Strigolactones interact with other phytohormones to modulate plant root growth and development

Strigolactones(SLs),which are biosynthesized mainly in roots,modulate various aspects of plant growth and development.Here,we review recent research on the role of SLs and their cross-regulation with auxin,cytokinin,and ethylene in the modulation of root growth and development.Under nutrientsufficient conditions,SLs regulate the elongation of primary roots and inhibit adventitious root formation in eudicot plants.SLs promote the elongation of seminal roots and increase the number of adventitious roots in grass plants in the short term,while inhibiting lateral root development in both grass and eudicot plants.The effects of SLs on the elongation of root hairs are variable and depend on plant species,growth conditions,and SL concentration.Nitrogen or phosphate deficiency induces the accumulation of endogenous SLs,modulates root growth and development.Genetic analyses indicate cross-regulation of SLs with auxin,cytokinin,and ethylene in regulation of root growth and development.We discuss the implications of these studies and consider their potential for exploiting the components of SL signaling for the design of crop plants with more efficient soil-resource utilization.

Exogenous strigolactones promote lateral root growth by reducing the endogenous auxin level in rapeseed

Strigolactones(SLs)are newly discovered plant hormones which regulate the normal development of different plant organs,especially root architecture.Lateral root formation of rapeseed seedlings before winter has great effects on the plant growth and seed yield.Here,we treated the seedlings of Zhongshuang 11(ZS11),an elite conventional rapeseed cultivar,with different concentrations of GR24(a synthetic analogue of strigolactones),and found that a low concentration(0.18μmol L–1)of GR24 could significantly increase the lateral root growth,shoot growth,and root/shoot ratio of seedlings.RNA-Seq analysis of lateral roots at 12 h,1 d,4 d,and 7 d after GR24 treatment showed that 2301,4626,1595,and 783 genes were significantly differentially expressed,respectively.Function enrichment analysis revealed that the plant hormone transduction pathway,tryptophan metabolism,and the phenylpropanoid biosynthesis pathway were over-represented.Moreover,transcription factors,including AP2/ERF,AUX/IAA,NAC,MYB,and WRKY,were up-regulated at 1 d after GR24 treatment.Metabolomics profiling further demonstrated that the amounts of various metabolites,such as indole-3-acetic acid(IAA)and cis-zeatin were drastically altered.In particular,the concentrations of endogenous IAA significantly decreased by 52.4 and 75.8%at 12 h and 1 d after GR24 treatment,respectively.Our study indicated that low concentrations of exogenous SLs could promote the lateral root growth of rapeseed through interaction with other phytohormones,which provides useful clues for the effects of SLs on root architecture and crop productivity.

Transcriptomic and metabolomic analyses reveal that exogenous strigolactones alleviate the response of melon root to cadmium stress

Strigolactones(SLs)are classified into plant hormones,playing a key role as a mediator of plant growth in response to several abiotic stresses.Cadmium(Cd),a common heavy metal and soil pollutant,can suppress plant growth and development.In this work,we explored the effects of exogenous SLs on root formation in response to Cd stress using melon seeds subjected to seven germination treatments:CK(control),Cd(300 μmol·L^(-1) CdCl_(2)),and SL1-SL5(CdCl_(2)-stressed seeds pretreated with 0.1,0.5,1,2,and 3 μmol·L^(-1) GR24 solutions).The results indicated that SLs increased the antioxidant enzyme activities and root vigor and decreased the malondialdehyde(MDA)contents in the roots of Cdstressed melon seedlings.Then we used transcriptomic and metabolomic analyses to explore the mechanisms by which exogenous SLs protect against Cd stress.There were 242 significant differentially expressed genes(DEGs)(78 upregulated,164 downregulated)and 247 significantly differentially expressed metabolites(DEMs)(222 upregulated,25 downregulated)between the Cd and SL3 treatments.SLs altered the expression of genes related to redox formation processes,including peroxidase(POD),lipoxygenase(LOX),glutamate dehydrogenase(GDH),and glutathione S-transferase(GST).In addition,we found that SLs regulated the expression of the MYB,AP2/ERF,bHLH,and WRKY transcription factor families.The combined transcriptomic and metabolomic analyses revealed that the DEGs and DEMs involved in Cd stress alleviation were mainly related to the gene expression of jasmonic acid(JA)and flavonoid biosynthesis.SLs might induce LOX-related genes to regulate JA biosynthesis.Moreover,SLs might promote flavonoid biosynthesis by regulating eleven flavonoid-related genes and eight metabolites.The results provide a new perspective for studying the adaptation of plants to Cd stress.

Functional identification of MdSMXL8.2,the homologous gene of strigolactones pathway repressor protein gene in Malus×domestica

A homologous gene of strigolactones repressor protein gene SMXL7/D53,MdSMXL8.2(GenBank accession No.:MD07G1222400),was cloned from‘Royal Gala’apple(Malus×domestica Borkh.)in this study.The sequence analysis revealed that the length of this gene was 3243 bp,which encoded 1080 amino acids,and had a protein molecular mass of∼110 kD.The phylogenetic tree analysis indicated that the MdSMXL8.2 exhibited the highest sequence similarity with Arabidopsis AtSMXL7.The protein conserved domain analysis revealed that the MdSMXL8.2 contained two ClpA domains.The prediction of the secondary and tertiary structures of the MdSMXL8.2 indicated that it contained 34.54%αhelix,3.43%β-sheet,and 11.76%extended chain.The in-silico analysis suggested that the promoter sequence of MdSMXL8.2 contained several typical cisacting elements,including abscisic acid(ABA),gibberellin(GA),ethylene,auxin,jasmonic acid(JA),salicylic acid(SA),drought,and heat stressresponsive elements.Quantitative real-time(qRT)-PCR analyses revealed that MdSMXL8.2 was expressed in different apple tissues,with the highest transcript level found in the stem.The expression of MdSMXL8.2 was significantly induced by exogenous ABA,PEG and mannitol,while exogenous NaCl significantly inhibited MdSMXL8.2 expression.The growing status of MdSMXL8.2-overexpressed Orin apple callus was worse than the wild type(WT)after NaCl treatment and had a higher malondialdehyde(MDA)content and relative conductance(REC).Additionally,MdSMXL8.2-overexpressed Arabidopsis exhibited shorter root length and a reduction in fresh weight under salt stress,indicating that MdSMXL8.2 negatively regulated salt tolerance in apples.

Parasitic Plants <i>Striga</i>and <i>Phelipanche</i>Dependent upon Exogenous Strigolactones for Germination Have Retained Genes for Strigolactone Biosynthesis

Strigolactones are plant hormones with multiple functions, including regulating various aspects of plant architecture such as shoot branching, facilitating the colonization of plant roots by arbuscular mycorrhizal fungi, and acting as seed germination stimulants for certain parasitic plants of the family Orobanchaceae. The obligate parasitic species Phelipanche aegyptiaca and Striga hermonthica require strigolactones for germination, while the facultative parasite Triphysaria versicolor does not. It has been hypothesized that P. aegyptiaca and S. hermonthica would have undergone evolutionary loss of strigolactone biosynthesis as a part of their mechanism to enable specific detection of exogenous strigolactones. We analyzed the transcriptomes of P. aegyptiaca, S. hermonthica and T. versicolor and identified genes known to act in strigolactone synthesis (D27, CCD7, CCD8, and MAX1), perception (MAX2 and D14) and transport (PDR12). These genes were then analyzed to assess likelihood of function. Transcripts of all strigolactone-related genes were found in P. aegyptiaca and S. hermonthica, and evidence points to their encoding functional proteins. Gene open reading frames were consistent with homologs from Arabidopsis and other strigolactone-producing plants, and all genes were expressed in parasite tissues. In general, the genes related to strigolactone synthesis and perception appeared to be evolving under codon-based selective constraints in strigolactone-dependent species. Bioassays of S. hermonthica root extracts indicated the presence of strigolactone class stimulants on germination of P. aegyptiaca seeds. Taken together, these results indicate that Phelipanche aegyptiaca and S. hermonthica have retained functional genes involved in strigolactone biosynthesis, suggesting that the parasites use both endogenous and exogenous strigolactones and have mechanisms to differentiate the two.

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 modulate cotton fiber elongation and secondary cell wall thickening

Cotton is one of the most important economic crops in the world,and it is a major source of fiber in the textile industry.Strigolactones(SLs)are a class of carotenoid-derived plant hormones involved in many processes of plant growth and development,although the functions of SL in fiber development remain largely unknown.Here,we found that the endogenous SLs were significantly higher in fibers at 20 days post-anthesis(DPA).Exogenous SLs significantly increased fiber length and cell wall thickness.Furthermore,we cloned three key SL biosynthetic genes,namely GhD27,GhMAX3,and GhMAX4,which were highly expressed in fibers,and subcellular localization analyses revealed that GhD27,GhMAX3,and GhMAX4 were localized in the chloroplast.The exogenous expression of GhD27,GhMAX3,and GhMAX4 complemented the physiological phenotypes of d27,max3,and max4 mutations in Arabidopsis,respectively.Knockdown of GhD27,GhMAX3,and GhMAX4 in cotton resulted in increased numbers of axillary buds and leaves,reduced fiber length,and significantly reduced fiber thickness.These findings revealed that SLs participate in plant growth,fiber elongation,and secondary cell wall formation in cotton.These results provide new and effective genetic resources for improving cotton fiber yield and plant architecture.

Low phosphorus promotes NSP1–NSP2 heterodimerization to enhance strigolactone biosynthesis and regulate shoot and root architecture in rice

Phosphorus is an essential macronutrient for plant development and metabolism,and plants have evolved ingenious mechanisms to overcome phosphate(Pi)starvation.However,the molecular mechanisms underlying the regulation of shoot and root architecture by low phosphorus conditions and the coordinated utilization of Pi and nitrogen remain largely unclear.Here,we show that Nodulation Signaling Pathway 1(NSP1)and NSP2 regulate rice tiller number by promoting the biosynthesis of strigolactones(SLs),a class of phytohormones with fundamental effects on plant architecture and environmental responses.We found that NSP1 and NSP2 are induced by Oryza sativa PHOSPHATE STARVATION RESPONSE2(OsPHR2)in response to low-Pi stress and form a complex to directly bind the promoters of SL biosynthesis genes,thus markedly increasing SL biosynthesis in rice.Interestingly,the NSP1/2–SL signaling module represses the expression of CROWN ROOTLESS 1(CRL1),a newly identified early SL-responsive gene in roots,to restrain lateral root density under Pi deficiency.We also demonstrated that GR24^(4DO) treatment under normal conditions inhibits the expression of OsNRTs and OsAMTs to suppress nitrogen absorption but enhances the expression of OsPTs to promote Pi absorption,thus facilitating the balance between nitrogen and phosphorus uptake in rice.Importantly,we found that NSP1p:NSP1 and NSP2p:NSP2 transgenic plants show improved agronomic traits and grain yield under low-and medium-phosphorus conditions.Taken together,these results revealed a novel regulatory mechanism of SL biosynthesis and signaling in response to Pi starvation,providing genetic resources for improving plant architecture and nutrient-use efficiency in low-Pi environments.

Strigolactone and gibberellin signaling coordinately regulate metabolic adaptations to changes in nitrogen availability in rice

Modern semi-dwarf rice varieties of the“Green Revolution”require a high supply of nitrogen(N)fertilizer to produce high yields.A better understanding of the interplay between N metabolism and plant developmental processes is required for improved N-use efficiency and agricultural sustainability.Here,we show that strigolactones(SLs)modulate root metabolic and developmental adaptations to low N availability for ensuring efficient uptake and translocation of available N.The key repressor DWARF 53(D53)of the SL signaling pathway interacts with the transcription factor GROWTH-REGULATING FACTOR 4(GRF4)and prevents GRF4 from binding to its target gene promoters.N limitation induces the accumulation of SLs,which in turn promotes SL-mediated degradation of D53,leading to the release of GRF4 and thus promoting the expression of genes associated with N metabolism.N limitation also induces degradation of the DELLA protein SLENDER RICE 1(SLR1)in an D14-and D53-dependent manner,effectively releasing GRF4 from competitive inhibition caused by SLR1.Collectively,our findings reveal a previously unrecognized mechanism underlying SL and gibberellin crosstalk in response to N availability,advancing our understanding of plant growth–metabolic coordination and facilitating the design of the strategies for improving N-use efficiency in high-yield crops.

AtMYBS1 negatively regulates heat tolerance by directly repressing the expression of MAX1 required for strigolactone biosynthesis in Arabidopsis

Heat stress caused by global warming requires the development of thermotolerant crops to sustain yield.It is necessary to understand the molecular mechanisms that underlie heat tolerance in plants.Strigolactones(SLs)are a class of carotenoid-derived phytohormones that regulate plant development and responses to abiotic or biotic stresses.Although SL biosynthesis and signaling processes are well established,genes that directly regulate SL biosynthesis have rarely been reported.Here,we report that the MYB-like transcription factor AtMYBS1/AtMYBL,whose gene expression is repressed by heat stress,functions as a negative regulator of heat tolerance by directly inhibiting SL biosynthesis in Arabidopsis.Overexpression of AtMYBS1 led to heat hypersensitivity,whereas atmybs1 mutants displayed increased heat tolerance.Expression of MAX1,a critical enzyme in SL biosynthesis,was induced by heat stress and downregulated in AtMYBS1-overexpression(OE)plants but upregulated in atmybs1 mutants.Overexpression of MAX1 in the AtMYBS1-OE background reversed the heat hypersensitivity of AtMYBS1-OE plants.Loss of MAX1 function in the atmyb1 background reversed the heat-tolerant phenotypes of atmyb1 mutants.Yeast one-hybrid assays,chromatin immunoprecipitation‒qPCR,and transgenic analyses demonstrated that AtMYBS1 directly represses MAX1 expression through the MYB binding site in the MAX1 promoter in vivo.The atmybs1d14 double mutant,like d14 mutants,exhibited hypersensitivity to heat stress,indicating the necessary role of SL signaling in AtMYBS1-regulated heat tolerance.Our findings provide new insights into the regulatory network of SL biosynthesis,facilitating the breeding of heat-tolerant crops to improve crop production in a warming world.

受多粗毛的茎枝内酯类多物质(Strigolactones)调控的植物侧枝生长

本文介绍受Strigolactones调控的植物侧枝生长的信号合成、信号转导机制以及它与生长素和细胞分裂素之间相互作用的研究进展。

Diverse Roles of Strigolactones in Plant Development

With the discovery of strigolactones as root exudate signals that trigger parasitic weed seed germination, and then as a branching inhibitor and plant hormone, the next phase of strigolactone research has quickly revealed this hormone class as a major player in optimizing plant growth and development. From the early stages of plant evolution, it seems that strigolactones were involved in enabling plants to modify growth in order to gain advantage in competi- tion with neighboring organisms for limited resources. For example, a moss plant can alter its growth in response to strigolactones emanating from a neighbor. Within a higher plant, strigolactones appear to be involved in controlling the balance of resource distribution via strategic modification of growth and development. Most notably, higher plants that encounter phosphate deficiency increase strigolactone production, which changes root growth and promotes fungal symbiosis to enhance phosphate intake. The shoot also changes by channeling resources away from unessential leaves and branches and into the main stem and root system. This hormonal response is a key adaption that radically alters whole-plant architecture in order to optimize growth and development under diverse environmental conditions.

Strigolactones are a new-defined class of plant hormones which inhibit shoot branching and mediate the interaction of plant-AM fungi and plant-parasitic weeds

Because plants are sessile organisms,the ability to adapt to a wide range of environmental conditions is critical for their survival.As a consequence,plants use hormones to regulate growth,mitigate biotic and abiotic stresses,and to communicate with other organisms.Many plant hormones function plei-otropically in vivo,and often work in tandem with other hormones that are chemically distinct.A newly-defined class of plant hormones,the strigolactones,cooperate with auxins and cytokinins to control shoot branching and the outgrowth of lateral buds.Strigolactones were originally identified as compounds that stimulated the germination of parasitic plant seeds,and were also demonstrated to induce hyphal branching in arbuscular mycorrhizal(AM) fungi.AM fungi form symbioses with higher plant roots and mainly facilitate the absorption of phosphate from the soil.Conforming to the classical definition of a plant hormone,strigolactones are produced in the roots and translocated to the shoots where they inhibit shoot outgrowth and branching.The biosynthesis of this class of compounds is regulated by soil nutrient availability,i.e.the plant will increase its production of strigolactones when the soil phosphate concentration is limited,and decrease production when phosphates are in ample supply.Strigolactones that affect plant shoot branching,AM fungal hyphal branching,and seed germination in parasitic plants facilitate chemical synthesis of similar compounds to control these and other biological processes by exogenous application.

Strigolactones and Brassinosteroids Antagonistically Regulate the Stability of the D53-OsBZR1 Complex to Determine FC1 Expression in Rice Tillering

Rice tillering,a key architecture trait determ ining grain yield,is highly regulated by a class of newly identified phytohorm ones,strigolactones(SLs).How ever,the whole SL signaling pathw ay from the receptor to dow nstream transcription factors to finally inhibit tillering remains unrevealed.In this study,we first found that brassinosteroids(BRs)strongly enhance tillering by prom oting bud outgrow th in rice,which is largely different from the function of BRs in Arabidopsis.Genetic and biochem ical analyses indicated that both the SL and BR signaling pathw ays control rice tillering by regulating the stability of D53 and/or the OsBZR1 RLA1-DLT module,a transcriptional complex in the rice BR signaling pathway.We further found that D53 interacts with OsBZR1 to inhibit the expression of FC1,a local inhibitor of tillering,and that this inhibition depends on direct DNA binding by OsBZR1,which recruits D53 to the FC1 promoter in rice buds.Taken together,these findings uncover a mechanism illustrating how SLs and BRs coordinately regulate rice tillering via the early responsive gene FC1.

Isolation of a Novel Lodging Resistance QTL Gene Involved in Strigolactone Signaling and Its Pyramiding with a QTL Gene Involved in Another Mechanism

Lodging has been a major roadblock to attaining increased crop productivity. In an attempt to understand the mechanism for culm strength in rice, we isolated an effective quantitative trait locus （QTL）, STRONG CULM3 （SCM3）, the causal gene of which is identical to rice TEOSINTE BRANCHED1 （OsTB1）, a gene previously reported to positively control strigolactone （SL） signaling. A near-isogenic line （NIL） carrying SCM3 showed enhanced culm strength and increased spikelet number despite the expected decrease in tiller number, indicating that SL also has a positive role in enhancing culm strength and spikelet number. We produced a pyramiding line carrying SCM3 and SCM2, another QTL encoding AP01 involved in panicle development. The NIL-SCM2＋SCM3 showed a much stronger culm than NIL-SCM2 and NIL-SCM3 and an increased spikelet number caused by the additive effect of these QTLs. We discuss the importance of utilizing suitable alleles of these STRONG CULM QTLs without inducing detrimental traits for breeding.

A Strigolactone Biosynthesis Gene Contributed to the Green Revolution in Rice

Plant architecture is a complex agronomic trait and a major factor of crop yield,which is affected by several important hormones.Strigolactones(SLs)are identified as a new class hormoneinhibiting branching in many plant species and have been shown to be involved in various developmental processes.Genetical and chemical modulation of the SL pathway is recognized as a promising approach to modify plant architecture.However,whether and how the genes involved in the SL pathway could be utilized in breeding still remain elusive.Here,we demonstrate that a partial loss-of-function allele of the SL biosynthesis gene,HIGH TILLERING AND DWARF 1/DWARF17(HTD1/D17),which encodes CAROTENOID CLEAVAGE DIOXYGENASE 7(CCD7),increases tiller number and improves grain yield in rice.We found that the HTD1 gene had been widely utilized and co-selected with Semidwarf 1(SD1),both contributing to the improvement of plant architecture in modern rice varieties since the Green Revolution in the 1960s.Understanding how phytohormone pathway genes regulate plant architecture and how they have been utilized and selected in breeding will lay the foundation for developing the rational approaches toward improving crop yield.

Confirming Stereochemical Structures of Strigolactones Produced by Rice and Tobacco

Major strigolactones （SLs） produced by rice （Oryza sativa L. cv. Nipponbare） and tobacco （Nicotiana tabacum L. cv. Michinoku No. 1） were purified and their stereochemical structures were determined by comparing with optically pure synthetic standards for their NMR and CD data and retention times and mass fragmentations in ESI-LC/MS and GC-MS. SLs purified from root exudates of rice plants were orobanchol, orobanchyl acetate, and ent-2＂-epi-5-deoxystr- igol. In addition to these SLs, 7-oxoorobanchyl acetate and the putative three methoxy-5~deoxystrigol isomers were detected by LC-MS/MS. The production of 7-oxoorobanchyl acetate seemed to occur in the early growth stage, as it was detected only in the root exudates collected during the first week of incubation. The root exudates of tobacco contained at least 11 SLs, including solanacol, solanacyl acetate, orobanchol, ent-2＂-epi-orobanchol, orobanchyl acetate, ent-2＇- epi-orobanchyl acetate, 5-deoxystrigol, ent-2＂-epi-5-deoxystrigol, and three isomers of putative didehydro-orobanchol whose structures remain to be clarified. Furthermore, two sorgolactone isomers but not sorgolactone were detected as minor SLs by LC-MS/MS analysis. It is intriguing to note that rice plants produced only orobanchol-type SLs, derived from ent-2＂-epi-5-deoxystrigol, but both orobanchol-type and strigol-type SLs, derived from 5-deoxystrigol were detected in tobacco plants.

KAI2-and MAX2-Mediated Responses to Karrikins and Strigolactones Are Largely Independent of HY5 in Arabidopsis Seedlings

Karrikins are butenolide compounds released from burning vegetation that stimulate seed germination and enhance seedling photomorphogenesis. Strigolactones are structurally similar plant hormones that regulate shoot and root development, and promote the germination of parasitic weed seeds. In Arabidopsis, the F-box protein MAX2 is required for responses to karrikins and strigolactones, and the a/~ hydrolase KAI2 is necessary for responses to karrikins. Both MAX2 and KAI2 are essential for normal light-dependent seedling development. The bZIP transcription factor HY5 acts downstream of multiple photoreceptors and promotes photomorphogenesis, but its relationship with MAX2 and KAI2 in terms of seedling development and responses to karrikins and strigolactones is poorly defined. Here, we dem- onstrate that HY5 action is genetically separable from that of MAX2 and KAI2. While by5 mutants have weak hypoco- tyl elongation responses to karrikins and the artificial strigolactone GR24, they have normal transcriptional responses, suggesting that HY5 is not involved in perception or action of karrikins or strigolactones. Furthermore, we show that overexpression of KAI2 is sufficient to enhance responses to both karrikins and GR24 in wild-type seedlings, and that KAI2 overexpression partially suppresses the hy5 long hypocotyl phenotype. These results suggest that KAI2 and MAX2 define a regulatory pathway that largely operates independently of HY5 to mediate seedling responses to abiotic signals such as smoke and light.

Strigolactone-Regulated Hypocotyl Elongation Is Dependent on Cryptochrome and Phytochrome Signaling Pathways in Arabidopsis

Seedling development including hypocotyl elongation is a critical phase in the plant life cycle. Light regula- tion of hypocotyl elongation is primarily mediated through the blue light photoreceptor cryptochrome and red/far-red light photoreceptor phytochrome signaling pathways, comprising regulators including COP1, HY5, and phytochrome- interacting factors （PIFs）. The novel phytohormones, strigolactones, also participate in regulating hypocotyl growth. However, how strigolactone coordinates with light and photoreceptors in the regulation of hypocotyl elongation is largely unclear. Here, we demonstrate that strigolactone inhibition of hypocotyl elongation is dependent on cryp- tochrome and phytochrome signaling pathways. The photoreceptor mutants cry1 cry2, phyA, and phyB are hyposensi- tive to strigolactone analog GR24 under the respective monochromatic light conditions, while cop1 and pifl pif3 pif4 pif5 （pifq） quadruple mutants are hypersensitive to GR24 in darkness. Genetic studies indicate that the enhanced respon- siveness of cop1 to GR24 is dependent on HY5 and MAX2, while that of pifq is independent of HY5. Further studies demonstrate that GR24 constitutively up-regulates HY5 expression in the dark and light, whereas GR24-promoted HY5 protein accumulation is light- and cryptochrome and phytochrome photoreceptor-dependent. These results suggest that the light dependency of strigolactone regulation of hypocotyl elongation is likely mediated through MAX2-dependent promotion of HY5 expression, light-dependent accumulation of HY5, and PIF-regulated components.

Strigolactones and the Regulation of Pea Symbioses in Response to Nitrate and Phosphate Deficiency

New roles for the recently identified group of plant hormones, the strigolactones, are currently under active investigation. One of their key roles is to regulate plant symbioses. These compounds act as a rhizosphere signal in arbuscular mycorrhizal symbioses and as a positive regulator of nodulation in legumes. The phosphorous and nitrogen status of the soil has emerged as a powerful regulator of strigolactone production. However, until now, the potential role of strigolactones in regulating mycorrhizal development and nodulation in response to nutrient deficiency has not been proven. In this paper, the role of strigolactone synthesis and response in regulating these symbioses is examined in pea （Pisum sativum L.）. Pea is well suited to this study, since there is a range of well-characterized strigolactone bio- synthesis and response mutants that is unique amongst legumes. Evidence is provided for a novel endogenous role for strigolactone response within the root during mycorrhizal development, in addition to the action of strigolactones on the fungal partner. The strigolactone response pathway that regulates mycorrhizal development also appears to dif- fer somewhat from the response pathway that regulates nodulation. Finally, studies with strigolactone-deficient pea mutants indicate that, despite strong regulation of strigolactone production by both nitrogen and phosphate, strigolac- tones are not required to regulate these symbioses in response to nutrient deficiency.