Influx and Efflux of Strigolactones Are Actively Regulated and Involve the Cell-Trafficking System

Strigolactones, from Plants to Human Health: Achievements and Challenges

Strigolactones (SLs) are a class of sesquiterpenoid plant hormones that play a role in the response of plants to various biotic and abiotic stresses. When released into the rhizosphere, they are perceived by both beneficial symbiotic mycorrhizal fungi and parasitic plants. Due to their multiple roles, SLs are potentially interesting agricultural targets. Indeed, the use of SLs as agrochemicals can favor sustainable agriculture via multiple mechanisms, including shaping root architecture, promoting ideal branching, stimulating nutrient assimilation, controlling parasitic weeds, mitigating drought and enhancing mycorrhization. Moreover, over the last few years, a number of studies have shed light onto the effects exerted by SLs on human cells and on their possible applications in medicine. For example, SLs have been demonstrated to play a key role in the control of pathways related to apoptosis and inflammation. The elucidation of the molecular mechanisms behind their action has inspired further investigations into their effects on human cells and their possible uses as anti-cancer and antimicrobial agents.

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.

The Control of Zealactone Biosynthesis and Exudation is Involved in the Response to Nitrogen in Maize Root

Nitrate acts as a signal in regulating plant development in response to environment. In particular nitric oxide, auxin and strigolactones (SLs) were supposed to cooperate to regulate the maize root response to this anion. In this study, a combined approach based on liquid chromatography-quadrupole/time-of-flight tandem mass spectrometry and on physiological and molecular analyses was adopted to specify the involvement of SLs in the maize response to N. Our results showed that N deficiency strongly induces SL exudation, likely through stimulating their biosynthesis. Nitrate provision early counteracts and also ammonium lowers SL exudation, but less markedly. Exudates obtained from N-starved and ammonium-provided seedlings stimulated Phelipanche germination, whereas when seeds were treated with exudates harvested from nitrate-provided plants no germination was observed. Furthermore, our findings support the idea that the inhibition of SL production observed in response to nitrate and ammonium would contribute to the regulation of lateral root development. Moreover, the transcriptional regulation of a gene encoding a putative maize WBC transporter, in response to various nitrogen supplies, together with its mRNA tissue localization, supported its role in SL allocation. Our results highlight the dual role of SLs as molecules able to signal outwards a nutritional need and as endogenous regulators of root architecture adjustments to N, thus synchronizing plant growth with nitrogen acquisition.

From synthetic control to natural products: a focus on N-heterocycles

Natural products containing a N-heterocycle motif are widespread in nature and medicinal plants, in particular, have proved to be a source of almost unlimited N-derived structures with high molecular diversity. Because of their intrinsic potential for use in both biomedical and agricultural applications, there is a general need for new compounds and for the synthesis of 'natural-inspired' analogues. Importantly, transition of a natural product from discovery to a 'market lead' is associated with an increasingly challenging demand for more of the compound, which cannot be met by isolation from natural plant sources, often due to low extraction yields and uneven availability of the plant source itself. Synthesis remains the most reliable approach to provide valuable products for the market. In this review, a comprehensive overview of our contribution to synthetic access to N-derived natural products is given. Major strengths of the proposed methodologies are discussed critically. © 2019 Society of Chemical Industry.

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.

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.

Heck functionalization of an asymmetric aza-BODIPY core: synthesis of far-red infrared probes for bioimaging applications

Heck functionalization of azadipyrromethenes (aza-DIPY) allows the introduction of suitable functional groups to convert aza-BODIPY in bioconjugate complexes.

Shaping Small Bioactive Molecules to Untangle Their Biological Function: A Focus on Fluorescent Plant Hormones

Modern biology overlaps with chemistry in explaining the structure and function of all cellular processes at the molecular level. Plant hormone research is perfectly located at the interface between these two disciplines, taking advantage of synthetic and computational chemistry as a tool to decipher the complex biological mechanisms regulating the action of plant hormones. These small signaling molecules regulate a wide range of developmental processes, adapting plant growth to ever changing environmental conditions. The synthesis of small bioactive molecules mimicking the activity of endogenous hormones allows us to unveil many molecular features of their functioning, giving rise to a new field, plant chemical biology. In this framework, fluorescence labeling of plant hormones is emerging as a successful strategy to track the fate of these challenging molecules inside living organisms. Thanks to the increasing availability of new fluorescent probes as well as advanced and innovative imaging technologies, we are now in a position to investigate many of the dynamic mechanisms through which plant hormones exert their action. Such a deep and detailed comprehension is mandatory for the development of new green technologies for practical applications. In this review, we summarize the results obtained so far concerning the fluorescent labeling of plant hormones, highlighting the basic steps leading to the design and synthesis of these compelling molecular tools and their applications.

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.

Strigolactone versus gibberellin signaling: reemerging concepts?

MAIN CONCLUSION

In this review, we compare knowledge about the recently discovered strigolactone signaling pathway and the well established gibberellin signaling pathway to identify gaps of knowledge and putative research directions in strigolactone biology. Communication between and inside cells is integral for the vitality of living organisms. Hormonal signaling cascades form a large part of this communication and an understanding of both their complexity and interactive nature is only beginning to emerge. In plants, the strigolactone (SL) signaling pathway is the most recent addition to the classically acting group of hormones and, although fundamental insights have been made, knowledge about the nature and impact of SL signaling is still cursory. This narrow understanding is in spite of the fact that SLs influence a specific spectrum of processes, which includes shoot branching and root system architecture in response, partly, to environmental stimuli. This makes these hormones ideal tools for understanding the coordination of plant growth processes, mechanisms of long-distance communication and developmental plasticity. Here, we summarize current knowledge about SL signaling and employ the well-characterized gibberellin (GA) signaling pathway as a scaffold to highlight emerging features as well as gaps in our knowledge in this context. GA signaling is particularly suitable for this comparison because both signaling cascades share key features of hormone perception and of immediate downstream events. Therefore, our comparative view demonstrates the possible level of complexity and regulatory interfaces of SL signaling.

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.

Smoke and Hormone Mirrors: Action and Evolution of Karrikin and Strigolactone Signaling

Karrikins and strigolactones are two classes of butenolide molecules that have diverse effects on plant growth. Karrikins are found in smoke and strigolactones are plant hormones, yet both molecules are likely recognized through highly similar signaling mechanisms. Here we review the most recent discoveries of karrikin and strigolactone perception and signal transduction. Two paralogous α/β hydrolases, KAI2 and D14, are respectively karrikin and strigolactone receptors. D14 acts with an F-box protein, MAX2, to target SMXL/D53 family proteins for proteasomal degradation, and genetic data suggest that KAI2 acts similarly. There are striking parallels in the signaling mechanisms of karrikins, strigolactones, and other plant hormones, including auxins, jasmonates, and gibberellins. Recent investigations of host perception in parasitic plants have demonstrated that strigolactone recognition can evolve following gene duplication of KAI2.

Stereochemical Assignment of Strigolactone Analogues Confirms Their Selective Biological Activity

Strigolactones (SLs) are new plant hormones with various developmental functions. They are also soil signaling chemicals that are required for establishing beneficial mycorrhizal plant/fungus symbiosis. In addition, SLs play an essential role in inducing seed germination in root-parasitic weeds, which are one of the seven most serious biological threats to food security. There are around 20 natural SLs that are produced by plants in very low quantities. Therefore, most of the knowledge on SL signal transduction and associated molecular events is based on the application of synthetic analogues. Stereochemistry plays a crucial role in the structure-activity relationship of SLs, as compounds with an unnatural D-ring configuration may induce biological effects that are unrelated to SLs. We have synthesized a series of strigolactone analogues, whose absolute configuration has been elucidated and related with their biological activity, thus confirming the high specificity of the response. Analogues bearing the R-configured butenolide moiety showed enhanced biological activity, which highlights the importance of this stereochemical motif.