Karrikins类天然产物的全合成及其衍生物的生物活性研究进展

Karrikins类天然产物是从植物燃烧烟雾中分离发现的新型植物信号分子,可在极低浓度(<10^(-9) mol/L)下促进植物种子的萌发,在农业、园艺等领域具有潜在应用价值。Karrikins类天然产物的新颖化学结构及其优异的生物活性,加之其在自然界中含量较低,其化学全合成、及其衍生物的构效关系引起了研究人员的极大关注。本文主要对Karrikins类天然产物及其衍生物的全合成及其生物活性研究进展进行总结、综述,可为该类天然产物的进一步研究提供参考。

Karrikins信号传导通路及功能研究进展

Karrikins是从野火烟中发现的一类具有促进某些植物种子(如拟南芥、野燕麦)萌发的信号分子。自2004年其结构首次被解析以来,目前已经发现6种不同形式的Karrikin,其活性各有不同。虽然Karrikins被发现的时间较短,但其已成为植物分子生物学领域的研究热点。研究发现,Karrikins除促进种子萌发以外,还具有调控植物光形态建成、叶片发生等过程等生物学功能;此外,Karrikins与植物激素独脚金内酯(Strigolactone)在结构、信号传导通路等方面具有非常高的相似性。本文从Karrikins的发现史、信号传导通路、生物学功能及生态学意义等方面综述了其最新的研究进展,并探讨了Karrikins领域未来的研究方向。

Environmentally adaptive reshaping of plant photomorphogenesis by karrikin and strigolactone signaling

Coordinated morphogenic adaptation of growing plants is critical for their survival and propagation under fluctuating environments.Plant morphogenic responses to light and warm temperatures,termed photomorphogenesis and thermomorphogenesis,respectively,have been extensively studied in recent decades.During photomorphogenesis,plants actively reshape their growth and developmental patterns to cope with changes in light regimes.Accordingly,photomorphogenesis is closely associated with diverse growth hormonal cues.Notably,accumulating evidence indicates that light-directed morphogenesis is profoundly affected by two recently identified phytochemicals,karrikins(KARs)and strigolactones(SLs).KARs and SLs are structurally related butenolides acting as signaling molecules during a variety of developmental steps,including seed germination.Their receptors and signaling mediators have been identified,and associated working mechanisms have been explored using gene-deficient mutants in various plant species.Of particular interest is that the KAR and SL signaling pathways play important roles in environmental responses,among which their linkages with photomorphogenesis are most comprehensively studied during seedling establishment.In this review,we focus on how the phytochemical and light signals converge on the optimization of morphogenic fitness.We also discuss molecular mechanisms underlying the signaling crosstalks with an aim of developing potential ways to improve crop productivity under climate changes.

Non-transcriptional regulatory activity of SMAX1 and SMXL2 mediates karrikin-regulated seedling response to red light in Arabidopsis

Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways.The transcriptional repressor proteins SUPPRESSOR OF MAX21(SMAX1),SMAX1-like2(SMXL2),and D53-like SMXLs mediate karrikin and strigolactone signaling by directly binding downstream genes or byinhibiting the activities of transcription factors.In this study,we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis.We discovered that SMAX1 and SMXL2 with mutations in their ethylene-responsefactor-associated amphiphilic repression(EAR)motif had undetectable or weak transcriptional repression activities but still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of the smax1 smxl2 mutant.SMAX1 and SMXL2 directly interact with PHYTOCHROME INTERACTION FACTOR4(PIF4)and PIF5 to enhance their protein stability by interacting with phytochrome B(phyB)and suppressing the association of phyB with PIF4 and PIF5.The karrikin-responsive genes were then identified by treatment with GR24ent-ssa,GR24 analog showing karrikin activity.Interestingly,INDOLE-3-ACETIC ACID INDUCIBLE 29(IAA29)expression was repressed by GR24^(ent-5D)streatment in a PIF4-and PIF5-dependent and EARindependent manner,whereas KARRIKIN UPREGULATED F-BOX 1(KUF1)expression was induced in a PIF4-and PIF5-independent and EAR-dependent manner.Furthermore,the non-transcriptional regulatory activity of SMAX1,which is independent of the EAR motif,had a global effect on gene expression.Taken together,these results indicate that non-transcriptional regulatory activities of SMAX1 and SMXL2 mediate karrikin-regulated seedling response to red light.

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.

Karrikin and Cyanohydrin Smoke Signals Provide Clues to New Endogenous Plant Signaling Compounds

Two new types of signaling compounds have been discovered in wildfire smoke due to their ability to stimulate seed germination. The first discovered were karrikins, which share some structural similarity with the strigol- actone class of plant hormones, and both signal through a common F-box protein. However, karrikins and strigolactones operate through otherwise distinct signaling pathways, each distinguished by a specific a/13 hydrolase protein. Genetic analysis suggests that plants contain endogenous compounds that signal specifically through the karrikin pathway. The other active compounds discovered in smoke are cyanohydrins that release germination-stimulating cyanide upon hydrolysis. Cyanohydrins occur widely in plants and have a role in defense against other organisms, but an additional role in endogenous cyanide signaling should also now be considered.

The MAX2-KAI2 module promotes salicylic acid-mediated immune responses in Arabidopsis

Arabidopsis MORE AXILLARY GROWTH2 MAX2)is a key component in the strigolactone(SL)and karrikin(KAR)signaling pathways and regulates the degradation of SUPPRESSOR OF MAX21/SMAX1-like(SMAX1/SMXL)proteins,which are transcriptional co-repressors that regulate plant architecture,as well as abiotic and biotic stress responses.The max2 mutation reduces resistance against Pseudomonas syringae pv.tomato(Pst).To uncover the mechanism of MAX2-mediated resistance,we evaluated the resistance of various SL and KAR signaling pathway mutants.The resistance of SL-deficient mutants and of dwarf 14(d14)was similar to that of the wild-type,whereas the resistance of the karrikin insensitive 2(kai2)mutant was compromised,demonstrating that the KAR signaling pathway,not the SL signaling pathway,positively regulates the immune response.We measured the resistance of smaxl and smxl mutants,as well as the double,triple,and quadruple mutants with max2,which revealed that both the smax1 mutant and smx16/7/8 triple mutant rescue the low resistance phenotype of max2 and that SMAX1 accumulation diminishes resistance.The susceptibility of smax1D,containing a degradation-insensitive form of SMAX1,further confirmed the SMAX1 function in the resistance.The relationship between the accumulation of SMAX1/SMXLs and disease resistance suggested that the inhibitory activity of SMAX1 to resistance requires SMXL6/7/8.Moreover,the exogenous application of KAR2 enhanced resistance against Pst,but KAR-induced resistance depended on salicylic acid(SA)signaling.Inhibition of karrikin signaling delayed SA-mediated defense responses and inhibited pathogen-induced protein biosynthesis.Together,we propose that the MAX2-KAI2-SMAX1 complex regulates resistance with the assistance of SMXL6/7/8 and SA signaling and that SMAX1/SMXLs possibly form a multimeric complex with their target transcription factors to fine tune immune responses.

独脚金内酯调控植物根系发育的分子机制研究的进展

独脚金内酯(strigolactones,SLs)是近年来发现的新型植物激素,参与调控植物生长发育过程,SLs在调控根系形态方面具有重要的作用。该文重点综述了SLs对植物主根、侧根、根毛及不定根的调节,特别是SLs与其他信号分子如生长素、乙烯、NO等的相互作用,以及SLs在氮磷胁迫条件下对根系调控的研究进展,为进一步深入了解SLs对植物生长和发育的调节奠定基础。

Downregulation of Rice DWARF 14 LIKE Suppress Mesocotyl Elongation via a Strigolactone Independent Pathway in the Dark

Strigolactones （SLs） are a class of plant hormones that control plant development in response to environmental conditions. In rice, mesocotyl elongation is regulated by SLs in the dark, while mesocotyls are longer in SL deficient or insensitive mutants. SLs are perceived by DWARF14 （D14）, which is a member of a small gene family. In this study, we examined the function of another D14 family gene in rice, D14 LIKE （D14L）, focusing on mesocotyl growth. The mesocotyls of D14L RNAi lines are longer than those of WT in the dark. This phenotype is enhanced when the D14L RNAi lines are combined with the d14 mutation, suggesting that D14 and D14L work independently to inhibit mesocotyl elongation. This phenotype is alleviated by the exogenous supply of GR24, a synthetic SL, suggesting that D14L is not necessary for SL signaling. D14L mRNA is predominantly expressed in vascular bundles and crown root primordia. Our results suggest that D14L and D14 confer their effects via an SL independent pathway and an SL signaling pathway respectively.