Apocarotenoids: Old and New Mediators of the Arbuscular Mycorrhizal Symbiosis

Regulation of apocarotenoids for quality improvement and biofortification of horticultural crops

BACKGROUND

Agro-food production and consumption impact climate change and human health. Bioactive secondary metabolites in horticulture crops make them an indispensable part of environmentally sustainable and healthy diet. Among them, apocarotenoids from carotenoid degradation are promising in promoting a preference for plant-based foods over other metabolites.

AIM OF REVIEW

In horticulture crops, carotenoids are vital for photosynthesis and antioxidant defense, but their enzymatic or oxidative metabolites, apocarotenoids, offer greater structural diversity and biological functions. They serve as pigments, scents, signaling molecules, and growth regulators in crop growth and development and provide antioxidant, nutraceutical, and pharmaceutical benefits to human health. The carotenoids as bioactive compounds are well understood. By contrast, much less is explored and reviewed about apocarotenoids.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Recently identified metabolic pathways and components of apocarotenoids are reviewed. Their significance for quality formation in horticulture crops, including the regulation of pigmentation, aroma, flavor, architecture, nutrition value, and broader ecological interactions is discussed. Additionally, this review specifically highlights two representative apocarotenoids, retinal and abscisic acid (ABA), that exhibit conserved yet distinct regulatory functions across plant and animal kingdoms. Comprehensive dissection of apocarotenoid metabolism and their regulatory mechanisms will enhance apocarotenoid biofortification and subsequent biotechnological exploitation in horticultural commodities. We put forward the perspective that apocarotenoids could enhance horticultural crop quality and then promote sensory- and health-driven dietary choices which will in turn increase consumption and production of horticultural plants and promote both human and ecosystem health.

Zaxinone Synthase overexpression modulates rice physiology and metabolism, enhancing nutrient uptake, growth and productivity

The rice Zaxinone Synthase (ZAS) gene encodes a carotenoid cleavage dioxygenase (CCD) that forms the apocarotenoid growth regulator zaxinone in vitro. Here, we generated and characterized constitutive ZAS-overexpressing rice lines, to better understand ZAS role in determining zaxinone content and regulating growth and architecture. ZAS overexpression enhanced endogenous zaxinone level, promoted root growth and increased the number of productive tillers, leading to about 30% higher grain yield per plant. Hormone analysis revealed a decrease in strigolactone (SL) content, which we confirmed by rescuing the high-tillering phenotype through application of a SL analogue. Metabolomics analysis revealed that ZAS overexpressing plants accumulate higher amounts of monosaccharide sugars, in line with transcriptome analysis. Moreover, transgenic plants showed higher carbon (C) assimilation rate and elevated root phosphate, nitrate and sulphate level, enhancing the tolerance towards low phosphate (Pi). Our study confirms ZAS as an important determinant of rice growth and architecture and shows that ZAS regulates hormone homoeostasis and a combination of physiological processes to promote growth and grain yield, which makes this gene an excellent candidate for sustainable crop improvement.

Effect of exogenous treatment with zaxinone and its mimics on rice root microbiota across different growth stages

Enhancing crops productivity to ensure food security is one of the major challenges encountering agriculture today. A promising solution is the use of biostimulants, which encompass molecules that enhance plant fitness, growth, and productivity. The regulatory metabolite zaxinone and its mimics (MiZax3 and MiZax5) showed promising results in improving the growth and yield of several crops. Here, the impact of their exogenous application on soil and rice root microbiota was investigated. Plants grown in native paddy soil were treated with zaxinone, MiZax3, and MiZax5 and the composition of bacterial and fungal communities in soil, rhizosphere, and endosphere at the tillering and the milky stage was assessed. Furthermore, shoot metabolome profile and nutrient content of the seeds were evaluated. Results show that treatment with zaxinone and its mimics predominantly influenced the root endosphere prokaryotic community, causing a partial depletion of plant-beneficial microbes at the tillering stage, followed by a recovery of the prokaryotic community structure during the milky stage. Our study provides new insights into the role of zaxinone and MiZax in the interplay between rice and its root-associated microbiota and paves the way for their practical application in the field as ecologically friendly biostimulants to enhance crop productivity.

Chromosome-scale pearl millet genomes reveal CLAMT1b as key determinant of strigolactone pattern and Striga susceptibility

The yield of pearl millet, a resilient cereal crop crucial for African food security, is severely impacted by the root parasitic weed Striga hermonthica, which requires host-released hormones, called strigolactones (SLs), for seed germination. Herein, we identify four SLs present in the Striga-susceptible line SOSAT-C88-P10 (P10) but absent in the resistant 29Aw (Aw). We generate chromosome-scale genome assemblies, including four gapless chromosomes for each line. The Striga-resistant Aw lacks a 0.7 Mb genome segment containing two putative CARLACTONOIC ACID METHYLTRANSFERASE1 (CLAMT1) genes, which may contribute to SL biosynthesis. Functional assays show that P10CLAMT1b produces the SL-biosynthesis intermediate methyl carlactonoate (MeCLA) and that MeCLA is the precursor of P10-specific SLs. Screening a diverse pearl millet panel confirms the pivotal role of the CLAMT1 section for SL diversity and Striga susceptibility. Our results reveal a reason for Striga susceptibility in pearl millet and pave the way for generating resistant lines through marker-assisted breeding or direct genetic modification.

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.

Independent regulation of strigolactones and blumenols during arbuscular mycorrhizal symbiosis in rice

The apocarotenoid strigolactones (SLs) facilitate pre-symbiotic communication between arbuscular mycorrhizal (AM) fungi and plants. Related blumenol-C-glucosides (blumenols), have also been associated with symbiosis, but the cues that are involved in the regulation of blumenol accumulation during AM symbiosis remain unclear. In rice, our analyses demonstrated a strict correlation between foliar blumenol abundance and intraradical fungal colonisation. More specifically, rice mutants affected at distinct stages of the interaction revealed that fungal cortex invasion was required for foliar blumenol accumulation. Plant phosphate status and D14L hormone signalling had no effect, contrasting their known role in induction of SLs. This a proportion of the SL biosynthetic enzymes, D27 and D17, are equally required for blumenol production. These results importantly clarify that, while there is a partially shared biosynthetic pathway between SL and blumenols, the dedicated induction of the related apocarotenoids occurs in response to cues acting at distinct stages during the root colonisation process. However, we reveal that neither SLs nor blumenols are essential for fungal invasion of rice roots.

Design, Synthesis and Biological Evaluation of Novel Phenyl-Substituted Naphthoic Acid Ethyl Ester Derivatives as Strigolactone Receptor Inhibitor

Strigolactones (SLs) are plant hormones that regulate several key agronomic traits, including shoot branching, leaf senescence, and stress tolerance. The artificial regulation of SL biosynthesis and signaling has been considered as a potent strategy in regulating plant architecture and combatting the infection of parasitic weeds to help improve crop yield. DL1b is a previously reported SL receptor inhibitor molecule that significantly promotes shoot branching. Here, we synthesized 18 novel compounds based on the structure of DL1b. We performed rice tillering activity assay and selected a novel small molecule, C6, as a candidate SL receptor inhibitor. In vitro bioassays demonstrated that C6 possesses various regulatory functions as an SL inhibitor, including inhibiting germination of the root parasitic seeds Phelipanche aegyptiaca, delaying leaf senescence and promoting hypocotyl elongation of Arabidopsis. ITC analysis and molecular docking experiments further confirmed that C6 can interact with SL receptor proteins, thereby interfering with the binding of SL to its receptor. Therefore, C6 is considered a novel SL receptor inhibitor with potential applications in plant architecture control and prevention of root parasitic weed infestation.

Strigolactone biosynthesis in rice can occur via a 9-cis-3-OH-10'-apo-β-carotenal intermediate

Strigolactones (SLs) play a crucial role in regulating plant architecture and mediating rhizosphere interactions. They are synthesized from all-trans-β-carotene converted into the intermediate carlactone (CL) via the intermediate 9-cis-β-apo-10'-carotenal. Recent studies indicate that plants can also synthesize 3-OH-CL from all-trans-β-zeaxanthin via the intermediate 9-cis-3-OH-β-apo-10'-carotenal. However, the question of whether plants can form bioactive SLs from 9-cis-3-OH-β-apo-10'-carotenal remains elusive. In this study, we supplied the 13 C-labeled 9-cis-3-OH-β-apo-10'-carotenal to rice seedlings and monitored the synthesis of SLs using liquid chromatography-mass spectrometry (LC-MS) and Striga bioassay. We further validated the biological activity of 9-cis-3-OH-β-apo-10'-carotenal-derived SLs using the ccd7/d17 SL-deficient mutant, which demonstrated increased Striga seed-germinating activity and partial rescue of tiller numbers and plant height. Our results establish 9-cis-3-OH-β-apo-10'-carotenal as a significant SL biosynthetic intermediate with implications for understanding plant hormonal functions and potential applications in agriculture.

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.

Identification and expression of strigolactone biosynthesis and signaling genes and the in vitro effects of strigolactones in olive (Olea europaea L.)

Strigolactones (SLs), synthesized in plant roots, play a dual role in modulating plant growth and development, and in inducing the germination of parasitic plant seeds and arbuscular mycorrhizal fungi in the rhizosphere. As phytohormones, SLs are crucial in regulating branching and shaping plant architecture. Despite the significant impact of branching strategies on the yield performance of fruit crops, limited research has been conducted on SLs in these crops. In our study, we identified the transcript sequences of SL biosynthesis and signaling genes in olive (Olea europaea L.) using rapid amplification of cDNA ends. We predicted the corresponding protein sequences, analyzed their characteristics, and conducted molecular docking with bioinformatics tools. Furthermore, we quantified the expression levels of these genes in various tissues using quantitative real-time PCR. Our findings demonstrate the predominant expression of SL biosynthesis and signaling genes (OeD27, OeMAX3, OeMAX4, OeMAX1, OeD14, and OeMAX2) in roots and lateral buds, highlighting their importance in branching. Treatment with rac-GR24, an SL analog, enhanced the germination frequency of olive seeds in vitro compared with untreated embryos. Conversely, inhibition of SL biosynthesis with TIS108 increased lateral bud formation in a hard-to-root cultivar, underscoring the role of SLs as phytohormones in olives. These results suggest that modifying SL biosynthesis and signaling pathways could offer novel approaches for olive breeding, with potential applicability to other fruit crops.

Integration of rice apocarotenoid profile and expression pattern of Carotenoid Cleavage Dioxygenases reveals a positive effect of β-ionone on mycorrhization

Carotenoids are susceptible to degrading processes initiated by oxidative cleavage reactions mediated by Carotenoid Cleavage Dioxygenases that break their backbone, leading to products called apocarotenoids. These carotenoid-derived metabolites include the phytohormones abscisic acid and strigolactones, and different signaling molecules and growth regulators, which are utilized by plants to coordinate many aspects of their life. Several apocarotenoids have been recruited for the communication between plants and arbuscular mycorrhizal (AM) fungi and as regulators of the establishment of AM symbiosis. However, our knowledge on their biosynthetic pathways and the regulation of their pattern during AM symbiosis is still limited. In this study, we generated a qualitative and quantitative profile of apocarotenoids in roots and shoots of rice plants exposed to high/low phosphate concentrations, and upon AM symbiosis in a time course experiment covering different stages of growth and AM development. To get deeper insights in the biology of apocarotenoids during this plant-fungal symbiosis, we complemented the metabolic profiles by determining the expression pattern of CCD genes, taking advantage of chemometric tools. This analysis revealed the specific profiles of CCD genes and apocarotenoids across different stages of AM symbiosis and phosphate supply conditions, identifying novel reliable markers at both local and systemic levels and indicating a promoting role of β-ionone in AM symbiosis establishment.

Characterization of a partially saturated and glycosylated apocarotenoid from wheat that is depleted upon leaf rust infection

Recent semi-targeted metabolomics studies have highlighted a number of metabolites in wheat that associate with leaf rust resistance genes and/or rust infection. Here, we report the structural characterization of a novel glycosylated and partially saturated apocarotenoid, reminiscent of a reduced form of mycorradicin, (6E,8E,10E)-4,9-dimethyl-12-oxo-12-((3,4,5-trihydroxy-6-(2-hydroxyethoxy)tetrahydro-2H-pyran-2-yl)methoxy)-3-((3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yl)oxy)dodeca-6,8,10-trienoic acid, isolated from Triticum aestivum L. (Poaceae) variety 'Thatcher' (Tc) flag leaves. While its accumulation was not associated with any of Lr34, Lr67 or Lr22a resistance genes, infection of Tc with leaf rust was found to deplete it, consistent with the idea of this metabolite being a glycosylated-storage form of an apocarotenoid of possible relevance to plant defense. A comparative analysis of wheat transcriptomic changes shows modulation of terpenoid, carotenoid, UDP-glycosyltransferase and glycosylase -related gene expression profiles, consistent with anticipated biosynthesis and degradation mechanisms. However, details of the exact nature of the relevant pathways remain to be validated in the future. Together these findings highlight another example of the breadth of unique metabolites underlying plant host-fungal pathogen interactions.

Molecular and Systems Biology Approaches for Harnessing the Symbiotic Interaction in Mycorrhizal Symbiosis for Grain and Oil Crop Cultivation

Mycorrhizal symbiosis, the mutually beneficial association between plants and fungi, has gained significant attention in recent years due to its widespread significance in agricultural productivity. Specifically, arbuscular mycorrhizal fungi (AMF) provide a range of benefits to grain and oil crops, including improved nutrient uptake, growth, and resistance to (a)biotic stressors. Harnessing this symbiotic interaction using molecular and systems biology approaches presents promising opportunities for sustainable and economically-viable agricultural practices. Research in this area aims to identify and manipulate specific genes and pathways involved in the symbiotic interaction, leading to improved cereal and oilseed crop yields and nutrient acquisition. This review provides an overview of the research frontier on utilizing molecular and systems biology approaches for harnessing the symbiotic interaction in mycorrhizal symbiosis for grain and oil crop cultivation. Moreover, we address the mechanistic insights and molecular determinants underpinning this exchange. We conclude with an overview of current efforts to harness mycorrhizal diversity to improve cereal and oilseed health through systems biology.

Does zaxinone counteract strigolactones in shaping rice architecture?

The cleavage of plant carotenoids leads to apocarotenoids, a group of metabolites including precursors of the hormones strigolactones (SLs) and abscisic acid, regulatory and signaling molecules. Zaxinone is a recently discovered apocarotenoid growth regulator that improves growth and suppress SL biosynthesis in rice (Oryza sativa). To test if zaxinone also counteracts the growth regulatory effects of SLs in rice, we co-supplied zaxinone and the synthetic SL analog rac-GR24 to the rice SL-deficient DWARF17 (d17) mutant. Results showed that co-application of GR24 and zaxinone still rescued d17 phenotype, indicating that zaxinone and GR24 act independently in regulating root and shoot growth and development in rice.

Comparative metabolomic study of fungal foliar endophytes and their long-lived host Astrocaryum sciophilum: a model for exploring the chemodiversity of host-microbe interactions

Introduction

In contrast to the dynamics observed in plant/pathogen interactions, endophytic fungi have the capacity to establish enduring associations within their hosts, leading to the development of a mutually beneficial relationship that relies on specialized chemical interactions. Research indicates that the presence of endophytic fungi has the ability to significantly modify the chemical makeup of the host organism. Our hypothesis proposes the existence of a reciprocal exchange of chemical signals between plants and fungi, facilitated by specialized chemical processes that could potentially manifest within the tissues of the host. This research aimed to precisely quantify the portion of the cumulative fungal endophytic community's metabolome detectable within host leaves, and tentatively evaluate its relevance to the host-endophyte interplay. The understory palm Astrocaryum sciophilum (Miq.) Pulle was used as a interesting host plant because of its notable resilience and prolonged life cycle, in a tropical ecosystem.

Method

Using advanced metabolome characterization, including UHPLC-HRMS/MS and molecular networking, the study explored enriched metabolomes of both host leaves and 15 endophytic fungi. The intention was to capture a metabolomic "snapshot" of both host and endophytic community, to achieve a thorough and detailed analysis.

Results and discussion

This approach yielded an extended MS-based molecular network, integrating diverse metadata for identifying host- and endophyte-derived metabolites. The exploration of such data (>24000 features in positive ionization mode) enabled effective metabolome comparison, yielding insights into cultivable endophyte chemodiversity and occurrence of common metabolites between the holobiont and its fungal communities. Surprisingly, a minor subset of features overlapped between host leaf and fungal samples despite significant plant metabolome enrichment. This indicated that fungal metabolic signatures produced in vitro remain sparingly detectable in the leaf. Several classes of primary metabolites were possibly shared. Specific fungal metabolites and/or compounds of their chemical classes were only occasionally discernible in the leaf, highlighting endophytes partial contribution to the overall holobiont metabolome. To our knowledge, the metabolomic study of a plant host and its microbiome has rarely been performed in such a comprehensive manner. The general analytical strategy proposed in this paper seems well-adapted for any study in the field of microbial- or microbiome-related MS and can be applied to most host-microbe interactions.

Signaling in Legume-Rhizobia Symbiosis

Legumes represent an important source of food protein for human nutrition and animal feed. Therefore, sustainable production of legume crops is an issue of global importance. It is well-known that legume-rhizobia symbiosis allows an increase in the productivity and resilience of legume crops. The efficiency of this mutualistic association strongly depends on precise regulation of the complex interactions between plant and rhizobia. Their molecular dialogue represents a complex multi-staged process, each step of which is critically important for the overall success of the symbiosis. In particular, understanding the details of the molecular mechanisms behind the nodule formation and functioning might give access to new legume cultivars with improved crop productivity. Therefore, here we provide a comprehensive literature overview on the dynamics of the signaling network underlying the development of the legume-rhizobia symbiosis. Thereby, we pay special attention to the new findings in the field, as well as the principal directions of the current and prospective research. For this, here we comprehensively address the principal signaling events involved in the nodule inception, development, functioning, and senescence.

Disruption of the rice 4-DEOXYOROBANCHOL HYDROXYLASE unravels specific functions of canonical strigolactones

Strigolactones (SLs) regulate many developmental processes, including shoot-branching/tillering, and mediate rhizospheric interactions. SLs originate from carlactone (CL) and are structurally diverse, divided into a canonical and a noncanonical subfamily. Rice contains two canonical SLs, 4-deoxyorobanchol (4DO) and orobanchol (Oro), which are common in different plant species. The cytochrome P450 OsMAX1-900 forms 4DO from CL through repeated oxygenation and ring closure, while the homologous enzyme OsMAX1-1400 hydroxylates 4DO into Oro. To better understand the biological function of 4DO and Oro, we generated CRISPR/Cas9 mutants disrupted in OsMAX1-1400 or in both OsMAX1-900 and OsMAX1-1400. The loss of OsMAX1-1400 activity led to a complete lack of Oro and an accumulation of its precursor 4DO. Moreover, Os1400 mutants showed shorter plant height, panicle and panicle base length, but no tillering phenotype. Hormone quantification and transcriptome analysis of Os1400 mutants revealed elevated auxin levels and changes in the expression of auxin-related, as well as of SL biosynthetic genes. Interestingly, the Os900/1400 double mutant lacking both Oro and 4DO did not show the observed Os1400 architectural phenotypes, indicating their being a result of 4DO accumulation. Treatment of wild-type plants with 4DO confirmed this assumption. A comparison of the Striga seed germinating activity and the mycorrhization of Os900, Os900/1400, and Os1400 loss-of-function mutants demonstrated that the germination activity positively correlates with 4DO content while disrupting OsMAX1-1400 has a negative impact on mycorrhizal symbiosis. Taken together, our paper deciphers the biological function of canonical SLs in rice and reveals their particular contributions to establishing architecture and rhizospheric communications.

Zaxinone mimics (MiZax) efficiently promote growth and production of potato and strawberry plants under desert climate conditions

Climate changes and the rapid expanding human population have become critical concerns for global food security. One of the promising solutions is the employment of plant growth regulators (PGRs) for increasing crop yield and overcoming adverse growth conditions, such as desert climate. Recently, the apocarotenoid zaxinone and its two mimics (MiZax3 and MiZax5) have shown a promising growth-promoting activity in cereals and vegetable crops under greenhouse and field conditions. Herein, we further investigated the effect of MiZax3 and MiZax5, at different concentrations (5 and 10 µM in 2021; 2.5 and 5 µM in 2022), on the growth and yield of the two valuable vegetable crops, potato and strawberry, in the Kingdom of Saudi of Arabia. Application of both MiZax significantly increased plant agronomic traits, yield components and total yield, in five independent field trials from 2021 to 2022. Remarkably, the amount of applied MiZax was far less than humic acid, a widely applied commercial compound used here for comparison. Hence, our results indicate that MiZax are very promising PGRs that can be applied to promote the growth and yield of vegetable crops even under desert conditions and at relatively low concentrations.

The apocarotenoid production in microbial biofactories: An overview

Carotenoids are a vast group of natural pigments that come in a variety of colors ranging from red to orange. Apocarotenoids are derived from these carotenoids, which are hormones, pigments, retinoids, and volatiles employed in the textiles, cosmetics, pharmaceutical, and food industries. Due to the high commercial value and poor natural host abundance, they are significantly undersupplied. Microbes like Saccharomyces cerevisiae and Escherichia coli act as heterologous hosts for apocarotenoid production. This article briefly reviews categories of apocarotenoids, their biosynthetic pathway commencing from the MVA and MEP, its significance, the tool enzymes for apocarotenoid biosynthesis like CCDs, their biotechnological production in microbial factories, and future perspectives.

New Series of Zaxinone Mimics (MiZax) for Fundamental and Applied Research

The apocarotenoid zaxinone is a recently discovered regulatory metabolite required for proper rice growth and development. In addition, zaxinone and its two mimics (MiZax3 and MiZax5) were shown to have a remarkable growth-promoting activity on crops and a capability to reduce infestation by the root parasitic plant Striga through decreasing strigolactone (SL) production, suggesting their potential for application in agriculture and horticulture. In the present study, we developed a new series of MiZax via structural modification of the two potent zaxinone mimics (MiZax3 and MiZax5) and evaluated their effect on plant growth and Striga infestation. In general, the structural modifications to MiZax3 and MiZax5 did not additionally improve their overall performance but caused an increase in certain activities. In conclusion, MiZax5 and especially MiZax3 remain the likely most efficient zaxinone mimics for controlling Striga infestation.

Disruption of the cytochrome CYP711A5 gene reveals MAX1 redundancy in rice strigolactone biosynthesis

Strigolactones (SLs) inhibit shoot branching/tillering and are secreted by plant roots as a signal to attract symbiotic mycorrhizal fungi in the rhizosphere, particularly under phosphate starvation. However, SLs are also hijacked by root parasitic weeds as inducer for the germination of their seeds. There are around 35 natural SLs divided, based on their structures, into canonical and non-canonical SLs. Cytochrome P450 enzymes of the 711 clade, such as MORE AXILLARY GROWTH1 (MAX1) in Arabidopsis, are a major driver of SL structural diversity. Monocots, such as rice, contain several MAX1 homologs that participate in SL biosynthesis. To investigate the function of OsMAX1-1900 in planta, we generated CRISPR/Cas9 mutants disrupted in the corresponding gene. Characterizing of the generated mutants at metabolite and phenotype level suggests that OsMAX1-1900 loss-of-function does neither affect the SL pattern nor rice architecture, indicating functional redundancy among rice MAX1 homologs.

Arbuscular mycorrhizal fungi-indicative blumenol-C-glucosides predict lipid accumulations and fitness in plants grown without competitors

Hydroxy- and carboxyblumenol C-glucosides specifically accumulate in roots and leaves of plants harboring arbuscular mycorrhizal fungi (AMF). To explore blumenol function in AMF relationships, we silenced an early key-gene in blumenol biosynthesis, CCD1 (carotenoid cleavage dioxygenase 1), in the ecological model plant, Nicotiana attenuata, and analyzed whole-plant performance in comparison with control and CCaMK-silenced plants, unable to form AMF associations. Root blumenol accumulations reflected a plant's Darwinian fitness, as estimated by capsule production, and were positively correlated with AMF-specific lipid accumulations in roots, with relationships that changed as plants matured when grown without competitors. When grown with wild-type competitors, transformed plants with decreased photosynthetic capacity or increased carbon flux to roots had blumenol accumulations that predicted plant fitness and genotype trends in AMF-specific lipids, but had similar levels of AMF-specific lipids between competing plants, likely reflecting AMF-networks. We propose that when grown in isolation, blumenol accumulations reflect AMF-specific lipid allocations and plant fitness. When grown with competitors, blumenol accumulations predict fitness outcomes, but not the more complicated AMF-specific lipid accumulations. RNA-seq analysis provided candidates for the final biosynthetic steps of these AMF-indicative blumenol C-glucosides; abrogation of these steps will provide valuable tools for understanding blumenol function in this context-dependent mutualism.

Terpenoid Transport in Plants: How Far from the Final Picture?

Contrary to the biosynthetic pathways of many terpenoids, which are well characterized and elucidated, their transport inside subcellular compartments and the secretion of reaction intermediates and final products at the short- (cell-to-cell), medium- (tissue-to-tissue), and long-distance (organ-to-organ) levels are still poorly understood, with some limited exceptions. In this review, we aim to describe the state of the art of the transport of several terpene classes that have important physiological and ecological roles or that represent high-value bioactive molecules. Among the tens of thousands of terpenoids identified in the plant kingdom, only less than 20 have been characterized from the point of view of their transport and localization. Most terpenoids are secreted in the apoplast or stored in the vacuoles by the action of ATP-binding cassette (ABC) transporters. However, little information is available regarding the movement of terpenoid biosynthetic intermediates from plastids and the endoplasmic reticulum to the cytosol. Through a description of the transport mechanisms of cytosol- or plastid-synthesized terpenes, we attempt to provide some hypotheses, suggestions, and general schemes about the trafficking of different substrates, intermediates, and final products, which might help develop novel strategies and approaches to allow for the future identification of terpenoid transporters that are still uncharacterized.

ZAXINONE SYNTHASE 2 regulates growth and arbuscular mycorrhizal symbiosis in rice

Carotenoid cleavage, catalyzed by CAROTENOID CLEAVAGE DIOXYGENASEs (CCDs), provides signaling molecules and precursors of plant hormones. Recently, we showed that zaxinone, a apocarotenoid metabolite formed by the CCD ZAXINONE SYNTHASE (ZAS), is a growth regulator required for normal rice (Oryza sativa) growth and development. The rice genome encodes three OsZAS homologs, called here OsZAS1b, OsZAS1c, and OsZAS2, with unknown functions. Here, we investigated the enzymatic activity, expression pattern, and subcellular localization of OsZAS2 and generated and characterized loss-of-function CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats and associated protein 9)-Oszas2 mutants. We show that OsZAS2 formed zaxinone in vitro. OsZAS2 was predominantly localized in plastids and mainly expressed under phosphate starvation. Moreover, OsZAS2 expression increased during mycorrhization, specifically in arbuscule-containing cells. Oszas2 mutants contained lower zaxinone content in roots and exhibited reduced root and shoot biomass, fewer tillers, and higher strigolactone (SL) levels. Exogenous zaxinone application repressed SL biosynthesis and partially rescued the growth retardation of the Oszas2 mutant. Consistent with the OsZAS2 expression pattern, Oszas2 mutants displayed a lower frequency of arbuscular mycorrhizal colonization. In conclusion, OsZAS2 is a zaxinone-forming enzyme that, similar to the previously reported OsZAS, determines rice growth, architecture, and SL content, and is required for optimal mycorrhization.

Metabolomics of plant root exudates: From sample preparation to data analysis

Plants release a set of chemical compounds, called exudates, into the rhizosphere, under normal conditions and in response to environmental stimuli and surrounding soil organisms. Plant root exudates play indispensable roles in inhibiting the growth of harmful microorganisms, while also promoting the growth of beneficial microbes and attracting symbiotic partners. Root exudates contain a complex array of primary and specialized metabolites. Some of these chemicals are only found in certain plant species for shaping the microbial community in the rhizosphere. Comprehensive understanding of plant root exudates has numerous applications from basic sciences to enhancing crop yield, production of stress-tolerant crops, and phytoremediation. This review summarizes the metabolomics workflow for determining the composition of root exudates, from sample preparation to data acquisition and analysis. We also discuss recent advances in the existing analytical methods and future perspectives of metabolite analysis.

Perspectives on the metabolism of strigolactone rhizospheric signals

Strigolactones (SLs) are a plant hormone regulating different processes in plant development and adjusting plant's architecture to nutrition availability. Moreover, SLs are released by plants to communicate with beneficial fungi in the rhizosphere where they are, however, abused as chemical cues inducing seed germination of root parasitic weeds, e.g. Striga spp., and guiding them towards host plants in their vicinity. Based on their structure, SLs are divided into canonical and non-canonical SLs. In this perspective, we describe the metabolism of root-released SLs and SL pattern in rice max1-900 mutants, which are affected in the biosynthesis of canonical SLs, and show the accumulation of two putative non-canonical SLs, CL+30 and CL+14. Using max1-900 and SL-deficient d17 rice mutants, we further investigated the metabolism of non-canonical SLs and their possible biological roles. Our results show that the presence and further metabolism of canonical and non-canonical SLs are particularly important for their role in rhizospheric interactions, such as that with root parasitic plants. Hence, we proposed that the root-released SLs are mainly responsible for rhizospheric communications and have low impact on plant architecture, which makes targeted manipulation of root-released SLs an option for rhizospheric engineering.

Canonical strigolactones are not the major determinant of tillering but important rhizospheric signals in rice

Strigolactones (SLs) are a plant hormone inhibiting shoot branching/tillering and a rhizospheric, chemical signal that triggers seed germination of the noxious root parasitic plant Striga and mediates symbiosis with beneficial arbuscular mycorrhizal fungi. Identifying specific roles of canonical and noncanonical SLs, the two SL subfamilies, is important for developing Striga-resistant cereals and for engineering plant architecture. Here, we report that rice mutants lacking canonical SLs do not show the shoot phenotypes known for SL-deficient plants, exhibiting only a delay in establishing arbuscular mycorrhizal symbiosis, but release exudates with a significantly decreased Striga seed-germinating activity. Blocking the biosynthesis of canonical SLs by TIS108, a specific enzyme inhibitor, significantly lowered Striga infestation without affecting rice growth. These results indicate that canonical SLs are not the determinant of shoot architecture and pave the way for increasing crop resistance by gene editing or chemical treatment.

Nutrient regulation of lipochitooligosaccharide recognition in plants via NSP1 and NSP2

Many plants associate with arbuscular mycorrhizal fungi for nutrient acquisition, while legumes also associate with nitrogen-fixing rhizobial bacteria. Both associations rely on symbiosis signaling and here we show that cereals can perceive lipochitooligosaccharides (LCOs) for activation of symbiosis signaling, surprisingly including Nod factors produced by nitrogen-fixing bacteria. However, legumes show stringent perception of specifically decorated LCOs, that is absent in cereals. LCO perception in plants is activated by nutrient starvation, through transcriptional regulation of Nodulation Signaling Pathway (NSP)1 and NSP2. These transcription factors induce expression of an LCO receptor and act through the control of strigolactone biosynthesis and the karrikin-like receptor DWARF14-LIKE. We conclude that LCO production and perception is coordinately regulated by nutrient starvation to promote engagement with mycorrhizal fungi. Our work has implications for the use of both mycorrhizal and rhizobial associations for sustainable productivity in cereals.

Carotenoid-derived bioactive metabolites shape plant root architecture to adapt to the rhizospheric environments

Roots are important plant organs for the uptake of water and nutrient elements. Plant root development is finely regulated by endogenous signals and environmental cues, which shapes the root system architecture to optimize the plant growth and adapt to the rhizospheric environments. Carotenoids are precursors of plant hormones strigolactones (SLs) and ABA, as well as multiple bioactive molecules. Numerous studies have demonstrated SLs and ABA as essential regulators of plant root growth and development. In addition, a lot carotenoid-derived bioactive metabolites are recently identified as plant root growth regulators, such as anchorene, β-cyclocitral, retinal and zaxinone. However, our knowledge on how these metabolites affect the root architecture to cope with various stressors and how they interact with each other during these processes is still quite limited. In the present review, we will briefly introduce the biosynthesis of carotenoid-derived root regulators and elaborate their biological functions on root development and architecture, focusing on their contribution to the rhizospheric environmental adaption of plants.

9-cis-β-Apo-10'-carotenal is the precursor of strigolactones in planta

¹³C-isotope feeding experiments demonstrate that the apocarotenoid 9-cis-β-apo-10'-carotenal is the precursor of several strigolactones in rice, providing a direct, in planta evidence for its role in strigolactone biosynthesis. Strigolactones (SLs) are plant hormone that regulates plant architecture and mediates rhizospheric communications. Previous in vitro studies using heterogously produced enzymes unraveled the conversion of all-trans-β-carotene via the intermediate 9-cis-β-apo-10'-carotenal into the SL precursor carlactone. However, a direct evidence for the formation of SLs from 9-cis-β-apo-10'-carotenal is still missing. To provide this evidence, we supplied rice seedlings with ¹³C-labeled 9-cis-β-apo-10'-carotenal and analyzed their SLs by LC-MS. Our results show that 9-cis-β-apo-10'-carotenal is the SL precursor in planta and reveal, for the first time, the application of labeled long-chain apocarotenoids as a promising approach to investigate apocarotenoid metabolism and the genesis of carotenoid-derived growth regulators and signaling molecules.

Zaxinone synthase controls arbuscular mycorrhizal colonization level in rice

The Oryza sativa (rice) carotenoid cleavage dioxygenase OsZAS was described to produce zaxinone, a plant growth-promoting apocarotenoid. A zas mutant line showed reduced arbuscular mycorrhizal (AM) colonization, but the mechanisms underlying this behavior are unknown. Here, we investigated how OsZAS and exogenous zaxinone treatment regulate mycorrhization. Micromolar exogenous supply of zaxinone rescued root growth but not the mycorrhizal defects of the zas mutant, and even reduced mycorrhization in wild-type and zas genotypes. The zas line did not display the increase in the level of strigolactones (SLs) that was observed in wild-type plants at 7 days post-inoculation with AM fungus. Moreover, exogenous treatment with the synthetic SL analog GR24 rescued the zas mutant mycorrhizal phenotype, indicating that the lower AM colonization rate of zas is caused by a deficiency in SLs at the early stages of the interaction, and indicating that during this phase OsZAS activity is required to induce SL production, possibly mediated by the Dwarf14-Like (D14L) signaling pathway. OsZAS is expressed in arbuscule-containing cells, and OsPT11prom::OsZAS transgenic lines, where OsZAS expression is driven by the OsPT11 promoter active in arbusculated cells, exhibit increased mycorrhization compared with the wild type. Overall, our results show that the genetic manipulation of OsZAS activity in planta leads to a different effect on AM symbiosis from that of exogenous zaxinone treatment, and demonstrate that OsZAS influences the extent of AM colonization, acting as a component of a regulatory network that involves SLs.

Tangerine tomato roots show increased accumulation of acyclic carotenoids, less abscisic acid, drought sensitivity, and impaired endomycorrhizal colonization

The Heirloom Golden tangerine tomato fruit variety is highly nutritious due to accumulation of tetra-cis-lycopene, that has a higher bioavailability and recognised health benefits in treating anti-inflammatory diseases compared to all-trans-lycopene isomers found in red tomatoes. We investigated if photoisomerization of tetra-cis-lycopene occurs in roots of the MicroTom tangerine (tang^mic) tomato and how this affects root to shoot biomass, mycorrhizal colonization, abscisic acid accumulation, and responses to drought. tang^mic plants grown in soil under glasshouse conditions displayed a reduction in height, number of flowers, fruit yield, and root length compared to wild-type (WT). Soil inoculation with Rhizophagus irregularis revealed fewer arbuscules and other fungal structures in the endodermal cells of roots in tang^mic relative to WT. The roots of tang^mic hyperaccumulated acyclic cis-carotenes, while only trace levels of xanthophylls and abscisic acid were detected. In response to a water deficit, leaves from the tang^mic plants displayed a rapid decline in maximum quantum yield of photosystem II compared to WT, indicating a defective root to shoot signalling response to drought. The lack of xanthophylls biosynthesis in tang^mic roots reduced abscisic acid levels, thereby likely impairing endomycorrhizal colonisation and drought-induced root to shoot signalling.

Protocol for characterizing strigolactones released by plant roots

The plant hormone strigolactones (SLs) are secreted by plant roots to act as rhizospheric signals. Here, we present a protocol for characterizing plant-released SLs. We first outline all necessary steps required for collection, processing, and analysis of plant root exudates using the C18 column for SL extraction, followed by liquid chromatography-mass spectrometry (LC-MS) for SL quantification. We then describe image processing by SeedQuant, an open-source artificial-intelligence-based software, for measuring the biological activity of SLs in inducing root parasitic plant seed germination. For complete details on the use and execution of this protocol, please refer to Wang et al. (2019) and Braguy et al. (2021).

Evaluation of the Biostimulant Activity of Zaxinone Mimics (MiZax) in Crop Plants

Global food security is a critical concern that needs practical solutions to feed the expanding human population. A promising approach is the employment of biostimulants to increase crop production. Biostimulants include compounds that boost plant growth. Recently, mimics of zaxinone (MiZax) were shown to have a promising growth-promoting effect in rice (Oryza sativa). In this study, we investigated the effect of MiZax on the growth and yield of three dicot horticultural plants, namely, tomato (Solanum lycopersicum), capsicum (Capsicum annuum), and squash (Cucurbita pepo) in different growth environments, as well as on the growth and development of the monocot date palm (Phoenix dactylifera), an important crop in the Middle East. The application of MiZax significantly enhanced plant height, flower, and branch numbers, fruit size, and total fruit yield in independent field trials from 2020 to 2021. Importantly, the amount of applied MiZax was far less than that used with the commercial compound humic acid, a widely used biostimulant in horticulture. Our results indicate that MiZax have significant application potential to improve the performance and productivity of horticultural crops.

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis

Plant-microorganism interactions at the rhizosphere level have a major impact on plant growth and plant tolerance and/or resistance to biotic and abiotic stresses. Of particular importance for forestry and agricultural systems is the cooperative and mutualistic interaction between plant roots and arbuscular mycorrhizal (AM) fungi from the phylum Glomeromycotina, since about 80% of terrestrial plant species can form AM symbiosis. The interaction is tightly regulated by both partners at the cellular, molecular and genetic levels, and it is highly dependent on environmental and biological variables. Recent studies have shown how fungal signals and their corresponding host plant receptor-mediated signalling regulate AM symbiosis. Host-generated symbiotic responses have been characterized and the molecular mechanisms enabling the regulation of fungal colonization and symbiosis functionality have been investigated. This review summarizes these and other recent relevant findings focusing on the molecular players and the signalling that regulate AM symbiosis. Future progress and knowledge about the underlying mechanisms for AM symbiosis regulation will be useful to facilitate agro-biotechnological procedures to improve AM colonization and/or efficiency.

Striga hermonthica Suicidal Germination Activity of Potent Strigolactone Analogs: Evaluation from Laboratory Bioassays to Field Trials

The obligate hemiparasite Striga hermonthica is one of the major global biotic threats to agriculture in sub-Saharan Africa, causing severe yield losses of cereals. The germination of Striga seeds relies on host-released signaling molecules, mainly strigolactones (SLs). This dependency opens up the possibility of deploying SL analogs as "suicidal germination agents" to reduce the accumulated seed bank of Striga in infested soils. Although several synthetic SL analogs have been developed for this purpose, the utility of these compounds in realizing the suicidal germination strategy for combating Striga is still largely unknown. Here, we evaluated the efficacy of three potent SL analogs (MP3, MP16, and Nijmegen-1) under laboratory, greenhouse, and farmer's field conditions. All investigated analogs showed around a 50% Striga germination rate, equivalent to a 50% reduction in infestation, which was comparable to the standard SL analog GR24. Importantly, MP16 had the maximum reduction of Striga emergence (97%) in the greenhouse experiment, while Nijmegen-1 appeared to be a promising candidate under field conditions, with a 43% and 60% reduction of Striga emergence in pearl millet and sorghum fields, respectively. These findings confirm that the selected SL analogs appear to make promising candidates as simple suicidal agents both under laboratory and real African field conditions, which may support us to improve suicidal germination technology to deplete the Striga seed bank in African agriculture.

Promiscuous activity of β-carotene hydroxylase CrtZ on epoxycarotenoids leads to the formation of rare carotenoids with 6-hydroxy-3-keto-ε-ends

Carotenoids with rare 6-hydroxy-3-keto-ε-end groups, such as piprixanthin, vitixanthin or cochloxanthin, found in manakin birds or plants, are rare carotenoids with high antioxidant activity. The same chemical structure is found in abscisic acid or blumenol, apocarotenoids found in plants or fungi. In this study, we serendipitously discovered that the promiscuous activity of the β-carotene hydroxylase CrtZ, a diiron-containing membrane protein, can catalyze the formation of 6-hydroxy-3-keto-ε-end by using epoxycarotenoids antheraxanthin or violaxanthin as substrate. We suggest that the reaction mechanism is similar to that of a rhodoxanthin biosynthetic enzyme. Our results provide further understanding of the reaction mechanism of diiron-containing β-carotene hydroxylases, as well as insight into the biosynthesis of natural compounds with 6-hydroxy-3-keto-ε-end carotenoid derivatives.

A New Formulation for Strigolactone Suicidal Germination Agents, towards Successful Striga Management

Striga hermonthica, a member of the Orobanchaceae family, is an obligate root parasite of staple cereal crops, which poses a tremendous threat to food security, contributing to malnutrition and poverty in many African countries. Depleting Striga seed reservoirs from infested soils is one of the crucial approaches to minimize subterranean damage to crops. The dependency of Striga germination on the host-released strigolactones (SLs) has prompted the development of the "Suicidal Germination" strategy to reduce the accumulated seed bank of Striga. The success of aforementioned strategy depends not only on the activity of the applied SL analogs, but also requires suitable application protocol with simple, efficient, and handy formulation for rain-fed African agriculture. Here, we developed a new formulation "Emulsifiable Concentration (EC)" for the two previously field-assessed SL analogs Methyl phenlactonoate 3 (MP3) and Nijmegen-1. The new EC formulation was evaluated for biological activities under lab, greenhouse, mini-field, and field conditions in comparison to the previously used Atlas G-1086 formulation. The EC formulation of SL analogs showed better activities on Striga germination with lower EC₅₀ and high stability under Lab conditions. Moreover, EC formulated SL analogs at 1.0 µM concentrations reduced 89-99% Striga emergence in greenhouse. The two EC formulated SL analogs showed also a considerable reduction in Striga emergence in mini-field and field experiments. In conclusion, we have successfully developed a desired formulation for applying SL analogs as suicidal agents for large-scale field application. The encouraging results presented in this study pave the way for integrating the suicidal germination approach in sustainable Striga management strategies for African agriculture.

Identifying Bixa orellana L. New Carotenoid Cleavage Dioxygenases 1 and 4 Potentially Involved in Bixin Biosynthesis

Carotene cleavage dioxygenases (CCDs) are a large family of Fe²⁺ dependent enzymes responsible for the production of a wide variety of apocarotenoids, such as bixin. Among the natural apocarotenoids, bixin is second in economic importance. It has a red-orange color and is produced mainly in the seeds of B. orellana. The biosynthesis of bixin aldehyde from the oxidative cleavage of lycopene at 5,6/5',6' bonds by a CCD is considered the first step of bixin biosynthesis. Eight BoCCD (BoCCD1-1, BoCCD1-3, BoCCD1-4, CCD4-1, BoCCD4-2, BoCCD4-3 and BoCCD4-4) genes potentially involved in the first step of B. orellana bixin biosynthesis have been identified. However, the cleavage activity upon lycopene to produce bixin aldehyde has only been demonstrated for BoCCD1-1 and BoCCD4-3. Using in vivo (Escherichia coli) and in vitro approaches, we determined that the other identified BoCCDs enzymes (BoCCD1-3, BoCCD1-4, BoCCD4-1, BoCCD4-2, and BoCCD4-4) also participate in the biosynthesis of bixin aldehyde from lycopene. The LC-ESI-QTOF-MS/MS analysis showed a peak corresponding to bixin aldehyde (m/z 349.1) in pACCRT-EIB E. coli cells that express the BoCCD1 and BoCCD4 proteins, which was confirmed by in vitro enzymatic assay. Interestingly, in the in vivo assay of BoCCD1-4, BoCCD4-1, BoCCD4-2, and BoCCD4-4, bixin aldehyde was oxidized to norbixin (m/z 380.2), the second product of the bixin biosynthesis pathway. In silico analysis also showed that BoCCD1 and BoCCD4 proteins encode functional dioxygenases that can use lycopene as substrate. The production of bixin aldehyde and norbixin was corroborated based on their ion fragmentation pattern, as well as by Fourier transform infrared (FTIR) spectroscopy. This work made it possible to clarify at the same time the first and second steps of the bixin biosynthesis pathway that had not been evaluated for a long time.

Rational design of Striga hermonthica-specific seed germination inhibitors

The obligate hemiparasitic weed Striga hermonthica grows on cereal roots and presents a severe threat to global food security by causing enormous yield losses, particularly in sub-Saharan Africa. The rapidly increasing Striga seed bank in infested soils provides a major obstacle in controlling this weed. Striga seeds require host-derived strigolactones (SLs) for germination, and corresponding antagonists could be used as germination inhibitors. Recently, we demonstrated that the common detergent Triton X-100 is a specific inhibitor of Striga seed germination by binding noncovalently to its receptor, S. hermonthica HYPO-SENSITIVE TO LIGHT 7 (ShHTL7), without blocking the rice (Oryza sativa) SL receptor DWARF14 (OsD14). Moreover, triazole ureas, the potent covalently binding antagonists of rice SL perception with much higher activity toward OsD14, showed inhibition of Striga but were less specific. Considering that Triton X-100 is not suitable for field application and by combining structural elements of Triton and triazole urea, we developed two hybrid compounds, KK023-N1 and KK023-N2, as potential Striga-specific germination inhibitors. Both compounds blocked the hydrolysis activity of ShHTL7 but did not affect that of OsD14. Binding of KK023-N1 diminished ShHTL7 interaction with S. hermonthica MORE AXILLARY BRANCHING 2, a major component in SL signal transduction, and increased ShHTL7 thermal specificity. Docking studies indicate that KK023-N1 binding is not covalent but is caused by hydrophobic interactions. Finally, in vitro and greenhouse tests revealed specific inhibition of Striga seed germination, which led to a 38% reduction in Striga infestation in pot experiments. These findings reveal that KK023-N1 is a potential candidate for combating Striga and a promising basis for rational design and development of further Striga-specific herbicides.

PHOSPHATE STARVATION RESPONSE transcription factors enable arbuscular mycorrhiza symbiosis

Arbuscular mycorrhiza (AM) is a widespread symbiosis between roots of the majority of land plants and Glomeromycotina fungi. AM is important for ecosystem health and functioning as the fungi critically support plant performance by providing essential mineral nutrients, particularly the poorly accessible phosphate, in exchange for organic carbon. AM fungi colonize the inside of roots and this is promoted at low but inhibited at high plant phosphate status, while the mechanistic basis for this phosphate-dependence remained obscure. Here we demonstrate that a major transcriptional regulator of phosphate starvation responses in rice PHOSPHATE STARVATION RESPONSE 2 (PHR2) regulates AM. Root colonization of phr2 mutants is drastically reduced, and PHR2 is required for root colonization, mycorrhizal phosphate uptake, and yield increase in field soil. PHR2 promotes AM by targeting genes required for pre-contact signaling, root colonization, and AM function. Thus, this important symbiosis is directly wired to the PHR2-controlled plant phosphate starvation response.

PHOSPHATE STARVATION RESPONSE transcription factors enable arbuscular mycorrhiza symbiosis

Arbuscular mycorrhiza (AM) is a widespread symbiosis between roots of the majority of land plants and Glomeromycotina fungi. AM is important for ecosystem health and functioning as the fungi critically support plant performance by providing essential mineral nutrients, particularly the poorly accessible phosphate, in exchange for organic carbon. AM fungi colonize the inside of roots and this is promoted at low but inhibited at high plant phosphate status, while the mechanistic basis for this phosphate-dependence remained obscure. Here we demonstrate that a major transcriptional regulator of phosphate starvation responses in rice PHOSPHATE STARVATION RESPONSE 2 (PHR2) regulates AM. Root colonization of phr2 mutants is drastically reduced, and PHR2 is required for root colonization, mycorrhizal phosphate uptake, and yield increase in field soil. PHR2 promotes AM by targeting genes required for pre-contact signaling, root colonization, and AM function. Thus, this important symbiosis is directly wired to the PHR2-controlled plant phosphate starvation response.

Plant carotenoids: recent advances and future perspectives

Carotenoids are isoprenoid metabolites synthesized de novo in all photosynthetic organisms. Carotenoids are essential for plants with diverse functions in photosynthesis, photoprotection, pigmentation, phytohormone synthesis, and signaling. They are also critically important for humans as precursors of vitamin A synthesis and as dietary antioxidants. The vital roles of carotenoids to plants and humans have prompted significant progress toward our understanding of carotenoid metabolism and regulation. New regulators and novel roles of carotenoid metabolites are continuously revealed. This review focuses on current status of carotenoid metabolism and highlights recent advances in comprehension of the intrinsic and multi-dimensional regulation of carotenoid accumulation. We also discuss the functional evolution of carotenoids, the agricultural and horticultural application, and some key areas for future research.

Abscisic acid supports colonization of Eucalyptus grandis roots by the mutualistic ectomycorrhizal fungus Pisolithus microcarpus

The pathways regulated in ectomycorrhizal (EcM) plant hosts during the establishment of symbiosis are not as well understood when compared to the functional stages of this mutualistic interaction. Our study used the EcM host Eucalyptus grandis to elucidate symbiosis-regulated pathways across the three phases of this interaction. Using a combination of RNA sequencing and metabolomics we studied both stage-specific and core responses of E. grandis during colonization by Pisolithus microcarpus. Using exogenous manipulation of the abscisic acid (ABA), we studied the role of this pathway during symbiosis establishment. Despite the mutualistic nature of this symbiosis, a large number of disease signalling TIR-NBS-LRR genes were induced. The transcriptional regulation in E. grandis was found to be dynamic across colonization with a small core of genes consistently regulated at all stages. Genes associated to the carotenoid/ABA pathway were found within this core and ABA concentrations increased during fungal integration into the root. Supplementation of ABA led to improved accommodation of P. microcarpus into E. grandis roots. The carotenoid pathway is a core response of an EcM host to its symbiont and highlights the need to understand the role of the stress hormone ABA in controlling host-EcM fungal interactions.

Exploring the Diversity and Regulation of Apocarotenoid Metabolic Pathways in Plants

In plants, carotenoids are subjected to enzyme-catalyzed oxidative cleavage reactions as well as to non-enzymatic degradation processes, which produce various carbonyl products called apocarotenoids. These conversions control carotenoid content in different tissues and give rise to apocarotenoid hormones and signaling molecules, which play important roles in plant growth and development, response to environmental stimuli, and in interactions with surrounding organisms. In addition, carotenoid cleavage gives rise to apocarotenoid pigments and volatiles that contribute to the color and flavor of many flowers and several fruits. Some apocarotenoid pigments, such as crocins and bixin, are widely utilized as colorants and additives in food and cosmetic industry and also have health-promoting properties. Considering the importance of this class of metabolites, investigation of apocarotenoid diversity and regulation has increasingly attracted the attention of plant biologists. Here, we provide an update on the plant apocarotenoid biosynthetic pathway, especially highlighting the diversity of the enzyme carotenoid cleavage dioxygenase 4 (CCD4) from different plant species with respect to substrate specificity and regioselectivity, which contribute to the formation of diverse apocarotenoid volatiles and pigments. In addition, we summarize the regulation of apocarotenoid metabolic pathway at transcriptional, post-translational, and epigenetic levels. Finally, we describe inter- and intraspecies variation in apocarotenoid production observed in many important horticulture crops and depict recent progress in elucidating the genetic basis of the natural variation in the composition and amount of apocarotenoids. We propose that the illustration of biochemical, genetic, and evolutionary background of apocarotenoid diversity would not only accelerate the discovery of unknown biosynthetic and regulatory genes of bioactive apocarotenoids but also enable the identification of genetic variation of causal genes for marker-assisted improvement of aroma and color of fruits and vegetables and CRISPR-based next-generation metabolic engineering of high-value apocarotenoids.

Carotenoids and Apocarotenoids in Planta: Their Role in Plant Development, Contribution to the Flavour and Aroma of Fruits and Flowers, and Their Nutraceutical Benefits

Carotenoids and apocarotenoids are diverse classes of compounds found in nature and are important natural pigments, nutraceuticals and flavour/aroma molecules. Improving the quality of crops is important for providing micronutrients to remote communities where dietary variation is often limited. Carotenoids have also been shown to have a significant impact on a number of human diseases, improving the survival rates of some cancers and slowing the progression of neurological illnesses. Furthermore, carotenoid-derived compounds can impact the flavour and aroma of crops and vegetables and are the origin of important developmental, as well as plant resistance compounds required for defence. In this review, we discuss the current research being undertaken to increase carotenoid content in plants and research the benefits to human health and the role of carotenoid derived volatiles on flavour and aroma of fruits and vegetables.

Multi-omics approaches explain the growth-promoting effect of the apocarotenoid growth regulator zaxinone in rice

The apocarotenoid zaxinone promotes growth and suppresses strigolactone biosynthesis in rice. To shed light on the mechanisms underlying its growth-promoting effect, we employed a combined omics approach integrating transcriptomics and metabolomics analysis of rice seedlings treated with zaxinone, and determined the resulting changes at the cellular and hormonal levels. Metabolites as well as transcripts analysis demonstrate that zaxinone application increased sugar content and triggered glycolysis, the tricarboxylic acid cycle and other sugar-related metabolic processes in rice roots. In addition, zaxinone treatment led to an increased root starch content and induced glycosylation of cytokinins. The transcriptomic, metabolic and hormonal changes were accompanied by striking alterations of roots at cellular level, which showed an increase in apex length, diameter, and the number of cells and cortex cell layers. Remarkably, zaxinone did not affect the metabolism of roots in a strigolactone deficient mutant, suggesting an essential role of strigolactone in the zaxinone growth-promoting activity. Taken together, our results unravel zaxinone as a global regulator of the transcriptome and metabolome, as well as of hormonal and cellular composition of rice roots. Moreover, they suggest that zaxinone promotes rice growth most likely by increasing sugar uptake and metabolism, and reinforce the potential of this compound in increasing rice performance.

Wang et al. report zaxinone as a global regulator of the transcriptome and metabolome, as well as of hormonal and cellular composition of rice roots. This study shows that zaxinone promotes rice growth by enhancing root sugar uptake and metabolism and modulation of cytokinin content, indicating the potential application of this compound in increasing rice performance.

The therapeutic potential of Crocus sativus Linn.: A comprehensive narrative review of clinical trials

Crocus sativus Linn. (Saffron) is valued worldwide for its potential use in the management of various degenerative disorders, including cardiovascular diseases (CVDs), diabetes, cancer, metabolic syndrome (MetS), neurodegenerative diseases, immune disorders, and sexual dysfunction. Previous reports, based on clinical trials, suggest that crocetin, crocin, picrocrocin, and safranal are the main bioactive components of saffron with antioxidant, anti-inflammatory, and anti-apoptotic effects. In this comprehensive narrative review, we studied the recent clinical trials, investigating the medicinal applications of saffron and/or its components. The present results can provide important insights into the potential of saffron in preventing and treating different disorders, with a special focus on the underlying cellular and molecular mechanisms. However, further high-quality studies are needed to firmly establish the clinical efficacy of saffron in treating some degenerative diseases.