Histidine-containing phosphotransfer domain extinction by RNA interference turns off a cytokinin signalling circuitry in Catharanthus roseus suspension cells

Identification of the AHP family reveals their critical response to cytokinin regulation during adventitious root formation in apple rootstock

Adventitious root (AR) formation is a bottleneck for vegetative proliferation. In this study, 13 AHP genes (MdAHPs) were identified in the apple genome. Phylogenetic analysis grouped them into 3 clusters (I, II, III), with 4, 4, and 5 genes respectively. The 13 MdAHPs family members were named MdAHP1 to MdAHP13 by chromosome positions. The physicochemical properties, phylogenetic relationship, motifs, and elements of their proteins were also analyzed. The amino acid quantity varied from 60~189 aa, isoelectric point lay between 4.10 and 8.93, and there were 3~7 protein-conserving motifs. Excluding MdAHP6, other members' promoter sequences behaved 2-4 CTK response elements. Additionally, the expression characteristics of MdAHPs family members at key stages of AR formation and in different tissues were also examined with exogenous 6-BA and Lov treatments. The results showed that MdAHP3 might be a key member in AR formation. GUS staining indicated that the activity of the MdAHP3 promoter was also significantly enhanced by CTK treatment. The protein interactions of MdAHP3/MdAHP1 and MdAHP3/MdAHP6 were verified. Compared with WT, 35S::MdAHP3 transgenic poplars inhibited AR formation. The above experimental results suggested that MdAHP3, as a key family member, interacts with MdAHP1 and MdAHP6 proteins to jointly mediate AR formation in apple rootstocks.

Uncovering a Phenomenon of Active Hormone Transcriptional Regulation during Early Somatic Embryogenesis in Medicago sativa

Somatic embryogenesis (SE) is a developmental process in which somatic cells undergo dedifferentiation to become plant stem cells, and redifferentiation to become a whole embryo. SE is a prerequisite for molecular breeding and is an excellent platform to study cell development in the majority of plant species. However, the molecular mechanism involved in M. sativa somatic embryonic induction, embryonic and maturation is unclear. This study was designed to examine the differentially expressed genes (DEGs) and miRNA roles during somatic embryonic induction, embryonic and maturation. The cut cotyledon (ICE), non-embryogenic callus (NEC), embryogenic callus (EC) and cotyledon embryo (CE) were selected for transcriptome and small RNA sequencing. The results showed that 17,251 DEGs, and 177 known and 110 novel miRNAs families were involved in embryonic induction (ICE to NEC), embryonic (NEC to EC), and maturation (EC to CE). Expression patterns and functional classification analysis showed several novel genes and miRNAs involved in SE. Moreover, embryonic induction is an active process of molecular regulation, and hormonal signal transduction related to pathways involved in the whole SE. Finally, a miRNA–target interaction network was proposed during M. sativa SE. This study provides novel perspectives to comprehend the molecular mechanisms in M. sativa SE.

The Journey from Two-Step to Multi-Step Phosphorelay Signaling Systems

BACKGROUND

The two-component signaling (TCS) system is an important signal transduction machinery in prokaryotes and eukaryotes, excluding animals, that uses a protein phosphorylation mechanism for signal transmission.

CONCLUSION

Prokaryotes have a primitive type of TCS machinery, which mainly comprises a membrane-bound sensory histidine kinase (HK) and its cognate cytoplasmic response regulator (RR). Hence, it is sometimes referred to as two-step phosphorelay (TSP). Eukaryotes have more sophisticated signaling machinery, with an extra component - a histidine-containing phosphotransfer (HPT) protein that shuttles between HK and RR to communicate signal baggage. As a result, the TSP has evolved from a two-step phosphorelay (His-Asp) in simple prokaryotes to a multi-step phosphorelay (MSP) cascade (His-Asp-His-Asp) in complex eukaryotic organisms, such as plants, to mediate the signaling network. This molecular evolution is also reflected in the form of considerable structural modifications in the domain architecture of the individual components of the TCS system. In this review, we present TCS system's evolutionary journey from the primitive TSP to advanced MSP type across the genera. This information will be highly useful in designing the future strategies of crop improvement based on the individual members of the TCS machinery.

A Novel AP2/ERF Transcription Factor CR1 Regulates the Accumulation of Vindoline and Serpentine in Catharanthus roseus

As one type of the most important alkaloids in the world, terpenoid indole alkaloids (TIAs) show a wide range of pharmaceutical activities that are beneficial for clinical treatments. Catharanthus roseus produces approximately 130 identified TIAs and is considered to be a model plant to study TIA biosynthesis. In order to increase the production of high medical value metabolites whose yields are extremely low in C. roseus, genetic engineering combined with transcriptional regulation has been applied in recent years. By using bioinformatics which is based on RNA sequencing (RNA-seq) data from methyl jasmonate (MeJA)-treated C. roseus as well as phylogenetic analysis, the present work aims to screen candidate genes that may be involved in the regulation of TIA biosynthesis, resulting in a novel AP2/ERF transcription factor, CR1 (Catharanthus roseus 1). Subsequently, virus-induced gene silencing (VIGS) of CR1 was carried out to identify the involvement of CR1 in the accumulations of several TIAs and quantitative real-time PCR (qRT-PCR) was then applied to detect the expression levels of 7 genes in the related biosynthetic pathway in silenced plants. The results show that all the 7 genes were upregulated in CR1-silenced plants. Furthermore, metabolite analyses indicate that silencing CR1 could increase the accumulations of vindoline and serpentine in C. roseus. These results suggest a novel negative regulator which may be involved in the TIAs biosynthetic pathway.

Phytochemical genomics of the Madagascar periwinkle: Unravelling the last twists of the alkaloid engine

The Madagascar periwinkle produces a large palette of Monoterpenoid Indole Alkaloids (MIAs), a class of complex alkaloids including some of the most valuable plant natural products with precious therapeutical values. Evolutionary pressure on one of the hotspots of biodiversity has obviously turned this endemic Malagasy plant into an innovative alkaloid engine. Catharanthus is a unique taxon producing vinblastine and vincristine, heterodimeric MIAs with complex stereochemistry, and also manufactures more than 100 different MIAs, some shared with the Apocynaceae, Loganiaceae and Rubiaceae members. For over 60 years, the quest for these powerful anticancer drugs has inspired biologists, chemists, and pharmacists to unravel the chemistry, biochemistry, therapeutic activity, cell and molecular biology of Catharanthus roseus. Recently, the "omics" technologies have fuelled rapid progress in deciphering the last secret of strictosidine biosynthesis, the central precursor opening biosynthetic routes to several thousand MIA compounds. Dedicated C. roseus transcriptome, proteome and metabolome databases, comprising organ-, tissue- and cell-specific libraries, and other phytogenomic resources, were developed for instance by PhytoMetaSyn, Medicinal Plant Genomic Resources and SmartCell consortium. Tissue specific library screening, orthology comparison in species with or without MIA-biochemical engines, clustering of gene expression profiles together with various functional validation strategies, largely contributed to enrich the toolbox for plant synthetic biology and metabolic engineering of MIA biosynthesis.

Down-regulation of the CrHPT1 histidine phosphotransfer protein prevents cytokinin-mediated up-regulation of CrDXR, and CrG10H transcript levels in periwinkle cell cultures

In Catharanthus roseus cell cultures, cytokinins (CK) improve monoterpenoid indole alkaloids (MIAs) accumulation. This metabolite production is correlated with an increase of transcripts corresponding to genes encoding both elements of the CK-signaling pathway and enzymes implicated in MIAs biosynthesis. However, it has not been demonstrated that the CK signal, leading to MIAs accumulation, comes through components identified as belonging to the CK-signaling pathway. In this work, we addressed this question, by transgenesis, using an inducible RNAi system targeting element of CK-signaling. In transgenic lines, the up-regulation by CK of two genes involved in MIA biosynthesis was abolished. These results demonstrate a relationship between the CK-signaling and the MIAs biosynthetic pathways.

A virus-induced gene silencing approach to understanding alkaloid metabolism in Catharanthus roseus

The anticancer agents vinblastine and vincristine are bisindole alkaloids derived from coupling vindoline and catharanthine, monoterpenoid indole alkaloids produced exclusively by the Madagascar periwinkle (Catharanthus roseus). Industrial production of vinblastine and vincristine currently relies on isolation from C. roseus leaves, a process that affords these compounds in 0.0003-0.01% yields. Metabolic engineering efforts to either improve alkaloid content or provide alternative sources of the bisindole alkaloids ultimately rely on the isolation and characterization of the genes involved. Several vindoline biosynthetic genes have been isolated, and the cellular and subcellular organization of the corresponding enzymes has been well studied. However, due to the leaf-specific localization of vindoline biosynthesis, and the lack of production of this precursor in cell suspension and hairy root cultures of C. roseus, further elucidation of this pathway demands the development of reverse genetics approaches to assay gene function in planta. The bipartite pTRV vector system is a Tobacco Rattle Virus-based virus-induced gene silencing (VIGS) platform that has provided efficient and effective means to assay gene function in diverse plant systems. A VIGS method was developed herein to investigate gene function in C. roseus plants using the pTRV vector system. The utility of this approach in understanding gene function in C. roseus leaves is demonstrated by silencing known vindoline biosynthetic genes previously characterized in vitro.

The more, the merrier: cytokinin signaling beyond Arabidopsis

The phytohormone cytokinin is a key player in many developmental processes and in the response of plants to biotic and abiotic stress. The cytokinin signal is perceived and transduced via a multistep variant of the bacterial two-component signaling system. Most of the research on cytokinin signaling has been done in the model plant Arabidopsis thaliana. Research on cytokinin signaling has expanded to a much broader range of plants species in recent years. This is due to the natural limitation of Arabidopsis as a model species for the investigation of processes like nodulation or wood formation. The rapidly increasing number of sequenced plant genomes also facilitates the use of other species in this line of research. This review summarizes what is known about the cytokinin signaling in the different organisms and highlights differences to Arabidopsis.

Silencing of tryptamine biosynthesis for production of nonnatural alkaloids in plant culture

Natural products have long served as both a source and inspiration for pharmaceuticals. Modifying the structure of a natural product often improves the biological activity of the compound. Metabolic engineering strategies to ferment "unnatural" products have been enormously successful in microbial organisms. However, despite the importance of plant derived natural products, metabolic engineering strategies to yield unnatural products from complex, lengthy plant pathways have not been widely explored. Here, we show that RNA mediated suppression of tryptamine biosynthesis in Catharanthus roseus hairy root culture eliminates all production of monoterpene indole alkaloids, a class of natural products derived from two starting substrates, tryptamine and secologanin. To exploit this chemically silent background, we introduced an unnatural tryptamine analog to the production media and demonstrated that the silenced plant culture could produce a variety of novel products derived from this unnatural starting substrate. The novel alkaloids were not contaminated by the presence of the natural alkaloids normally present in C. roseus. Suppression of tryptamine biosynthesis therefore did not appear to adversely affect expression of downstream biosynthetic enzymes. Targeted suppression of substrate biosynthesis therefore appears to be a viable strategy for programming a plant alkaloid pathway to more effectively produce desirable unnatural products. Moreover, although tryptamine is widely found among plants, this silenced line demonstrates that tryptamine does not play an essential role in growth or development in C. roseus root culture. Silencing the biosynthesis of an early starting substrate enhances our ability to harness the rich diversity of plant based natural products.

Cytokinin signaling during root development

The cytokinin class of phytohormones regulates division and differentiation of plant cells. They are perceived and signaled by a phosphorelay mechanism similar to those observed in prokaryotes. Research into the components of phosphorelay had previously been marred by genetic redundancy. However, recent studies have addressed this with the creation of high-order mutants. In addition, several new elements regulating cytokinin signaling have been identified. This has uncovered many roles in diverse developmental and physiological processes. In this review, we look at these processes specifically in the context of root development. We focus on the formation and maintenance of the root apical meristem, primary and secondary vascular development, lateral root emergence and development, and root nodulation. We believe that the root is an ideal organ with which to investigate cytokinin signaling in a wider context.

Cytokinin signaling: two-components and more

The hormone cytokinin is perceived via a phosphorelay that is similar to the two-component systems with which bacteria sense and respond to environmental stimuli. Characterization of two-component elements in Arabidopsis, maize and rice shows that cytokinin responses are mediated via partially redundant two-component protein families: histidine kinases, histidine phosphotransfer proteins and response regulators, as well as by novel players, such as the cytokinin response factors. Recent studies in the model plant Arabidopsis have revealed that cytokinin regulates these signaling components through a variety of mechanisms, including modulating transcription, controlling phosphorelay and regulating protein localization and stability. Genetic analyses of cytokinin signaling components have clarified the roles of cytokinin signaling in development and revealed novel functions for this 'classic' phytohormone.

Signaling via Histidine-Containing Phosphotransfer Proteins in Arabidopsis

The Arabidopsis genome encodes a number of proteins with similarity to two-component phosphorelay signaling elements, including hybrid receptor histidine kinases, two classes of response regulator proteins (type-A and type-B ARRs) and a family of six histidine-containing phosphotransfer proteins (AHPs), five of which contain a conserved His residue that is required for phosphorelay signaling. The current model for cytokinin signaling includes a multistep phosphorelay: three histidine kinases and at least five type-B ARRs have been shown to act as positive regulators of cytokinin signaling, while a number of type-A ARRs, and AHP6, act as negative regulators of the pathway. In our recent Plant Cell paper, we provided genetic evidence that at least four AHPs can act as positive regulators of cytokinin signaling, affecting responses to cytokinin in the root and the shoot. In this addendum, we discuss the role of AHPs in cytokinin signaling and speculate on their potential interactions with other signaling pathways in Arabidopsis.

Involvement of hormones and KNOXI genes in early Arabidopsis seedling development

Plant hormones control plant development by modulating the expression of regulatory genes, including homeobox-containing KNOXI genes. However, much remains to be elucidated about the interactions involved. Therefore, hormonal regulation of KNOXI gene expression was investigated using hormone applications and an inducible transgenic ipt expression system to increase endogenous cytokinin (CK) levels. Treatments with auxin, abscisic acid (ABA), cytokinins, ethylene, and gibberellin (GA) did not result in ectopic expression of the BP (BREVIPEDICELLUS) gene. However, BP expression was strongly reduced by ABA, increased by auxin treatment (correlating with the initiation of lateral root meristems, which strongly express BP), and did not significantly respond to short-term treatments with the other hormones in whole seedlings. Following short-term ipt activation, organ-specific differential regulation of KNOXI gene expression was observed. While several KNOXI genes were transiently up-regulated to low levels, STM was selectively repressed (especially at low light) in hypocotyls. In cotyledons, activation of CK-responsive genes preceded ipt induction, suggesting that CKs are transported more rapidly than the inducing agent (dexamethasone). Long-term increases in CK levels induced raised levels of several KNOXI transcripts in hypocotyls, correlating with the radial expansion of vascular tissues, the main domains of KNOXI gene expression, suggesting that CKs had little effect on KNOXI promoter activity. No alterations in hormone sensitivity were observed in a bp null mutant. Constitutive BP overexpression caused reductions in the length and number of lateral roots, while the primary root remained unaffected. The transgenic seedlings displayed weak, but significant, alterations in sensitivity to ABA, CK, and 1-amino-cyclopropane-1-carboxylic acid.

The Arabidopsis histidine phosphotransfer proteins are redundant positive regulators of cytokinin signaling

Arabidopsis thaliana histidine phosphotransfer proteins (AHPs) are similar to bacterial and yeast histidine phosphotransfer proteins (HPts), which act in multistep phosphorelay signaling pathways. A phosphorelay pathway is the current model for cytokinin signaling. To assess the role of AHPs in cytokinin signaling, we isolated T-DNA insertions in the five AHP genes that are predicted to encode functional HPts and constructed multiple insertion mutants, including an ahp1,2,3,4,5 quintuple mutant. Single ahp mutants were indistinguishable from wild-type seedlings in cytokinin response assays. However, various higher-order mutants displayed reduced sensitivity to cytokinin in diverse cytokinin assays, indicating both a positive role for AHPs in cytokinin signaling and functional overlap among the AHPs. In contrast with the other four AHPs, AHP4 may play a negative role in some cytokinin responses. The quintuple ahp mutant showed various abnormalities in growth and development, including reduced fertility, increased seed size, reduced vascular development, and a shortened primary root. These data indicate that most of the AHPs are redundant, positive regulators of cytokinin signaling and affect multiple aspects of plant development.

Molecular cloning and expression of a cDNA encoding a hybrid histidine kinase receptor in tropical periwinkle Catharanthus roseus

Signalling pathways involving histidine kinase receptors (HKRs) are widely used by prokaryotes and fungi to regulate a large palette of biological processes. In plants, HKRs are known to be implicated in cytokinin, ethylene, and osmosensing transduction pathways. In this work, a full length cDNA named CRCIK was isolated from the tropical species CATHARANTHUS ROSEUS (L.) G. Don. It encodes a 1205 amino acid protein that belongs to the hybrid HKR family. The deduced amino acid sequence shows the highest homology with AtHK1, an osmosensing HKR in ARABIDOPSIS THALIANA. In return, CrCIK protein shares very low identity with the other 10 ARABIDOPSIS HKRs. Southern blot analysis indicates that the CRCIK corresponding gene is either present in multiple copies or has very close homologues in the genome of the tropical periwinkle. The gene is widely expressed in the plant. In C. ROSEUS C20D cell suspension, it is slightly induced after exposure to low temperature, pointing to a putative role in cold-shock signal transduction.

Osmotic stress sensing in Populus: components identification of a phosphorelay system

To study the Populus response to an osmotic stress, we have isolated one cDNA encoding a histidine-aspartate kinase (HK1) and four cDNAs encoding histidine-containing phosphotransfer proteins (HPts), HPt1-4. The predicted HK1 protein shares a typical structure with ATHK1 and SLN1 osmosensors. The 4 HPTs are characterized by the histidine phosphotransfer domain. We have shown that HK1 is upregulated during an osmotic stress in hydroponic culture. We have detected an interaction between HK1 and HPt2, using the yeast two-hybrid system. These results suggest the existence of a multi-step phosphorelay pathway probably involved in osmotic stress sensing in Populus.

Cytokinin signaling

Cytokinins influence many aspects of plant growth and development. The current model for cytokinin signaling is a multi-step phosphorelay similar to the prokaryotic two-component systems that are used in responses to environmental stimuli. Recently, progress has been made in improving our understanding of the molecular mechanism that underlies cytokinin signaling. Molecular and genetic analyses of loss-of-function mutants indicate that the two-component elements that are involved in cytokinin signaling have redundant and overlapping functions. These elements regulate both the shoot and root meristems, are required for the development of fertile flowers, and modulate the response to varying nutrient levels.

CaaX-prenyltransferases are essential for expression of genes involvedin the early stages of monoterpenoid biosynthetic pathway in Catharanthus roseus cells

CaaX-prenyltransferases (CaaX-PTases) catalyse the covalent attachment of isoprenyl groups to conserved cysteine residues located at the C-terminal CaaX motif of a protein substrate. This post-translational modification is required for the function and/or subcellular localization of some transcription factors and components of signal transduction and membrane trafficking machinery. CaaX-PTases, including protein farnesyltransferase (PFT) and type-I protein geranylgeranyltransferase (PGGT-I), are heterodimeric enzymes composed of a common alpha subunit and a specific beta subunit. We have established RNA interference cell lines targeting the beta subunits of PFT and PGGT-I, respectively, in the Catharanthus roseus C20D cell line, which synthesizes monoterpenoid indole alkaloids in response to auxin depletion from the culture medium. In both types of RNAi cell lines, expression of a subset of genes involved in the early stage of monoterpenoid biosynthetic pathway (ESMB genes), including the MEP pathway, is strongly decreased. The role of CaaX-PTases in ESMB gene regulation was confirmed by using the general prenyltransferase inhibitor s-perillyl alcohol (SP) and the specific PFT inhibitor Manumycin A on the wild type line. Furthermore, supplementation of SP inhibited cells with monoterpenoid intermediates downstream of the steps encoded by the ESMB genes restores monoterpenoid indole alkaloids biosynthesis. We conclude that protein targets for both PFT and PGGT-I are required for the expression of ESMB genes and monoterpenoid biosynthesis in C. roseus, this represents a non previously described role for protein prenyltransferase in plants.