Reactive Oxygen Species (ROS) Generation Is Indispensable for Haustorium Formation of the Root Parasitic Plant Striga hermonthica

Striga hermonthica induces lignin deposition at the root tip to facilitate prehaustorium formation and obligate parasitism

Striga hermonthica, an obligate parasitic plant that causes severe agricultural damage, recognizes its hosts by sensing haustorium-inducing factors (HIFs). Perception of HIFs induces the rapid transformation of S. hermonthica radicles into prehaustoria, structures that enable host invasion and mature into haustoria. HIFs consist of various aromatic compounds, including quinones, lignin monomers, and flavonoids. However, the downstream molecular pathways that orchestrate these developmental events are largely unknown. Here, we report that S. hermonthica root-tip cells rapidly deposit lignin, a major cell wall component, in response to HIFs. In addition to enhancing lignin levels, HIFs strongly induce genes involved in lignin monomer biosynthesis and polymerization, including several respiratory burst oxidase homologs (RBOHs) and class III peroxidases. Disruption of lignin monomer biosynthesis compromises prehaustorium formation, whereas HIF-induced class III peroxidases facilitate the process by promoting lignification. Our study demonstrates that cell wall lignification is a converged cellular process downstream of various HIFs that guides root meristematic cells in prehaustorium development.

Effects of Exogenous Phenolic Acids on Haustorium Induction of Cistanche deserticola Seeds Based on Host Metabolome Data

Cistanche deserticola, a holoparasitic plant widely used in traditional Chinese medicine, relies on chemical signals from its host plant, Haloxylon ammodendron, to initiate seed germination and haustorium induction. This study employed UPLC-MS/MS to analyze the root metabolites of H. ammodendron. The results showed that 11 substances such as phenolic acids, flavonoids, and alkaloids were mainly contained in the roots of H. ammodendron, among which phenolic acids accounted for the largest proportion, accounting for 18.00% in winter samples and 16.11% in autumn samples. Based on the reported exogenous substances that promote haustorium induction in C. deserticola and the differential metabolites in H. ammodendron roots, we selected seven exogenous signaling substances: 2,6-dimethoxy-1,4-benzoquinone, resorcinol, ferulic acid, syringic acid, vanillic acid, vanillin, and pelargonidin. Through concentration-gradient experiments (0.1-100 μM), we assessed their effects on haustorium induction in C. deserticola seeds. The results showed that among the seven substances, syringic acid, vanillic acid, and vanillin had the best impact on promoting the haustorium induction of C. deserticola seeds. Vanillic acid had the best impact at the concentration of 10 μmol/L, and the highest haustorium induction rate was 50.2%. There was no significant difference in the concentrations of vanillin and syringic acid. The results showed that phenolic acids in the host root system stimulated haustoria induction in C. deserticola seeds, with different substances requiring different optimal concentrations. This study not only identifies specific phenolic acids that enhance C. deserticola productivity but also establishes a chemical ecology framework for investigating host-parasite interactions in other root parasitic species.

Harnessing plant resistance against Striga spp. parasitism in major cereal crops for enhanced crop production and food security in Sub-Saharan Africa: a review

Given their long-lasting seed viability, 15–20-year lifespan and their high seed production levels, a significant impact of parasitic plant Striga spp. on African food production is inevitable. Over the last decades, climate change has increasingly favoured the adaptability, spread and virulence of major Striga species, S. hermonthica and S. asiatica, across arable land in Sub-Saharan Africa (SSA). These parasitic weeds are causing important yield losses on several staple food crops and endangering food and nutritional security in many SSA countries. Losses caused by Striga spp. are amplified by low soil fertility and recurrent droughts. The impact of Striga parasitism has been characterized through different phenotypic and genotypic traits assessment of their host plants. Among all control strategies, host-plant resistance remains the most pro-poor, easy-to-adopt, sustainable and eco-friendly control strategy against Striga parasitism. This review highlights the impact of Striga parasitism on food security in SSA and reports recent results related to the genetic basis of different agronomic, pheno-physiological and biochemical traits associated with the resistance to Striga in major African cereal food crops.

Host root exudates initiate a foraging preference by the root parasite Santalum album

Haustoria of root-parasitic plants draw nutrients from the roots of host species. While recent studies have assessed host preferences of parasitic plants, how root-exuded chemicals can mediate host tropism and selection by root-parasitic plants is poorly understood. Under greenhouse conditions, we performed two pot experiments to determine whether the root parasite Santalum album selectively forages for superior hosts (N2-fixing Acacia confusa Merr. or Dalbergia odorifera T. Chen) rather than for inferior hosts (non-N2-fixing Bischofia polycarpa (levl.) Airy Shaw or Dracontomelon duperreranum Pierre), and whether S. album uses host root exudates and/or specific chemicals in these root exudates to locate and trigger haustorium formation. Lateral roots and haustoria of S. album seedlings exhibited greater growth in the direction of D. odorifera roots than toward roots from the other three hosts. Comparative metabolic analysis revealed that D. odorifera root exudates were enriched in isoflavonoid, flavonoid and flavone/flavonol biosynthesis pathways, and that the relative contents of flavonoids were significantly greater in the root exudates of D. odorifera than in those of the other three hosts. Root exudates from D. odorifera significantly promoted S. album root growth, haustorium formation and reactive oxygen species accumulation in haustoria. Our results demonstrate that the key step in plant parasitism by S. album is based on root exudation by a host plant; the exudates function as a metabolite signal that activate lateral root growth and haustorium formation. Our results also indicate that flavonoids in the root exudates could play an important role in S. album foraging activity. Information on the responses of root parasites to host root exudates and/or haustorium-inducing chemicals may be useful for selecting superior host species to plant with valuable species of root parasites.

Phenolic signals for prehaustorium formation in Striga hermonthica

Striga hermonthica is a root parasitic plant that causes considerable crop yield losses. To parasitize host plants, parasitic plants develop a specialized organ called the haustorium that functions in host invasion and nutrient absorption. The initiation of a prehaustorium, the primitive haustorium structure before host invasion, requires the perception of host-derived compounds, collectively called haustorium-inducing factors (HIFs). HIFs comprise quinones, phenolics, flavonoids and cytokinins for S. hermonthica; however, the signaling pathways from various HIFs leading to prehaustorium formation remain largely uncharacterized. It has been proposed that quinones serve as direct signaling molecules for prehaustorium induction and phenolic compounds originating from the host cell wall are the oxidative precursors, but the overlap and distinction of their downstream signaling remain unknown. Here we show that quinone and phenolic-triggered prehaustorium induction in S. hermonthica occurs through partially divergent signaling pathways. We found that ASBr, an inhibitor of acetosyringone in virulence gene induction in the soil bacterium Agrobacterium, compromised prehaustorium formation in S. hermonthica. In addition, LGR-991, a competitive inhibitor of cytokinin receptors, inhibited phenolic-triggered but not quinone-triggered prehaustorium formation, demonstrating divergent signaling pathways of phenolics and quinones for prehaustorium formation. Comparisons of genome-wide transcriptional activation in response to either phenolic or quinone-type HIFs revealed markedly distinct gene expression patterns specifically at the early initiation stage. While quinone DMBQ triggered rapid and massive transcriptional changes in genes at early stages, only limited numbers of genes were induced by phenolic syringic acid. The number of genes that are commonly upregulated by DMBQ and syringic acid is gradually increased, and many genes involved in oxidoreduction and cell wall modification are upregulated at the later stages by both HIFs. Our results show kinetic and signaling differences in quinone and phenolic HIFs, providing useful insights for understanding how parasitic plants interpret different host signals for successful parasitism.

Cytokinins Induce Prehaustoria Coordinately with Quinone Signals in the Parasitic Plant Striga hermonthica

Orobanchaceae parasitic plants are major threats to global food security, causing severe agricultural damage worldwide. Parasitic plants derive water and nutrients from their host plants through multicellular organs called haustoria. The formation of a prehaustorium, a primitive haustorial structure, is provoked by host-derived haustorium-inducing factors (HIFs). Quinones, including 2,6-dimethoxy-p-benzoquinone (DMBQ), are of the most potent HIFs for various species in Orobanchaceae, but except non-photosynthetic holoparasites, Phelipanche and Orobanche spp. Instead, cytokinin (CK) phytohormones were reported to induce prehaustoria in Phelipanche ramosa. However, little is known about whether CKs act as HIFs in the other parasitic species to date. Moreover, the signaling pathways for quinones and CKs in prehaustorium induction are not well understood. This study shows that CKs act as HIFs in the obligate parasite Striga hermonthica but not in the facultative parasite Phtheirospermum japonicum. Using chemical inhibitors and marker gene expression analysis, we demonstrate that CKs activate prehaustorium formation through a CK-specific signaling pathway that overlaps with the quinone HIF pathway at downstream in S. hermonthica. Moreover, host root exudates activated S. hermonthica CK biosynthesis and signaling genes, and DMBQ and CK inhibitors perturbed the prehaustorium-inducing activity of exudates, indicating that host root exudates include CKs. Our study reveals the importance of CKs for prehaustorium formation in obligate parasitic plants.

Emerging technologies for the chemical control of root parasitic weeds

Parasitic plants in the Orobanchaceae family include devastating weed species, such as Striga, Orobanche, and Phelipanche, which parasitize major crops, drastically reduces crop yields and cause economic losses of over a billion US dollars worldwide. Advances in basic research on molecular and cellular processes responsible for parasitic relationships has now achieved steady progress through advances in genome analysis, biochemical analysis and structural biology. On the basis of these advances it is now possible to develop chemicals that control parasitism and reduce agricultural damage. In this review we summarized the recent development of chemicals that can control each step of parasitism from strigolactone biosynthesis in host plants to haustorium formation.

A PLETHORA/PIN-FORMED/auxin network mediates prehaustorium formation in the parasitic plant Striga hermonthica

The parasitic plant Striga (Striga hermonthica) invades the host root through the formation of a haustorium and has detrimental impacts on cereal crops. The haustorium results from the prehaustorium, which is derived directly from the differentiation of the Striga radicle. The molecular mechanisms leading to radicle differentiation shortly after germination remain unclear. In this study, we determined the developmental programs that regulate terminal prehaustorium formation in S. hermonthica at cellular resolution. We showed that shortly after germination, cells in the root meristem undergo multiplanar divisions. During growth, the meristematic activity declines and associates with reduced expression of the stem cell regulator PLETHORA1 and the cell cycle genes CYCLINB1 and HISTONE H4. We also observed a basal localization of the PIN-FORMED (PIN) proteins and a decrease in auxin levels in the meristem. Using the structural layout of the root meristem and the polarity of outer-membrane PIN proteins, we constructed a mathematical model of auxin transport that explains the auxin distribution patterns observed during S. hermonthica root growth. Our results reveal a fundamental molecular and cellular framework governing the switch of S. hermonthica roots to form the invasive prehaustoria.

BcMettl4-Mediated DNA Adenine N6-Methylation Is Critical for Virulence of Botrytis cinerea

DNA adenine N⁶-methylation (6mA) plays a critical role in various biological functions, but its occurrence and functions in filamentous plant pathogens are largely unexplored. Botrytis cinerea is an important pathogenic fungus worldwide. A systematic analysis of 6mA in B. cinerea was performed in this study, revealing that 6mA is widely distributed in the genome of this fungus. The 2 kb regions flanking many genes, particularly the upstream promoter regions, were susceptible to methylation. The role of BcMettl4, a 6mA methyltransferase, in the virulence of B. cinerea was investigated. BcMETTL4 disruption and point mutations of its catalytic motif “DPPW” both resulted in significant 6mA reduction in the genomic DNA and in reduced virulence of B. cinerea. RNA-Seq analysis revealed a total of 13 downregulated genes in the disruption mutant ΔBcMettl4 in which methylation occurred at the promoter sites. These were involved in oxidoreduction, secretory pathways, autophagy and carbohydrate metabolism. Two of these genes, BcFDH and BcMFS2, were independently disrupted. Knockout of BcFDH led to reduced sclerotium formation, while disruption of BcMFS2 resulted in dramatically decreased conidium formation and pathogenicity. These observations indicated that 6mA provides potential epigenetic markers in B. cinerea and that BcMettl4 regulates virulence in this important plant pathogen.

Nitrogen represses haustoria formation through abscisic acid in the parasitic plant Phtheirospermum japonicum

Parasitic plants are globally prevalent pathogens that withdraw nutrients from their host plants using an organ known as the haustorium. The external environment including nutrient availability affects the extent of parasitism and to understand this phenomenon, we investigated the role of nutrients and found that nitrogen is sufficient to repress haustoria formation in the root parasite Phtheirospermum japonicum. Nitrogen increases levels of abscisic acid (ABA) in P. japonicum and prevents the activation of hundreds of genes including cell cycle and xylem development genes. Blocking ABA signaling overcomes nitrogen’s inhibitory effects indicating that nitrogen represses haustoria formation by increasing ABA. The effect of nitrogen appears more widespread since nitrogen also inhibits haustoria in the obligate root parasite Striga hermonthica. Together, our data show that nitrogen acts as a haustoria repressing factor and suggests a mechanism whereby parasitic plants use nitrogen availability in the external environment to regulate the extent of parasitism.

Parasitic plants obtain nutrients from their hosts. Here the authors show that nitrogen sufficiency suppresses parasitism in the root parasite Phtheirospermum japonicum by increasing levels of the phytohormone ABA suggesting that the degree of parasitism is regulated by nutrient availability.

Genome Editing for Sustainable Agriculture in Africa

Sustainable intensification of agriculture in Africa is essential for accomplishing food and nutritional security and addressing the rising concerns of climate change. There is an urgent need to close the yield gap in staple crops and enhance food production to feed the growing population. In order to meet the increasing demand for food, more efficient approaches to produce food are needed. All the tools available in the toolbox, including modern biotechnology and traditional, need to be applied for crop improvement. The full potential of new breeding tools such as genome editing needs to be exploited in addition to conventional technologies. Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas)-based genome editing has rapidly become the most prevalent genetic engineering approach for developing improved crop varieties because of its simplicity, efficiency, specificity, and easy to use. Genome editing improves crop variety by modifying its endogenous genome free of any foreign gene. Hence, genome-edited crops with no foreign gene integration are not regulated as genetically modified organisms (GMOs) in several countries. Researchers are using CRISPR/Cas-based genome editing for improving African staple crops for biotic and abiotic stress resistance and improved nutritional quality. Many products, such as disease-resistant banana, maize resistant to lethal necrosis, and sorghum resistant to the parasitic plant Striga and enhanced quality, are under development for African farmers. There is a need for creating an enabling environment in Africa with science-based regulatory guidelines for the release and adoption of the products developed using CRISPR/Cas9-mediated genome editing. Some progress has been made in this regard. Nigeria and Kenya have recently published the national biosafety guidelines for the regulation of gene editing. This article summarizes recent advances in developments of tools, potential applications of genome editing for improving staple crops, and regulatory policies in Africa.

Living with a giant, flowering parasite: metabolic differences between Tetrastigma loheri Gagnep. (Vitaceae) shoots uninfected and infected with Rafflesia (Rafflesiaceae) and potential applications for propagation

Metabolites in Rafflesia-infected and non-infected Tetrastigma were compared which may have applications in Rafflesia propagation. Benzylisoquinoline alkaloids, here reported for the first time in Vitaceae, were abundant in non-infected shoots and may be a form of defense. In Rafflesia-infected shoots, oxylipins, which mediate immune response, were elevated. Endemic to the forests of Southeast Asia, Rafflesia (Rafflesiaceae) is a genus of holoparasitic plants producing the largest flowers in the world, yet completely dependent on its host, the tropical grape vine, Tetrastigma. Rafflesia species are threatened with extinction, making them an iconic symbol of plant conservation. Thus far, propagation has proved challenging, greatly decreasing efficacy of conservation efforts. This study compared the metabolites in the shoots of Rafflesia-infected and non-infected Tetrastigma loheri to examine how Rafflesia infection affects host metabolomics and elucidate the Rafflesia infection process. Results from LC-MS-based untargeted metabolomics analysis showed benzylisoquinoline alkaloids were naturally more abundant in non-infected shoots and are here reported for the first time in the genus Tetrastigma, and in the grape family, Vitaceae. These metabolites have been implicated in plant defense mechanisms and may prevent a Rafflesia infection. In Rafflesia-infected shoots, oxygenated fatty acids, or oxylipins, and a flavonoid, previously shown involved in plant immune response, were significantly elevated. This study provides a preliminary assessment of metabolites that differ between Rafflesia-infected and non-infected Tetrastigma hosts and may have applications in Rafflesia propagation to meet conservation goals.

How to resist parasitic plants: pre- and post-attachment strategies

The lifecycle of parasitic plants can be divided into pre-attachment and post-attachment phases that equate to free living and parasitic stages. Similarly, plant resistance to parasitic plants can be defined as pre-attachment and post-attachment resistance. Parasitic plants rely on host cues for successful host invasion. During pre-attachment resistance, changes in the composition of host signals can disrupt parasitic plant development and ultimately host invasion. Recent studies have only now begun to elucidate the genetic elements in the host that promote pre-attachment resistance. In comparison, new research points to post-attachment resistance using the common molecular mechanisms utilized by the plant immune system during plant-pathogen interactions. In kind, parasitic plants secrete proteinaceous and RNA-based effectors post-attachment to subvert the host immune system.

Molecular dissection of haustorium development in Orobanchaceae parasitic plants

Characterizing molecular aspects of haustorium development by parasitic plants in the Orobanchaceae family has identified hormone signaling/transport and specific genes as major players.

New pre-attachment Striga resistant sorghum adapted to African agro-ecologies

BACKGROUND

Pre-attachment resistance to the parasitic plants Striga hermonthica and S. asiatica occurs in sorghum mutants designated low germination stimulant 1 (lgs1). However, only a few of these mutants have been identified and their resistance validated. Additionally, pre-attachment resistance in sorghum beyond lgs1 mutants has not been explored. We used lgs1-specific markers to identify new lgs1-like mutants in a diverse global sorghum collection. The sorghum collection was also evaluated for pre-attachment resistance against Striga using an in vitro assay that measured Striga germination activity and radicle growth.

RESULTS

From a total of 177 sorghum accessions, 60 recorded mean germination levels of below 42%, which is comparable with the previously identified lgs1-like sorghum (SRN39 and IS9830) used as controls in this study. Furthermore, 32 of these accessions recorded Striga radicle lengths comparable or lower than the controls (0.42 mm). Thirty-eight accessions contained the lgs1 mutation and although overall, lgs1 mutants had considerably reduced Striga germination, some low inducers of Striga germination were wild-type for lgs1. Germination was positively but weakly correlated with radicle length pointing to additional radicle growth inhibitory activity.

CONCLUSIONS

lgs1 mutations, alongside other mechanisms for low Striga germination stimulation, are prevalent in sorghum, and poor Striga radicle growth is suggestive of host-derived inhibition. As an outcome, our study makes available multiple Striga-resistant sorghum with adaptability to diverse agro-ecological regions in sub-Saharan Africa making immediate deployment possible. © 2021 Society of Chemical Industry.

Molecular actors of seed germination and haustoriogenesis in parasitic weeds

One-sentence summary Recent advances provide insight into the molecular mechanisms underlying host-dependent seed germination and haustorium formation in parasitic plants.

Subtilase activity in intrusive cells mediates haustorium maturation in parasitic plants

Parasitic plants that infect crops are devastating to agriculture throughout the world. These parasites develop a unique inducible organ called the haustorium that connects the vascular systems of the parasite and host to establish a flow of water and nutrients. Upon contact with the host, the haustorial epidermal cells at the interface with the host differentiate into specific cells called intrusive cells that grow endophytically toward the host vasculature. Following this, some of the intrusive cells re-differentiate to form a xylem bridge (XB) that connects the vasculatures of the parasite and host. Despite the prominent role of intrusive cells in host infection, the molecular mechanisms mediating parasitism in the intrusive cells remain poorly understood. In this study, we investigated differential gene expression in the intrusive cells of the facultative parasite Phtheirospermum japonicum in the family Orobanchaceae by RNA-sequencing of laser-microdissected haustoria. We then used promoter analyses to identify genes that are specifically induced in intrusive cells, and promoter fusions with genes encoding fluorescent proteins to develop intrusive cell-specific markers. Four of the identified intrusive cell-specific genes encode subtilisin-like serine proteases (SBTs), whose biological functions in parasitic plants are unknown. Expression of SBT inhibitors in intrusive cells inhibited both intrusive cell and XB development and reduced auxin response levels adjacent to the area of XB development. Therefore, we propose that subtilase activity plays an important role in haustorium development in P. japonicum.

Adaptation of the parasitic plant lifecycle: germination is controlled by essential host signaling molecules

Parasitic plants are plants that connect with a haustorium to the vasculature of another, host, plant from which they absorb water, assimilates, and nutrients. Because of this parasitic lifestyle, parasitic plants need to coordinate their lifecycle with that of their host. Parasitic plants have evolved a number of host detection/host response mechanisms of which the germination in response to chemical host signals in one of the major families of parasitic plants, the Orobanchaceae, is a striking example. In this update review, we discuss these germination stimulants. We review the different compound classes that function as germination stimulants, how they are produced, and in which host plants. We discuss why they are reliable signals, how parasitic plants have evolved mechanisms that detect and respond to them, and whether they play a role in host specificity. The advances in the knowledge underlying this signaling relationship between host and parasitic plant have greatly improved our understanding of the evolution of plant parasitism and are facilitating the development of more effective control measures in cases where these parasitic plants have developed into weeds.

Using biotechnological approaches to develop crop resistance to root parasitic weeds

New transgenic and biotechnological approaches may serve as a key component in achieving crop resistance to root parasitic weeds. Root parasitic weeds inflict severe damage to numerous crops, reducing yield quantity and quality. A lack of new sources of resistance limits our ability to manage newly developing, more virulent races. Having no effective means to control the parasites in most crops, innovative biotechnological solutions are needed. Several novel biotechnological strategies using regulatory RNA molecules, the CRISPR/Cas9 system, and T-DNA insertions have been acknowledged for engineering resistance against parasitic weeds. Significant breakthroughs have been made over the years in deciphering the plant genome and its functions, including the genomes of parasitic weeds. However, the basis of biotechnological strategies to generate host resistance to root parasitic weeds needs to be further developed. Gene-silencing and editing tools should be used to target key processes of host-parasite interactions, such as strigolactone biosynthesis and signaling, haustorium development, and degradation and penetration of the host cell wall. In this review, we summarize and discuss the main areas of research leading to the discovery and functional analysis of genes involved in host-induced gene silencing that target key parasite genes, transgenic host modification, and host gene editing to generate sustainable resistance to root parasitic weeds.

Effects of Benzoquinones on Radicles of Orobanche and Phelipanche Species

The holoparasitic broomrape weeds (Orobanche and Phelipanche species) cause severe yield losses throughout North Africa, the Middle East, and Southern and Eastern Europe. These parasitic weeds form an haustorium at the tip of their radicles to infect the crop upon detection of the host-derived haustorium-inducing factors. Until now, the haustorial induction in the broomrapes remains less studied than in other parasitic plant species. Known haustorium-inducing factors active in hemiparasites, such as Striga and Triphysaria species, were reported to be inefficient for the induction of haustoria in broomrape radicles. In this work, the haustorium-inducing activity of p-benzoquinone and 2,6-dimethoxy-p-benzoquinone (BQ and DMBQ) on radicles of three different broomrapes, namely Orobanche cumana, Orobanche minor and Phelipanche ramosa, is reported. Additional allelopathic effects of benzoquinones on radicle growth and radicle necrosis were studied. The results of this work suggest that benzoquinones play a role in the induction of haustorium in broomrapes. Although dependent on the broomrape species assayed and the concentration of quinones used in the test, the activity of BQ appeared to be stronger than that of DMBQ. The redox property represented by p-benzoquinone, which operates in several physiological processes of plants, insects and animals, is invoked to explain this different activity. This work confirms the usefulness of benzoquinones as haustorium-inducing factors for holoparasitic plant research. The findings of this work could facilitate future studies in the infection process, such as host-plant recognition and haustorial formation.

Orobanchaceae parasite-host interactions

Parasitic plants in the family Orobanchaceae, such as Striga, Orobanche and Phelipanche, often cause significant damage to agricultural crops. The Orobanchaceae family comprises more than 2000 species in about 100 genera, providing an excellent system for studying the molecular basis of parasitism and its evolution. Notably, the establishment of model Orobanchaceae parasites, such as Triphysaria versicolor and Phtheirospermum japonicum, that can infect the model host Arabidopsis, has greatly facilitated transgenic analyses of genes important for parasitism. In addition, recent genomic and transcriptomic analyses of several Orobanchaceae parasites have revealed fascinating molecular insights into the evolution of parasitism and strategies for adaptation in this family. This review highlights recent progress in understanding how Orobanchaceae parasites attack their hosts and how the hosts mount a defense against the threats.

Allelopathic Effect of Quercetin, a Flavonoid from Fagopyrum esculentum Roots in the Radicle Growth of Phelipanche ramosa: Quercetin Natural and Semisynthetic Analogues Were Used for a Structure-Activity Relationship Investigation

Allelopathic potential of buckwheat roots on the radicle growth of the broomrape weed species Orobanche cumana and Phelipanche ramosa was studied. Buckwheat root exudates induced a significant growth inhibition in P. ramosa radicles but radicles of O. cumana were not affected. Among the metabolites present in the root organic extract we identified the flavonol quercetin and the stilbene p-coumaric acid methyl ester with only quercetin showing inhibitory effect on P. ramosa. The activity of quercetin was compared with other two similar flavanoids, the flavone apigenin and the dihydroflavanol 3-O-acetylpadmatin extracted respectively from Lavandula stoechas and Dittrichia viscosa plants. In this comparative assay only 3-O-acetylpadmatin besides quercetin, showed inhibition activity of radicle growth while apigenin was inactive. These results indicated that the presence of two ortho-free hydroxy groups of C ring, like catechol, could be an important feature to impart activity while the carbon skeleton of B ring and substituents of both A and B rings are not essential. Besides reduction of radicle growth, haustorium induction was observed at the tip of P. ramosa radicles treated with quercetin which swelled and a layer of papillae was formed. Activity of quercetin on haustorium induction in P. ramosa was assayed in comparison with the known haustorium-inducing factor 2,6-dimethoxy-p-benzoquinone (DMBQ) and a three partial methyl ether derivatives semisynthetized from quercetin. Results indicated that P. ramosa haustorium was induced by DMBQ at concentrations of 1–0.5 mM and quercetin and its derivatives at concentration range 0.1–0.05 mM.

Reactive Oxygen Species in Plant Interactions With Aphids

Reactive oxygen species (ROS) such as hydrogen peroxide and superoxide are produced in plants in response to many biotic and abiotic stressors, and they can enhance stress adaptation in certain circumstances or mediate symptom development in others. The roles of ROS in plant-pathogen interactions have been extensively studied, but far less is known about their involvement in plant-insect interactions. A growing body of evidence, however, indicates that ROS accumulate in response to aphids, an economically damaging group of phloem-feeding insects. This review will cover the current state of knowledge about when, where, and how ROS accumulate in response to aphids, which salivary effectors modify ROS levels in plants, and how microbial associates influence ROS induction by aphids. We will also explore the potential adaptive significance of intra- and extracellular oxidative responses to aphid infestation in compatible and incompatible interactions and highlight knowledge gaps that deserve further exploration.

Kin Recognition in the Parasitic Plant Triphysaria versicolor Is Mediated Through Root Exudates

Triphysaria is a facultative parasitic plant in the Orobanchaceae that parasitizes the roots of a wide range of host plants including Arabidopsis, Medicago, rice and maize. The important exception to this broad host range is that Triphysaria rarely parasitize other Triphysaria. We explored self and kin recognition in Triphysaria versicolor and showed that exudates collected from roots of host species, Arabidopsis thaliana and Medicago truncatula, induced haustorium development when applied to the roots of Triphysaria seedlings in vitro while those collected from Triphysaria did not. In mixed exudate experiments, Triphysaria exudates did not inhibit the haustorium-inducing activity of those from host roots. Interestingly, when roots of Triphysaria seedlings were treated with either horseradish peroxidase or fungal laccase, the extracts showed haustorium-inducing factor (HIF) activity, suggesting that Triphysaria roots contain the proper substrates for producing HIFs. Transgenic Triphysaria roots overexpressing a fungal laccase gene TvLCC1 showed an increased responsiveness to a known HIF, 2,6-dimethoxy benzoquinone (DMBQ), in developing haustoria. Our results indicate kin recognition in Triphysaria is associated with the lack of active HIFs in root exudates. Treatment of Triphysaria roots with enzymatic oxidases activates or releases molecules that are HIFs. This study shows that exogenously applied oxidases can activate HIFs in Triphysaria roots that had no previous HIF activity. Further studies are necessary to determine if differential oxidase activities in host and parasite roots account for the kin recognition in haustorium development.

BcSas2-Mediated Histone H4K16 Acetylation Is Critical for Virulence and Oxidative Stress Response of Botrytis cinerea

Histone acetyltransferase plays a critical role in transcriptional regulation by increasing accessibility of target genes to transcriptional activators. Botrytis cinerea is an important necrotrophic fungal pathogen with worldwide distribution and a very wide host range, but little is known of how the fungus regulates the transition from saprophytic growth to infectious growth. Here, the function of BcSas2, a histone acetyltransferase of B. cinerea, was investigated. Deletion of the BcSAS2 gene resulted in significantly reduced acetylation levels of histone H4, particularly of H4K16ac. The deletion mutant ΔBcSas2.1 was not only less pathogenic but also more sensitive to oxidative stress than the wild-type strain. RNA-Seq analysis revealed that a total of 13 B. cinerea genes associated with pathogenicity were down-regulated in the ΔBcSas2.1 mutant. Independent knockouts of two of these genes, BcXYGA (xyloglucanase) and BcCAT (catalase), led to dramatically decreased virulence and hypersensitivity to oxidative stress, respectively. Chromatin immunoprecipitation followed by quantitative PCR confirmed that BcSas2 bound directly to the promoter regions of both these pathogenicity-related genes. These observations indicated that BcSas2 regulated the transcription of pathogenicity-related genes by controlling the acetylation level of H4K16, thereby affecting the virulence and oxidative sensitivity of B. cinerea.

Common Mechanisms of Developmental Reprogramming in Plants-Lessons From Regeneration, Symbiosis, and Parasitism

Most plants are exquisitely sensitive to their environment and adapt by reprogramming post-embryonic development. The systematic understanding of molecular mechanisms regulating developmental reprogramming has been underexplored because abiotic and biotic stimuli that lead to reprogramming of post-embryonic development vary and the outcomes are highly species-specific. In this review, we discuss the diversity and similarities of developmental reprogramming processes by summarizing recent key findings in reprogrammed development: plant regeneration, nodule organogenesis in symbiosis, and haustorial formation in parasitism. We highlight the potentially shared molecular mechanisms across the different developmental programs, especially a core network module mediated by the AUXIN RESPONSIVE FACTOR (ARF) and the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors. This allows us to propose a new holistic concept that will provide insights into the nature of plant development, catalyzing the fusion of subdisciplines in plant developmental biology.

Nitro aspirin (NCX4040) induces apoptosis in PC3 metastatic prostate cancer cells via hydrogen peroxide (H2O2)-mediated oxidative stress

Non-steroidal anti-inflammatory drugs (NSAID) have shown promise as anticancer agents by inducing cell death apart from their antipyretic, anti-inflammatory and anti-thrombogenic effects. In our current study, we investigated the oxidative stress mediated cell death mechanism of a NSAID derivative NCX4040 (a nitric oxide (NO) releasing form of aspirin) in castration-resistant prostate cancer (CRPC) PC3 cell line. Our data revealed that NCX4040 is more potent than its parent compound aspirin or NO releasing compound DETA NONOate. NCX4040 significantly induced hydrogen peroxide formation with ensuing oxidative stress and mitochondrial depolarization resulting in lipid peroxidation, cell cycle arrest, inhibition of colony growth and induction of apoptosis in PC3 cells. Moreover, NCX4040 inhibited migration potential of PC3 cells by depolymerizing F-actin and promoting anoikis. Interestingly, elevated levels of NADPH oxidase 1 (NOX1), superoxide dismutase (SOD) 1 and 2 were observed upon NCX4040 treatment. However, down regulation of anti-apoptotic markers B-cell lymphoma 2 (Bcl2) and anti-oxidant thioredoxin reductase 1 (TXNRD1) expression were observed. In addition, NCX4040 down regulated cyclin D1 expression in PC3 cells further supporting the anticancer effect of NCX4040. Western blot analysis revealed that significant down regulation of key anti-apoptotic markers such as cellular inhibitor of apoptosis protein-1 (cIAP1), X-linked inhibitor of apoptosis (XIAP), survivin, and Cellular-Myc (c-Myc). On the other hand, NCX4040-treated cells showed upregulation of phosho histone H2AX (pH2AX), cleaved caspase3 and cleaved Poly [ADP-ribose] polymerase 1 (PARP1). Taken together, our data demonstrate that NCX4040 treatment enhances free radical formation which in turn induces oxidative stress leading to mitochondrial mediated cell death in metastatic PC3 cells.

Small-Molecule Screens Reveal Novel Haustorium Inhibitors in the Root Parasitic Plant Triphysaria versicolor

Root parasitic weeds in Orobanchaceae pose a tremendous threat to agriculture worldwide. We used an in vitro assay to screen libraries of small molecules for those capable of inhibiting or enhancing haustorium development in the parasitic plant Triphysaria versicolor. Several redox-modifying molecules and one structural analog of 2,6-dimethoxybenzoquine (DMBQ) inhibited haustorium development in the presence of the haustorium-inducing factor DMBQ, some of these without apparent growth inhibition to the root. Triphysaria seedlings were able to acclimate to some of these redox inhibitors. Transcript levels of four early-stage haustorium genes were differentially influenced by inhibitors. These novel haustorium inhibitors highlight the importance of redox cycling for haustorium development and suggest the potential of controlling parasitic weeds by interrupting early-stage redox-signaling pathways.

Haustorium Inducing Factors for Parasitic Orobanchaceae

Parasitic plants in the Orobanchaceae family include devastating weed species, such as Striga, Orobanche, and Phelipanche, which infest important crops and cause economic losses of over a billion US dollars worldwide, yet the molecular and cellular processes responsible for such parasitic relationships remain largely unknown. Parasitic species of the Orobanchaceae family form specialized invasion organs called haustoria on their roots to enable the invasion of host root tissues. The process of forming haustoria can be divided into two steps, prehaustorium formation and haustorium maturation, the processes occurring before and after host attachment, respectively. Prehaustorium formation is provoked by host-derived signal molecules, collectively called haustorium-inducing factors (HIFs). Cell wall-related quinones and phenolics have been known for a long time to induce haustoria in many Orobanchaceae species. Although such phenolics are widely produced in plants, structural specificities exist among these molecules that modulate their competency to induce haustoria in different parasitic plant species. In addition, the plant hormone cytokinins, structurally distinct from phenolic compounds, also trigger prehaustorium formation in Orobanchaceae. Recent findings demonstrate their involvement as rhizopsheric HIFs for Orobanche and Phelipanche species and thus address new activities for cytokinins in haustorium formation in Orobanchaceae, as well as in rhizospheric signaling. This review highlights haustorium-inducing signals in the Orobanchaceae family in the context of their host origin, action mechanisms, and species specificity.