Cytokinins play opposite roles in lateral root formation, and nematode and Rhizobial symbioses

Cytokinin: From autoclaved DNA to two-component signaling

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

Light-sensitive short hypocotyl genes confer symbiotic nodule identity in the legume Medicago truncatula

Legumes produce specialized root nodules that are distinct from lateral roots in morphology and function, with nodules intracellularly hosting nitrogen-fixing bacteria. We have previously shown that a lateral root program underpins nodule initiation, but there must be additional developmental regulators that confer nodule identity. Here, we show two members of the LIGHT-SENSITIVE SHORT HYPOCOTYL (LSH) transcription factor family, predominantly known to define shoot meristem complexity and organ boundaries, function as regulators of nodule organ identity. In parallel to the root initiation program, LSH1/LSH2 recruit a program into the root cortex that mediates the divergence into nodules, in particular with cell divisions in the mid-cortex. This includes regulation of auxin and cytokinin, promotion of NODULE ROOT1/2 and Nuclear Factor YA1, and suppression of the lateral root program. A principal outcome of LSH1/LSH2 function is the production of cells able to accommodate nitrogen-fixing bacteria, a key feature unique to nodules.

Transcriptional alterations of peanut root during interaction with growth-promoting Tsukamurella tyrosinosolvens strain P9

The plant growth-promoting rhizobacterium Tsukamurella tyrosinosolvens P9 can improve peanut growth. In this study, a co-culture system of strain P9 and peanut was established to analyze the transcriptome of peanut roots interacting with P9 for 24 and 72 h. During the early stage of co-culturing, genes related to mitogen-activated protein kinase (MAPK) and Ca2+ signal transduction, ethylene synthesis, and cell wall pectin degradation were induced, and the up-regulation of phenylpropanoid derivative, flavonoid, and isoflavone synthesis enhanced the defense response of peanut. The enhanced expression of genes associated with photosynthesis and carbon fixation, circadian rhythm regulation, indoleacetic acid (IAA) synthesis, and cytokinin decomposition promoted root growth and development. At the late stage of co-culturing, ethylene synthesis was reduced, whereas Ca2+ signal transduction, isoquinoline alkaloid synthesis, and ascorbate and aldarate metabolism were up-regulated, thereby maintaining root ROS homeostasis. Sugar decomposition and oxidative phosphorylation and nitrogen and fatty acid metabolism were induced, and peanut growth was significantly promoted. Finally, the gene expression of seedlings inoculated with strain P9 exhibited temporal differences. The results of our study, which explored transcriptional alterations of peanut root during interacting with P9, provide a basis for elucidating the growth-promoting mechanism of this bacterial strain in peanut.

CEP hormones at the nexus of nutrient acquisition and allocation, root development, and plant-microbe interactions

A growing understanding is emerging of the roles of peptide hormones in local and long-distance signalling that coordinates plant growth and development as well as responses to the environment. C-TERMINALLY ENCODED PEPTIDE (CEP) signalling triggered by its interaction with CEP RECEPTOR 1 (CEPR1) is known to play roles in systemic nitrogen (N) demand signalling, legume nodulation, and root system architecture. Recent research provides further insight into how CEP signalling operates, which involves diverse downstream targets and interactions with other hormone pathways. Additionally, there is emerging evidence of CEP signalling playing roles in N allocation, root responses to carbon levels, the uptake of other soil nutrients such as phosphorus and sulfur, root responses to arbuscular mycorrhizal fungi, plant immunity, and reproductive development. These findings suggest that CEP signalling more broadly coordinates growth across the whole plant in response to diverse environmental cues. Moreover, CEP signalling and function appear to be conserved in angiosperms. We review recent advances in CEP biology with a focus on soil nutrient uptake, root system architecture and organogenesis, and roles in plant-microbe interactions. Furthermore, we address knowledge gaps and future directions in this research field.

Combination of Bacillus and Low Fertigation Input Promoted the Growth and Productivity of Chinese Cabbage and Enriched Beneficial Rhizosphere Bacteria Lechevalieria

Long-term overfertilization increases soil salinity and disease occurrence and reduces crop yield. Integrated application of microbial agents with low fertigation input might be a sustainable and cost-effective strategy. Herein, the promoting effects of Bacillus velezensis B006 on the growth of Chinese cabbage under different fertigation conditions in field trials were studied and the underlying mechanisms were revealed. In comparison with normal fertigation (water potential of -30 kPa and soluble N, P, K of 29.75, 8.26, 21.48 Kg hm-2) without B006 application, the combination of B. velezensis B006 and reduced fertigation input (-50 kPa and N, P, K of 11.75, 3.26, 6.48 Kg hm-2) promoted cabbage growth and root development, restrained the occurrence of soft rot disease, and improved the yield. High-performance liquid chromatography (HPLC) analyses indicated that B006 application promoted the production of indole-3-acetic acid and salicylic acid in cabbage roots, which are closely related to plant growth. Rhizosphere microbiota analyses indicated that the combination of low fertigation input and B006 application promoted the enrichment of Streptomyces, Lechevalieria, Promicromonospora, and Aeromicrobium and the abundance of Lechevalieria was positively correlated with the root length and vitality. This suggested that the integrated application of reduced fertigation and Bacillus is highly efficient to improve soil ecology and productivity and will benefit the sustainable development of crop cultivation in a cost-effective way.

Carpel-specific down-regulation of GhCKXs in cotton significantly enhances seed and fiber yield

Cytokinin is considered to be an important driver of seed yield. To increase the yield of cotton while avoiding the negative consequences caused by constitutive overproduction of cytokinin, we down-regulated specifically the carpel genes for cytokinin oxidase/dehydrogenase (CKX), a key negative regulator of cytokinin levels, in transgenic cotton. The carpel-specific down-regulation of CKXs significantly enhanced cytokinin levels in the carpels. The elevated cytokinin promoted the expression of carpel- and ovule-development-associated genes, GhSTK2, GhAG1, and GhSHP, boosting ovule formation and thus producing more seeds in the ovary. Field experiments showed that the carpel-specific increase of cytokinin significantly increased both seed yield and fiber yield of cotton, without resulting in detrimental phenotypes. Our study details the regulatory mechanism of cytokinin signaling for seed development, and provides an effective and feasible strategy for yield improvement of seed crops.

Right time, right place: The dynamic role of hormones in rhizobial infection and nodulation of legumes

Many legume plants form beneficial associations with rhizobial bacteria that are hosted in new plant root organs, nodules, in which atmospheric nitrogen is fixed. This association requires the precise coordination of two separate programs, infection in the epidermis and nodule organogenesis in the cortex. There is extensive literature indicating key roles for plant hormones during nodulation, but a detailed analysis of the spatial and temporal roles of plant hormones during the different stages of nodulation is required. This review analyses the current literature on hormone regulation of infection and organogenesis to reveal the differential roles and interactions of auxin, cytokinin, brassinosteroids, ethylene, and gibberellins during epidermal infection and cortical nodule initiation, development, and function. With the exception of auxin, all of these hormones suppress infection events. By contrast, there is evidence that all of these hormones promote nodule organogenesis, except ethylene, which suppresses nodule initiation. This differential role for many of the hormones between the epidermal and cortical programs is striking. Future work is required to fully examine hormone interactions and create a robust model that integrates this knowledge into our understanding of nodulation pathways.

Transcriptome Analysis Reveals Key Pathways and Candidate Genes Controlling Seed Development and Size in Ricebean (Vigna umbellata)

Ricebean (Vigna umbellata) is a lesser known pulse with well-recognized potential. Recently, it has emerged as a legume with endowed nutritional potential because of high concentration of quality protein and other vital nutrients in its seeds. However, the genes and pathways involved in regulating seed development and size are not understood in this crop. In our study, we analyzed the transcriptome of two genotypes with contrasting grain size (IC426787: large seeded and IC552985: small seeded) at two different time points, namely, 5 and 10 days post-anthesis (DPA). The bold seeded genotype across the time points (B5_B10) revealed 6,928 differentially expressed genes (DEGs), whereas the small seeded genotype across the time point (S5_S10) contributed to 14,544 DEGs. We have also identified several candidate genes for seed development–related traits like seed size and 100-seed weight. On the basis of similarity search and domain analysis, some candidate genes (PHO1, cytokinin dehydrogenase, A-type cytokinin, and ARR response negative regulator) related to 100-seed weight and seed size showed downregulation in the small seeded genotype. The MapMan and KEGG analysis confirmed that auxin and cytokinin pathways varied in both the contrasting genotypes and can therefore be the regulators of the seed size and other seed development–related traits in ricebeans. A total of 51 genes encoding SCF^TIR1/AFB, Aux/IAA, ARFs, E3 ubiquitin transferase enzyme, and 26S proteasome showing distinct expression dynamics in bold and small genotypes were also identified. We have also validated randomly selected SSR markers in eight accessions of the Vigna species (V. umbellata: 6; Vigna radiata: 1; and Vigna mungo: 1). Cross-species transferability pattern of ricebean–derived SSR markers was higher in V. radiata (73.08%) than V. mungo (50%). To the best of our knowledge, this is the first transcriptomic study conducted in this crop to understand the molecular basis of any trait. It would provide us a comprehensive understanding of the complex transcriptome dynamics during the seed development and gene regulatory mechanism of the seed size determination in ricebeans.

SHORT-ROOT paralogs mediate feedforward regulation of D-type cyclin to promote nodule formation in soybean

Nitrogen fixation in soybean takes place in root nodules that arise from de novo cell divisions in the root cortex. Although several early nodulin genes have been identified, the mechanism behind the stimulation of cortical cell division during nodulation has not been fully resolved. Here we provide evidence that two paralogs of soybean SHORT-ROOT (GmSHR) play vital roles in soybean nodulation. Expression of GmSHR4 and GmSHR5 (GmSHR4/5) is induced in cortical cells at the beginning of nodulation, when the first cell divisions occur. The expression level of GmSHR4/5 is positively associated with cortical cell division and nodulation. Knockdown of GmSHR5 inhibits cell division in outer cortical layers during nodulation. Knockdown of both paralogs disrupts the cell division throughout the cortex, resulting in poorly organized nodule primordia with delayed vascular tissue formation. GmSHR4/5 function by enhancing cytokinin signaling and activating early nodulin genes. Interestingly, D-type cyclins act downstream of GmSHR4/5, and GmSHR4/5 form a feedforward loop regulating D-type cyclins. Overexpression of D-type cyclins in soybean roots also enhanced nodulation. Collectively, we conclude that the GmSHR4/5-mediated pathway represents a vital module that triggers cytokinin signaling and activates D-type cyclins during nodulation in soybean.

Auxin-Cytokinin Balance Shapes Maize Root Architecture by Controlling Primary Root Elongation and Lateral Root Development

The root system is responsible for water and nutrients uptake from the soil, and therefore, its extension is basic for an efficient acquisition. The maize root system is formed by different types of roots, and the lateral root branching substantially increases the surface for nutrient uptake. Therefore, the regulation of lateral root formation is fundamental in the development of root functions. Root architecture is basically controlled by auxin and cytokinins, which antagonize in the formation of lateral roots (LR) along the primary root axis, with auxin, a stimulator, and cytokinins inhibitors of LR development. This interaction has been analyzed in several zones along the primary root where LRs in different developmental stages were located. The root has been divided into several zones, such as meristem, elongation zone, and mature zone, according to the developmental processes occurring in each one. As Arabidopsis root elongated more slowly than maize root, these zones are shorter, and its delimitation is more difficult. However, these zones have previously been delimitated clearly in maize, and therefore, they analyze the effect of exogenous hormones in several LR developmental stages. The inhibitory effect of cytokinin on lateral root formation was observed in already elongated primary root zones in which initial events to form new lateral roots are taking place. Contrarily, auxin increased LR formation in the primary root segments elongated in the presence of the hormone. The inhibitory effect of cytokinin was reversed by auxin in a concentration-dependent manner when both hormones were combined. However, auxin is unable to recover LR development in primary root zones that have been previously elongated only in the presence of cytokinin. This antagonistic auxin-cytokinin effect on LR development depended on the balance between both hormones, which controls the root system architecture and determines the formation of LR during the process of initiation.

Meta-analysis of transcriptomic studies of cytokinin-treated rice roots defines a core set of cytokinin response genes

Cytokinins regulate diverse aspects of plant growth and development, primarily through modulation of gene expression. The cytokinin-responsive transcriptome has been thoroughly described in dicots, especially Arabidopsis, but much less so in monocots. Here, we present a meta-analysis of five different transcriptomic analyses of rice (Oryza sativa) roots treated with cytokinin, including three previously unpublished experiments. We developed a treatment method in which hormone is added to the media of rice seedlings grown in sterile hydroponic culture under a continuous airflow, which resulted in minimal perturbation of the seedlings, thus greatly reducing changes in gene expression in the absence of exogenous hormone. We defined a core set of 205 upregulated and 86 downregulated genes that were differentially expressed in at least three of the transcriptomic datasets. This core set includes genes encoding the type-A response regulators (RRs) and cytokinin oxidases/dehydrogenases, which have been shown to be primary cytokinin response genes. GO analysis revealed that the upregulated genes were enriched for terms related to cytokinin/hormone signaling and metabolism, while the downregulated genes were significantly enriched for genes encoding transporters. Variations of type-B RR binding motifs were significantly enriched in the promoters of the upregulated genes, as were binding sites for other potential partner transcription factors. The promoters of the downregulated genes were generally enriched for distinct cis-acting motifs and did not include the type-B RR binding motif. This analysis provides insight into the molecular mechanisms underlying cytokinin action in a monocot and provides a useful foundation for future studies of this hormone in rice and other cereals.

Transcriptome analysis of resistant and susceptible mulberry responses to Meloidogyne enterolobii infection

BACKGROUND

Mulberry (Morus alba L.) is an important sericulture crop; however, root-knot nematode infection seriously limits its production. Understanding the mechanism of interaction between mulberry and nematode is important for control of infection.

RESULTS

Using sequencing and de novo transcriptome assembly, we identified 55,894 unigenes from root samples of resistant and susceptible mulberry cultivars at different stages after infection with the nematode Meloidogyne enterolobii; 33,987 of these were annotated in the Nr, SWISS-PROT, KEGG, and KOG databases. Gene ontology and pathway enrichment analyses of differentially expressed genes (DEGs) revealed key genes involved in hormone metabolic processes, plant hormone signal transduction, flavonoid biosynthesis, phenylpropanoid biosynthesis, and peroxisomal and photosynthetic pathways. Analysis of key trends in co-expression networks indicated that expression of unigenes 0,015,083, 0,073,272, 0,004,006, and 0,000,628 was positively correlated with resistance to M. enterolobii. Unigene 0015083 encodes tabersonine 16-O-methyltransferase (16OMT), which is involved in alkaloid biosynthesis. Unigene 0073272 encodes a transcription factor contributing to nitric oxide accumulation during plant immune responses. Unigenes 0,004,006 and 0,000,628 encode ERF and MYB transcription factors, respectively, involved in plant hormone signaling. We verified the accuracy of transcriptome sequencing results by RT-qPCR of 21 DEGs.

CONCLUSIONS

The results of this study increase our understanding of the resistance mechanisms and candidate genes involved in mulberry-M. enterolobii interaction. Thus, our data will contribute to the development of effective control measures against this pathogen.

Sedentary Plant-Parasitic Nematodes Alter Auxin Homeostasis via Multiple Strategies

Sedentary endoparasites such as cyst and root-knot nematodes infect many important food crops and are major agro-economical pests worldwide. These plant-parasitic nematodes exploit endogenous molecular and physiological pathways in the roots of their host to establish unique feeding structures. These structures function as highly active transfer cells and metabolic sinks and are essential for the parasites' growth and reproduction. Plant hormones like indole-3-acetic acid (IAA) are a fundamental component in the formation of these feeding complexes. However, their underlying molecular and biochemical mechanisms are still elusive despite recent advances in the field. This review presents a comprehensive overview of known functions of various auxins in plant-parasitic nematode infection sites, based on a systematic analysis of current literature. We evaluate multiple aspects involved in auxin homeostasis in plants, including anabolism, catabolism, transport, and signalling. From these analyses, a picture emerges that plant-parasitic nematodes have evolved multiple strategies to manipulate auxin homeostasis to establish a successful parasitic relationship with their host. Additionally, there appears to be a potential role for auxins other than IAA in plant-parasitic nematode infections that might be of interest to be further elucidated.

Ectopic expression of GmHP08 enhances resistance of transgenic Arabidopsis toward drought stress

Ectopic expression of Glycine max two-component system member GmHP08 in Arabidopsis enhanced drought tolerance of transgenic plants, possibly via ABA-dependent pathways. Phosphorelay by two-component system (TCS) is a signal transduction mechanism which has been evolutionarily conserved in both prokaryotic and eukaryotic organisms. Previous studies have provided lines of evidence on the involvement of TCS genes in plant perception and responses to environmental stimuli. In this research, drought-associated functions of GmHP08, a TCS member from soybean (Glycine max L.), were investigated via its ectopic expression in Arabidopsis system. Results from the drought survival assay showed that GmHP08-transgenic plants exhibited higher survival rates compared with their wild-type (WT) counterparts, indicating better drought resistance of the former group. Analyses revealed that the transgenic plants outperformed the WT in various regards, i.e. capability of water retention, prevention of hydrogen peroxide accumulation and enhancement of antioxidant enzymatic activities under water-deficit conditions. Additionally, the expression of stress-marker genes, especially antioxidant enzyme-encoding genes, in the transgenic plants were found greater than that of the WT plants. In contrary, the expression of SAG13 gene, one of the senescence-associated genes, and of several abscisic acid (ABA)-related genes was repressed. Data from this study also revealed that the ectopic expression lines at germination and early seedling development stages were hypersensitive to exogenous ABA treatment. Taken together, our results demonstrated that GmHP08 could play an important role in mediating plant response to drought, possibly via an ABA-dependent manner.

Interaction of Symbiotic Rhizobia and Parasitic Root-Knot Nematodes in Legume Roots: From Molecular Regulation to Field Application

Legumes form two types of root organs in response to signals from microbes, namely, nodules and root galls. In the field, these interactions occur concurrently and often interact with each other. The outcomes of these interactions vary and can depend on natural variation in rhizobia and nematode populations in the soil as well as abiotic conditions. While rhizobia are symbionts that contribute fixed nitrogen to their hosts, parasitic root-knot nematodes (RKN) cause galls as feeding structures that consume plant resources without a contribution to the plant. Yet, the two interactions share similarities, including rhizosphere signaling, repression of host defense responses, activation of host cell division, and differentiation, nutrient exchange, and alteration of root architecture. Rhizobia activate changes in defense and development through Nod factor signaling, with additional functions of effector proteins and exopolysaccharides. RKN inject large numbers of protein effectors into plant cells that directly suppress immune signaling and manipulate developmental pathways. This review examines the molecular control of legume interactions with rhizobia and RKN to elucidate shared and distinct mechanisms of these root-microbe interactions. Many of the molecular pathways targeted by both organisms overlap, yet recent discoveries have singled out differences in the spatial control of expression of developmental regulators that may have enabled activation of cortical cell division during nodulation in legumes. The interaction of legumes with symbionts and parasites highlights the importance of a comprehensive view of root-microbe interactions for future crop management and breeding strategies.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Promoter P PSP1-5-BnPSP-1 From Ramie (Boehmeria nivea L. Gaud.) Can Drive Phloem-Specific GUS Expression in Arabidopsis thaliana

Isolation of phloem-specific promoters is one of the basic conditions for improving the fiber development and resistance of ramie phloem using genetic engineering. In this study, we isolated a ramie endogenous promoter (named P_PSP1-BnPSP-1) and analyzed the function of its truncated fragments in Arabidopsis. The results show that P_PSP1-BnPSP-1 can drive the GUS reporter gene to be specifically expressed in the veins of Arabidopsis. After hormone and simulated drought treatment of the independent Arabidopsis lines carrying P_PSP1-BnPSP-1 and its truncated fragments, only P_PSP1–5-BnPSP-1 (−600 to −1 bp region of P_PSP1-BnPSP-1) is stably expressed and exhibits phloem specificity. Our findings suggest that P_PSP1–5-BnPSP-1 can be used as a phloem specific promoter for further research.

Unlocking the potential of plant growth-promoting rhizobacteria on soil health and the sustainability of agricultural systems

The concept of soil health refers to specific soil properties and the ability to support and sustain crop growth and productivity, while maintaining long-term environmental quality. The key components of healthy soil are high populations of organisms that promote plant growth, such as the plant growth promoting rhizobacteria (PGPR). PGPR plays multiple beneficial and ecological roles in the rhizosphere soil. Among the roles of PGPR in agroecosystems are the nutrient cycling and uptake, inhibition of potential phytopathogens growth, stimulation of plant innate immunity, and direct enhancement of plant growth by producing phytohormones or other metabolites. Other important roles of PGPR are their environmental cleanup capacities (soil bioremediation). In this work, we review recent literature concerning the diverse mechanisms of PGPR in maintaining healthy conditions of agricultural soils, thus reducing (or eliminating) the toxic agrochemicals dependence. In conclusion, this review provides comprehensive knowledge on the current PGPR basic mechanisms and applications as biocontrol agents, plant growth stimulators and soil rhizoremediators, with the final goal of having more agroecological practices for sustainable agriculture.

Root-specific expression of chickpea cytokinin oxidase/dehydrogenase 6 leads to enhanced root growth, drought tolerance and yield without compromising nodulation

Cytokinin group of phytohormones regulate root elongation and branching during post-embryonic development. Cytokinin-degrading enzymes cytokinin oxidases/dehydrogenases (CKXs) have been deployed to investigate biological activities of cytokinin and to engineer root growth. We expressed chickpea cytokinin oxidase 6 (CaCKX6) under the control of a chickpea root-specific promoter of CaWRKY31 in Arabidopsis thaliana and chickpea having determinate and indeterminate growth patterns, respectively, to study the effect of cytokinin depletion on root growth and drought tolerance. Root-specific expression of CaCKX6 led to a significant increase in lateral root number and root biomass in Arabidopsis and chickpea without any penalty to vegetative and reproductive growth of shoot. Transgenic chickpea lines showed increased CKX activity in root. Soil-grown advanced chickpea transgenic lines exhibited higher root-to-shoot biomass ratio and enhanced long-term drought tolerance. These chickpea lines were not compromised in root nodulation and nitrogen fixation. The seed yield in some lines was up to 25% higher with no penalty in protein content. Transgenic chickpea seeds possessed higher levels of zinc, iron, potassium and copper. Our results demonstrated the potential of cytokinin level manipulation in increasing lateral root number and root biomass for agronomic trait improvement in an edible legume crop with indeterminate growth habit.

Infection of Medicago truncatula by the Root-Knot Nematode Meloidogyne javanica Does Not Require Early Nodulation Genes

Because of the developmental similarities between root nodules induced by symbiotic rhizobia and root galls formed by parasitic nematodes, we investigated the involvement of nodulation genes in the infection of Medicago truncatula by the root knot nematode (RKN), Meloidogyne javanica. We found that gall formation, including giant cell formation, pericycle and cortical cell division, as well as egg laying, occurred successfully in the non-nodulating mutants nfp1 (nod factor perception1), nin1 (nodule inception1) and nsp2 (nodulation signaling pathway2) and the cytokinin perception mutant cre1 (cytokinin receptor1). Gall and egg formation were significantly reduced in the ethylene insensitive, hypernodulating mutant skl (sickle), and to a lesser extent, in the low nodulation, abscisic acid insensitive mutant latd/nip (lateral root-organ defective/numerous infections and polyphenolics). Despite its supernodulation phenotype, the sunn4 (super numeric nodules4) mutant, which has lost the ability to autoregulate nodule numbers, did not form excessive numbers of galls. Co-inoculation of roots with nematodes and rhizobia significantly reduced nodule numbers compared to rhizobia-only inoculated roots, but only in the hypernodulation mutant skl. Thus, this effect is likely to be influenced by ethylene signaling, but is not likely explained by resource competition between galls and nodules. Co-inoculation with rhizobia also reduced gall numbers compared to nematode-only infected roots, but only in the wild type. Therefore, the protective effect of rhizobia on nematode infection does not clearly depend on nodule number or on Nod factor signaling. Our study demonstrates that early nodulation genes that are essential for successful nodule development are not necessary for nematode-induced gall formation, that gall formation is not under autoregulation of nodulation control, and that ethylene signaling plays a positive role in successful RKN parasitism in M. truncatula.

Mutational analysis indicates that abnormalities in rhizobial infection and subsequent plant cell and bacteroid differentiation in pea (Pisum sativum) nodules coincide with abnormal cytokinin responses and localization

BACKGROUND AND AIMS

Recent findings indicate that Nod factor signalling is tightly interconnected with phytohormonal regulation that affects the development of nodules. Since the mechanisms of this interaction are still far from understood, here the distribution of cytokinin and auxin in pea (Pisum sativum) nodules was investigated. In addition, the effect of certain mutations blocking rhizobial infection and subsequent plant cell and bacteroid differentiation on cytokinin distribution in nodules was analysed.

METHODS

Patterns of cytokinin and auxin in pea nodules were profiled using both responsive genetic constructs and antibodies.

KEY RESULTS

In wild-type nodules, cytokinins were found in the meristem, infection zone and apical part of the nitrogen fixation zone, whereas auxin localization was restricted to the meristem and peripheral tissues. We found significantly altered cytokinin distribution in sym33 and sym40 pea mutants defective in IPD3/CYCLOPS and EFD transcription factors, respectively. In the sym33 mutants impaired in bacterial accommodation and subsequent nodule differentiation, cytokinin localization was mostly limited to the meristem. In addition, we found significantly decreased expression of LOG1 and A-type RR11 as well as KNOX3 and NIN genes in the sym33 mutants, which correlated with low cellular cytokinin levels. In the sym40 mutant, cytokinins were detected in the nodule infection zone but, in contrast to the wild type, they were absent in infection droplets.

CONCLUSIONS

In conclusion, our findings suggest that enhanced cytokinin accumulation during the late stages of symbiosis development may be associated with bacterial penetration into the plant cells and subsequent plant cell and bacteroid differentiation.

Plant tumors: a hundred years of study

The review provides information on the mechanisms underlying the development of spontaneous and pathogen-induced tumors in higher plants. The activation of meristem-specific regulators in plant tumors of various origins suggests the meristem-like nature of abnormal plant hyperplasia. Plant tumor formation has more than a century of research history. The study of this phenomenon has led to a number of important discoveries, including the development of the Agrobacterium-mediated transformation technique and the discovery of horizontal gene transfer from bacteria to plants. There are two main groups of plant tumors: pathogen-induced tumors (e.g., tumors induced by bacteria, viruses, fungi, insects, etc.), and spontaneous ones, which are formed in the absence of any pathogen in plants with certain genotypes (e.g., interspecific hybrids, inbred lines, and mutants). The causes of the transition of plant cells to tumor growth are different from those in animals, and they include the disturbance of phytohormonal balance and the acquisition of meristematic characteristics by differentiated cells. The aim of this review is to discuss the mechanisms underlying the development of most known examples of plant tumors.

Mechanism of Allium Crops Bulb Enlargement in Response to Photoperiod: A Review

The photoperiod marks a varied set of behaviors in plants, including bulbing. Bulbing is controlled by inner signals, which can be stimulated or subdued by the ecological environment. It had been broadly stated that phytohormones control the plant development, and they are considered to play a significant part in the bulb formation. The past decade has witnessed significant progress in understanding and advancement about the photoperiodic initiation of bulbing in plants. A noticeable query is to what degree the mechanisms discovered in bulb crops are also shared by other species and what other qualities are also dependent on photoperiod. The FLOWERING LOCUS T (FT) protein has a role in flowering; however, the FT genes were afterward reported to play further functions in other biological developments (e.g., bulbing). This is predominantly applicable in photoperiodic regulation, where the FT genes seem to have experienced significant development at the practical level and play a novel part in the switch of bulb formation in Alliums. The neofunctionalization of FT homologs in the photoperiodic environments detects these proteins as a new class of primary signaling mechanisms that control the growth and organogenesis in these agronomic-related species. In the present review, we report the underlying mechanisms regulating the photoperiodic-mediated bulb enlargement in Allium species. Therefore, the present review aims to systematically review the published literature on the bulbing mechanism of Allium crops in response to photoperiod. We also provide evidence showing that the bulbing transitions are controlled by phytohormones signaling and FT-like paralogues that respond to independent environmental cues (photoperiod), and we also show that an autorelay mechanism involving FT modulates the expression of the bulbing-control gene. Although a large number of studies have been conducted, several limitations and research gaps have been identified that need to be addressed in future studies.

Molecular Changes Concomitant with Vascular System Development in Mature Galls Induced by Root-Knot Nematodes in the Model Tree Host Populus tremula × P. alba

One of the most striking features occurring in the root-knot nematode Meloidogyne incognita induced galls is the reorganization of the vascular tissues. During the interaction of the model tree species Populus and M. incognita, a pronounced xylem proliferation was previously described in mature galls. To better characterise changes in expression of genes possibly involved in the induction and the formation of the de novo developed vascular tissues occurring in poplar galls, a comparative transcript profiling of 21-day-old galls versus uninfected root of poplar was performed. Genes coding for transcription factors associated with procambium maintenance and vascular differentiation were shown to be differentially regulated, together with genes partaking in phytohormones biosynthesis and signalling. Specific signatures of transcripts associated to primary cell wall biosynthesis and remodelling, as well as secondary cell wall formation (cellulose, xylan and lignin) were revealed in the galls. Ultimately, we show that molecules derived from the monolignol and salicylic acid pathways and related to secondary cell wall deposition accumulate in mature galls.

New Insights into Multistep-Phosphorelay (MSP)/ Two-Component System (TCS) Regulation: Are Plants and Bacteria that Different?

The Arabidopsis multistep-phosphorelay (MSP) is a signaling mechanism based on a phosphorelay that involves three different types of proteins: Histidine kinases, phosphotransfer proteins, and response regulators. Its bacterial equivalent, the two-component system (TCS), is the most predominant device for signal transduction in prokaryotes. The TCS has been extensively studied and is thus generally well-understood. In contrast, the MSP in plants was first described in 1993. Although great advances have been made, MSP is far from being completely comprehended. Focusing on the model organism Arabidopsis thaliana, this review summarized recent studies that have revealed many similarities with bacterial TCSs regarding how TCS/MSP signaling is regulated by protein phosphorylation and dephosphorylation, protein degradation, and dimerization. Thus, comparison with better-understood bacterial systems might be relevant for an improved study of the Arabidopsis MSP.

NODULE INCEPTION Recruits the Lateral Root Developmental Program for Symbiotic Nodule Organogenesis in Medicago truncatula

To overcome nitrogen deficiencies in the soil, legumes enter symbioses with rhizobial bacteria that convert atmospheric nitrogen into ammonium. Rhizobia are accommodated as endosymbionts within lateral root organs called nodules that initiate from the inner layers of Medicago truncatula roots in response to rhizobial perception. In contrast, lateral roots emerge from predefined founder cells as an adaptive response to environmental stimuli, including water and nutrient availability. CYTOKININ RESPONSE 1 (CRE1)-mediated signaling in the pericycle and in the cortex is necessary and sufficient for nodulation, whereas cytokinin is antagonistic to lateral root development, with cre1 showing increased lateral root emergence and decreased nodulation. To better understand the relatedness between nodule and lateral root development, we undertook a comparative analysis of these two root developmental programs. Here, we demonstrate that despite differential induction, lateral roots and nodules share overlapping developmental programs, with mutants in LOB-DOMAIN PROTEIN 16 (LBD16) showing equivalent defects in nodule and lateral root initiation. The cytokinin-inducible transcription factor NODULE INCEPTION (NIN) allows induction of this program during nodulation through activation of LBD16 that promotes auxin biosynthesis via transcriptional induction of STYLISH (STY) and YUCCAs (YUC). We conclude that cytokinin facilitates local auxin accumulation through NIN promotion of LBD16, which activates a nodule developmental program overlapping with that induced during lateral root initiation.

Effective approaches to study the plant-root knot nematode interaction

Plant-parasitic nematodes cause major agricultural losses worldwide. Examining the molecular mechanisms underlying plant-nematode interactions and how plants respond to different invading pathogens is attracting major attention to reduce the expanding gap between agricultural production and the needs of the growing world population. This review summarizes the most recent developments in plant-nematode interactions and the diverse approaches used to improve plant resistance against root knot nematode (RKN). We will emphasize the recent rapid advances in genome sequencing technologies, small interfering RNA techniques (RNAi) and targeted genome editing which are contributing to the significant progress in understanding the plant-nematode interaction mechanisms. Also, molecular approaches to improve plant resistance against nematodes are considered.

Interactions of endoparasitic and ectoparasitic nematodes within the plant root system

Root-knot and cyst nematodes have sophisticated mechanisms to invade their plant hosts to reprogram the plant developmental program to induce feeding structures essential for nematode survival and reproduction. This has a detrimental effect on the plant as this sedentary endoparasitic interaction affects the growth and yields of many crop plants. However, other migratory endoparasitic nematodes that do not establish root feeding sites are as aggressive on many crop plants. With new information gained from the genome and transcriptomes of the migratory endoparasitic nematode, Pratylenchus spp., this review compares the different lifestyles and the pathogenic interactions these nematodes have with their plant host. Pratylenchus spp. utilises a common arsenal of effectors involved in plant cell wall degradation and the manipulation of plant host innate immunity. The absence of specific cell reprogramming effector genes may explain its migratory endoparasitic lifestyle, making it relevant to pest management approaches in Australia.

Root enhancement in cytokinin-deficient oilseed rape causes leaf mineral enrichment, increases the chlorophyll concentration under nutrient limitation and enhances the phytoremediation capacity

BACKGROUND

Cytokinin is a negative regulator of root growth, and a reduction of the cytokinin content or signalling causes the formation a larger root system in model plants, improves their growth under drought and nutrient limitation and causes increased accumulation of elements in the shoot. Roots are an important but understudied target of plant breeding. Here we have therefore explored whether root enhancement by lowering the cytokinin content can also be achieved in oilseed rape (Brassica napus L.) plants.

RESULTS

Transgenic plants overexpressing the CKX2 gene of Arabidopsis thaliana encoding a cytokinin-degrading cytokinin oxidase/dehydrogenase showed higher CKX activity and a strongly reduced cytokinin content. Cytokinin deficiency led to the formation of a larger root system under different growth conditions, which was mainly due to an increased number of lateral and adventitious roots. In contrast, shoot growth was comparable to wild type, which caused an enhanced root-to-shoot ratio. Transgenic plants accumulated in their leaves higher concentrations of macro- and microelements including P, Ca, Mg, S, Zn, Cu, Mo and Mn. They formed more chlorophyll under Mg- and S-deficiency and accumulated a larger amount of Cd and Zn from contaminated medium and soil.

CONCLUSIONS

These findings demonstrate the usefulness of ectopic CKX gene expression to achieve root enhancement in oilseed rape and underpin the functional relevance of a larger root system. Furthermore, the lack of major developmental consequences on shoot growth in cytokinin-deficient oilseed rape indicates species-specific differences of CKX gene and/or cytokinin action.

ARR22 overexpression can suppress plant Two-Component Regulatory Systems

In plants, several developmental processes are co-coordinated by cytokinins via phosphorylation dependent processes of the Two-Component System (TCS). An outstanding challenge is to track phosphorelay flow from cytokinin perception to its molecular outputs, of which gene activation plays a major role. To address this issue, a kinetic-based reporter system was expounded to track TCS phosphorelay activity in vivo that can distinguish between basal and cytokinin dependent effects of overexpressed TCS members. The TCS phosphorelay can be positively activated by cytokinin and inhibited by pharmaceuticals or naturally interfering components. In this case we took advantage of the phosphohistidine-phosphatase Arabidopsis Response Regulator (ARR) 22 and investigated its phosphocompetition with other TCS members in regulating promoters of ARR5 and WUS in Arabidopsis thaliana cell culture protoplasts. In congruency with the proposed function of ARR22, overexpression of ARR22 blocked the activation of all B-type ARRs in this study in a TCS dependent manner. Furthermore, this effect could not be mimicked by A-type response regulator overexpression or compensated by AHP overexpression. Compared to other reporter assays, ours mimicked effects previously observed only in transgenic plants for all of the TCS proteins studied, suggesting that it is possible to expose phosphocompetition. Thus, our approach can be used to investigate gene signaling networks involving the TCS by leveraging ARR22 as a TCS inhibitor along with B-type ARR overexpression.

Cytokinin - A Developing Story

In the past decade tremendous advances have been made in understanding the biosynthesis, perception, and signaling pathways of the plant hormone cytokinin. It also became clear that interfering with any of these steps greatly impacts all on stages of growth and development. This has recently spurted renewed effort to understand how cytokinin signaling affects developmental processes. As a result, new insights on the role of cytokinin signaling and the downstream targets during, for example, shoot apical meristem, flower, female gametophyte, stomata and vascular development are being unraveled. In this review we aim to give a comprehensive overview of recent findings on how cytokinin influences growth and development in plants, and highlight areas for future research.

Parasitic nematodes manipulate plant development to establish feeding sites

Cyst and root-knot nematodes, the two economically most important groups of plant parasitic nematodes, induce neoplastic feeding sites in the roots of their host plants. The formation of feeding sites is accompanied by large-scale transcriptomic, metabolomic, and structural changes in host plants. However, the mechanisms that lead to such remarkable changes have remained poorly understood until recently. Now, genomic and genetic analyses have greatly enhanced our understanding of all aspects of plant-nematode interaction. Here, we review some of the recent advances in understanding cyst and root-knot nematode parasitism. In particular, we highlight new findings on the role of plant hormones and small RNAs in nematode feeding site formation and function. Finally, we touch on our emerging understanding of the function of nematode-associated secretions.

Quantitative 3D imaging of cell level auxin and cytokinin response ratios in soybean roots and nodules

Legume-Rhizobium symbiosis results in root nodules where rhizobia fix atmospheric nitrogen into plant usable forms in exchange for plant-derived carbohydrates. The development of these specialized root organs involves a set of carefully orchestrated plant hormone signalling. In particular, a spatio-temporal balance between auxin and cytokinin appears to be crucial for proper nodule development. We put together a construct that carried nuclear localized fluorescence sensors for auxin and cytokinin and used two photon induced fluorescence microscopy for concurrent quantitative 3-dimensional imaging to determine cellular level auxin and cytokinin outputs and ratios in root and nodule tissues of soybean. The use of nuclear localization signals on the markers and nuclei segmentation during image processing enabled accurate monitoring of outputs in 3D image volumes. The ratiometric method used here largely compensates for variations in individual outputs due to sample turbidity and scattering, an inherent issue when imaging thick root and nodule samples typical of many legumes. Overlays of determined auxin/cytokinin ratios on specific root zones and cell types accurately reflected those predicted based on previously reported outputs for each hormone individually. Importantly, distinct auxin/cytokinin ratios corresponded to distinct nodule cell types indicating a key role for these hormones in nodule cell type identity.

A C3HC4-type RING finger protein regulates rhizobial infection and nodule organogenesis in Lotus japonicus

During the establishment of rhizobia-legume symbiosis, the cytokinin receptor LHK1 (Lotus Histidine Kinase 1) is essential for nodule formation. However, the mechanism by which cytokinin signaling regulates symbiosis remains largely unknown. In this study, an LHK1-interacting protein, LjCZF1, was identified and further characterized. LjCZF1 is a C3HC4-type RING finger protein that is highly conserved in plants. LjCZF1 specifically interacted with LHK1 in yeast two-hybrid, in vitro pull-down and co-immunoprecipitation assays conducted in tobacco. Phosphomimetic mutation of the potential threonine (T167D) phosphorylation site enhanced the interaction between LjCZF1 and LHK1, whereas phosphorylation mutation (T167A) eliminated this interaction. Transcript abundance of LjCZF1 was up-regulated significantly after inoculation with rhizobia. The LORE1 insertion mutant and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated knockout mutant Lotus japonicus plants demonstrated significantly reduced number of infection threads and nodules. In contrast, plants over-expressing LjCZF1 exhibited increased numbers of infection threads and nodules. Collectively, these data support the notion that LjCZF1 is a positive regulator of symbiotic nodulation, possibly through interaction with LHK1.

Local and Systemic Effect of Cytokinins on Soybean Nodulation and Regulation of Their Isopentenyl Transferase (IPT) Biosynthesis Genes Following Rhizobia Inoculation

Cytokinins are important regulators of cell proliferation and differentiation in plant development. Here, a role for this phytohormone group in soybean nodulation is shown through the exogenous application of cytokinins (6-benzylaminopurine, N6-(Δ2-isopentenyl)-adenine and trans-zeatin) via either root drenching or a petiole feeding technique. Overall, nodule numbers were reduced by treatment with high cytokinin concentrations, but increased with lower concentrations. This was especially evident when feeding the solutions directly into the vasculature via petiole feeding. These findings highlight the importance of cytokinin in nodule development. To further investigate the role of cytokinin in controlling nodule numbers, the IPT gene family involved in cytokinin biosynthesis was characterized in soybean. Bioinformatic analyses identified 17 IPT genes in the soybean genome and homeologous duplicate gene partners were subsequently identified including GmIPT5 and GmIPT6, the orthologs of LjIPT3. Expression of GmIPT5 was upregulated in the shoot in response to nodulation, but this was independent of a functional copy of the autoregulation of nodulation (AON) receptor, GmNARK, which suggests it is unlikely to have a role in the negative feedback system called AON. Legumes also control nodule numbers in the presence of soil nitrogen through nitrate-dependent regulation of nodulation, a locally acting pathway in soybean. Upon nitrate treatment to the root, the tandem duplicates GmIPT3 and GmIPT15 were upregulated in expression indicating a role for these genes in the plant's response to soil nitrogen, potentially including the nitrate-dependent regulation of legume nodulation pathway. Additional roles for cytokinin and their IPT biosynthetic genes in nodulation and the control of nodule numbers are discussed.

Hormone modulation of legume-rhizobial symbiosis

Leguminous plants can establish symbiotic associations with diazotropic rhizobia to form nitrogen-fixating nodules, which are classified as determinate or indeterminate based on the persistence of nodule meristem. The formation of nitrogen-fixing nodules requires coordinating rhizobial infection and root nodule organogenesis. The formation of an infection thread and the extent of nodule formation are largely under plant control, but vary with environmental conditions and the physiological state of the host plants. Many achievements in these two areas have been made in recent decades. Phytohormone signaling pathways have gradually emerged as important regulators of root nodule symbiosis. Cytokinin, strigolactones (SLs) and local accumulation of auxin can promote nodule development. Ethylene, jasmonic acid (JA), abscisic acid (ABA) and gibberellic acid (GA) all negatively regulate infection thread formation and nodule development. However, salicylic acid (SA) and brassinosteroids (BRs) have different effects on the formation of these two nodule types. Some peptide hormones are also involved in nodulation. This review summarizes recent findings on the roles of these plant hormones in legume-rhizobial symbiosis, and we propose that DELLA proteins may function as a node to integrate plant hormones to regulate nodulation.

Same tune, different song-cytokinins as virulence factors in plant-pathogen interactions?

Virulence factors are molecules that enable plant pathogens to infect and colonize host tissues successfully. These molecules co-evolve with host genes to ensure functionality and to avoid recognition by the host immune system. Some pathogens also produce the plant growth hormone cytokinin (CK) and other plant hormones that contribute to virulence without being subjected to the molecular arms race. Here, we summarize recent findings regarding the role of CKs during infection and the establishment of plant diseases. We discuss commonalities and differences in CK biosynthesis, perception, and activity in infections by different phytopathogenic bacteria, fungi, nematodes and parasitic plants. Finally, we attempt to answer the question if CKs can be classified as bona fide virulence factors.

Genome-Wide Identification of TCP Family Transcription Factors in Medicago truncatula Reveals Significant Roles of miR319-Targeted TCPs in Nodule Development

TCP proteins, the plant-specific transcription factors, are involved in the regulation of multiple aspects of plant development among different species, such as leaf development, branching, and flower symmetry. However, thus far, the roles of TCPs in legume, especially in nodulation are still not clear. In this study, a genome-wide analysis of TCP genes was carried out to discover their evolution and function in Medicago truncatula. In total, 21 MtTCPs were identified and classified into class I and class II, and the class II MtTCPs were further divided into two subclasses, CIN and CYC/TB1. The expression profiles of MtTCPs are dramatically different. The universal expression of class I MtTCPs was detected in all organs. However, the MtTCPs in CIN subclass were highly expressed in leaf and most of the members in CYC/TB1 subclass were highly expressed in flower. Such organ-specific expression patterns of MtTCPs suggest their different roles in plant development. In addition, most MtTCPs were down-regulated during the nodule development, except for the putative MtmiR319 targets, MtTCP3, MtTCP4, and MtTCP10A. Overexpression of MtmiR319A significantly reduced the expression level of MtTCP3/4/10A/10B and resulted in the decreased nodule number, indicating the important roles of MtmiR319-targeted MtTCPs in nodulation. Taken together, this study systematically analyzes the MtTCP gene family at a genome-wide level and their possible functions in nodulation, which lay the basis for further explorations of MtmiR319/MtTCPs module in association with nodule development in M. truncatula.

Multiple Nodulation Genes Are Up-Regulated During Establishment of Reniform Nematode Feeding Sites in Soybean

The semi-endoparastic reniform nematode (Rotylenchulus reniformis) infects over 300 plant species. Females penetrate host roots and induce formation of complex, multinucleate feeding sites called syncytia. While anatomical changes associated with reniform nematode infection are well documented, little is known about their molecular basis. We grew soybean (Glycine max) in a split-root growth system, inoculated half of each root system with R. reniformis, and quantified gene expression in infected and control root tissue at four dates after inoculation. Over 6,000 genes were differentially expressed between inoculated and control roots on at least one date (false discovery rate [FDR] = 0.01, |log2FC| ≥ 1), and 507 gene sets were significantly enriched or depleted in inoculated roots (FDR = 0.05). Numerous genes up-regulated during syncytium formation had previously been associated with rhizobia nodulation. These included the nodule-initiating transcription factors CYCLOPS, NSP1, NSP2, and NIN, as well as multiple nodulins associated with the plant-derived peribacteroid membrane. Nodulation-related NIP aquaporins and SWEET sugar transporters were induced, as were plant CLAVATA3/ESR-related (CLE) signaling proteins and cell cycle regulators such as CCS52A and E2F. Nodulins and nodule-associated genes may have ancestral functions in normal root development and mycorrhization that have been co-opted by both parasitic nematodes and rhizobial bacteria to promote feeding site and nodule formation.

Parallel adaptations and common host cell responses enabling feeding of obligate and facultative plant parasitic nematodes

Parallel adaptations enabling the use of plant cells as the primary food source have occurred multiple times in distinct nematode clades. The hallmark of all extant obligate and facultative plant-feeding nematodes is the presence of an oral stylet, which is required for penetration of plant cell walls, delivery of pharyngeal gland secretions into host cells and selective uptake of plant assimilates. Plant parasites from different clades, and even within a single clade, display a large diversity in feeding behaviours ranging from short feeding cycles on single cells to prolonged feeding on highly sophisticated host cell complexes. Despite these differences, feeding of nematodes frequently (but certainly not always) induces common responses in host cells (e.g. endopolyploidization and cellular hypertrophy). It is thought that these host cell responses are brought about by the interplay of effectors and other biological active compounds in stylet secretions of feeding nematodes, but this has only been studied for the most advanced sedentary plant parasites. In fact, these responses are thought to be fundamental for prolonged feeding of sedentary plant parasites on host cells. However, as we discuss in this review, some of these common plant responses to independent lineages of plant parasitic nematodes might also be generic reactions to cell stress and as such their onset may not require specific inputs from plant parasitic nematodes. Sedentary plant parasitic nematodes may utilize effectors and their ability to synthesize other biologically active compounds to tailor these common responses for prolonged feeding on host cells.

Auxin transport, metabolism, and signalling during nodule initiation: indeterminate and determinate nodules

Most legumes can form a unique type of lateral organ on their roots: root nodules. These structures host symbiotic nitrogen-fixing bacteria called rhizobia. Several different types of nodules can be found in nature, but the two best-studied types are called indeterminate and determinate nodules. These two types differ with respect to the presence or absence of a persistent nodule meristem, which consistently correlates with the cortical cell layers giving rise to the nodule primordia. Similar to other plant developmental processes, auxin signalling overlaps with the site of organ initiation and meristem activity. Here, we review how auxin contributes to early nodule development. We focus on changes in auxin transport, signalling, and metabolism during nodule initiation, describing both experimental evidence and computer modelling. We discuss how indeterminate and determinate nodules may differ in their mechanisms for generating localized auxin response maxima and highlight outstanding questions for future research.

The Multiple Faces of the Medicago-Sinorhizobium Symbiosis

Medicago truncatula is able to perform a symbiotic association with Sinorhizobium spp. This interaction leads to the formation of a new root organ, the nodule, in which bacteria infect the host cells and fix atmospheric nitrogen for the plant benefit. Multiple and complex processes are essential for the success of this interaction from the recognition phase to nodule formation and functioning, and a wide range of plant host genes is required to orchestrate this phenomenon. Thanks to direct and reverse genetic as well as transcriptomic approaches, numerous genes involved in this symbiosis have been described and improve our understanding of this fantastic association. Herein we propose to update the recent molecular knowledge of how M. truncatula associates to its symbiotic partner Sinorhizobium spp.

Divergent expression of cytokinin biosynthesis, signaling and catabolism genes underlying differences in feeding sites induced by cyst and root-knot nematodes

Cyst and root-knot nematodes are obligate parasites of economic importance with a remarkable ability to reprogram root cells into unique metabolically active feeding sites. Previous studies have suggested a role for cytokinin in feeding site formation induced by these two types of nematodes, but the mechanistic details have not yet been described. Using Arabidopsis as a host plant species, we conducted a comparative analysis of cytokinin genes in response to the beet cyst nematode (BCN), Heterodera schachtii, and the root-knot nematode (RKN), Meloidogyne incognita. We identified distinct differences in the expression of cytokinin biosynthesis, catabolism and signaling genes in response to infection by BCN and RKN, suggesting differential manipulation of the cytokinin pathway by these two nematode species. Furthermore, we evaluated Arabidopsis histidine kinase receptor mutant lines ahk2/3, ahk2/4 and ahk3/4 in response to RKN infection. Similar to our previous studies with BCN, these lines were significantly less susceptible to RKN without compromising nematode penetration, suggesting a requirement of cytokinin signaling in RKN feeding site formation. Moreover, an analysis of ahk double mutants using CycB1;1:GUS/ahk introgressed lines revealed contrasting differences in the cytokinin receptors mediating cell cycle activation in feeding sites induced by BCN and RKN.

Cytokinins in Symbiotic Nodulation: When, Where, What For?

Substantial progress has been made in the understanding of early stages of the symbiotic interaction between legume plants and rhizobium bacteria. Those include the specific recognition of symbiotic partners, the initiation of bacterial infection in root hair cells, and the inception of a specific organ in the root cortex, the nodule. Increasingly complex regulatory networks have been uncovered in which cytokinin (CK) phytohormones play essential roles in different aspects of early symbiotic stages. Intriguingly, these roles can be either positive or negative, cell autonomous or non-cell autonomous, and vary, depending on time, root tissues, and possibly legume species. Recent developments on CK symbiotic functions and interconnections with other signaling pathways during nodule initiation are the focus of this review.

Gene expression analysis in Musa acuminata during compatible interactions with Meloidogyne incognita

BACKGROUND AND AIMS

Endoparasitic root-knot nematodes (RKNs) ( Meloidogyne spp.) cause considerable losses in banana ( Musa spp.), with Meloidogyne incognita a predominant species in Cavendish sub-group bananas. This study investigates the root transcriptome in Musa acuminata genotypes 4297-06 (AA) and Cavendish Grande Naine (CAV; AAA) during early compatible interactions with M. incognita .

METHODS

Roots were analysed by brightfield light microscopy over a 35 d period to examine nematode penetration and morphological cell transformation. RNA samples were extracted 3, 7 and 10 days after inoculation (DAI) with nematode J2 juveniles, and cDNA libraries were sequenced using lllumina HiSeq technology. Sequences were mapped to the M. acuminata ssp. malaccensis var. Pahang genome sequence, differentially expressed genes (DEGs) identified and transcript representation determined by gene set enrichment and pathway mapping.

KEY RESULTS

Microscopic analysis revealed a life cycle of M. incognita completing in 24 d in CAV and 27 d in 4279-06. Comparable numbers of DEGs were up- and downregulated in each genotype, with potential involvement of many in early host defence responses involving reactive oxygen species and jasmonate/ethylene signalling. DEGs revealed concomitant auxin metabolism and cell wall modification processes likely to be involved in giant cell formation. Notable transcripts related to host defence included those coding for leucine-rich repeat receptor-like serine/threonine-protein kinases, peroxidases, thaumatin-like pathogenesis-related proteins, and DREB, ERF, MYB, NAC and WRKY transcription factors. Transcripts related to giant cell development included indole acetic acid-amido synthetase GH3.8 genes, involved in auxin metabolism, as well as genes encoding expansins and hydrolases, involved in cell wall modification.

CONCLUSIONS

Expression analysis in M. acuminata during compatible interactions with RKNs provides insights into genes modulated during infection and giant cell formation. Increased understanding of both defence responses to limit parasitism during compatible interactions and effector-targeted host genes in this complex interaction will facilitate the development of genetic improvement measures for RKNs.

How Auxin and Cytokinin Phytohormones Modulate Root Microbe Interactions

A large range of microorganisms can associate with plants, resulting in neutral, friendly or hostile interactions. The ability of plants to recognize compatible and incompatible microorganisms and to limit or promote their colonization is therefore crucial for their survival. Elaborated communication networks determine the degree of association between the host plant and the invading microorganism. Central to these regulations of plant microbe interactions, phytohormones modulate microorganism plant associations and coordinate cellular and metabolic responses associated to the progression of microorganisms across different plant tissues. We review here hormonal regulations, focusing on auxin and cytokinin phytohormones, involved in the interactions between plant roots and soil microorganisms, including bacterial and fungi associations, either beneficial (symbiotic) or detrimental (pathogenic). The aim is to highlight similarities and differences in cytokinin/auxin functions amongst various compatible versus incompatible associations.

Poplar-Root Knot Nematode Interaction: A Model for Perennial Woody Species

Plant root-knot nematode (RKN) interaction studies are performed on several host plant models. Though RKN interact with trees, no perennial woody model has been explored so far. Here, we show that poplar (Populus tremula × P. alba) grown in vitro is susceptible to Meloidogyne incognita, allowing this nematode to penetrate, to induce feeding sites, and to successfully complete its life cycle. Quantitative reverse transcription-polymerase chain reaction analysis was performed to study changes in poplar gene expression in galls compared with noninfected roots. Three genes (expansin A, histone 3.1, and asparagine synthase), selected as gall development marker genes, followed, during poplar-nematode interaction, a similar expression pattern to what was described for other plant hosts. Downregulation of four genes implicated in the monolignol biosynthesis pathway was evidenced in galls, suggesting a shift in the phenolic profile within galls developed on poplar roots. Raman microspectroscopy demonstrated that cell walls of giant cells were not lignified but mainly composed of pectin and cellulose. The data presented here suggest that RKN exercise conserved strategies to reproduce and to invade perennial plant species and that poplar is a suitable model host to study specific traits of tree-nematode interactions.

HAWAIIAN SKIRT regulates the quiescent center-independent meristem activity in Arabidopsis roots

Root apical meristem (RAM) drives post-embryonic root growth by constantly supplying cells through mitosis. It is composed of stem cells and their derivatives, the transit-amplifying (TA) cells. Stem cell organization and its maintenance in the RAM are well characterized, however, their relationships with TA cells remain unclear. SHORTROOT (SHR) is critical for root development. It patterns cell types and promotes the post-embryonic root growth. Defective root growth in the shr has been ascribed to the lack of quiescent center (QC), which maintains the surrounding stem cells. However, our recent investigation indicated that SHR maintains TA cells independently of QC by modulating PHABULOSA (PHB) through miRNA165/6. PHB controls TA cell activity by modulating cytokinin levels and type B Arabidopsis Response Regulator activity, in a dosage-dependent manner. To further understand TA cell regulation, we conducted a shr suppressor screen. With an extensive mutagenesis screen followed by genome sequencing of a pooled F2 population, we discovered two suppressor alleles with mutations in HAWAIIAN SKIRT (HWS). HWS, encoding an F-box protein with kelch domain, is expressed, partly depending on SHR, in the root cap and in the pericycle of the differentiation zone. Interestingly, root growth in the shr hws was more active than the wild-type roots for the first 7 days after germination, without recovering QC. Contrary to shr phb, shr hws did not show a recovery of cytokinin signaling. These indicate that HWS affects QC-independent TA cell activities through a pathway distinctive from PHB.

The Role of Cytokinin During Infection of Arabidopsis thaliana by the Cyst Nematode Heterodera schachtii

Plant-parasitic cyst nematodes induce the formation of hypermetabolic feeding sites, termed syncytia, as their sole source of nutrients. The formation of the syncytium is orchestrated by the nematode, in part, by modulation of phytohormone responses, including cytokinin. In response to infection by the nematode Heterodera schachtii, cytokinin signaling is transiently induced at the site of infection and in the developing syncytium. Arabidopsis lines with reduced cytokinin sensitivity show reduced susceptibility to nematode infection, indicating that cytokinin signaling is required for optimal nematode development. Furthermore, lines with increased cytokinin sensitivity also exhibit reduced nematode susceptibility. To ascertain why cytokinin hypersensitivity reduces nematode parasitism, we examined the transcriptomes in wild type and a cytokinin-hypersensitive type-A arr Arabidopsis mutant in response to H. schachtii infection. Genes involved in the response to biotic stress and defense response were elevated in the type-A arr mutant in the absence of nematodes and were hyperinduced following H. schachtii infection, which suggests that the Arabidopsis type-A arr mutants impede nematode development because they are primed to respond to pathogen infection. These results suggest that cytokinin signaling is required for optimal H. schachtii parasitism of Arabidopsis but that elevated cytokinin signaling triggers a heightened immune response to nematode infection.