The role of strigolactones in root development

Strigolactones: A promising tool for nutrient acquisition through arbuscular mycorrhizal fungi symbiosis and abiotic stress tolerance

Strigolactones (SLs) constitute essential phytohormones that control pathogen defense, resilience to phosphate deficiency and abiotic stresses. Furthermore, SLs are released into the soil by roots, especially in conditions in which there is inadequate phosphate or nitrogen available. SLs have the aptitude to stimulate the root parasite plants and symbiotic cooperation with arbuscular mycorrhizal (AM) fungi in rhizosphere. The use of mineral resources, especially phosphorus (P), by host plants is accelerated by AMF, which also improves plant growth and resilience to a series of biotic and abiotic stresses. Thus, these SL treatments that promote rhizobial symbiosis are substitutes for artificial fertilizers and other chemicals, supporting ecologically friendly farming practices. Moreover, SLs have become a fascinating target for abiotic stress adaptation in plants, with an array of uses in sustainable agriculture. In this review, the biological activity has been summarized that SLs as a signaling hormone for AMF symbiosis, nutrient acquisition, and abiotic stress tolerance through interaction with other hormones. Furthermore, the processes behind the alterations in the microbial population caused by SL are clarified, emphasizing the interplay with other signaling mechanisms. This review covers the latest developments in SL studies as well as the properties of SLs on microbial populations, plant hormone transductions, interactions and abiotic stress tolerance.

Distinguishing the functions of canonical strigolactones as rhizospheric signals

Strigolactones (SLs) act as regulators of plant architecture as well as signals in rhizospheric communications. Reduced availability of minerals, particularly phosphorus, leads to an increase in the formation and release of SLs that enable adaptation of root and shoot architecture to nutrient limitation and, simultaneously, attract arbuscular mycorrhizal fungi (AMF) for establishing beneficial symbiosis. Based on their chemical structure, SLs are designated as either canonical or non-canonical; however, the question of whether the two classes are also distinguished in their biological functions remained largely elusive until recently. In this review we summarize the latest advances in SL biosynthesis and highlight new findings pointing to rhizospheric signaling as the major function of canonical SLs.

Adaptive Responses of Hormones to Nitrogen Deficiency in Citrus sinensis Leaves and Roots

Some citrus orchards in China often experience nitrogen (N) deficiency. For the first time, targeted metabolomics was used to examine N-deficient effects on hormones in sweet orange (Citrus sinensis (L.) Osbeck cv. Xuegan) leaves and roots. The purpose was to validate the hypothesis that hormones play a role in N deficiency tolerance by regulating root/shoot dry weight ratio (R/S), root system architecture (RSA), and leaf and root senescence. N deficiency-induced decreases in gibberellins and indole-3-acetic acid (IAA) levels and increases in cis(+)-12-oxophytodienoic acid (OPDA) levels, ethylene production, and salicylic acid (SA) biosynthesis might contribute to reduced growth and accelerated senescence in leaves. The increased ethylene formation in N-deficient leaves might be caused by increased 1-aminocyclopropanecarboxylic acid and OPDA and decreased abscisic acid (ABA). N deficiency increased R/S, altered RSA, and delayed root senescence by lowering cytokinins, jasmonic acid, OPDA, and ABA levels and ethylene and SA biosynthesis, increasing 5-deoxystrigol levels, and maintaining IAA and gibberellin homeostasis. The unchanged IAA concentration in N-deficient roots involved increased leaf-to-root IAA transport. The different responses of leaf and root hormones to N deficiency might be involved in the regulation of R/S, RSA, and leaf and root senescence, thus improving N use efficiency, N remobilization efficiency, and the ability to acquire N, and hence conferring N deficiency tolerance.

Strigolactones alleviate the toxicity of polystyrene nanoplastics (PS-NPs) in maize (Zea mays L.)

Nanoplastics are widely used across various fields, yet their uptake can potentially exert adverse effects on plant growth and development, ultimately reducing yields. While there is growing awareness of the phytotoxicity caused by nanoplastics, our understanding of effective strategies to prevent nanoplastic accumulation in plants remains limited. This study explores the role of strigolactones (SLs) in mitigating the toxicity of polystyrene nanoplastics (PS-NPs) in Zea mays L. (maize). SLs application markedly inhibited PS-NPs accumulation in maize roots, thus enhancing the root weight, shoot weight and shoot length of maize. Physiological analysis showed that SLs application activated the activities of antioxidant defence enzymes, superoxide dismutase and catalase, to decrease the malondialdehyde content and electrolyte leakage and alleviate the accumulation of H2O2 and O2.- induced by PS-NPs in maize plants. Transcriptomic analyses revealed that SLs application induced transcriptional reprogramming by regulating the expression of genes related to MAPK, plant hormones and plant-pathogen interaction signal pathways in maize treated with PS-NPs. Notably, the expression of genes, such as ZmAUX/IAA and ZmGID1, associated with phytohormones in maize treated with PS-NPs underwent significant changes. In addition, SLs induced metabolic dynamics changes related to amino acid biosynthesis, ABC transporters, cysteine and methionine metabolism in maize treated with PS-NPs. In summary, these results strongly reveal that SLs could serve as a strategy to mitigate the accumulation and alleviate the stress of PS-NPs in maize, which appears to be a potential approach for mitigating the phytotoxicity induced by PS-NPs in maize.

The dosage- and size-dependent effects of micro- and nanoplastics in lettuce roots and leaves at the growth, photosynthetic, and metabolomics levels

The occurrence of microplastics (MPs) and nanoplastics (NPs) in soils potentially induce morphological, physiological, and biochemical alterations in plants. The present study investigated the effects of MPs/NPs on lettuce (Lactuca sativa L. var. capitata) plants by focusing on (i) four different particle sizes of polyethylene micro- and nanoplastics, at (ii) four concentrations. Photosynthetic activity, morphological changes in plants, and metabolomic shifts in roots and leaves were investigated. Our findings revealed that particle size plays a pivotal role in influencing various growth traits of lettuce (biomass, color segmentation, greening index, leaf area, and photosynthetic activity), physiological parameters (including maximum quantum yield - Fv/Fmmax, or quantum yield in the steady-state Fv/FmLss, NPQLss, RfdLss, FtLss, FqLss), and metabolomic signatures. Smaller plastic sizes demonstrated a dose-dependent impact on aboveground plant structures, resulting in an overall elicitation of biosynthetic processes. Conversely, larger plastic size had a major impact on root metabolomics, leading to a negative modulation of biosynthetic processes. Specifically, the biosynthesis of secondary metabolites, phytohormone crosstalk, and the metabolism of lipids and fatty acids were among the most affected processes. In addition, nitrogen-containing compounds accumulated following plastic treatments. Our results highlighted a tight correlation between the qPCR analysis of genes associated with the soil nitrogen cycle (such as NifH, NirK, and NosZ), available nitrogen pools in soil (including NO3- and NH4), N-containing metabolites and morpho-physiological parameters of lettuce plants subjected to MPs/NPs. These findings underscore the intricate relationship between specific plastic contaminations, nitrogen dynamics, and plant performance.

Modulating root system architecture: cross-talk between auxin and phytohormones

Root architecture is an important agronomic trait that plays an essential role in water uptake, soil compactions, nutrient recycling, plant-microbe interactions, and hormone-mediated signaling pathways. Recently, significant advancements have been made in understanding how the complex interactions of phytohormones regulate the dynamic organization of root architecture in crops. Moreover, phytohormones, particularly auxin, act as internal regulators of root development in soil, starting from the early organogenesis to the formation of root hair (RH) through diverse signaling mechanisms. However, a considerable gap remains in understanding the hormonal cross-talk during various developmental stages of roots. This review examines the dynamic aspects of phytohormone signaling, cross-talk mechanisms, and the activation of transcription factors (TFs) throughout various developmental stages of the root life cycle. Understanding these developmental processes, together with hormonal signaling and molecular engineering in crops, can improve our knowledge of root development under various environmental conditions.

Strigolactone and abscisic acid synthesis and signaling pathways are enhanced in the wheat oligo-tillering mutant ot1

Tiller number greatly contributes to grain yield in wheat. Using ethylmethanesulfonate mutagenesis, we previously discovered the oligo-tillering mutant ot1. The tiller number was significantly lower in ot1 than in the corresponding wild type from the early tillering stage until the heading stage. Compared to the wild type, the thousand-grain weight and grain length were increased by 15.41% and 31.44%, respectively, whereas the plant height and spike length were decreased by 26.13% and 37.25%, respectively. Transcriptomic analysis was conducted at the regreening and jointing stages to identify differential expressed genes (DEGs). Functional enrichment analysis with the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases showed differential expression of genes associated with ADP binding, transmembrane transport, and transcriptional regulation during tiller development. Differences in tiller number in ot1 led to the upregulation of genes in the strigolactone (SL) and abscisic acid (ABA) pathways. Specifically, the SL biosynthesis genes DWARF (D27), D17, D10, and MORE AXILLARY GROWTH 1 (MAX1) were upregulated by 3.37- to 8.23-fold; the SL signal transduction genes D14 and D53 were upregulated by 1.81- and 1.32-fold, respectively; the ABA biosynthesis genes 9-CIS-EPOXICAROTENOID DIOXIGENASE 3 (NCED3) and NCED5 were upregulated by 1.66- and 3.4-fold, respectively; and SNF1-REGULATED PROTEIN KINASE2 (SnRK2) and PROTEIN PHOSPHATASE 2C (PP2C) genes were upregulated by 1.30- to 4.79-fold. This suggested that the tiller number reduction in ot1 was due to alterations in plant hormone pathways. Genes known to promote tillering growth were upregulated, whereas those known to inhibit tillering growth were downregulated. For example, PIN-FORMED 9 (PIN9), which promotes tiller development, was upregulated by 8.23-fold in ot1; Ideal Plant Architecture 1 (IPA1), which inhibits tiller development, was downregulated by 1.74-fold. There were no significant differences in the expression levels of TILLER NUMBER 1 (TN1) or TEOSINTE BRANCHED 1 (TB1), indicating that the tiller reduction in ot1 was not controlled by known genes. Our findings provide valuable data for subsequent research into the genetic bases and regulatory mechanisms of wheat tillering.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01450-3.

Modeling the effects of strigolactone levels on maize root system architecture

Maize is the most in-demand staple crop globally. Its production relies strongly on the use of fertilizers for the supply of nitrogen, phosphorus, and potassium, which the plant absorbs through its roots, together with water. The architecture of maize roots is determinant in modulating how the plant interacts with the microbiome and extracts nutrients and water from the soil. As such, attempts to use synthetic biology and modulate that architecture to make the plant more resilient to drought and parasitic plants are underway. These attempts often try to modulate the biosynthesis of hormones that determine root architecture and growth. Experiments are laborious and time-consuming, creating the need for simulation platforms that can integrate metabolic models and 3D root growth models and predict the effects of synthetic biology interventions on both, hormone levels and root system architectures. Here, we present an example of such a platform that is built using Mathematica. First, we develop a root model, and use it to simulate the growth of many unique 3D maize root system architectures (RSAs). Then, we couple this model to a metabolic model that simulates the biosynthesis of strigolactones, hormones that modulate root growth and development. The coupling allows us to simulate the effect of changing strigolactone levels on the architecture of the roots. We then integrate the two models in a simulation platform, where we also add the functionality to analyze the effect of strigolactone levels on root phenotype. Finally, using in silico experiments, we show that our models can reproduce both the phenotype of wild type maize, and the effect that varying strigolactone levels have on changing the architecture of maize roots.

The D14-SDEL1-SPX4 cascade integrates the strigolactone and phosphate signalling networks in rice

Modern agriculture needs large quantities of phosphate (Pi) fertilisers to obtain high yields. Information on how plants sense and adapt to Pi is required to enhance phosphorus-use efficiency (PUE) and thereby promote agricultural sustainability. Here, we show that strigolactones (SLs) regulate rice root developmental and metabolic adaptations to low Pi, by promoting efficient Pi uptake and translocation from roots to shoots. Low Pi stress triggers the synthesis of SLs, which dissociate the Pi central signalling module of SPX domain-containing protein (SPX4) and PHOSPHATE STARVATION RESPONSE protein (PHR2), leading to the release of PHR2 into the nucleus and activating the expression of Pi-starvation-induced genes including Pi transporters. The SL synthetic analogue GR24 enhances the interaction between the SL receptor DWARF 14 (D14) and a RING-finger ubiquitin E3 ligase (SDEL1). The sdel mutants have a reduced response to Pi starvation relative to wild-type plants, leading to insensitive root adaptation to Pi. Also, SLs induce the degradation of SPX4 via forming the D14-SDEL1-SPX4 complex. Our findings reveal a novel mechanism underlying crosstalk between the SL and Pi signalling networks in response to Pi fluctuations, which will enable breeding of high-PUE crop plants.

Genome-wide association study in two-row spring barley landraces identifies QTL associated with plantlets root system architecture traits in well-watered and osmotic stress conditions

Water availability is undoubtedly one of the most important environmental factors affecting crop production. Drought causes a gradual deprivation of water in the soil from top to deep layers and can occur at diverse stages of plant development. Roots are the first organs that perceive water deficit in soil and their adaptive development contributes to drought adaptation. Domestication has contributed to a bottleneck in genetic diversity. Wild species or landraces represent a pool of genetic diversity that has not been exploited yet in breeding program. In this study, we used a collection of 230 two-row spring barley landraces to detect phenotypic variation in root system plasticity in response to drought and to identify new quantitative trait loci (QTL) involved in root system architecture under diverse growth conditions. For this purpose, young seedlings grown for 21 days in pouches under control and osmotic-stress conditions were phenotyped and genotyped using the barley 50k iSelect SNP array, and genome-wide association studies (GWAS) were conducted using three different GWAS methods (MLM GAPIT, FarmCPU, and BLINK) to detect genotype/phenotype associations. In total, 276 significant marker-trait associations (MTAs; p-value (FDR)< 0.05) were identified for root (14 and 12 traits under osmotic-stress and control conditions, respectively) and for three shoot traits under both conditions. In total, 52 QTL (multi-trait or identified by at least two different GWAS approaches) were investigated to identify genes representing promising candidates with a role in root development and adaptation to drought stress.

Liriodendron chinense LcMAX1 regulates primary root growth and shoot branching in Arabidopsis thaliana

Strigolactones (SLs) play prominent roles in regulating shoot branching and root architecture in model plants. However, their roles in non-model (particularly woody) plants remain unclear. Liriodendron chinense is a timber tree species widely planted in southern China. The outturn percentage and wood quality of L. chinense are greatly affected by the branching characteristics of its shoot, and the rooting ability of the cuttings is key for its vegetative propagation. Here, we isolated and analyzed the function of the MORE AXILLARY GROWTH 1 (LcMAX1) gene, which is involved in L. chinense SL biosynthesis. RT-qPCR showed that LcMAX1 was highly expressed in the roots and axillary buds. LcMAX1 was located in the endoplasmic reticulum (ER) and nucleus. LcMAX1 ectopic expression promoted primary root growth, whereas there were no phenotypic differences in shoot branching between transgenic and wild-type (WT) A. thaliana plants. LcMAX1 overexpression in the max1 mutant restored them to the WT A. thaliana phenotypes. Additionally, AtPIN1, AtPIN2, and AtBRC1 expressions were significantly upregulated in transgenic A. thaliana and the max1 mutant. It was therefore speculated that LcMAX1 promotes primary root growth by regulating expression of auxin transport-related genes in A. thaliana, and LcMAX1 inhibits shoot branching by upregulating expression of AtBRC1 in the max1 mutant. Altogether, these results demonstrated that the root development and shoot branching functions of LcMAX1 were similar to those of AtMAX1. Our findings provide a foundation for obtaining further insights into root and branch development in L. chinense.

Chromosome-level genome assembly of a xerophytic plant, Haloxylon ammodendron

Haloxylon ammodendron is a xerophytic perennial shrub or small tree that has a high ecological value in anti-desertification due to its high tolerance to drought and salt stress. Here, we report a high-quality, chromosome-level genome assembly of H. ammodendron by integrating PacBio's high-fidelity sequencing and Hi-C technology. The assembled genome size was 685.4 Mb, of which 99.6% was assigned to nine pseudochromosomes with a contig N50 value of 23.6 Mb. Evolutionary analysis showed that both the recent substantial amplification of long terminal repeat retrotransposons and tandem gene duplication may have contributed to its genome size expansion and arid adaptation. An ample amount of low-GC genes was closely related to functions that may contribute to the desert adaptation of H. ammodendron. Gene family clustering together with gene expression analysis identified differentially expressed genes that may play important roles in the direct response of H. ammodendron to water-deficit stress. We also identified several genes possibly related to the degraded scaly leaves and well-developed root system of H. ammodendron. The reference-level genome assembly presented here will provide a valuable genomic resource for studying the genome evolution of xerophytic plants, as well as for further genetic breeding studies of H. ammodendron.

Dissecting the metabolic reprogramming of maize root under nitrogen-deficient stress conditions

The growth and development of maize (Zea mays L.) largely depends on its nutrient uptake through the root. Hence, studying its growth, response, and associated metabolic reprogramming to stress conditions is becoming an important research direction. A genome-scale metabolic model (GSM) for the maize root was developed to study its metabolic reprogramming under nitrogen stress conditions. The model was reconstructed based on the available information from KEGG, UniProt, and MaizeCyc. Transcriptomics data derived from the roots of hydroponically grown maize plants were used to incorporate regulatory constraints in the model and simulate nitrogen-non-limiting (N+) and nitrogen-deficient (N-) condition. Model-predicted flux-sum variability analysis achieved 70% accuracy compared with the experimental change of metabolite levels. In addition to predicting important metabolic reprogramming in central carbon, fatty acid, amino acid, and other secondary metabolism, maize root GSM predicted several metabolites (l-methionine, l-asparagine, l-lysine, cholesterol, and l-pipecolate) playing a regulatory role in the root biomass growth. Furthermore, this study revealed eight phosphatidylcholine and phosphatidylglycerol metabolites which, even though not coupled with biomass production, played a key role in the increased biomass production under N-deficient conditions. Overall, the omics-integrated GSM provides a promising tool to facilitate stress condition analysis for maize root and engineer better stress-tolerant maize genotypes.

Involvement of strigolactone hormone in root development, influence and interaction with mycorrhizal fungi in plant: Mini-review

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Arbuscular mycorrhizal fungi (AMF) and plant symbiosis.

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Role AMF in root development and plant growth promotion.

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AMF influence and plant response under strigolactone (SL) and SL-GR24 application.

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Effects and functions of SL in root development and interaction with AMF.

Arbuscular mycorrhizal fungi (AMF) and plant symbiosis is the old, fascinating and beneficial relation that exist on earth for the plants. In this review, we have elaborated that the strigolactones (SLs) are released from the roots and function with root parasite, seeds and symbiotic AMF as contact chemicals. They are transported through the xylem in the plants and can regulate plant architecture, seed germination, nodule formation, increase the primary root length, influence the root hairs and physiological reactions to non-living agents by regulating their metabolism. SLs first evolved in ancient plant lineages as regulators of the basic production processes and then took a new role to maintain the growing biological complexities of terrestrial plant. SLs belongs to a diversified category of butenolide‐bearing plant hormones related to various processes of agricultural concern. SLs also arouses the development of spores, the divergence and enlargement of hyphae of AMF, metabolism of mitochondria, reprogramming of transcription process, and generation of chitin oligosaccharides which further stimulate the early response of symbiosis in the host plant, results from better communication in plant and ability of coexistence with these fungi. The required nutrients are transferred from the roots to the shoots, which affect the physiological, biochemical, and morphological characteristics of the plant. On the other hand, the plant provides organic carbon in the form of sugars and lipids to the fungi, which they use as a source of energy and for carried out different anabolic pathways. SLs also lead to alteration in the dynamic and structure of actin in the root region as well as changes the auxin's transporter localization in the plasma membrane. Thus, this study reveals the functions that SLs play in the growth of roots, as well as their effect and interaction with AMF that promote plant growth.

KAI2 promotes Arabidopsis root hair elongation at low external phosphate by controlling local accumulation of AUX1 and PIN2

Root hair (RH) growth to increase the absorptive root surface area is a key adaptation of plants to limiting phosphate availability in soils. Despite the importance of this trait, especially for seedling survival, little is known about the molecular events connecting phosphate starvation sensing and RH growth regulation. KARRIKIN INSENSITIVE2 (KAI2), an α/β-hydrolase receptor of a yet-unknown plant hormone ("KAI2-ligand" [KL]), is required for RH elongation.¹ KAI2 interacts with the F-box protein MORE AXILLIARY BRANCHING2 (MAX2) to target regulatory proteins of the SUPPRESSOR of MAX2 1 (SMAX1) family for degradation.² Here, we demonstrate that P_i starvation increases KL signaling in Arabidopsis roots through transcriptional activation of KAI2 and MAX2. Both genes are required for RH elongation under these conditions, while smax1 smxl2 mutants have constitutively long RHs, even at high P_i availability. Attenuated RH elongation in kai2 mutants is explained by reduced shootward auxin transport from the root tip resulting in reduced auxin signaling in the RH zone, caused by an inability to increase localized accumulation of the auxin importer AUXIN TRANSPORTER PROTEIN1 (AUX1) and the auxin exporter PIN-FORMED2 (PIN2) upon P_i starvation. Consistent with AUX1 and PIN2 accumulation being mediated via ethylene signaling,³ expression of 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE 7 (ACS7) is increased at low P_i in a KAI2-dependent manner, and treatment with an ethylene precursor restores RH elongation of acs7, but not of aux1 and pin2. Thus, KAI2 signaling is increased by phosphate starvation to trigger an ethylene- AUX1/PIN2-auxin cascade required for RH elongation.

Digital RNA-seq transcriptome plus tissue anatomy analyses reveal the developmental mechanism of the calabash-shaped root in Tetrastigma hemsleyanum

Tetrastigma hemsleyanum Diels & Gilg ex Diels is a liana plant with promising medicinal and ornamental values. Its calabash-shaped roots (CRs) are served as a traditional Chinese herb. However, it takes a long growth period to form CRs. In this study, three types of architectural roots, including fibrous roots (FRs), bar-shaped roots (BRs) and CRs, were employed as materials, and the characteristics of histo-anatomy and digital RNA-seq transcriptome profiles were analyzed. Among the three types of roots, the vascular bundles in FRs were intact, while some of the vascular bundles degenerated in BRs, and only few traces of vascular bundles existed in CRs. Meanwhile, no obvious cell inclusions were found in the cytoplasm of FRs, while a few inclusions were found in BRs, and abundant inclusions were detected in CRs, which might be the main source of medicinal components in roots. The transcriptome profiles and qRT-PCR validation indicated that seven upregulated genes, encoding xyloglucan glycosyltransferase, ACC oxidase, CYP711A1, SHORT-ROOT transcript factor, galacturonosyltransferas, WAT1 and WRKY, and two downregulated genes, encoding LRR receptor-like serine/threonine-protein kinase and CYP83B1, were probably involved in the formation and development of CRs. In addition, Gene Ontology terms of intrinsic component of membrane, integral component of membrane, cell periphery, membrane part, plasma membrane, membrane, intrinsic component of plasma membrane, cellular chemical homeostasis and plasma membrane part were probably related to the formation of CRs. Kyoto Encyclopedia of Genes and Genomes pathways related to the development of CRs probably included MAPK signaling pathway-plant, plant hormone signal transduction and circadian rhythm-plant. Our finding suggested a probable mode for the formation of CRs.

Bioassays for the Effects of Strigolactones and Other Small Molecules on Root and Root Hair Development

Growth and development of plant roots are highly dynamic and adaptable to environmental conditions. They are under the control of several plant hormone signaling pathways, and therefore root developmental responses can be used as bioassays to study the action of plant hormones and other small molecules. In this chapter, we present different procedures to measure root traits of the model plant Arabidopsis thaliana. We explain methods for phenotypic analysis of lateral root development, primary root length, root skewing and straightness, and root hair density and length. We describe optimal growth conditions for Arabidopsis seedlings for reproducible root and root hair developmental outputs; and how to acquire images and measure the different traits using image analysis with relatively low-tech equipment. We provide guidelines for a semiautomatic image analysis of primary root length, root skewing, and root straightness in Fiji and a script to automate the calculation of root angle deviation from the vertical and root straightness. By including mutants defective in strigolactone (SL) or KAI2 ligand (KL) synthesis and/or signaling, these methods can be used as bioassays for different SLs or SL-like molecules. In addition, the techniques described here can be used for studying seedling root system architecture, root skewing, and root hair development in any context.

Diverse Roles of MAX1 Homologues in Rice

Cytochrome P450 enzymes encoded by MORE AXILLARY GROWTH1 (MAX1)-like genes produce most of the structural diversity of strigolactones during the final steps of strigolactone biosynthesis. The diverse copies of MAX1 in Oryza sativa provide a resource to investigate why plants produce such a wide range of strigolactones. Here we performed in silico analyses of transcription factors and microRNAs that may regulate each rice MAX1, and compared the results with available data about MAX1 expression profiles and genes co-expressed with MAX1 genes. Data suggest that distinct mechanisms regulate the expression of each MAX1. Moreover, there may be novel functions for MAX1 homologues, such as the regulation of flower development or responses to heavy metals. In addition, individual MAX1s could be involved in specific functions, such as the regulation of seed development or wax synthesis in rice. Our analysis reveals potential new avenues of strigolactone research that may otherwise not be obvious.

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.

Role of Strigolactones: Signalling and Crosstalk with Other Phytohormones

Plant hormones play important roles in controlling how plants grow and develop. While metabolism provides the energy needed for plant survival, hormones regulate the pace of plant growth. Strigolactones (SLs) were recently defined as new phytohormones that regulate plant metabolism and, in turn, plant growth and development. This group of phytohormones is derived from carotenoids and has been implicated in a wide range of physiological functions including regulation of plant architecture (inhibition of bud outgrowth and shoot branching), photomorphogenesis, seed germination, nodulation, and physiological reactions to abiotic factors. SLs also induce hyphal branching in germinating spores of arbuscular mycorrhizal fungi (AMF), a process that is important for initiating the connection between host plant roots and AMF. This review outlines the physiological roles of SLs and discusses the significance of interactions between SLs and other phytohormones to plant metabolic responses.

Root exudates: from plant to rhizosphere and beyond

This article describes the composition of root exudates, how these metabolites are released to the rhizosphere and their importance in the recruitment of beneficial microbiota that alleviate plant stress. Metabolites secreted to the rhizosphere by roots are involved in several processes. By modulating the composition of the root exudates, plants can modify soil properties to adapt and ensure their survival under adverse conditions. They use several strategies such as (1) changing soil pH to solubilize nutrients into assimilable forms, (2) chelating toxic compounds, (3) attracting beneficial microbiota, or (4) releasing toxic substances for pathogens, etc. In this work, the composition of root exudates as well as the different mechanisms of root exudation have been reviewed. Existing methodologies to collect root exudates, indicating their advantages and disadvantages, are also described. Factors affecting root exudation have been exposed, including physical, chemical, and biological agents which can produce qualitative and quantitative changes in exudate composition. Finally, since root exudates play an important role in the recruitment of mycorrhizal fungi and plant growth-promoting rhizobacteria (PGPR), the mechanisms of interaction between plants and the beneficial microbiota have been highlighted.

Strigolactones and their crosstalk with other phytohormones

BACKGROUND

Strigolactones (SLs) are a diverse class of butenolide-bearing phytohormones derived from the catabolism of carotenoids. They are associated with an increasing number of emerging regulatory roles in plant growth and development, including seed germination, root and shoot architecture patterning, nutrient acquisition, symbiotic and parasitic interactions, as well as mediation of plant responses to abiotic and biotic cues.

SCOPE

Here, we provide a concise overview of SL biosynthesis, signal transduction pathways and SL-mediated plant responses with a detailed discourse on the crosstalk(s) that exist between SLs/components of SL signalling and other phytohormones such as auxins, cytokinins, gibberellins, abscisic acid, ethylene, jasmonates and salicylic acid.

CONCLUSION

SLs elicit their control on physiological and morphological processes via a direct or indirect influence on the activities of other hormones and/or integrants of signalling cascades of other growth regulators. These, among many others, include modulation of hormone content, transport and distribution within plant tissues, interference with or complete dependence on downstream signal components of other phytohormones, as well as acting synergistically or antagonistically with other hormones to elicit plant responses. Although much has been done to evince the effects of SL interactions with other hormones at the cell and whole plant levels, research attention must be channelled towards elucidating the precise molecular events that underlie these processes. More especially in the case of abscisic acid, cytokinins, gibberellin, jasmonates and salicylic acid for which very little has been reported about their hormonal crosstalk with SLs.

SMAX1/SMXL2 regulate root and root hair development downstream of KAI2-mediated signalling in Arabidopsis

Karrikins are smoke-derived compounds presumed to mimic endogenous signalling molecules (KAI2-ligand, KL), whose signalling pathway is closely related to that of strigolactones (SLs), important regulators of plant development. Both karrikins/KLs and SLs are perceived by closely related α/β hydrolase receptors (KAI2 and D14 respectively), and signalling through both receptors requires the F-box protein MAX2. Furthermore, both pathways trigger proteasome-mediated degradation of related SMAX1-LIKE (SMXL) proteins, to influence development. It has previously been suggested in multiple studies that SLs are important regulators of root and root hair development in Arabidopsis, but these conclusions are based on phenotypes observed in the non-specific max2 mutants and by use of racemic-GR24, a mixture of stereoisomers that activates both D14 and KAI2 signalling pathways. Here, we demonstrate that the majority of the effects on Arabidopsis root development previously attributed to SL signalling are actually mediated by the KAI2 signalling pathway. Using mutants defective in SL or KL synthesis and/or perception, we show that KAI2-mediated signalling alone regulates root hair density and root hair length as well as root skewing, straightness and diameter, while both KAI2 and D14 pathways regulate lateral root density and epidermal cell length. We test the key hypothesis that KAI2 signals by a non-canonical receptor-target mechanism in the context of root development. Our results provide no evidence for this, and we instead show that all effects of KAI2 in the root can be explained by canonical SMAX1/SMXL2 activity. However, we do find evidence for non-canonical GR24 ligand-receptor interactions in D14/KAI2-mediated root hair development. Overall, our results demonstrate that the KAI2 signalling pathway is an important new regulator of root hair and root development in Arabidopsis and lay an important basis for research into a molecular understanding of how very similar and partially overlapping hormone signalling pathways regulate different phenotypic outputs.

Impairment in karrikin but not strigolactone sensing enhances root skewing in Arabidopsis thaliana

Roots form highly complex systems varying in growth direction and branching pattern to forage for nutrients efficiently. Here mutations in the KAI2 (KARRIKIN INSENSITIVE) α/β-fold hydrolase and the MAX2 (MORE AXILLARY GROWTH 2) F-box leucine-rich protein, which together perceive karrikins (smoke-derived butenolides), caused alteration in root skewing in Arabidopsis thaliana. This phenotype was independent of endogenous strigolactones perception by the D14 α/β-fold hydrolase and MAX2. Thus, KAI2/MAX2 effect on root growth may be through the perception of endogenous KAI2-ligands (KLs), which have yet to be identified. Upon perception of a ligand, a KAI2/MAX2 complex is formed together with additional target proteins before ubiquitination and degradation through the 26S proteasome. Using a genetic approach, we show that SMAX1 (SUPPRESSOR OF MAX2-1)/SMXL2 and SMXL6,7,8 (SUPPRESSOR OF MAX2-1-LIKE) are also likely degradation targets for the KAI2/MAX2 complex in the context of root skewing. In A. thaliana therefore, KAI2 and MAX2 act to limit root skewing, while kai2's gravitropic and mechano-sensing responses remained largely unaffected. Many proteins are involved in root skewing, and we investigated the link between MAX2 and two members of the SKS/SKU family. Though KLs are yet to be identified in plants, our data support the hypothesis that they are present and can affect root skewing.

Methylation at the C-3' in D-Ring of Strigolactone Analogs Reduces Biological Activity in Root Parasitic Plants and Rice

Strigolactones (SLs) regulate plant development and induce seed germination in obligate root parasitic weeds, e.g. Striga spp. Because organic synthesis of natural SLs is laborious, there is a large need for easy-to-synthesize and efficient analogs. Here, we investigated the effect of a structural modification of the D-ring, a conserved structural element in SLs. We synthesized and investigated the activity of two analogs, MP13 and MP26, which differ from previously published AR8 and AR36 only in the absence of methylation at C-3'. The de-methylated MP13 and MP26 were much more efficient in regulating plant development and inducing Striga seed germination, compared with AR8. Hydrolysis assays performed with purified Striga SL receptor and docking of AR8 and MP13 to the corresponding active site confirmed and explained the higher activity. Field trials performed in a naturally Striga-infested African farmer's field unraveled MP13 as a promising candidate for combating Striga by inducing germination in host's absence. Our findings demonstrate that methylation of the C-3' in D-ring in SL analogs has a negative impact on their activity and identify MP13 and, particularly, MP26 as potent SL analogs with simple structures, which can be employed to control Striga, a major threat to global food security.

Regulation of Root Development and Architecture by Strigolactones under Optimal and Nutrient Deficiency Conditions

Strigolactones (SLs) constitute a group of plant hormones which are involved in multiple aspects of plant growth and development. Beside their role in shoot and root development and plant architecture in general, SLs are also involved in plant responses to nutrient deficiency by promoting interactions with symbiotic organisms and via promotion of root elongation. Recent observations on the cross talk between SLs and other hormones demonstrate that the inhibition of adventitious root formation by ethylene is independent of SLs. Additionally, it was shown that root exposure to SLs leads to the accumulation of secondary metabolites, such as flavonols or antioxidants. These data suggest pleiotropic effects of SLs, that influence root development. The discovery that the commonly used synthetic SL analogue racGR24 might also mimic the function of other plant growth regulators, such as karrikins, has led us to consider the previously published publications under the new aspects. This review summarizes present knowledge about the function of SLs in shaping root systems under optimal and nutrient deficiency conditions. Results which appear inconsistent with the various aspects of root development are singled out.

Strigolactones cross the kingdoms: plants, fungi, and bacteria in the arbuscular mycorrhizal symbiosis

Strigolactones (SLs) first evolved as regulators of simple developmental processes in very ancient plant lineages, and then assumed new roles to sustain the increasing biological complexity of land plants. Their versatility is also shown by the fact that during evolution they have been exploited, once released in the rhizosphere, as a communication system towards plant-interacting organisms even belonging to different kingdoms. Here, we reviewed the impact of SLs on soil microbes, paying particular attention to arbuscular mycorrhizal fungi (AMF). SLs induce several responses in AMF, including spore germination, hyphal branching, mitochondrial metabolism, transcriptional reprogramming, and production of chitin oligosaccharides which, in turn, stimulate early symbiotic responses in the host plant. In the specific case study of the AMF Gigaspora margarita, SLs are also perceived, directly or indirectly, by the well-characterized population of endobacteria, with an increase of bacterial divisions and the activation of specific transcriptional responses. The dynamics of SLs during AM root colonization were also surveyed. Although not essential for the establishment of this mutualistic association, SLs act as positive regulators as they are relevant to achieve the full extent of colonization. This possibly occurs through a complex crosstalk with other hormones such as auxin, abscisic acid, and gibberellins.

Perception and Signaling of Strigolactones

Strigolactones (SLs), a recently discovered class of phytohormones, are important regulators of plant growth and development. While the biosynthetic pathway of these molecules is well documented, until recently there was not much known about the molecular mechanisms underlying SL perception and signal transduction in plants. Certain aspects of their perception and signaling, including the hormone-mediated interaction between receptor and F-box protein, degradation of suppressor proteins and activation of transcription factors, are also found in other phytohormones. However, some of SL signaling features seem to be specific for the SL signaling pathway. These include the enzymatic activity of the SL receptor and its destabilization caused by SLs. This review summarizes the current knowledge about SL signaling pathway in plants.