the potential role of SLs as regulators of early Pi starvation signalling in plants. A short-term pulse of the synthetic SL analogue 2′-epi-GR24 promoted SL accumulation and the expression of Pi starvation markers in tomato and wheat under Pi deprivation. 2′-epi-GR24 application also increased SL production and the expression of Pi starvation markers under normal Pi conditions, being its effect dependent on the endogenous SL levels. Remarkably, 2′-epi-GR24 also impacted the root metabolic profile under these conditions, promoting the levels of metabolites associated to plant responses to Pi limitation, thus partially mimicking the pattern observed under Pi deprivation. The results suggest an endogenous role for SLs as Pi starvation signals. In agreement with this idea, SL-deficient plants were less sensitive to this stress. Based on the results, we propose that SLs may act as early modulators of plant responses to P starvation.     

Strigolactones activate different hormonal pathways for regulation of root development in response to phosphate growth conditions

Background

Strigolactones (SLs) – a group of plant hormones and their derivatives – have been found to play a role in the regulation of root development, in addition to their role in suppression of lateral shoot branching: they alter root architecture and affect root-hair elongation, and SL signalling is necessary for the root response to low phosphate (Pi) conditions. These effects of SLs have been shown to be associated with differential activation of the auxin and ethylene signalling pathways.

Scope

The present review highlights recent findings on the activity of SLs as regulators of root development, in particular in response to low Pi stress, and discusses the different hormonal networks putatively acting with SLs in the root''s Pi response.

Conclusions

SLs are suggested to be key regulators of the adaptive responses to low Pi in the root by modulating the balance between auxin and ethylene signalling. Consequently, they impact different developmental programmes responsible for the changes in root system architecture under differential Pi supply.     

The role of strigolactones in root development

Strigolactones (SLs) and their derivatives were recently defined as novel phytohormones that orchestrate shoot and root growth. Levels of SLs, which are produced mainly by plant roots, increase under low nitrogen and phosphate levels to regulate plant responses. Here, we summarize recent work on SL biology by describing their role in the regulation of root development and hormonal crosstalk during root deve-lopment. SLs promote the elongation of seminal/primary roots and adventitious roots (ARs) and they repress lateral root formation. In addition, auxin signaling acts downstream of SLs. AR formation is positively or negatively regulated by SLs depending largely on the plant species and experimental conditions. The relationship between SLs and auxin during AR formation appears to be complex. Most notably, this hormonal response is a key adaption that radically alters rice root architecture in response to nitrogen- and phosphate-deficient conditions.     

Stereospecific reduction of the butenolide in strigolactones in plants

Reductive metabolism of strigolactones (SLs) in several plants was investigated. Analysis of aquaculture filtrates of cowpea and sorghum each fed with four stereoisomers of GR24, the most widely used synthetic SL, revealed stereospecific reduction of the double bond at C-3′ and C-4′ in the butenolide D-ring with preference for an unnatural 2′S configuration. The cowpea metabolite converted from 2′-epi-GR24 and the sorghum metabolite converted from ent-GR24 had the methyl group at C-4′ in the trans configuration with the substituent at C-2′, different from the cis configuration of the synthetic H₂-GR24 reduced with Pd/C catalyst. The plants also reduced the double bond in the D-ring of 5-deoxystrigol isomers with a similar preference. The metabolites and synthetic H₂-GR24 stereoisomers were much less active than were the GR24 stereoisomers in inducing seed germination of the root parasitic weeds Striga hermonthica, Orobanche crenata, and O. minor. These results provide additional evidence of the importance of the D-ring for bioactivity of SLs.     

Expression of <Emphasis Type="Italic">MAX2</Emphasis> under <Emphasis Type="Italic">SCARECROW</Emphasis> promoter enhances the strigolactone/MAX2 dependent response of <Emphasis Type="Italic">Arabidopsis</Emphasis> roots to low-phosphate conditions

Main conclusion

MAX2/strigolactone signaling in the endodermis and/or quiescent center of the root is partiallysufficient to exert changes in F-actin density and cellular trafficking in the root epidermis, and alter gene expression during plant response to low Pi conditions.Strigolactones (SLs) are a new group of plant hormones that regulate different developmental processes in the plant via MAX2, an F-box protein that interacts with their receptor. SLs and MAX2 are necessary for the marked increase in root-hair (RH) density in seedlings under conditions of phosphate (Pi) deprivation. This marked elevation was associated with an active reduction in actin-filament density and endosomal movement in root epidermal cells. Also, expression of MAX2 under the SCARECROW (SCR) promoter was sufficient to confer SL sensitivity in roots, suggesting that SL signaling pathways act through a root-specific, yet non-cell-autonomous regulatory mode of action. Here we show evidence for a non-cell autonomous signaling of SL/MAX2, originating from the root endodermis, and necessary for seedling response to conditions of Pi deprivation. SCR-derived expression of MAX2 in max2-1 mutant background promoted the root low Pi response, whereas supplementation of the synthetic SL GR24 to these SCR:MAX2 expressing lines further enhanced this response. Moreover, the SCR:MAX2 expression led to changes in actin density and endosome movement in epidermal cells and in TIR1 and PHO2 gene expression. These results demonstrate that MAX2 signaling in the endodermis and/or quiescent center is partially sufficient to exert changes in F-actin density and cellular trafficking in the epidermis, and alter gene expression under low Pi conditions.

     

Exogenous Jasmonic Acid and Cytokinin Antagonistically Regulate Rice Flag Leaf Senescence by Mediating Chlorophyll Degradation,Membrane Deterioration,and Senescence-Associated Genes Expression

Although it is well known that jasmonic acid (JA) and cytokinin (CK) are involved in regulating leaf senescence, the antagonistic mechanisms of JA and CK on leaf senescence are still unknown. To explore the antagonistic effects of JA and CK on leaf senescence, we treated detached rice flag leaves with JA and CK under dark conditions, and evaluated their chlorophyll contents, membrane deterioration, and expression levels of chlorophyll-degradation-related genes (CDRGs) and senescence-associated genes (SAGs). Our results demonstrated that exogenous application of JA promoted chlorophyll degradation by enhancing the expression levels of CDRGs, promoted membrane deterioration by accelerating the increases in lipid peroxidation and membrane permeability, enhanced the expression levels of SAGs, and consequently accelerated rice flag leaf senescence. On the other hand, exogenous application of CK retarded chlorophyll degradation by down-regulating the expression levels of CDRGs, retarded membrane deterioration by retarding the increases in lipid peroxidation and membrane permeability, down-regulated the expression levels of SAGs, and consequently delayed rice flag leaf senescence. Furthermore, the senescence-accelerating effect of a certain concentration of JA was nullified by the senescence-retarding effect of a certain concentration of CK. These results suggested that exogenous applications of JA and CK were able to antagonistically regulate flag leaf senescence by mediating chlorophyll degradation, membrane deterioration, and SAGs expression. In addition, our results suggested that the progression of flag leaf senescence might not only depend on the level of JA or CK but also depend on the balance between JA and CK.     

On the outside looking in: roles of endogenous and exogenous strigolactones

A collection of small molecules called strigolactones (SLs) act as both endogenous hormones to control plant development and as ecological communication cues between organisms. SL signalling overlaps with that of a class of smoke-derived compounds, karrikins (KARs), which have distinct yet overlapping developmental effects on plants. Although the roles of SLs in shoot and root development, in the promotion of arbuscular mycorrhizal (AM) fungal branching and in parasitic plant germination have been well characterized, recent data have illustrated broader roles for these compounds in the rhizosphere. Here, we review the known roles of SLs in development, growth of AM fungi and germination of parasitic plants to develop a framework for understanding the use of SLs as molecules of communication in the rhizosphere. It appears, for example, that there are many connections between SLs and phosphate utilization. Low phosphate levels regulate SL metabolism and, in turn, SLs sculpt root and shoot architecture to coordinate growth and optimize phosphate uptake from the environment. Plant-exuded SLs attract fungal symbionts to deliver inorganic phosphate (Pi) to the host. These and other examples suggest the boundary between exogenous and endogenous SL functions can be easily blurred and a more holistic view of these small molecules is likely to be required to fully understand SL biology. Related to this, we summarize and discuss evidence for a primitive role of SLs in moss as a quorum sensing-like molecule, providing a unifying concept of SLs as endogenous and exogenous signalling molecules.     

Actions of strigolactone GR24 and <Emphasis Type="Italic">DRM1</Emphasis> gene expression on Arabidopsis root architecture

Strigolactones are mostly known for their influence on apical dominance, but new insights suggest that they may be involved in many other biological events including root development. DRM1 gene is ubiquitary expressed in plants but its role is not well known. In our experiments, the strigolactone analogue GR24 stimulated the expression of DRM1.1, DRM1.2, DRM1.4 splicing variants and inhibited root branching in 5-day-old Arabidopsis thaliana (L.) Heynh. seedlings. On the other hand, the expression of these splicing variants was lower in 10-day-old GR24-treated roots. DRM1.6 gene expression differently responded to GR24 than other DRM1 splicing variants, however, there was no clear relationship between DRM1.6 expression and root length. Our results suggest that strigolactones and the expression of DRM1 gene play interactive roles in root branching.