Strigolactones are transported from roots to shoots, although not through the xylem

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.

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.

Hyphal Growth and Conidia Germination Are Induced by Phytohormones in the Root Colonizing and Plant Growth Promoting Fungus Metarhizium guizhouense

Beneficial associations are very important for plants and soil-dwelling microorganisms in different ecological niches, where communication by chemical signals is relevant. Among the chemical signals, the release of phytohormones by plants is important to establish beneficial associations with fungi, and a recently described association is that of the entomopathogenic ascomycete fungus Metarhizium with plants. Here, we evaluated the effect of four different phytohormones, synthetic strigolactone (GR24), sorgolactone (SorL), 3-indolacetic acid (IAA) and gibberellic acid (GA3), on the fungus Metarhizium guizhouense strain HA11-2, where the germination rate and hyphal elongation were determined at three different times. All phytohormones had a positive effect on germination, with GA3 showing the greatest effect, and for hyphal length, on average, the group treated with synthetic strigolactone GR24 showed greater average hyphal length at 10 h of induction. This work expands the knowledge of the effect of phytohormones on the fungus M. guizhouense, as possible chemical signals for the rapid establishment of the fungus-plant association.

Strigolactones in Rhizosphere Communication: Multiple Molecules With Diverse Functions

Strigolactones (SLs) are root-secreted small molecules that influence organisms living in the rhizosphere. While SLs are known as germination stimulants for root parasitic plants and as hyphal branching factors for arbuscular mycorrhizal fungi, recent studies have also identified them as chemoattractants for parasitic plants, sensors of neighboring plants and key players in shaping the microbiome community. Furthermore, the discovery of structurally diverged SLs, including so-called canonical and non-canonical SLs in various plant species, raises the question of whether the same SLs are responsible for their diverse functions 'in planta' and the rhizosphere or whether different molecules play different roles. Emerging evidence supports the latter, with each SL exhibiting different activities as rhizosphere signals and plant hormones. The evolution of D14/KAI2 receptors has enabled the perception of various SLs or SL-like compounds to control downstream signaling, highlighting the complex interplay between plants and their rhizosphere environment. This review summarizes the recent advances in our understanding of the diverse functions of SLs in the rhizosphere.

Strigolactones and Shoot Branching: What Is the Real Hormone and How Does It Work?

There have been substantial advances in our understanding of many aspects of strigolactone regulation of branching since the discovery of strigolactones as phytohormones. These include further insights into the network of phytohormones and other signals that regulate branching, as well as deep insights into strigolactone biosynthesis, metabolism, transport, perception and downstream signaling. In this review, we provide an update on recent advances in our understanding of how the strigolactone pathway co-ordinately and dynamically regulates bud outgrowth and pose some important outstanding questions that are yet to be resolved.

Strigolactones can be a potential tool to fight environmental stresses in arid lands

BACKGROUND

Environmental stresses pose a significant threat to plant growth and ecosystem productivity, particularly in arid lands that are more susceptible to climate change. Strigolactones (SLs), carotenoid-derived plant hormones, have emerged as a potential tool for mitigating environmental stresses.

METHODS

This review aimed to gather information on SLs' role in enhancing plant tolerance to ecological stresses and their possible use in improving the resistance mechanisms of arid land plant species to intense aridity in the face of climate change.

RESULTS

Roots exude SLs under different environmental stresses, including macronutrient deficiency, especially phosphorus (P), which facilitates a symbiotic association with arbuscular mycorrhiza fungi (AMF). SLs, in association with AMF, improve root system architecture, nutrient acquisition, water uptake, stomatal conductance, antioxidant mechanisms, morphological traits, and overall stress tolerance in plants. Transcriptomic analysis revealed that SL-mediated acclimatization to abiotic stresses involves multiple hormonal pathways, including abscisic acid (ABA), cytokinins (CK), gibberellic acid (GA), and auxin. However, most of the experiments have been conducted on crops, and little attention has been paid to the dominant vegetation in arid lands that plays a crucial role in reducing soil erosion, desertification, and land degradation. All the environmental gradients (nutrient starvation, drought, salinity, and temperature) that trigger SL biosynthesis/exudation prevail in arid regions. The above-mentioned functions of SLs can potentially be used to improve vegetation restoration and sustainable agriculture.

CONCLUSIONS

Present review concluded that knowledge on SL-mediated tolerance in plants is developed, but still in-depth research is needed on downstream signaling components in plants, SL molecular mechanisms and physiological interactions, efficient methods of synthetic SLs production, and their effective application in field conditions. This review also invites researchers to explore the possible application of SLs in improving the survival rate of indigenous vegetation in arid lands, which can potentially help combat land degradation problems.

Environmental strigolactone drives early growth responses to neighboring plants and soil volume in pea

There has been a dramatic recent increase in the understanding of the mechanisms by which plants detect their neighbors,1 including by touch,2 reflected light,3 volatile organic chemicals, and root exudates.4,5 The importance of root exudates remains ill-defined because of confounding experimental variables6,7 and difficulties disentangling neighbor detection in shoot and roots.8-10 There is evidence that root exudates allow distinction between kin and non-kin neighbors,11-13 but identification of specific exudates that function in neighbor detection and/or kin recognition remain elusive.1 Strigolactones (SLs), which are exuded into the soil in significant quantities in flowering plants to promote recruitment of arbuscular mycorrhizal fungi (AMF),14 seem intuitive candidates to act as plant-plant signals, since they also act as hormones in plants,15-17 with dramatic effects on shoot growth18,19 and milder effects on root development.20 Here, using pea, we test whether SLs act as either cues or signals for neighbor detection. We show that peas detect neighbors early in the life cycle through their root systems, resulting in strong changes in shoot biomass and branching, and that this requires SL biosynthesis. We demonstrate that uptake and detection of SLs exuded by neighboring plants are needed for this early neighbor detection, and that plants that cannot exude SLs are outcompeted by neighboring plants and fail to adjust growth to their soil volume. We conclude that plants both exude SLs as signals to modulate neighbor growth and detect environmental SLs as a cue for neighbor presence; collectively, this allows plants to proactively adjust their shoot growth according to neighbor density.

How Strigolactone Shapes Shoot Architecture

Despite its central role in the control of plant architecture, strigolactone has been recognized as a phytohormone only 15 years ago. Together with auxin, it regulates shoot branching in response to genetically encoded programs, as well as environmental cues. A central determinant of shoot architecture is apical dominance, i.e., the tendency of the main shoot apex to inhibit the outgrowth of axillary buds. Hence, the execution of apical dominance requires long-distance communication between the shoot apex and all axillary meristems. While the role of strigolactone and auxin in apical dominance appears to be conserved among flowering plants, the mechanisms involved in bud activation may be more divergent, and include not only hormonal pathways but also sugar signaling. Here, we discuss how spatial aspects of SL biosynthesis, transport, and sensing may relate to apical dominance, and we consider the mechanisms acting locally in axillary buds during dormancy and bud activation.

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

Wheat plants sense substrate volume and root density to proactively modulate shoot growth

Plants must carefully coordinate their growth and development with respect to prevailing environmental conditions. To do this, plants can use a range of nutritional and non-nutritional information that allows them to proactively modulate their growth to avoid resource limitations. As is well-known to gardeners and horticulturists alike, substrate volume strongly influences plant growth, and maybe a key source of non-nutritional information for plants. However, the mechanisms by which these substrate volume effects occur remain unclear. Here, we show that wheat plants proactively modulate their shoot growth with respect to substrate volume, independent of nutrient availability. We show that these effects occur in two phases; in the first phase, the dilution of a mobile 'substrate volume-sensing signal' (SVS) allows plants to match their shoot (but not root) growth to the total size of the substrate, irrespective of how much of this they can occupy with their roots. In the second phase, the dilution of a less mobile 'root density-sensing signal' (RDS) allows plants to match root growth to actual rooting volume, with corresponding effects on shoot growth. We show that the effects of soil volume and plant density are largely interchangeable and that plants may use both SVS and RDS to detect their neighbours and to integrate growth responses to both volume and the presence of neighbours. Our work demonstrates the remarkable ability of plants to make proactive decisions about their growth, and has implications for mitigating the effects of dense sowing of crops in agricultural practice.

Strigolactone biosynthesis, transport and perception

Strigolactones (SLs) are plant hormones that regulate diverse developmental processes and environmental responses. They are also known to be root-derived chemical signals that regulate symbiotic and parasitic interactions with arbuscular mycorrhizal fungi and root parasitic plants, respectively. Since the discovery of the hormonal function of SLs in 2008, there has been much progress in the SL research field. In particular, a number of breakthroughs have been achieved in our understanding of SL biosynthesis, transport and perception. The discovery of the hormonal function of SL was quite valuable not only as the identification of a new class of plant hormones, but also as the discovery of the long-sought-after SL biosynthetic and response mutants. These mutants in several plant species provided us the genetic resources to address fundamental questions regarding SL biosynthesis and perception. Such mutants were further characterized later, and biochemical analyses of these genetically identified factors have uncovered the outline of SL biosynthesis and perception so far. Moreover, new genes involved in SL transport have been discovered through reverse genetic analyses. In this review, we summarize recent advances in SL research with a focus on biosynthesis, transport and perception.

Application of Strigolactones to Plant Roots to Influence Formation of Symbioses

Strigolactones play a potent role in the rhizosphere as a signal to symbiotic microbes including arbuscular mycorrhizal fungi and rhizobial bacteria. This chapter outlines guidelines for application of strigolactones to pea roots to influence symbiotic relationships, and includes careful consideration of type of strigolactones applied, solvent use, frequency of application and nutrient regime to optimize experimental conditions.

There and back again: An evolutionary perspective on long-distance coordination of plant growth and development

Vascular plants, unlike bryophytes, have a strong root-shoot dichotomy in which the tissue systems are mutually interdependent; roots are completely dependent on shoots for photosynthetic sugars, and shoots are completely dependent on roots for water and mineral nutrients. Long-distance communication between shoot and root is therefore critical for the growth, development and survival of vascular plants, especially with regard to variable environmental conditions. However, this long-distance signalling does not appear an ancestral feature of land plants, and has likely arisen in vascular plants to service the radical alterations in body-plan seen in this taxon. In this review, we examine the defined hormonal root-to-shoot and shoot-to-root signalling pathways that coordinate the growth of vascular plants, with a particular view to understanding how these pathways may have evolved. We highlight the completely divergent roles of isopentenyl-adenine and trans-zeatin cytokinin species in long-distance signalling, and ask whether cytokinin can really be considered as a single class of hormones in the light of recent research. We also discuss the puzzlingly sparse evidence for auxin as a shoot-to-root signal, the evolutionary re-purposing of strigolactones and gibberellins as hormonal signals, and speculate on the possible role of sugars as long-distance signals. We conclude by discussing the 'design principles' of long-distance signalling in vascular plants.

Differential role of MAX2 and strigolactones in pathogen, ozone, and stomatal responses

Strigolactones are a group of phytohormones that control developmental processes including shoot branching and various plant-environment interactions in plants. We previously showed that the strigolactone perception mutant more axillary branches 2 (max2) has increased susceptibility to plant pathogenic bacteria. Here we show that both strigolactone biosynthesis (max3 and max4) and perception mutants (max2 and dwarf14) are significantly more sensitive to Pseudomonas syringae DC3000. Moreover, in response to P. syringae infection, high levels of SA accumulated in max2 and this mutant was ozone sensitive. Further analysis of gene expression revealed no major role for strigolactone in regulation of defense gene expression. In contrast, guard cell function was clearly impaired in max2 and depending on the assay used, also in max3, max4, and d14 mutants. We analyzed stomatal responses to stimuli that cause stomatal closure. While the response to abscisic acid (ABA) was not impaired in any of the mutants, the response to darkness and high CO2 was impaired in max2 and d14-1 mutants, and to CO2 also in strigolactone synthesis (max3, max4) mutants. To position the role of MAX2 in the guard cell signaling network, max2 was crossed with mutants defective in ABA biosynthesis or signaling. This revealed that MAX2 acts in a signaling pathway that functions in parallel to the guard cell ABA signaling pathway. We propose that the impaired defense responses of max2 are related to higher stomatal conductance that allows increased entry of bacteria or air pollutants like ozone. Furthermore, as MAX2 appears to act in a specific branch of guard cell signaling (related to CO2 signaling), this protein could be one of the components that allow guard cells to distinguish between different environmental conditions.

Strigolactone-Based Node-to-Bud Signaling May Restrain Shoot Branching in Hybrid Aspen

The biosynthesis and roles of strigolactones (SLs) have been investigated in herbaceous plants, but so far, their role in trees has received little attention. In this study, we analyzed the presence, spatial/temporal expression and role of SL pathway genes in Populus tremula � Populus tremuloides. In this proleptic species, axillary buds (AXBs) become para-dormant at the bud maturation point, providing an unambiguous starting point to study AXB activation. We identified previously undescribed Populus homologs of DWARF27 (D27), LATERAL BRANCHING OXIDOREDUCTASE (LBO) and DWARF53-like (D53-like) and analyzed the relative expression of all SL pathway genes in root tips and shoot tissues. We found that, although AXBs expressed MORE AXILLARY GROWTH1 (MAX1) and LBO, they did not express MAX3 and MAX4, whereas nodal bark expressed high levels of all SL biosynthesis genes. By contrast, expression of the SL perception and signaling genes MAX2, D14 and D53 was high in AXBs relative to nodal bark and roots. This suggests that AXBs are reliant on the associated nodes for the import of SLs and SL precursors. Activation of AXBs was initiated by decapitation and single-node isolation. This rapidly downregulated SL pathway genes downstream of MAX4, although later these genes were upregulated coincidently with primordia formation. GR24-feeding counteracted all activation-related changes in SL gene expression but did not prevent AXB outgrowth showing that SL is ineffective once AXBs are activated. The results indicate that nodes rather than roots supply SLs and its precursors to AXBs, and that SLs may restrain embryonic shoot elongation during AXB formation and para-dormancy in intact plants.

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.

Petunia PLEIOTROPIC DRUG RESISTANCE 1 Is a Strigolactone Short-Distance Transporter with Long-Distance Outcomes

Phytohormones of the strigolactone (SL) family have been characterized as negative regulators of lateral bud outgrowth and triggers of symbioses between plants and mycorrhizal fungi. SLs and their precursors are synthesized in root tips as well as along shoot and root vasculature; they either move shoot-wards and regulate plant architecture or are exuded from roots into the soil to establish mycorrhizal symbiosis. Owing to the difficulty in quantification of SL in shoot tissues because of low abundance, it is not yet clear how SL distribution in plants is regulated at short- and long-distances from SL biosynthetic and target tissues. To address this question, we grafted wild-type scions and rootstocks from different petunia mutants for SL biosynthesis/transport and investigated SL activity by quantifying lateral bud outgrowth in the main shoot. Based on these results, we show that (i) the previously reported petunia SL transporter PLEIOTROPIC DRUG RESISTANCE 1 (PDR1) directly accounts for short-distance SL transport and (ii) long-distance transport of SLs seems to be partially and not directly dependent on PDR1. These data suggest that the root-to-shoot transport of SLs occurs either via the vasculature bundle through transporters other than PDR1 or involves SL precursors that are not substrates of PDR1.

Strigolactones in plant adaptation to abiotic stresses: An emerging avenue of plant research

Phytohormones play central roles in boosting plant tolerance to environmental stresses, which negatively affect plant productivity and threaten future food security. Strigolactones (SLs), a class of carotenoid-derived phytohormones, were initially discovered as an "ecological signal" for parasitic seed germination and establishment of symbiotic relationship between plants and beneficial microbes. Subsequent characterizations have described their functional roles in various developmental processes, including root development, shoot branching, reproductive development, and leaf senescence. SLs have recently drawn much attention due to their essential roles in the regulation of various physiological and molecular processes during the adaptation of plants to abiotic stresses. Reports suggest that the production of SLs in plants is strictly regulated and dependent on the type of stresses that plants confront at various stages of development. Recently, evidence for crosstalk between SLs and other phytohormones, such as abscisic acid, in responses to abiotic stresses suggests that SLs actively participate within regulatory networks of plant stress adaptation that are governed by phytohormones. Moreover, the prospective roles of SLs in the management of plant growth and development under adverse environmental conditions have been suggested. In this review, we provide a comprehensive discussion pertaining to SL-mediated plant responses and adaptation to abiotic stresses.

Seed germination in parasitic plants: what insights can we expect from strigolactone research?

Obligate root-parasitic plants belonging to the Orobanchaceae family are deadly pests for major crops all over the world. Because these heterotrophic plants severely damage their hosts even before emerging from the soil, there is an unequivocal need to design early and efficient methods for their control. The germination process of these species has probably undergone numerous selective pressure events in the course of evolution, in that the perception of host-derived molecules is a necessary condition for seeds to germinate. Although most of these molecules belong to the strigolactones, structurally different molecules have been identified. Since strigolactones are also classified as novel plant hormones that regulate several physiological processes other than germination, the use of autotrophic model plant species has allowed the identification of many actors involved in the strigolactone biosynthesis, perception, and signal transduction pathways. Nevertheless, many questions remain to be answered regarding the germination process of parasitic plants. For instance, how did parasitic plants evolve to germinate in response to a wide variety of molecules, while autotrophic plants do not? What particular features are associated with their lack of spontaneous germination? In this review, we attempt to illustrate to what extent conclusions from research into strigolactones could be applied to better understand the biology of parasitic plants.

Spatial regulation of strigolactone function

Strigolactones are plant hormones that control many aspects of plant development and environmental responses. Despite recent and rapid progress in the biochemical and molecular understanding of strigolactone biosynthesis, transport, and signaling, our knowledge about where strigolactones are produced and where they act is fragmented. In this review, we summarize current knowledge about these aspects of strigolactones, obtained from mutant phenotypes, grafting experiments, gene expression patterns, and protein localization studies. We also discuss the potential of new imaging technologies to reveal the spatial regulation of strigolactone function.

Strigolactones: mediators of osmotic stress responses with a potential for agrochemical manipulation of crop resilience

After quickly touching upon general aspects of strigolactone biology and functions, including structure, synthesis, and perception, this review focuses on the role and regulation of the strigolactone pathway during osmotic stress, in light of the most recent research developments. We discuss available data on organ-specific dynamics of strigolactone synthesis and interaction with abscisic acid in the acclimatization response, with emphasis on the ecophysiological implications of the effects on the stomatal closure process. We highlight the importance of considering roots and shoots separately as well as combined versus individual stress treatments; and of performing reciprocal grafting experiments to work out organ contributions and long-distance signalling events and components under more realistic conditions. Finally, we elaborate on the question of if and how synthetic or natural strigolactones, alone or in combination with crop management strategies such as grafting, hold potential to maximize crop resilience to abiotic stresses.

Which are the major players, canonical or non-canonical strigolactones?

Strigolactones (SLs) can be classified into two structurally distinct groups: canonical and non-canonical SLs. Canonical SLs contain the ABCD ring system, and non-canonical SLs lack the A, B, or C ring but have the enol ether-D ring moiety, which is essential for biological activities. The simplest non-canonical SL is the SL biosynthetic intermediate carlactone. In plants, carlactone and its oxidized metabolites, such as carlactonoic acid and methyl carlactonoate, are present in root and shoot tissues. In some plant species, including black oat (Avena strigosa), sunflower (Helianthus annuus), and maize (Zea mays), non-canonical SLs in the root exudates are major germination stimulants. Various plant species, such as tomato (Solanum lycopersicum), Arabidopsis, and poplar (Populus spp.), release carlactonoic acid into the rhizosphere. These observations suggest that both canonical and non-canonical SLs act as host-recognition signals in the rhizosphere. In contrast, the limited distribution of canonical SLs in the plant kingdom, and the structure-specific and stereospecific transportation of canonical SLs from roots to shoots, suggest that plant hormones inhibiting shoot branching are not canonical SLs but, rather, are non-canonical SLs.

Changes in the allocation of endogenous strigolactone improve plant biomass production on phosphate-poor soils

Strigolactones (SLs) are carotenoid-derived phytohormones shaping plant architecture and inducing the symbiosis with endomycorrhizal fungi. In Petunia hybrida, SL transport within the plant and towards the rhizosphere is driven by the ABCG-class protein PDR1. PDR1 expression is regulated by phytohormones and by the soil phosphate abundance, and thus SL transport integrates plant development with nutrient conditions. We overexpressed PDR1 (PDR1 OE) to investigate whether increased endogenous SL transport is sufficient to improve plant nutrition and productivity. Phosphorus quantification and nondestructive X-ray computed tomography were applied. Morphological and gene expression changes were quantified at cellular and whole tissue levels via time-lapse microscopy and quantitative PCR. PDR1 OE significantly enhanced phosphate uptake and plant biomass production on phosphate-poor soils. PDR1 OE plants showed increased lateral root formation, extended root hair elongation, faster mycorrhization and reduced leaf senescence. PDR1 overexpression allowed considerable SL biosynthesis by releasing SL biosynthetic genes from an SL-dependent negative feedback. The increased endogenous SL transport/biosynthesis in PDR1 OE plants is a powerful tool to improve plant growth on phosphate-poor soils. We propose PDR1 as an as yet unexplored trait to be investigated for crop production. The overexpression of PDR1 is a valuable strategy to investigate SL functions and transport routes.

Functional characterization of soybean strigolactone biosynthesis and signaling genes in Arabidopsis MAX mutants and GmMAX3 in soybean nodulation

BACKGROUND

Strigolactones (SLs) play important roles in controlling root growth, shoot branching, and plant-symbionts interaction. Despite the importance, the components of SL biosynthesis and signaling have not been unequivocally explored in soybean.

RESULTS

Here we identified the putative components of SL synthetic enzymes and signaling proteins in soybean genome. Soybean genome contains conserved MORE AXILLARY BRANCHING (MAX) orthologs, GmMAX1s, GmMAX2s, GmMAX3s, and GmMAX4s. The tissue expression patterns are coincident with SL synthesis in roots and signaling in other tissues under normal conditions. GmMAX1a, GmMAX2a, GmMAX3b, and GmMAX4a expression in their Arabidopsis orthologs' mutants not only restored most characteristic phenotypes, such as shoot branching and shoot height, leaf shape, primary root length, and root hair growth, but also restored the significantly changed hormone contents, such as reduced JA and ABA contents in all mutant leaves, but increased auxin levels in atmax1, atmax3 and atmax4 mutants. Overexpression of these GmMAXs also altered the hormone contents in wild-type Arabidopsis. GmMAX3b was further characterized in soybean nodulation with overexpression and knockdown transgenic hairy roots. GmMAX3b overexpression (GmMAX3b-OE) lines exhibited increased nodule number while GmMAX3b knockdown (GmMAX3b-KD) decreased the nodule number in transgenic hairy roots. The expression levels of several key nodulation genes were also altered in GmMAX3b transgenic hairy roots. GmMAX3b overexpression hairy roots had reduced ABA, but increased JA levels, with no significantly changed auxin content, while the contrast changes were observed in GmMAX3b-KD lines. Global gene expression in GmMAX3b-OE or GmMAX3b-KD hairy roots also revealed that altered expression of GmMAX3b in soybean hairy roots changed several subsets of genes involved in hormone biosynthesis and signaling and transcriptional regulation of nodulation processes.

CONCLUSIONS

This study not only revealed the conservation of SL biosynthesis and signaling in soybean, but also showed possible interactions between SL and other hormone synthesis and signaling during controlling plant development and soybean nodulation. GmMAX3b-mediated SL biosynthesis and signaling may be involved in soybean nodulation by affecting both root hair formation and its interaction with rhizobia.

Strigolactones in the Rhizosphere: Friend or Foe?

Strigolactones are well-known endogenous plant hormones that play a major role in planta by influencing different physiological processes. Moreover, ex planta, strigolactones are important signaling molecules in root exudates and function as host detection cues to launch mutualistic interactions with arbuscular mycorrhizal fungi in the rhizosphere. However, parasitic plants belonging to the Orobanchaceae family hijacked this communication system to stimulate their seed germination when in close proximity to the roots of a suitable host. As a result, the secretion of strigolactones by the plant can have both favorable and detrimental outcomes. Here, we discuss these dual positive and negative effects of strigolactones and we provide a detailed overview on the role of these molecules in the complex dialogs between plants and different organisms in the rhizosphere.

Strigolactones in Plant Interactions with Beneficial and Detrimental Organisms: The Yin and Yang

Strigolactones (SLs) are plant hormones that have important roles as modulators of plant development. They were originally described as ex planta signaling molecules in the rhizosphere that induce the germination of parasitic plants, a role that was later linked to encouraging the beneficial symbiosis with arbuscular mycorrhizal (AM) fungi. Recently, the focus has shifted to examining the role of SLs in plant-microbe interactions, and has revealed roles for SLs in the association of legumes with nitrogen-fixing rhizobacteria and in interactions with disease-causing pathogens. Here, we examine the role of SLs in plant interactions with beneficial and detrimental organisms, and propose possible future biotechnological applications.

Methyl zealactonoate, a novel germination stimulant for root parasitic weeds produced by maize

One of the germination stimulants for root parasitic weeds produced by maize (Zea mays) was isolated and named methyl zealactonoate (1). Its structure was determined to be methyl (2E,3E)-4-((RS)-3,3-dimethyl-2-(3-methylbut-2-en-2-yl)-5-oxotetrahydrofuran-2-yl)-2-((((R)-4-methyl-5-oxo-2,5-dihydrofran-2-yl)oxy)methylene)but-3-enoate using by 1D and 2D NMR spectroscopy and ESI and EI-MS spectrometry. Feeding experiments with 13C-carlactone (CL), a biosynthetic intermediate for strigolactones, confirmed that 1 is produced from CL in maize. Methyl zealactonoate strongly elicits Striga hermonthica and Phelipanche ramosa seed germination, while Orobanche minor seeds are 100-fold less sensitive to this stimulant.

Strigolactone Signaling and Evolution

Strigolactones are a structurally diverse class of plant hormones that control many aspects of shoot and root growth. Strigolactones are also exuded by plants into the rhizosphere, where they promote symbiotic interactions with arbuscular mycorrhizal fungi and germination of root parasitic plants in the Orobanchaceae family. Therefore, understanding how strigolactones are made, transported, and perceived may lead to agricultural innovations as well as a deeper knowledge of how plants function. Substantial progress has been made in these areas over the past decade. In this review, we focus on the molecular mechanisms, core developmental roles, and evolutionary history of strigolactone signaling. We also propose potential translational applications of strigolactone research to agriculture.

Structural diversity of strigolactones and their distribution in the plant kingdom

Strigolactones (SLs) are plant secondary metabolites that were first identified as germination stimulants for the root parasitic weeds witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.). In the rhizosphere, SLs also promote root colonization by arbuscular mycorrhizal fungi. In plants, SLs as a novel class of plant hormones regulate various aspects of plant growth and development. Herein I discuss structural diversity of naturally occurring SLs and their distribution in the plant kingdom.

Parasitic weed management by using strigolactone-degrading fungi

BACKGROUND

Seed germination is a key phase of the parasitic plant life cycle that is stimulated by the secondary metabolites, mainly strigolactones (SLs), secreted by the host roots. Interventions during this stage would be particularly suitable for parasitic weed management practices, as blocking these chemical signals would prevent seed germination and thus parasite attack. Four fungal strains with different ecological functions were considered for their possible ability to metabolise SLs: Fusarium oxysporum and F. solani, biocontrol agents of Phelipanche ramosa; Trichoderma harzianum, a potential biopesticide; Botrytis cinerea, a phytopathogenic fungus. Four different SLs [the natural strigol, 5-deoxystrigol (5DS) and 4-deoxyorobanchol (4DO), and the synthetic analogue GR24] were added to fungal cultures, followed by determination of the SL content by liquid chromatography-tandem mass spectrometry.

RESULTS

Differences were observed among microorganisms, treatments and SLs used. T. harzianum and F. oxysporum were the most capable of reducing the SL content; considering the whole set of fungi used, 5DS and 4DO proved to be the most degradable SLs.

CONCLUSIONS

Beneficial microscopic fungi could differently be used for biocontrolling parasitic weeds, acting as a 'physiological' barrier, by preventing the germination of their seeds through the ability to biotransform the stimulatory signals. © 2016 Society of Chemical Industry.

Strigolactones, super hormones in the fight against Striga

Strigolactones are plant hormones that control diverse aspects of plant growth, but are also exuded into soil as recruitment signals for arbuscular mycorrhizal fungi interactions. Highly damaging parasitic weeds in the Orobanchaceae family have coopted strigolactones as germination cues that indicate the presence of a host. Recent studies have established how strigolactones are actively transported within and out of plants. Key components of the strigolactone signaling system have been identified, including strigolactone receptors in angiosperms and parasites, as well as downstream targets that are polyubiquitinated and proteolyzed following strigolactone perception. The basis for protein-protein interactions among these signaling components has also been explored. We propose several strategies to translate current knowledge of strigolactone transport and signaling into parasite control methods.

The importance of strigolactone transport regulation for symbiotic signaling and shoot branching

This review presents the role of strigolactone transport in regulating plant root and shoot architecture, plant-fungal symbiosis and the crosstalk with several phytohormone pathways. The authors, based on their data and recently published results, suggest that long-distance, as well local strigolactone transport might occur in a cell-to-cell manner rather than via the xylem stream. Strigolactones (SLs) are recently characterized carotenoid-derived phytohormones. They play multiple roles in plant architecture and, once exuded from roots to soil, in plant-rhizosphere interactions. Above ground SLs regulate plant developmental processes, such as lateral bud outgrowth, internode elongation and stem secondary growth. Below ground, SLs are involved in lateral root initiation, main root elongation and the establishment of the plant-fungal symbiosis known as mycorrhiza. Much has been discovered on players and patterns of SL biosynthesis and signaling and shown to be largely conserved among different plant species, however little is known about SL distribution in plants and its transport from the root to the soil. At present, the only characterized SL transporters are the ABCG protein PLEIOTROPIC DRUG RESISTANCE 1 from Petunia axillaris (PDR1) and, in less detail, its close homologue from Nicotiana tabacum PLEIOTROPIC DRUG RESISTANCE 6 (PDR6). PDR1 is a plasma membrane-localized SL cellular exporter, expressed in root cortex and shoot axils. Its expression level is regulated by its own substrate, but also by the phytohormone auxin, soil nutrient conditions (mainly phosphate availability) and mycorrhization levels. Hence, PDR1 integrates information from nutrient availability and hormonal signaling, thus synchronizing plant growth with nutrient uptake. In this review we discuss the effects of PDR1 de-regulation on plant development and mycorrhization, the possible cross-talk between SLs and other phytohormone transporters and finally the need for SL transporters in different plant species.

The Whats, the Wheres and the Hows of strigolactone action in the roots

Strigolactones control various aspects of plant development, including root architecture. Here, we review how strigolactones act in the root and survey the strigolactone specificity of signaling components that affect root development. Strigolactones are a group of secondary metabolites produced in plants that have been assigned multiple roles, of which the most recent is hormonal activity. Over the last decade, these compounds have been shown to regulate various aspects of plant development, such as shoot branching and leaf senescence, but a growing body of literature suggests that these hormones play an equally important role in the root. In this review, we present all known root phenotypes linked to strigolactones. We examine the expression and presence of the main players in biosynthesis and signaling of these hormones and bring together the available information that allows us to explain how strigolactones act to modulate the root system architecture.

Structure- and stereospecific transport of strigolactones from roots to shoots

Strigolactones (SLs) are carotenoid-derived signaling molecules that mediate symbiotic and parasitic communications in the rhizosphere and plant hormones that regulate the growth and development of plants through crosstalk with other hormones. Natural SLs are classified into two groups based on the stereochemistry of the B-C ring junction. Rice and sorghum plants, both gramineous crops, produce orobanchol-type and strigol-type SLs, respectively, while tobacco plants produce both types. In the present study, we demonstrate that such species-specific phenomena in SL production also occur in the transport of exogenous SLs from roots to shoots. In rice plants, strigol-type SLs such as 5-deoxystrigol have been reported to actively inhibit tiller bud outgrowth, whereas root-applied strigol-type SLs could not be detected in shoots harvested 20 hr after treatment, indicating that metabolites of SLs or other signaling compounds downstream of SLs-but not SLs themselves-are the true inhibitors of tiller bud outgrowth.

Strigolactone derivatives for potential crop enhancement applications

New technologies able to mitigate the main abiotic stresses (i.e., drought, salinity, cold and heat) represent a substantial opportunity to contribute to a sustainable increase of agricultural production. In this context, the recently discovered phytohormone strigolactone is an important area of study which can underpin the quest for new anti-stress technologies. The pleiotropic roles played by strigolactones in plant growth/development and in plant adaptation to environmental changes can pave the way for new innovative crop enhancement applications. Although a significant scientific effort has been dedicated to the strigolactone subject, an updated review with emphasis on the crop protection perspective was missing. This paper aims to analyze the advancement in different areas of the strigolactone domain and the implications for agronomical applications.