Do strigolactones contribute to plant defence?

Do strigolactones play a role in the ascent and attachment behavior of Pisum sativum?

Strigolactones (SLs) are signaling compounds made by plants. They play a crucial role in acting as long-distance signals from root to shoot to coordinate shoot growth with root environmental conditions. Here, we test whether and how SLs play a role in the climbing behavior of pea plants by studying the circumnutation of the tendrils using three-dimensional (3D) kinematical analysis. To assess this, we compare the typical behavior of P. sativum, a wild-type plant that produces and perceives SLs, with mutants defective in SLs synthesis or signaling, known as ramosus(rms) mutants. The results indicate that mutant plants seem unable to locate and grasp a potential support. Their movement appears to be disoriented and much less energized. We contend that this research opens new avenues for exploring SLs' role in plant behavior, a novel lens through which the role of SLs in root-to-shoot communication can be observed and analyzed.

Verticillium dahliae effector Vd06254 disrupts cotton defence response by interfering with GhMYC3-GhCCD8-mediated hormonal crosstalk between jasmonic acid and strigolactones

Verticillium dahliae is among the most destructive plant pathogens, posing a significant threat to global cotton production. Cotton plants have developed sophisticated immune networks to inhibit V. dahliae colonization. Ingeniously, V. dahliae employs numerous virulent effectors to surmount plant immune responses. However, the pathogenic mechanisms of V. dahliae-derived effectors remain elusive. In this study, we demonstrate that the Vd06254 effector from V. dahliae disrupts the synergistic interaction between jasmonic acid (JA) and strigolactones (SL), thereby suppressing cotton immunity. Ectopic expression of Vd06254 enhanced susceptibility to both viral and V. dahliae infections in Nicotiana benthamiana and cotton, respectively. Vd06254 directly interacts with the JA pathway regulator GhMYC3. The nuclear localization signal (NLS) was found to be essential for the virulence of Vd06254 and its interaction with GhMYC3. Additionally, overexpression and knockout of GhMYC3 in cotton modified the plant's resistance to V. dahliae. Our findings further reveal that GhMYC3 inhibits the expression of GhCCD8 by binding to its promoter, potentially regulating SL homeostasis in cotton through a negative feedback loop. This repression was enhanced by Vd06254, highlighting its crucial role in modulating cotton immunity and illustrating how V. dahliae effectors reprogram cotton transcription to disrupt this regulatory mechanism.

Plant Signaling Hormones and Transcription Factors: Key Regulators of Plant Responses to Growth, Development, and Stress

Plants constantly encounter a wide range of biotic and abiotic stresses that adversely affect their growth, development, and productivity. Phytohormones such as abscisic acid, jasmonic acid, salicylic acid, and ethylene serve as crucial regulators, integrating internal and external signals to mediate stress responses while also coordinating key developmental processes, including seed germination, root and shoot growth, flowering, and senescence. Transcription factors (TFs) such as WRKY, NAC, MYB, and AP2/ERF play complementary roles by orchestrating complex transcriptional reprogramming, modulating stress-responsive genes, and facilitating physiological adaptations. Recent advances have deepened our understanding of hormonal networks and transcription factor families, revealing their intricate crosstalk in shaping plant resilience and development. Additionally, the synthesis, transport, and signaling of these molecules, along with their interactions with stress-responsive pathways, have emerged as critical areas of study. The integration of cutting-edge biotechnological tools, such as CRISPR-mediated gene editing and omics approaches, provides new opportunities to fine-tune these regulatory networks for enhanced crop resilience. By leveraging insights into transcriptional regulation and hormone signaling, these advancements provide a foundation for developing stress-tolerant, high-yielding crop varieties tailored to the challenges of climate change.

Phytohormones: Heart of plants' signaling network under biotic, abiotic, and climate change stresses

Industrialization has made the world increasingly unstable, subjecting plants to various constraints. As a consequence, plants are constantly experiencing biological, environmental, and climatic constraints, necessitating defense mechanisms to ensure their survival. Plants are vulnerable to various biotic factors, including insects, pathogens (bacterial, fungal, viral, and nematodes), weeds, and herbivores. They also face different abiotic and climate change challenges such as drought (regulated by genes like GH3, DREB, ZIFL1;3, etc), salinity, heavy metals, metalloids, ultraviolet radiations (UV), ozone (O3), low and high temperature (chilling/cold/freezing/heat), carbon dioxide (CO2), chlorofluorocarbons (CFCs), and flooding/hypoxia/anoxia. Different transcriptional factors, such as KNOX1, PYK10, and NRP1, regulate these abiotic and climate change stresses. Different phytohormones such as auxin (regulated by components AUX/IAA3, PIN, indole-glucosinolate, indole-3-acetaldoxine), gibberellin (key elements involved in the synthesis and signaling such as DELLA, GA3ox, RhHB1), cytokinin (signaling through ARR5), ethylene (involved transcription factors like AP2/ERF), abscisic acid (signaling regulated through SnRK2), salicylic acid, jasmonic acid (regulated by JAZ1/TIFYIOA), brassinosteroids, nitric oxide, and strigolactones (synthetic precursor being GR24) control plants' maturation in normal and stressed conditions by regulating various metabolic and physiological plant activities. Phytohormonal interactions and their synergy are often assessed by different techniques and assays such as CRISPR/Cas9, ELISA, RIA, luciferase, GAL4, and mEmerald GFP. Their synthesis and signaling are regulated by various genes (such as YUCCA1, YUCCA5, GA3ox, etc), transporters (PIN, such as PIN, ABCB, NPF, etc), and receptors (such as PLY4, PLY5, BZR1/BES1, MYC2, etc) and have different precursors such as L-arginine, L-tryptophan, phenylalanine, linolenic acid, S-adenosylmethionine, geranylgeranyl diphosphate. This review comprehensively analyses the breakthrough in phytohormones and their signaling in regulating plants' growth and maturation. Their significance in combating the biotic, abiotic, and climate change stresses, improving stress adaptation to identify novel strategies enhancing plant resilience, sustainable agriculture, and ensuring food security.

Strigolactone enhances tea plant adaptation to drought and Phyllosticta theicola petch by regulating caffeine content via CsbHLH80

Strigolactones (SLs) play crucial roles in both plant growth and stress responses. However, their impact on the secondary metabolites of woody plants remains elusive. Here, we found that exogenous strigolactone analogue GR24 positively regulates tea plant flavor secondary metabolites, concurrently inhibiting caffeine biosynthesis and promoting the accumulation of caffeine catabolic pathway products. In this process, SL directly or indirectly inhibits the expression of CsSAMSs by inducing CsbHLH80, thereby reducing caffeine biosynthesis. Furthermore, CsbHLH80 enhances caffeine degradation, leading to increased allantoin. Under normal conditions, heightened allantoin reduces abscisic acid (ABA) accumulation. This inhibition reverses under drought stress. Increased ABA significantly enhances tea plant tolerance to both drought and Phyllosticta theicola Petch. In summary, this study offers novel insights for improving tea plant adaptation and quality in arid regions, particularly emphasizing the selection of stress-tolerant varieties and the refinement of production measures with a focus on high-quality production and environmentally friendly biological control methods.

Callose and Salicylic Acid Are Key Determinants of Strigolactone-Mediated Disease Resistance in Arabidopsis

Research has demonstrated that strigolactones (SLs) mediate plant disease resistance; however, the basal mechanism is unclear. Here, we provide key genetic evidence supporting how SLs mediate plant disease resistance. Exogenous application of the SL analog, rac-GR24, increased Arabidopsis thaliana resistance to virulent Pseudomonas syringae. SL-biosynthetic mutants and overexpression lines of more axillary growth 1 (MAX1, an SL-biosynthetic gene) enhanced and reduced bacterial susceptibility, respectively. In addition, rac-GR24 promoted bacterial pattern flg22-induced callose deposition and hydrogen peroxide production. SL-biosynthetic mutants displayed reduced callose deposition but not hydrogen peroxide production under flg22 treatment. Moreover, rac-GR24 did not affect avirulent effector-induced cell death between Col-0 and SL-biosynthetic mutants. Furthermore, rac-GR24 increased the free salicylic acid (SA) content and significantly promoted the expression of pathogenesis-related gene 1 related to SA signaling. Importantly, rac-GR24- and MAX1-induced bacterial resistance disappeared completely in Arabidopsis plants lacking both callose synthase and SA. Taken together, our data revealed that callose and SA are two important determinants in SL-mediated plant disease resistance, at least in Arabidopsis.

Biosynthesis and Signaling of Strigolactones Act Synergistically With That of ABA and JA to Enhance Verticillium dahliae Resistance in Cotton (Gossypium hirsutum L.)

Verticillium wilt (VW) caused by the soil-borne fungal pathogen Verticillium dahliae reduces cotton productivity and quality. Numerous studies have explored the genetic and molecular mechanisms regulating VW resistance in cotton, but the role and mechanism of strigolactone (SL) is still elusive. We investigated the function of SL in cotton's immune response to V. dahliae infection by exogenously applying SL analog, blocking or enhancing biosynthesis of endogenous SLs in combination with comparative transcriptome analysis and by exploring cross-talk between SL and other phytohormones. Silencing GhDWARF27 and applying the SL analog GR24 or overexpressing GhDWARF27 decreased and enhanced V. dahliae resistance, respectively. Transcriptome analysis revealed SL-mediated activation of abscisic acid (ABA) and jasmonic acid (JA) biosynthesis and signaling pathways. Enhanced ABA biosynthesis and signaling led to increased activity of antioxidant enzymes and reduced buildup of excess reactive oxygen species. Enhanced JA biosynthesis and signaling facilitated transcription of JA-dependent disease resistance genes. One of the components of the SL signal transduction pathway, GhD53, was found to interact with GhNCED5 and GhLOX2, the key enzymes of ABA and JA biosynthesis, respectively. We revealed the molecular mechanism underlying SL-enabled V. dahliae resistance and provided potential solutions for improving VW resistance in cotton.

Enhanced disease resistance against Botrytis cinerea by strigolactone-mediated immune priming in Arabidopsis thaliana

Strigolactones (SLs) are a class of plant hormones that play several roles in plants, such as suppressing shoot branching and promoting arbuscular mycorrhizal symbiosis. The positive regulation of plant disease resistance by SLs has recently been demonstrated by analyses using SL-related mutants. In Arabidopsis, SL-mediated signaling has been reported to modulate salicylic acid-mediated disease resistance, in which the priming of plant immunity plays an important role. In this study, we analyzed the effect of the synthetic SL analogue rac-GR24 on resistance against necrotrophic pathogen Botrytis cinerea. In rac-GR24-treated plants, disease resistance against B. cinerea was enhanced in an ethylene- and camalexin-dependent manners. Expression of the ethylene-related genes and the camalexin biosynthetic gene and camalexin accumulation after pathogen infection were enhanced by immune priming in rac-GR24-treated plants. These suggest that SL-mediated immune priming is effective for many types of resistance mechanisms in plant self-defense systems.

The role of strigolactones in resistance to environmental stress in plants

Abiotic stress impairs plant growth and development, thereby causing low yield and inferior quality of crops. Increasing studies reported that strigolactones (SL) are plant hormones that enhance plant stress resistance by regulating plant physiological processes and gene expressions. In this review, we introduce the response and regulatory role of SL in salt, drought, light, heat, cold and cadmium stresses in plants. This review also discusses how SL alleviate the damage of abiotic stress in plants, furthermore, introducing the mechanisms of SL enhancing plant stress resistance at the genetic level. Under abiotic stress, the exogenous SL analog GR24 can induce the biosynthesis of SL in plants, and endogenous SL can alleviate the damage caused by abiotic stress. SL enhanced the stress resistance of plants by protecting photosynthesis, enhancing the antioxidant capacity of plants and promoting the symbiosis between plants and arbuscular mycorrhiza (AM). SL interact with abscisic acid (ABA), salicylic acid (SA), auxin, cytokinin (CK), jasmonic acid (JA), hydrogen peroxide (H2O2) and other signal molecules to jointly regulate plant stress resistance. Lastly, both the importance of SL and their challenges for future work are outlined in order to further elucidate the specific mechanisms underlying the roles of SL in plant responses to abiotic stress.

Harnessing Phytohormones: Advancing Plant Growth and Defence Strategies for Sustainable Agriculture

Phytohormones, pivotal regulators of plant growth and development, are increasingly recognized for their multifaceted roles in enhancing crop resilience against environmental stresses. In this review, we provide a comprehensive synthesis of current research on utilizing phytohormones to enhance crop productivity and fortify their defence mechanisms. Initially, we introduce the significance of phytohormones in orchestrating plant growth, followed by their potential utilization in bolstering crop defences against diverse environmental stressors. Our focus then shifts to an in-depth exploration of phytohormones and their pivotal roles in mediating plant defence responses against biotic stressors, particularly insect pests. Furthermore, we highlight the potential impact of phytohormones on agricultural production while underscoring the existing research gaps and limitations hindering their widespread implementation in agricultural practices. Despite the accumulating body of research in this field, the integration of phytohormones into agriculture remains limited. To address this discrepancy, we propose a comprehensive framework for investigating the intricate interplay between phytohormones and sustainable agriculture. This framework advocates for the adoption of novel technologies and methodologies to facilitate the effective deployment of phytohormones in agricultural settings and also emphasizes the need to address existing research limitations through rigorous field studies. By outlining a roadmap for advancing the utilization of phytohormones in agriculture, this review aims to catalyse transformative changes in agricultural practices, fostering sustainability and resilience in agricultural settings.

An ecotoxicological assessment of a strigolactone mimic used as the active ingredient in a plant biostimulant formulation

A risk assessment on the aquatic toxicity of the plant biostimulant strigolactone mimic (2-(4-methyl-5-oxo-2,5-dihydro-furan-2-yloxy)-benzo[de]isoquinoline-1,3-dione (SL-6) was performed using a suite of standardised bioassays representing different trophic groups and acute and chronic endpoints. In freshwater, three trophic groups of algae, crustacea and fish were used. Whilst in seawater, algae (unicellular and macroalgae), Crustacea and Mollusca were employed. In addition, the genotoxicity of SL-6 was determined with the comet assessment performed on unicellular marine algae, oysters, and fish embryos. This was the first time ecotoxicity tests have been performed on SL-6. In freshwater, the lowest LOEC was measured in the unicellular algae at 0.31 mg/L SL-6. Although, similar LOEC values were found for embryo malformations and impacts on hatching rate in zebrafish (LOEC 0.31-0.33 mg/L). Consistent malformations of pericardial and yolk sac oedemas were identified in the zebrafish embryos at 0.31 mg/L. In marine species, the lowest LOEC was found for both Tisbe battagliai mortality and microalgae growth at an SL-6 concentration of 1.0 mg/L. Significant genotoxicity was observed above control levels at 0.0031 mg/L SL-6 in the unicellular algae and 0.001 mg/L SL-6 in the oyster and zebrafish larvae. When applying the simple risk assessment, based on the lowest NOECs and appropriate assessment factors, the calculated predicted no effect concentration (PNEC), for the ecotoxicity and the genotoxicity tests were 1.0 µg/L and 0.01 µg/L respectively.

D27-like carotenoid isomerases: at the crossroads of strigolactone and abscisic acid biosynthesis

Strigolactones and abscisic acid (ABA) are apocarotenoid-derived plant hormones. Their biosynthesis starts with the conversion of trans-carotenes into cis forms, which serve as direct precursors. Iron-containing DWARF27 isomerases were shown to catalyse or contribute to the trans/cis conversions of these precursor molecules. D27 converts trans-β-carotene into 9-cis-β-carotene, which is the first committed step in strigolactone biosynthesis. Recent studies found that its paralogue, D27-LIKE1, also catalyses this conversion. A crucial step in ABA biosynthesis is the oxidative cleavage of 9-cis-violaxanthin and/or 9-cis-neoxanthin, which are formed from their trans isomers by unknown isomerases. Several lines of evidence point out that D27-like proteins directly or indirectly contribute to 9-cis-violaxanthin conversion, and eventually ABA biosynthesis. Apparently, the diversity of D27-like enzymatic activity is essential for the optimization of cis/trans ratios, and hence act to maintain apocarotenoid precursor pools. In this review, we discuss the functional divergence and redundancy of D27 paralogues and their potential direct contribution to ABA precursor biosynthesis. We provide updates on their gene expression regulation and alleged Fe-S cluster binding feature. Finally, we conclude that the functional divergence of these paralogues is not fully understood and we provide an outlook on potential directions in research.

Cracking the enigma: understanding strigolactone signalling in the rhizosphere

The rhizosphere is a complex physical and chemical interface between plants and their underground environment, both biotic and abiotic. Plants exude a large number of chemicals into the rhizosphere in order to manipulate these biotic and abiotic components. Among such chemicals are strigolactones, ancient signalling molecules that in flowering plants act as both internal hormones and external rhizosphere signals. Plants exude strigolactones to communicate with their preferred symbiotic partners and neighbouring plants, but at least some classes of parasitic organisms are able to 'crack' these private messages and eavesdrop on the signals. In this review, we examine the intentional consequences of strigolactone exudation, and also the unintentional consequences caused by eavesdroppers. We examine the molecular mechanisms by which strigolactones act within the rhizosphere, and attempt to understand the enigma of the strigolactone molecular diversity synthesized and exuded into the rhizosphere by plants. We conclude by looking at the prospects of using improved understanding of strigolactones in agricultural contexts.

The Overexpression of Zea mays Strigolactone Receptor Gene D14 Enhances Drought Resistance in Arabidopsis thaliana L

Strigolactones (SLs) represent a recently identified class of plant hormones that are crucial for plant tillering and mycorrhizal symbiosis. The D14 gene, an essential receptor within the SLs signaling pathway, has been well-examined in crops, like rice (Oryza sativa L.) and Arabidopsis (Arabidopsis thaliana L.), yet the research on its influence in maize (Zea mays L.) remains scarce. This study successfully clones and establishes Arabidopsis D14 gene overexpression lines (OE lines). When compared with the wild type (WT), the OE lines exhibited significantly longer primary roots during germination. By seven weeks of age, these lines showed reductions in plant height and tillering, alongside slight decreases in rosette and leaf sizes, coupled with early aging symptoms. Fluorescence-based quantitative assays indicated notable hormonal fluctuations in OE lines versus the WT, implying that D14 overexpression disrupts plant hormonal homeostasis. The OE lines, exposed to cold, drought, and sodium chloride stressors during germination, displayed an especially pronounced resistance to drought. The drought resistance of OE lines, as evident from dehydration-rehydration assays, outmatched that of the WT lines. Additionally, under drought conditions, the OE lines accumulated less reactive oxygen species (ROS) as revealed by the assessment of the related physiological and biochemical parameters. Upon confronting the pathogens Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), post-infection, fluorescence quantitative investigations showed a significant boost in the salicylic acid (SA)-related gene expression in OE lines compared to their WT counterparts. Overall, our findings designate the SL receptor D14 as a key upregulator of drought tolerance and a regulator in the biotic stress response, thereby advancing our understanding of the maize SL signaling pathway by elucidating the function of the pivotal D14 gene.

Strigolactone deficiency induces jasmonate, sugar and flavonoid phytoalexin accumulation enhancing rice defense against the blast fungus Pyricularia oryzae

Strigolactones (SLs) are carotenoid-derived phytohormones that regulate plant growth and development. While root-secreted SLs are well-known to facilitate plant symbiosis with beneficial microbes, the role of SLs in plant interactions with pathogenic microbes remains largely unexplored. Using genetic and biochemical approaches, we demonstrate a negative role of SLs in rice (Oryza sativa) defense against the blast fungus Pyricularia oryzae (syn. Magnaporthe oryzae). We found that SL biosynthesis and perception mutants, and wild-type (WT) plants after chemical inhibition of SLs, were less susceptible to P. oryzae. Strigolactone deficiency also resulted in a higher accumulation of jasmonates, soluble sugars and flavonoid phytoalexins in rice leaves. Likewise, in response to P. oryzae infection, SL signaling was downregulated, while jasmonate and sugar content increased markedly. The jar1 mutant unable to synthesize jasmonoyl-l-isoleucine, and the coi1-18 RNAi line perturbed in jasmonate signaling, both accumulated lower levels of sugars. However, when WT seedlings were sprayed with glucose or sucrose, jasmonate accumulation increased, suggesting a reciprocal positive interplay between jasmonates and sugars. Finally, we showed that functional jasmonate signaling is necessary for SL deficiency to induce rice defense against P. oryzae. We conclude that a reduction in rice SL content reduces P. oryzae susceptibility by activating jasmonate and sugar signaling pathways, and flavonoid phytoalexin accumulation.

Strigolactones as promising biomolecule for oxidative stress management: A comprehensive review

Strigolactones, which are a group of plant hormones, have emerged as promising biomolecules for effectively managing oxidative stress in plants. Oxidative stress occurs when the production of reactive oxygen species (ROS) exceeds the plant's ability to detoxify or scavenge these harmful molecules. An elevation in reactive oxygen species (ROS) levels often occurs in response to a range of stressors in plants. These stressors encompass both biotic factors, such as fungal, viral, or nematode attacks, as well as abiotic challenges like intense light exposure, drought, salinity, and pathogenic assaults. This ROS surge can ultimately lead to cellular harm and damage. One of the key ways in which strigolactones help mitigate oxidative stress is by stimulating the synthesis and accumulation of antioxidants. These antioxidants act as scavengers of ROS, neutralizing their harmful effects. Additionally, strigolactones also regulate stomatal closure, which reduces water loss and helps alleviate oxidative stress during conditions of drought stress or water deficiencies. By understanding and harnessing the capabilities of strigolactones, it becomes possible to enhance crop productivity and enable plants to withstand environmental stresses in the face of a changing climate. This comprehensive review provides an in-depth exploration of the various roles of strigolactones in plant growth, development, and response to various stresses, with a specific emphasis on their involvement in managing oxidative stress. Strigolactones also play a critical role in detoxifying ROS while regulating the expression of genes related to antioxidant defense pathways, striking a balance between ROS detoxification and production.

The Complex Interplay between Arbuscular Mycorrhizal Fungi and Strigolactone: Mechanisms, Sinergies, Applications and Future Directions

Plants, the cornerstone of life on Earth, are constantly struggling with a number of challenges arising from both biotic and abiotic stressors. To overcome these adverse factors, plants have evolved complex defense mechanisms involving both a number of cell signaling pathways and a complex network of interactions with microorganisms. Among these interactions, the relationship between symbiotic arbuscular mycorrhizal fungi (AMF) and strigolactones (SLs) stands as an important interplay that has a significant impact on increased resistance to environmental stresses and improved nutrient uptake and the subsequent enhanced plant growth. AMF establishes mutualistic partnerships with plants by colonizing root systems, and offers a range of benefits, such as increased nutrient absorption, improved water uptake and increased resistance to both biotic and abiotic stresses. SLs play a fundamental role in shaping root architecture, promoting the growth of lateral roots and regulating plant defense responses. AMF can promote the production and release of SLs by plants, which in turn promote symbiotic interactions due to their role as signaling molecules with the ability to attract beneficial microbes. The complete knowledge of this synergy has the potential to develop applications to optimize agricultural practices, improve nutrient use efficiency and ultimately increase crop yields. This review explores the roles played by AMF and SLs in plant development and stress tolerance, highlighting their individual contributions and the synergistic nature of their interaction.

Strigolactones GR-24 and Nijmegen Applications Result in Reduced Susceptibility of Tobacco and Grapevine Plantlets to Botrytis cinerea Infection

Priming agents are plant defence-inducing compounds which can prompt a state of protection but may also aid in plant growth and interactions with beneficial microbes. The synthetic strigolactones (±)-GR24 and Nijmegen-1 were evaluated as potential priming agents for induced resistance against Botrytis cinerea in tobacco and grapevine plants. The growth and stress response profiles of B. cinerea to strigolactones were also investigated. Soil drench treatment with strigolactones induced resistance in greenhouse-grown tobacco plants and restricted lesion development. The mode of action appeared to function by priming redox-associated compounds to produce an anti-oxidant protective response for limiting the infection. The results obtained in the in vitro assays mirrored that of the greenhouse-grown plants. Exposure of B. cinerea to the strigolactones resulted in increased hyphal branching, with (±)-GR24 stimulating a stronger effect than Nijmegen-1 by affecting colony diameter and radial growth. An oxidative stress response was observed, with B. cinerea exhibiting increased ROS and SOD levels when grown with strigolactones. This study identified the application of strigolactones as potential priming agents to induce disease resistance in both tobacco and grapevine plants. In addition, strigolactones may alter the ROS homeostasis of B. cinerea, resulting in both morphological and physiological changes, thereby reducing virulence.

Strigolactones positively regulate Verticillium wilt resistance in cotton via crosstalk with other hormones

Verticillium wilt caused by Verticillium dahliae is a serious vascular disease in cotton (Gossypium spp.). V. dahliae induces the expression of the CAROTENOID CLEAVAGE DIOXYGENASE 7 (GauCCD7) gene involved in strigolactone (SL) biosynthesis in Gossypium australe, suggesting a role for SLs in Verticillium wilt resistance. We found that the SL analog rac-GR24 enhanced while the SL biosynthesis inhibitor TIS108 decreased cotton resistance to Verticillium wilt. Knock-down of GbCCD7 and GbCCD8b genes in island cotton (Gossypium barbadense) decreased resistance, whereas overexpression of GbCCD8b in upland cotton (Gossypium hirsutum) increased resistance to Verticillium wilt. Additionally, Arabidopsis (Arabidopsis thaliana) SL mutants defective in CCD7 and CCD8 putative orthologs were susceptible, whereas both Arabidopsis GbCCD7- and GbCCD8b-overexpressing plants were more resistant to Verticillium wilt than wild-type (WT) plants. Transcriptome analyses showed that several genes related to the jasmonic acid (JA)- and abscisic acid (ABA)-signaling pathways, such as MYELOCYTOMATOSIS 2 (GbMYC2) and ABA-INSENSITIVE 5, respectively, were upregulated in the roots of WT cotton plants in responses to rac-GR24 and V. dahliae infection but downregulated in the roots of both GbCCD7- and GbCCD8b-silenced cotton plants. Furthermore, GbMYC2 suppressed the expression of GbCCD7 and GbCCD8b by binding to their promoters, which might regulate the homeostasis of SLs in cotton through a negative feedback loop. We also found that GbCCD7- and GbCCD8b-silenced cotton plants were impaired in V. dahliae-induced reactive oxygen species (ROS) accumulation. Taken together, our results suggest that SLs positively regulate cotton resistance to Verticillium wilt through crosstalk with the JA- and ABA-signaling pathways and by inducing ROS accumulation.

Genome-Wide Identification and Expression Analysis of CCO Gene Family in Liriodendron chinense

Carotenoid cleavage oxygenase (CCO) is an enzyme that can catalyze carotenoids to volatile aromatic substances and participate in the biosynthesis of two important phytohormones, i.e., abscisic acid (ABA) and strigolactone (SL). However, the genome-wide identification and analysis of the CCO gene family in the rare and endangered woody plant Liriodendron chinense has not been reported. Here, we performed a genome-wide analysis of the CCO gene family in the L. chinense genome and examined its expression pattern during different developmental processes and in response to various abiotic stresses. A total of 10 LcCCO genes were identified and divided into 6 subfamilies according to the phylogenetic analysis. Subcellular localization prediction showed that most of the LcCCO proteins were located in the cytoplasm. Gene replication analysis showed that segmental and tandem duplication contributed to the expansion of this gene family in the L. chinense genome. Cis-element prediction showed that cis-elements related to plant hormones, stress and light response were widely distributed in the promoter regions of LcCCO genes. Gene expression profile analysis showed that LcNCED3b was extensively involved in somatic embryogenesis, especially the somatic embryo maturation, as well as in response to heat and cold stress in leaves. Furthermore, qRT-PCR analysis showed that LcNCED3b obviously responded to drought stress in roots and leaves. This study provides a comprehensive overview of the LcCCO gene family and a potential gene target for the optimization of the somatic embryogenesis system and resistance breeding in the valuable forest tree L. chinense.

Karrikin increases tomato cold tolerance via strigolactone and the abscisic acid signaling network

As a class of biostimulants, karrikins (KARs) were first identified from plant-derived smoke to regulate plant growth, development, and stress tolerance. However, the roles of KARs in plant cold tolerance and their crosstalk with strigolactones (SLs) and abscisic acid (ABA) remain elusive. We studied the interaction among KAR, SLs, and ABA in cold acclimatization with KAI2-, MAX1-, SnRK2.5-silenced, or cosilenced plant materials. KAI2 is involved in smoke-water- (SW-) and KAR-mediated cold tolerance. MAX1 acts downstream of KAR in cold acclimation. ABA biosynthesis and sensitivity are regulated by KAR and SLs, which improve cold acclimation through the SnRK2.5 component. The physiological mechanisms of SW and KAR in improving growth, yield, and tolerance under a long-term sublow temperature environment were also studied. SW and KAR were shown to improve tomato growth and yield under sublow temperature conditions by regulating nutritional uptake, leaf temperature control, photosynthetic defense, ROS scavenging, and CBF transcriptional activation. Together, SW, which functions via the KAR-mediated SL and ABA signaling network, has potential application value for increasing cold tolerance in tomato production.

Genome-wide identification of MAXs genes for strigolactones synthesis/signaling in solanaceous plants and analysis of their potential functions in tobacco

The more axillary growth (MAX) gene family is a group of key genes involved in the synthesis and signal transduction of strigolactones (SLs) in plants. Although MAX genes play vital roles in plant growth and development, characterization of the MAX gene family has been limited in solanaceous crops, especially in tobacco. In this study, 74 members of the MAX family were identified in representative Solanaceae crops and classified into four groups. The physicochemical properties, gene structure, conserved protein structural domains, cis-acting elements, and expression patterns could be clearly distinguished between the biosynthetic and signal transduction subfamilies; furthermore, MAX genes in tobacco were found to be actively involved in the regulation of meristem development by responding to hormones. MAX genes involved in SL biosynthesis were more responsive to abiotic stresses than genes involved in SL signaling. Tobacco MAX genes may play an active role in stress resistance. The results of this study provide a basis for future in-depth analysis of the molecular mechanisms of MAX genes in tobacco meristem development and stress resistance.

Specific roles of strigolactones in plant physiology and remediation of heavy metals from contaminated soil

Strigolactones (SLs) have been implicated in various developmental processes of the plant, including the response against several abiotic stresses. It is well known as a class of endogenous phytohormones that regulates shoot branching, secondary growth and root morphology. This hormone facilitates plants in responding to nitrogen and phosphorus starvation by shaping the above and below ground structural design. SLs actively participate within regulatory networks of plant stress adaptation that are governed by phytohormones. Heavy metals (HMs) in soil are considered a serious environmental problem that causes various harmful effects on plants. SLs along with other plant hormones imply the role in plant architecture is far from being fully understood. Strategy to remove/remediation of HMs from the soil with the help of SLs has not been defined yet. Therefore, the present review aims to comprehensively provide an overview of SLs role in fine-tuning plant architectures, relation with other plant hormones under abiotic stress, and remediation of HMs contaminated soil using SLs.

The Effects of High CO2 and Strigolactones on Shoot Branching and Aphid-Plant Compatibility Control in Pea

Elevated atmospheric CO2 concentrations (eCO2) regulate plant architecture and susceptibility to insects. We explored the mechanisms underpinning these responses in wild type (WT) peas and mutants defective in either strigolactone (SL) synthesis or signaling. All genotypes had increased shoot height and branching, dry weights and carbohydrate levels under eCO2, demonstrating that SLs are not required for shoot acclimation to eCO2. Since shoot levels of jasmonic acid (JA) and salicylic acid (SA) tended to be lower in SL signaling mutants than the WT under ambient conditions, we compared pea aphid performance on these lines under both CO2 conditions. Aphid fecundity was increased in the SL mutants compared to the WT under both ambient and eCO2 conditions. Aphid infestation significantly decreased levels of JA, isopentenyladenine, trans-zeatin and gibberellin A4 and increased ethylene precursor ACC, gibberellin A1, gibberellic acid (GA3) and SA accumulation in all lines. However, GA3 levels were increased less in the SL signaling mutants than the WT. These studies provide new insights into phytohormone responses in this specific aphid/host interaction and suggest that SLs and gibberellins are part of the network of phytohormones that participate in host susceptibility.

Novel Approaches for Sustainable Horticultural Crop Production: Advances and Prospects

Reduction of plant growth, yield and quality due to diverse environmental constrains along with climate change significantly limit the sustainable production of horticultural crops. In this review, we highlight the prospective impacts that are positive challenges for the application of beneficial microbial endophytes, nanomaterials (NMs), exogenous phytohormones strigolactones (SLs) and new breeding techniques (CRISPR), as well as controlled environment horticulture (CEH) using artificial light in sustainable production of horticultural crops. The benefits of such applications are often evaluated by measuring their impact on the metabolic, morphological and biochemical parameters of a variety of cultures, which typically results in higher yields with efficient use of resources when applied in greenhouse or field conditions. Endophytic microbes that promote plant growth play a key role in the adapting of plants to habitat, thereby improving their yield and prolonging their protection from biotic and abiotic stresses. Focusing on quality control, we considered the effects of the applications of microbial endophytes, a novel class of phytohormones SLs, as well as NMs and CEH using artificial light on horticultural commodities. In addition, the genomic editing of plants using CRISPR, including its role in modulating gene expression/transcription factors in improving crop production and tolerance, was also reviewed.

Overexpression of Sweet Potato Carotenoid Cleavage Dioxygenase 4 (IbCCD4) Decreased Salt Tolerance in Arabidopsis thaliana

Salt stress has a serious impact on normal plant growth and yield. Carotenoid cleavage dioxygenase (CCD) degrades carotenoids to produce apocarotenoids, which are involved in plant responses to biotic and abiotic stresses. This study shows that the expression of sweet potato IbCCD4 was significantly induced by salt and dehydration stress. The heterologous expression of IbCCD4 in Arabidopsis was induced to confirm its salt tolerance. Under 200 mM NaCl treatment, compared to wild-type plants, the rosette leaves of IbCCD4-overexpressing Arabidopsis showed increased anthocyanins and carotenoid contents, an increased expression of most genes in the carotenoid metabolic pathway, and increased malondialdehyde (MDA) levels. IbCCD4-overexpressing lines also showed a decreased expression of resistance-related genes and a lower activity of three antioxidant enzymes: peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT). These results indicate that IbCCD4 reduced salt tolerance in Arabidopsis, which contributes to the understanding of the role of IbCCD4 in salt stress.

Effects of Strigolactone on Torreya grandis Gene Expression and Soil Microbial Community Structure Under Simulated Nitrogen Deposition

Nitrogen enters the terrestrial ecosystem through deposition. High nitrogen levels can affect physical and chemical properties of soil and inhibit normal growth and reproduction of forest plants. Nitrogen modulates the composition of soil microorganisms. Strigolactones inhibits plant branching, promotes root growth, nutrient absorption, and promotes arbuscular fungal mycelia branching. Plants are subjected to increasing atmospheric nitrogen deposition. Therefore, it is imperative to explore the relationship between strigolactone and nitrogen deposition of plants and abundance of soil microorganisms. In the present study, the effects of strigolactone on genetic responses and soil microorganisms of Torreya grandis, under simulated nitrogen deposition were explored using high-throughput sequencing techniques. T. grandis is a subtropical economic tree species in China. A total of 4,008 differentially expressed genes were identified in additional N deposition and GR24 treatment. These genes were associated with multiple GO terms and metabolic pathways. GO enrichment analysis showed that several DEGs were associated with enrichment of the transporter activity term. Both additional nitrogen deposition and GR24 treatment modulated the content of nutrient elements. The content of K reduced in leaves after additional N deposition treatment. The content of P increased in leaves after GR24 treatment. A total of 20 families and 29 DEGs associated with transporters were identified. These transporters may be regulated by transcription factors. A total of 1,402,819 clean reads and 1,778 amplicon sequence variants (ASVs) were generated through Bacterial 16S rRNA sequencing. Random forest classification revealed that Legionella, Lacunisphaera, Klebsiella, Bryobacter, and Janthinobacterium were significantly enriched in the soil in the additional N deposition group and the GR24 treatment group. Co-occurrence network analysis showed significant differences in composition of soil microbial community under different treatments. These results indicate a relationship between N deposition and strigolactones effect. The results provide new insights on the role of strigolactones in plants and composition of soil microorganisms under nitrogen deposition.

Strigolactones Modulate Salicylic Acid-Mediated Disease Resistance in Arabidopsis thaliana

Strigolactones are low-molecular-weight phytohormones that play several roles in plants, such as regulation of shoot branching and interactions with arbuscular mycorrhizal fungi and parasitic weeds. Recently, strigolactones have been shown to be involved in plant responses to abiotic and biotic stress conditions. Herein, we analyzed the effects of strigolactones on systemic acquired resistance induced through salicylic acid-mediated signaling. We observed that the systemic acquired resistance inducer enhanced disease resistance in strigolactone-signaling and biosynthesis-deficient mutants. However, the amount of endogenous salicylic acid and the expression levels of salicylic acid-responsive genes were lower in strigolactone signaling-deficient max2 mutants than in wildtype plants. In both the wildtype and strigolactone biosynthesis-deficient mutants, the strigolactone analog GR24 enhanced disease resistance, whereas treatment with a strigolactone biosynthesis inhibitor suppressed disease resistance in the wildtype. Before inoculation of wildtype plants with pathogenic bacteria, treatment with GR24 did not induce defense-related genes; however, salicylic acid-responsive defense genes were rapidly induced after pathogenic infection. These findings suggest that strigolactones have a priming effect on Arabidopsis thaliana by inducing salicylic acid-mediated disease resistance.

Transcriptomic analysis reveals phytohormone and photosynthetic molecular mechanisms of a submerged macrophyte in response to microcystin-LR stress

Cyanobacterial blooms impose a substantial risk for submerged macrophytes in aquatic environments. This study investigated the cellular and transcriptomic responses of Vallisneria natans to microcystin-LR (MCLR) exposure, as well as abscisic acid (ABA) and strigolactone (SL), which are the major compounds in signaling networks that regulate plant defense. The results revealed that MCLR significantly (p <0.05) decreased the photosynthetic pigments and significantly (p < 0.05) increased the contents of the ABA and SL stress-related phytohormones under MCLR stress. Related genes involved in the photosynthetic pathways were down-regulated, including psbO, psbP, psbQ and psbR. In the SL biosynthetic pathway of roots under MCLR stress, related genes, such as D27 and CCD7, were down-regulated, while the CCD8 and MAX1 genes were up-regulated. In the ABA synthetic pathway, the genes LUT5, ZEP, NCED, ABA2 and AAO3 were up-regulated. Furthermore, a reduction in the content of SL enriched ABA after 3 days under MCLR stress. The potential molecular mechanism of the interactions between SL and ABA were confirmed with the relative up- and down-regulated genes in the pathway, and ABA could play a major role in plant physiology under MCLR stress. This study provides valuable information to understand the stress-related mechanisms of response of submerged macrophytes to cyanobacterial blooms.

Physiological mechanism of strigolactone enhancing tolerance to low light stress in cucumber seedlings

Strigolactone is a newly discovered type of plant hormone that has multiple roles in modulating plant responses to abiotic stress. Herein, we aimed to investigate the effects of exogenous GR24 (a synthetic analogue of strigolactone) on plant growth, photosynthetic characteristics, carbohydrate levels, endogenous strigolactone content and antioxidant metabolism in cucumber seedlings under low light stress. The results showed that the application of 10 μM GR24 can increase the photosynthetic efficiency and plant biomass of low light-stressed cucumber seedlings. GR24 increased the accumulation of carbohydrates and the synthesis of sucrose-related enzyme activities, enhanced antioxidant enzyme activities and antioxidant substance contents, and reduced the levels of H₂O₂ and MDA in cucumber seedlings under low light stress. These results indicate that exogenous GR24 might alleviate low light stress-induced growth inhibition by regulating the assimilation of carbon and antioxidants and endogenous strigolactone contents, thereby enhancing the tolerance of cucumber seedlings to low light stress.

Hormonomic Changes Driving the Negative Impact of Broomrape on Plant Host Interactions with Arbuscular Mycorrhizal Fungi

Belowground interactions of plants with other organisms in the rhizosphere rely on extensive small-molecule communication. Chemical signals released from host plant roots ensure the development of beneficial arbuscular mycorrhizal (AM) fungi which in turn modulate host plant growth and stress tolerance. However, parasitic plants have adopted the capacity to sense the same signaling molecules and to trigger their own seed germination in the immediate vicinity of host roots. The contribution of AM fungi and parasitic plants to the regulation of phytohormone levels in host plant roots and root exudates remains largely obscure. Here, we studied the hormonome in the model system comprising tobacco as a host plant, Phelipanche spp. as a holoparasitic plant, and the AM fungus Rhizophagus irregularis. Co-cultivation of tobacco with broomrape and AM fungi alone or in combination led to characteristic changes in the levels of endogenous and exuded abscisic acid, indole-3-acetic acid, cytokinins, salicylic acid, and orobanchol-type strigolactones. The hormonal content in exudates of broomrape-infested mycorrhizal roots resembled that in exudates of infested non-mycorrhizal roots and differed from that observed in exudates of non-infested mycorrhizal roots. Moreover, we observed a significant reduction in AM colonization of infested tobacco plants, pointing to a dominant role of the holoparasite within the tripartite system.

Biofertilizers: An ecofriendly technology for nutrient recycling and environmental sustainability

Modern intensive agricultural practices face numerous challenges that pose major threats to global food security. In order to address the nutritional requirements of the ever-increasing world population, chemical fertilizers and pesticides are applied on large scale to increase crop production. However, the injudicious use of agrochemicals has resulted in environmental pollution leading to public health hazards. Moreover, agriculture soils are continuously losing their quality and physical properties as well as their chemical (imbalance of nutrients) and biological health. Plant-associated microbes with their plant growth- promoting traits have enormous potential to solve these challenges and play a crucial role in enhancing plant biomass and crop yield. The beneficial mechanisms of plant growth improvement include enhanced nutrient availability, phytohormone modulation, biocontrol of phytopathogens and amelioration of biotic and abiotic stresses. Solid-based or liquid bioinoculant formulation comprises inoculum preparation, addition of cell protectants such as glycerol, lactose, starch, a good carrier material, proper packaging and best delivery methods. Recent developments of formulation include entrapment/microencapsulation, nano-immobilization of microbial bioinoculants and biofilm-based biofertilizers. This review critically examines the current state-of-art on use of microbial strains as biofertilizers and the important roles performed by these beneficial microbes in maintaining soil fertility and enhancing crop productivity.

Strigolactones positively regulate abscisic acid-dependent heat and cold tolerance in tomato

Strigolactones are carotenoid-derived phytohormones that impact plant growth and development in diverse ways. However, the roles of strigolactones in the responses to temperature stresses are largely unknown. Here, we demonstrated that strigolactone biosynthesis is induced in tomato (Solanum lycopersicum) by heat and cold stresses. Compromised strigolactone biosynthesis or signaling negatively affected heat and cold tolerance, while application of the synthetic strigolactone analog GR245DS enhanced heat and cold tolerance. Strigolactone-mediated heat and cold tolerance was associated with the induction of abscisic acid (ABA), heat shock protein 70 (HSP70) accumulation, C-REPEAT BINDING FACTOR 1 (CBF1) transcription, and antioxidant enzyme activity. Importantly, a deficiency in ABA biosynthesis compromised the GR245DS effects on heat and cold stresses and abolished the GR245DS-induced transcription of HSP70, CBF1, and antioxidant-related genes. These results support that strigolactones positively regulate tomato heat and cold tolerance and that they do so at least partially by the induction of CBFs and HSPs and the antioxidant response in an ABA-dependent manner.

Harmony but Not Uniformity: Role of Strigolactone in Plants

Strigolactones (SLs) represent an important new plant hormone class marked by their multifunctional roles in plants and rhizosphere interactions, which stimulate hyphal branching in arbuscular mycorrhizal fungi (AMF) and seed germination of root parasitic plants. SLs have been broadly implicated in regulating root growth, shoot architecture, leaf senescence, nodulation, and legume-symbionts interaction, as well as a response to various external stimuli, such as abiotic and biotic stresses. These functional properties of SLs enable the genetic engineering of crop plants to improve crop yield and productivity. In this review, the conservation and divergence of SL pathways and its biological processes in multiple plant species have been extensively discussed with a particular emphasis on its interactions with other different phytohormones. These interactions may shed further light on the regulatory networks underlying plant growth, development, and stress responses, ultimately providing certain strategies for promoting crop yield and productivity with the challenges of global climate and environmental changes.

The Impact of Phosphorus on Plant Immunity

Phosphorus (P) is the second most essential macronutrient in terms of limiting plant growth. The genes involved in P acquisition, transport, storage, utilization and respective regulation have been extensively studied. In addition, significant attention has been given to the crosstalk between P and other environmental stresses. In this review, we summarize recent discoveries pertaining to the emerging function of P in plant immunity. The roles of external soil P availability, internal cellular P in plants, P starvation signaling machinery and phosphate transporters in biotic interactions are discussed. We also highlight the impact of several phytohormones on the signaling convergence between cellular P and immune responses. This information may serve as a foundation for dissecting the molecular interaction between nutrient responses and plant immunity.

Evaluation of the Effect of Strigolactones and Synthetic Analogs on Fungi

Strigolactones (SLs) are components of root exudates as a consequence of active release from the roots into the soil. Notably, they have been described as stimulants of seed germination in parasitic plants and of the presymbiotic growth in arbuscular mycorrhizal (AM) fungi, which are a crucial component of the plant root beneficial microbiota. SLs have therefore the potential to influence other microbes that proliferate in the soil around the roots and may interact with plants. A direct effect of SL analogs on the in vitro growth of a number of saprotrophic or plant pathogenic fungi was indeed reported.Here we describe a standardized method to evaluate the effect of SLs or their synthetic analogs on AM and filamentous fungi. For AM fungi, we propose a spore germination assay since it is more straightforward than the hyphal branching assay and it does not require deep expertise and skills. For filamentous fungi that can grow in axenic cultures, we describe the assay based on SLs embedded in the solid medium or dissolved in liquid cultures where the fungus is inoculated to evaluate the effect on growth, hyphal branching or conidia germination. These assays are of help to test the activity of natural SLs as well as of newly designed SL analogs for basic and applied research.

Chemotropic Assay for Testing Fungal Response to Strigolactones and Strigolactone-Like Compounds

Current knowledge on the mechanism of strigolactones (SLs) as signaling molecules during specific interactions in the rhizosphere is mainly related to the control of germination of parasitic weed seeds and hyphal branching of arbuscular mycorrhizal fungi. Thus, the role of plant secreted SLs in regulating the growth and development of root-colonizing fungi still remains controversial. Fusarium oxysporum can sense and respond to extracellular signals through oriented germ tube emergence and redirectioning of hyphal growth toward gradients of nutrients, sex pheromones, or plant root exudates. However, chemoattractant activity of SLs against microorganisms living in the soil has not been tested so far. Here we propose a quantitative chemotropic assay to understand if and how soil fungi could sense gradients of SLs and SLs-like sources. In the example case of F. oxysporum, hyphae of fungal representative mutants preferentially grow toward the synthetic SL analog GR24; and this chemotropic response requires conserved elements of the fungal invasive growth mitogen-activated protein kinase (MAPK) cascade.

Genome-wide identification, characterization and expression analysis of the carotenoid cleavage oxygenase (CCO) gene family in Saccharum

Carotenoid cleavage oxygenases (CCOs) play crucial roles in plant growth and development, as well as in the response to phytohormonal, biotic and abiotic stresses. However, comprehensive and systematic research on the CCO gene family has not yet been conducted in Saccharum. In this study, 47 SsCCO and 14 ShCCO genes were identified and characterized in Saccharum spontaneum and Saccharum spp. R570 cultivar, respectively. The SsCCOs consisted of 38 SsCCDs and 9 SsNCEDs, while ShCCOs contained 11 ShCCDs and 3 ShNCEDs. The SsCCO family could be divided into 7 groups, while ShCCO family into 5 groups. The genes/proteins contained similar compositions within the same group, and the evolutionary mechanisms differed between S. spontaneum and R570. Gene Ontology annotation implied that CCOs were involved in many physiological and biochemical processes. Additionally, 41 SsCCOs were regulated by 19 miRNA families, and 8 ShCCOs by 9 miRNA families. Cis-regulatory elements analysis suggested that CCO genes functioned in the process of growth and development or under the phytohormonal, biotic and abiotic stresses. qRT-PCR analysis indicated that nine CCO genes from different groups exhibited similar expression patterns under abscisic acid treatment, while more divergent profiles were observed in response to Sporisorium scitamineum and cold stresses. Herein, comparative genomics analysis of the CCO gene family between S. spontaneum and R570 was conducted to investigate its evolution and functions. This is the first report on the CCO gene family in S. spontaneum and R570, thus providing valuable information and facilitating further investigation into its function in the future.

Regulation of Plant Mineral Nutrition by Signal Molecules

Microbes operate their metabolic activities at a unicellular level. However, it has been revealed that a few metabolic activities only prove beneficial to microbes if operated at high cell densities. These cell density-dependent activities termed quorum sensing (QS) operate through specific chemical signals. In Gram-negative bacteria, the most widely reported QS signals are acylhomoserine lactones. In contrast, a novel QS-like system has been elucidated, regulating communication between microbes and plants through strigolactones. These systems regulate bioprocesses, which affect the health of plants, animals, and human beings. This mini-review presents recent developments in the QS and QS-like signal molecules in promoting plant health.

Overexpression of a Cytochrome P450 Monooxygenase Involved in Orobanchol Biosynthesis Increases Susceptibility to Fusarium Head Blight

Fusarium Head Blight (FHB) is a cereal disease caused primarily by the ascomycete fungus Fusarium graminearum with public health issues due to the production of mycotoxins including deoxynivalenol (DON). Genetic resistance is an efficient protection means and numerous quantitative trait loci have been identified, some of them related to the production of resistance metabolites. In this study, we have functionally characterized the Brachypodium distachyon BdCYP711A29 gene encoding a cytochrome P450 monooxygenase (CYP). We showed that BdCYP711A29 belongs to an oligogenic family of five members. However, following infection by F. graminearum, BdCYP711A29 is the only copy strongly transcriptionally induced in a DON-dependent manner. The BdCYP711A29 protein is homologous to the Arabidopsis thaliana MAX1 and Oryza sativa MAX1-like CYPs representing key components of the strigolactone biosynthesis. We show that BdCYP711A29 is likely involved in orobanchol biosynthesis. Alteration of the BdCYP711A29 sequence or expression alone does not modify plant architecture, most likely because of functional redundancy with the other copies. B. distachyon lines overexpressing BdCYP711A29 exhibit an increased susceptibility to F. graminearum, although no significant changes in defense gene expression were detected. We demonstrate that both orobanchol and exudates of Bd711A29 overexpressing lines stimulate the germination of F. graminearum macroconidia. We therefore hypothesize that orobanchol is a susceptibility factor to FHB.

Roles of Phytohormones and Their Signaling Pathways in Leaf Development and Stress Responses

Phytohormones participate in various processes over the course of a plant's lifecycle. In addition to the five classical phytohormones (auxins, cytokinins, gibberellins, abscisic acid, and ethylene), phytohormones such as brassinosteroids, jasmonic acid, salicylic acid, strigolactones, and peptides also play important roles in plant growth and stress responses. Given the highly interconnected nature of phytohormones during plant development and stress responses, it is challenging to study the biological function of a single phytohormone in isolation. In the current Review, we describe the combined functions and signaling cascades (especially the shared points and pathways) of various phytohormones in leaf development, in particular, during leaf primordium initiation and the establishment of leaf polarity and leaf morphology as well as leaf development under various stress conditions. We propose a model incorporating the roles of multiple phytohormones in leaf development and stress responses to illustrate the underlying combinatorial signaling pathways. This model provides a reference for breeding stress-resistant crops.

Plant apocarotenoids: from retrograde signaling to interspecific communication

Carotenoids are isoprenoid compounds synthesized by all photosynthetic and some non-photosynthetic organisms. They are essential for photosynthesis and contribute to many other aspects of a plant's life. The oxidative breakdown of carotenoids gives rise to the formation of a diverse family of essential metabolites called apocarotenoids. This metabolic process either takes place spontaneously through reactive oxygen species or is catalyzed by enzymes generally belonging to the CAROTENOID CLEAVAGE DIOXYGENASE family. Apocarotenoids include the phytohormones abscisic acid and strigolactones (SLs), signaling molecules and growth regulators. Abscisic acid and SLs are vital in regulating plant growth, development and stress response. SLs are also an essential component in plants' rhizospheric communication with symbionts and parasites. Other apocarotenoid small molecules, such as blumenols, mycorradicins, zaxinone, anchorene, β-cyclocitral, β-cyclogeranic acid, β-ionone and loliolide, are involved in plant growth and development, and/or contribute to different processes, including arbuscular mycorrhiza symbiosis, abiotic stress response, plant-plant and plant-herbivore interactions and plastid retrograde signaling. There are also indications for the presence of structurally unidentified linear cis-carotene-derived apocarotenoids, which are presumed to modulate plastid biogenesis and leaf morphology, among other developmental processes. Here, we provide an overview on the biology of old, recently discovered and supposed plant apocarotenoid signaling molecules, describing their biosynthesis, developmental and physiological functions, and role as a messenger in plant communication.

6-Benzylaminopurine (6-BA) ameliorates drought stress response in tall fescue via the influencing of biochemicals and strigolactone-signaling genes

Drought is a major agricultural and societal concern that causes farmers worldwide billions of dollars in annual losses. By revealing the as-of-yet unknown details of the biochemical and phytohormonal crosstalk occurring in drought-stressed plants, novel strategies can be pioneered to enhance drought tolerance in crop plants. Toward this goal, exogenous treatments containing the synthetic cytokinin 6-Benzylaminopurine (6-BA) were applied to the perennial monocot grass Festuca arundinacea (Tall Fescue). These plants were subjected to three irrigation levels: 100% ± 5%, 50% ± 5%, and 25% ± 5% of field capacity, at which a number of morpho-physiological and biochemical responses were evaluated. Furthermore, to elucidate the crosstalk between cytokinin (CK) and strigolactone (SL), we evaluated the activities of several SL-responsive genes. Drought conditions were shown to have widespread effects on morpho-physiological and biochemical indices. However, foliar application of 6-BA on tall fescue largely ameliorated drought stress symptoms. Water-soluble carbohydrates also declined significantly in response to CK over the course of drought progression, with virtually no change to starch content. Severe drought stress also upregulated a number of SL-response genes in the leaves of plants, indicating a correlation between the degree of drought severity and the quantity of SLs in tall fescue. Furthermore, the drought‒mediated induction of SL-signaling genes (including FaD14 and FaMax2) was inhibited in response to exogenous application of 6-BA, implying that 6-BA is a drought-dependent suppressor of SL-signaling genes. However, our results also hint at the existence of an as-of-yet poorly-characterized system of complex phytohormonal responses coordinated from multiple signaling pathways in response to drought.

Genome-wide analysis of H3K4me3 and H3K27me3 modifications due to Lr28 for leaf rust resistance in bread wheat (Triticum aestivum)

Present study revealed a complex relationship among histone H3 methylation (examined using H3K4/K27me3 marks), cytosine DNA methylation and differential gene expression during Lr28 mediated leaf rust resistance in wheat. During the present study, genome-wide histone modifications were examined in a pair of near isogenic lines (NILs) (with and without Lr28 in the background of cv. HD2329). The two histone marks used included H3K4me3 (an activation mark) and H3K27me3 (a repression mark). The results were compared with levels of expression (using RNA-seq) and DNA methylation (MeDIP) data obtained using the same pair of NILs. Some of the salient features of the present study include the following: (i) large scale differential binding sites (DBS) were available for only H3K4me3 in the susceptible cultivar, but for both H3K4me3 and H3K27me3 in its resistant NIL; (ii) DBSs for H3K27me3 mark were more abundant (> 80%) in intergenic regions, whereas DBSs for H3K4me3 were distributed in all genomic regions including exons, introns, intergenic, TTS (transcription termination sites) and promoters; (iii) fourteen (14) genes associated with DBSs showed co-localization for both the marks; (iv) only a small fraction (7% for H3K4me3 and 12% for H3K27me3) of genes associated with DBSs matched with the levels of gene expression inferred from RNA-seq data; (v) validation studies using qRT-PCR were conducted on 26 selected representative genes; results for only 11 genes could be validated. The proteins encoded by important genes involved in promoting infection included domains generally carried by R gene proteins such as Mlo like protein, protein kinases and purple acid phosphatase. Similarly, proteins encoded by genes involved in resistance included those carrying domains for lectin kinase, R gene, aspartyl protease, etc. Overall, the results suggest a very complex network of downstream genes that are expressed during compatible and incompatible interactions; some of the genes identified during the present study may be used in future validation studies involving RNAi/overexpression approaches.

Strigolactone signaling regulates specialized metabolism in tobacco stems and interactions with stem-feeding herbivores

Plants are attacked by herbivores, which often specialize on different tissues, and in response, have evolved sophisticated resistance strategies that involve different types of chemical defenses frequently targeted to different tissues. Most known phytohormones have been implicated in regulating these defenses, with jasmonates (JAs) playing a pivotal role in complex regulatory networks of signaling interactions, often generically referred to as "cross talk." The newly identified class of phytohormones, strigolactones (SLs), known to regulate the shoot architecture, remain unstudied with regard to plant-herbivore interactions. We explored the role of SL signaling in resistance to a specialist weevil (Trichobaris mucorea) herbivore of the native tobacco, Nicotiana attenuata, that attacks the root-shoot junction (RSJ), the part of the plant most strongly influenced by alterations in SL signaling (increased branching). As SL signaling shares molecular components, such as the core F-box protein MORE AXILLARY GROWTH 2 (MAX2), with another new class of phytohormones, the karrikins (KARs), which promote seed germination and seedling growth, we generated transformed lines, individually silenced in the expression of NaMAX2, DWARF 14 (NaD14: the receptor for SL) and CAROTENOID CLEAVAGE DIOXYGENASE 7 (NaCCD7: a key enzyme in SL biosynthesis), and KARRIKIN INSENSITIVE 2 (NaKAI2: the KAR receptor). The mature stems of all transgenic lines impaired in the SL, but not the KAR signaling pathway, overaccumulated anthocyanins, as did the stems of plants attacked by the larvae of weevil, which burrow into the RSJs to feed on the pith of N. attenuata stems. T. mucorea larvae grew larger in the plants silenced in the SL pathway, but again, not in the KAI2-silenced plants. These phenotypes were associated with elevated JA and auxin (indole-3-acetic acid [IAA]) levels and significant changes in the accumulation of defensive compounds, including phenolamides and nicotine. The overaccumulation of phenolamides and anthocyanins in the SL pathway-silenced plants likely resulted from antagonism between the SL and JA pathway in N. attenuata. We show that the repressors of SL signaling, suppressor of max2-like (NaSMXL6/7), and JA signaling, jasmonate zim-domain (NaJAZs), physically interact, promoting NaJAZb degradation and releasing JASMONATE INSENSITIVE 1 (JIN1/MYC2) (NaMYC2), a critical transcription factor promoting JA responses. However, the increased performance of T. mucorea larvae resulted from lower pith nicotine levels, which were inhibited by increased IAA levels in SL pathway-silenced plants. This inference was confirmed by decapitation and auxin transport inhibitor treatments that decreased pith IAA and increased nicotine levels. In summary, SL signaling tunes specific sectors of specialized metabolism in stems, such as phenylpropanoid and nicotine biosynthesis, by tailoring the cross talk among phytohormones, including JA and IAA, to mediate herbivore resistance of stems. The metabolic consequences of the interplay of SL, JA, and IAA signaling revealed here could provide a mechanism for the commonly observed pattern of herbivore tolerance/resistance trade-offs.

Science and application of strigolactones

Strigolactones (SLs) represent a class of plant hormones that regulate developmental processes and play a role in the response of plants to various biotic and abiotic stresses. Both in planta hormonal roles and ex planta signalling effects of SLs are potentially interesting agricultural targets. In this review, we explore various aspects of SL function and highlight distinct areas of agriculture that may benefit from the use of synthetic SL analogues, and we identify possible bottlenecks. Our objective is to identify where the contributions of science and stakeholders are still needed to achieve harnessing the benefits of SLs for a sustainable agriculture of the near future.

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.