Ethylene Controls Adventitious Root Initiation Sites in <Emphasis Type="Italic">Arabidopsis</Emphasis> Hypocotyls Independently of Strigolactones

Adventitious root formation is essential for cutting propagation of diverse species; however, until recently little was known about its regulation. Strigolactones and ethylene have both been shown to inhibit adventitious roots and it has been suggested that ethylene interacts with strigolactones in root hair elongation. We have investigated the interaction between strigolactones and ethylene in regulating adventitious root formation in intact seedlings of Arabidopsis thaliana. We used strigolactone mutants together with 1-aminocyclopropane-1-carboxylic acid (ACC) (ethylene precursor) treatments and ethylene mutants together with GR24 (strigolactone agonist) treatments. Importantly, we conducted a detailed mapping of adventitious root initiation along the hypocotyl and measured ethylene production in strigolactone mutants. ACC treatments resulted in a slight increase in adventitious root formation at low doses and a decrease at higher doses, in both wild-type and strigolactone mutants. Furthermore, the distribution of adventitious roots dramatically changed to the top third of the hypocotyl in a dose-dependent manner with ACC treatments in both wild-type and strigolactone mutants. The ethylene mutants all responded to treatments with GR24. Wild type and max4 (strigolactone-deficient mutant) produced the same amount of ethylene, while emanation from max2 (strigolactone-insensitive mutant) was lower. We conclude that strigolactones and ethylene act largely independently in regulating adventitious root formation with ethylene controlling the distribution of root initiation sites. This role for ethylene may have implications for flood response because both ethylene and adventitious root development are crucial for flood tolerance.     

Physiological Effects of the Synthetic Strigolactone Analog GR24 on Root System Architecture in Arabidopsis: Another Belowground Role for Strigolactones?

In this study, the role of the recently identified class of phytohormones, strigolactones, in shaping root architecture was addressed. Primary root lengths of strigolactone-deficient and -insensitive Arabidopsis (Arabidopsis thaliana) plants were shorter than those of wild-type plants. This was accompanied by a reduction in meristem cell number, which could be rescued by application of the synthetic strigolactone analog GR24 in all genotypes except in the strigolactone-insensitive mutant. Upon GR24 treatment, cells in the transition zone showed a gradual increase in cell length, resulting in a vague transition point and an increase in transition zone size. PIN1/3/7-green fluorescent protein intensities in provascular tissue of the primary root tip were decreased, whereas PIN3-green fluorescent protein intensity in the columella was not affected. During phosphate-sufficient conditions, GR24 application to the roots suppressed lateral root primordial development and lateral root forming potential, leading to a reduction in lateral root density. Moreover, auxin levels in leaf tissue were reduced. When auxin levels were increased by exogenous application of naphthylacetic acid, GR24 application had a stimulatory effect on lateral root development instead. Similarly, under phosphate-limiting conditions, endogenous strigolactones present in wild-type plants stimulated a more rapid outgrowth of lateral root primordia when compared with strigolactone-deficient mutants. These results suggest that strigolactones are able to modulate local auxin levels and that the net result of strigolactone action is dependent on the auxin status of the plant. We postulate that the tightly balanced auxin-strigolactone interaction is the basis for the mechanism of the regulation of the plants' root-to-shoot ratio.     

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.     

Biosynthetic considerations could assist the structure elucidation of host plant produced rhizosphere signalling compounds (strigolactones) for arbuscular mycorrhizal fungi and parasitic plants

Parasitic plants cause devastating losses to crop yields in several parts of the world. The root parasites, Striga and Orobanche species, use chemical signalling molecules that are exuded by the roots of plants in extremely low concentrations, and that can induce germination of the seeds of these parasites, to detect the vicinity of a suitable host. The majority of the so far identified germination stimulants belong to the strigolactones. It was recently discovered that this class of compounds can also induce hyphal branching in the symbiotic arbuscular mycorrhizal fungi, a process involved in root colonisation. The elucidation of the structure of new strigolactones is hindered by their low abundance and instability. In the present paper, we have used existing knowledge on the structure of strigolactones and combined it with recently obtained insight in the biosynthetic origin of these signalling compounds. This enabled us to postulate structures for strigolactones that have been isolated but for which so far the structure has not been elucidated, but also to propose structures of strigolactones that may be discovered in the future. Considering the strongly increased importance of the strigolactones, we expect that more groups will look for these compounds and also in systems so far not exploited. This could lead to the discovery of new strigolactones for which we expect the present biogenetic considerations will facilitate identification and structure elucidation.     

The potential roles of strigolactones and brassinosteroids in the autoregulation of nodulation pathway

Background and Aims

The number of nodules formed on a legume root system is under the strict genetic control of the autoregulation of nodulation (AON) pathway. Plant hormones are thought to play a role in AON; however, the involvement of two hormones recently described as having a largely positive role in nodulation, strigolactones and brassinosteroids, has not been examined in the AON process.

Methods

A genetic approach was used to examine if strigolactones or brassinosteroids interact with the AON system in pea (Pisum sativum). Double mutants between shoot-acting (Psclv2, Psnark) and root-acting (Psrdn1) mutants of the AON pathway and strigolactone-deficient (Psccd8) or brassinosteroid-deficient (lk) mutants were generated and assessed for various aspects of nodulation. Strigolactone production by AON mutant roots was also investigated.

Key Results

Supernodulation of the roots was observed in both brassinosteroid- and strigolactone-deficient AON double-mutant plants. This is despite the fact that the shoots of these plants displayed classic strigolactone-deficient (increased shoot branching) or brassinosteroid-deficient (extreme dwarf) phenotypes. No consistent effect of disruption of the AON pathway on strigolactone production was found, but root-acting Psrdn1 mutants did produce significantly more strigolactones.

Conclusions

No evidence was found that strigolactones or brassinosteroids act downstream of the AON genes examined. While in pea the AON mutants are epistatic to brassinosteroid and strigolactone synthesis genes, we argue that these hormones are likely to act independently of the AON system, having a role in the promotion of nodule formation.     

Genetic variation in strigolactone production and tillering in rice and its effect on Striga hermonthica infection

Tillering in cereals is a complex process in the regulation of which also signals from the roots in the form of strigolactones play an important role. The strigolactones are signalling molecules that are secreted into the rhizosphere where they act as germination stimulants for root parasitic plants and hyphal branching factors for arbuscular mycorrhizal fungi. On the other hand, they are also transported from the roots to the shoot where they inhibit tillering or branching. In the present study, the genetic variation in strigolactone production and tillering phenotype was studied in twenty rice varieties collected from all over the world and correlated with S. hermonthica infection. Rice cultivars like IAC 165, IAC 1246, Gangweondo and Kinko produced high amounts of the strigolactones orobanchol, 2′-epi-5-deoxystrigol and three methoxy-5-deoxystrigol isomers and displayed low amounts of tillers. These varieties induced high S. hermonthica germination, attachment, emergence as well as dry biomass. In contrast, rice cultivars such as Super Basmati, TN 1, Anakila and Agee displayed high tillering in combination with low production of the aforementioned strigolactones. These varieties induced only low S. hermonthica germination, attachment, emergence and dry biomass. Statistical analysis across all the varieties confirmed a positive correlation between strigolactone production and S. hermonthica infection and a negative relationship with tillering. These results show that genetic variation in tillering capacity is the result of genetic variation in strigolactone production and hence could be a helpful tool in selecting rice cultivars that are less susceptible to S. hermonthica infection.     

Strigolactones promote nodulation in pea

Strigolactones are recently defined plant hormones with roles in mycorrhizal symbiosis and shoot and root architecture. Their potential role in controlling nodulation, the related symbiosis between legumes and Rhizobium bacteria, was explored using the strigolactone-deficient rms1 mutant in pea (Pisum sativum L.). This work indicates that endogenous strigolactones are positive regulators of nodulation in pea, required for optimal nodule number but not for nodule formation per se. rms1 mutant root exudates and root tissue are almost completely deficient in strigolactones, and rms1 mutant plants have approximately 40% fewer nodules than wild-type plants. Treatment with the synthetic strigolactone GR24 elevated nodule number in wild-type pea plants and also elevated nodule number in rms1 mutant plants to a level similar to that seen in untreated wild-type plants. Grafting studies revealed that nodule number and strigolactone levels in root tissue of rms1 roots were unaffected by grafting to wild-type scions indicating that strigolactones in the root, but not shoot-derived factors, regulate nodule number and provide the first direct evidence that the shoot does not make a major contribution to root strigolactone levels.     

Phosphorus deficiency in red clover promotes exudation of orobanchol,the signal for mycorrhizal symbionts and germination stimulant for root parasites

Plant derived sesquiterpene strigolactones, which have previously been characterized as germination stimulants for root parasitic plants, have recently been identified as the branching factors which induce hyphal branching morphogenesis, a critical step in host recognition by arbuscular mycorrhizal (AM) fungi. We show here that, in red clover plants (Trifolium pratense L.), which is known as a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Mg, Ca) in the culture medium significantly promotes the release of a strigolactone, orobanchol, by the roots of this plant. In red clover plants, the level of orobanchol exudation appeared to be regulated by P availability and was in good agreement with germination stimulation activity of the root exudates. This implies that under P deficiency, plant roots attract not only symbiotic fungi but also root parasitic plants through the release of strigolactones. This is the first report demonstrating that nutrient availability influences both symbiotic and parasitic interactions in the rhizosphere.     

Arbuscular mycorrhiza: the mother of plant root endosymbioses

Arbuscular mycorrhiza (AM), a symbiosis between plants and members of an ancient phylum of fungi, the Glomeromycota, improves the supply of water and nutrients, such as phosphate and nitrogen, to the host plant. In return, up to 20% of plant-fixed carbon is transferred to the fungus. Nutrient transport occurs through symbiotic structures inside plant root cells known as arbuscules. AM development is accompanied by an exchange of signalling molecules between the symbionts. A novel class of plant hormones known as strigolactones are exuded by the plant roots. On the one hand, strigolactones stimulate fungal metabolism and branching. On the other hand, they also trigger seed germination of parasitic plants. Fungi release signalling molecules, in the form of 'Myc factors' that trigger symbiotic root responses. Plant genes required for AM development have been characterized. During evolution, the genetic programme for AM has been recruited for other plant root symbioses: functional adaptation of a plant receptor kinase that is essential for AM symbiosis paved the way for nitrogen-fixing bacteria to form intracellular symbioses with plant cells.     

Strigolactones suppress adventitious rooting in Arabidopsis and pea

Adventitious root formation is essential for the propagation of many commercially important plant species and involves the formation of roots from nonroot tissues such as stems or leaves. Here, we demonstrate that the plant hormone strigolactone suppresses adventitious root formation in Arabidopsis (Arabidopsis thaliana) and pea (Pisum sativum). Strigolactone-deficient and response mutants of both species have enhanced adventitious rooting. CYCLIN B1 expression, an early marker for the initiation of adventitious root primordia in Arabidopsis, is enhanced in more axillary growth2 (max2), a strigolactone response mutant, suggesting that strigolactones restrain the number of adventitious roots by inhibiting the very first formative divisions of the founder cells. Strigolactones and cytokinins appear to act independently to suppress adventitious rooting, as cytokinin mutants are strigolactone responsive and strigolactone mutants are cytokinin responsive. In contrast, the interaction between the strigolactone and auxin signaling pathways in regulating adventitious rooting appears to be more complex. Strigolactone can at least partially revert the stimulatory effect of auxin on adventitious rooting, and auxin can further increase the number of adventitious roots in max mutants. We present a model depicting the interaction of strigolactones, cytokinins, and auxin in regulating adventitious root formation.     

The microRNA miR171h modulates arbuscular mycorrhizal colonization of Medicago truncatula by targeting NSP2

Most land plants live symbiotically with arbuscular mycorrhizal fungi. Establishment of this symbiosis requires signals produced by both partners: strigolactones in root exudates stimulate pre‐symbiotic growth of the fungus, which releases lipochito‐oligosaccharides (Myc‐LCOs) that prepare the plant for symbiosis. Here, we have investigated the events downstream of this early signaling in the roots. We report that expression of miR171h, a microRNA that targets NSP2, is up‐regulated in the elongation zone of the root during colonization by Rhizophagus irregularis (formerly Glomus intraradices) and in response to Myc‐LCOs. Fungal colonization was much reduced by over‐expressing miR171h in roots, mimicking the phenotype of nsp2 mutants. Conversely, in plants expressing an NSP2 mRNA resistant to miR171h cleavage, fungal colonization was much increased and extended into the elongation zone of the roots. Finally, phylogenetic analyses revealed that miR171h regulation of NSP2 is probably conserved among mycotrophic plants. Our findings suggest a regulatory mechanism, triggered by Myc‐LCOs, that prevents over‐colonization of roots by arbuscular mycorrhizal fungi by a mechanism involving miRNA‐mediated negative regulation of NSP2.     

Gene for a protein capable of enhancing lateral root formation.

Analysis of genes preferentially expressed in hairy roots caused by infection with Agrobacterium rhizogenes has provided insights into the regulation of lateral root formation. A hairy root preferential cDNA, HR7, has been cloned from hairy roots of Hyoscyamus niger. HR7 encodes a novel protein partially homologous to a metallocarboxypeptidase inhibitor and is expressed exclusively in the primordium and base of lateral roots in hairy roots. Overexpression of HR7 in transgenic roots of H. niger dramatically enhances the frequency of lateral root formation. The results of this study indicate that expression of HR7 plays a critical role in initiating lateral root formation.     

Molecular mechanism of adventitious root formation in rice

Adventitious roots account for the majority of the rice root system and play an irreplaceable role in rice growth and development. Rice adventitious roots are formed by division of the innermost ground meristem cells in the central cylinder, and some lateral roots are observable in the adventitious root system. Multiple hormones have been implicated in the regulation of root development. Auxin is involved in the initiation of adventitious roots, whereas cytokinin inhibits adventitious root initiation, but promotes adventitious root elongation. Other phytohormones such as nitric oxide, ethylene, brassinosteroid, jasmonic acid and gibberellin may be also involved in regulating adventitious root initiation and development. Additionally, more than 600 root development related quantitative trait loci (QTLs) have been located by QTL analysis of root traits.     

Fast moves in arbuscular mycorrhizal symbiotic signalling

Exciting research looking at early events in arbuscular mycorrhizal symbioses has shown how the fungus and plant get together. Kohki Akiyama et al. have demonstrated that strigolactones in root exudates are fungal germ tube branching factors, and Arnaud Besserer et al. found that these compounds rapidly induce fungal mitochondrial activity. Andrea Genre et al. have shown that subsequent development of appressoria on host roots induces construction of a transient prepenetration apparatus inside epidermal cells that is reminiscent of nodulation infection.     

Do Strigolactones Regulate Bud Winter Dormancy and Charactrisitc Secondary Metabolism in Tea?

Tea (Camellia sinensis [L.] O. Kuntze.) is an important cash crop, which mainly uses tender shoots and young leaves for manufacturing. Due to the marketing characteristic that earlier made tea has higher price, the time of the breaking of winter dormancy buds in spring is extremely important in tea industry. Strigolactones are a group of carotenoids-derived metabolites which regulates bud outgrowth, shoot branching, tiller angle and environmental stress responses. The role of strigolactones in tea plant was briefly summarized in the current review, with an emphasis of the association of strigolactones on bud ecodormancy and shoot branching. The involvement of strigolactones on the biosynthesis of the tea characteristic metabolites flavonoids, caffeine and theanine were also discussed. Moreover, recent advances on the biosynthesis of strigolactones and its regulation by microRNAs and environmental stresses were also presented. This review provides a basis for future investigations underlying the mechanisms of strigolactones on bud winter dormancy and tea secondary metabolism.     

Strigolactones, signals for parasitic plants and arbuscular mycorrhizal fungi

Although strigolactones play a critical role as rhizospheric signaling molecules for the establishment of arbuscular mycorrhizal (AM) symbiosis and for seed germination of parasitic weeds, scarce data are available about interactions between AM fungi and strigolactones. In the present work, we present background data on strigolactones from studies on their seed germination activity on the parasitic weeds Orobanche and Striga, the importance of nitrogen and phosphorus for this seed germination activity, and what this could mean for AM fungi. We also present results on the susceptibility of plants to AM fungi and the possible involvement of strigolactones in this AM susceptibility and discuss the role of strigolactones for the formation and the regulation of the AM symbiosis as well as the possible implication of these compounds as plant signals in other soil-borne plant–microbe interactions.     

Do strigolactones contribute to plant defence?

Strigolactones are multifunctional molecules involved in several processes outside and within the plant. As signalling molecules in the rhizosphere, they favour the establishment of arbuscular mycorrhizal symbiosis, but they also act as host detection cues for root parasitic plants. As phytohormones, they are involved in the regulation of plant architecture, adventitious rooting, secondary growth and reproductive development, and novel roles are emerging continuously. In the present study, the possible involvement of strigolactones in plant defence responses was investigated. For this purpose, the resistance/susceptibility of the strigolactone‐deficient tomato mutant Slccd8 against the foliar fungal pathogens Botrytis cinerea and Alternaria alternata was assessed. Slccd8 was more susceptible to both pathogens, pointing to a new role for strigolactones in plant defence. A reduction in the content of the defence‐related hormones jasmonic acid, salicylic acid and abscisic acid was detected by high‐performance liquid chromatography coupled to tandem mass spectrometry in the Slccd8 mutant, suggesting that hormone homeostasis is altered in the mutant. Moreover, the expression level of the jasmonate‐dependent gene PinII, involved in the resistance of tomato to B. cinerea, was lower than in the corresponding wild‐type. We propose here that strigolactones play a role in the regulation of plant defences through their interaction with other defence‐related hormones, especially with the jasmonic acid signalling pathway.     

Regulation of Root Growth by Plant Hormones—Roles for Auxin and Gibberellin

Plant hormones are important biotic factors to regulate root growth. Among the seven kinds of plant hormones, auxin and gibberellin (GA) are strong accelerators of shoot growth, but these are not always accelerators for root growth. The classical views of root-growth regulation by auxin and gibberellin are summarized and current theory of the regulation mechanism is described in this review. The concentration-dependent deceleration of root growth is a key to understanding the auxin action on roots, since the endogenous concentration of indole-3-acetic acid (IAA) is inversely proportional to the growth rate. As massive IAA is transported from shoots to roots by polar transport, the influx speed of IAA mainly controls IAA levels in root cells. The classical view of IAA transport in roots has been supported by recent discoveries of IAA-carrier proteins such as AUX1, PINs and MDRs. The role of plasma membrane-located H⁺-ATPase and its regulation by IAA has also been described for the acid growth phenomenon caused by the acidification of root cell walls.

Compared to auxins, GA functions in roots are less remarkable. Nevertheless, GA also plays an indispensable role in the normal development of roots, since artificial GA-depletion causes abnormal expansion and suppression of root elongation. The GA-requirement for normal root growth was unveiled by the use of chemical inhibitors and mutants of GA biosynthesis. GA function that keeps root morphology long and slender is ascribed to the arrangement of cortical microtubules, cellulose microfibrils and unknown additional factor(s). Cross talks among plant hormones were recently found in the signal transduction pathways mainly in aerial organs. GA and IAA de-repress gene expression by degrading the gene-repressing proteins via the ubiquitin-mediated proteasome system. Another interaction of IAA and GA in growth regulation is the enhancement of GA₁ level by IAA. Since the final biochemical steps of growth regulation take place in cell walls, possible cross talks are also conceivable in cell wall formation and modification.     

Regulation of root morphogenesis in arbuscular mycorrhizae: what role do fungal exudates,phosphate, sugars and hormones play in lateral root formation?

Background

Arbuscular mycorrhizae (AMs) form a widespread root–fungus symbiosis that improves plant phosphate (Pi) acquisition and modifies the physiology and development of host plants. Increased branching is recognized as a general feature of AM roots, and has been interpreted as a means of increasing suitable sites for colonization. Fungal exudates, which are involved in the dialogue between AM fungi and their host during the pre-colonization phase, play a well-documented role in lateral root (LR) formation. In addition, the increased Pi content of AM plants, in relation to Pi-starved controls, as well as changes in the delivery of carbohydrates to the roots and modulation of phytohormone concentration, transport and sensitivity, are probably involved in increasing root system branching.

Scope

This review discusses the possible causes of increased branching in AM plants. The differential root responses to Pi, sugars and hormones of potential AM host species are also highlighted and discussed in comparison with those of the non-host Arabidopsis thaliana.

Conclusions

Fungal exudates are probably the main compounds regulating AM root morphogenesis during the first colonization steps, while a complex network of interactions governs root development in established AMs. Colonization and high Pi act synergistically to increase root branching, and sugar transport towards the arbusculated cells may contribute to LR formation. In addition, AM colonization and high Pi generally increase auxin and cytokinin and decrease ethylene and strigolactone levels. With the exception of cytokinins, which seem to regulate mainly the root:shoot biomass ratio, these hormones play a leading role in governing root morphogenesis, with strigolactones and ethylene blocking LR formation in the non-colonized, Pi-starved plants, and auxin inducing them in colonized plants, or in plants grown under high Pi conditions.