Spatiotemporal changes in the role of cytokinin during root development

Antifungal mechanisms and characteristics of Pseudomonas fluorescens: Promoting peanut growth and combating Fusarium oxysporum-induced root rot

Continuous cropping of peanuts presents significant challenges to sustainable production due to soil-borne diseases like root rot caused by Fusarium species. In this study, field inoculation experiments treatments and in vitro agar plate confrontation tests were conducted, including non-inoculated controls (CK), inoculation with Pseudomonas fluorescens (PF), Fusarium oxysporum (FO), and co-inoculation with both (PF + FO). The aim was to explore the antifungal mechanisms of Pseudomonas fluorescens in mitigating root rot and enhancing peanut yield. The results indicated that PF and PF + FO significantly enhanced peanut root activity, as well as superoxide dismutase, catalase, and glutathione S-transferase activities, while simultaneously decreasing the accumulation of reactive oxygen species and malondialdehyde contents, compared to FO treatment. Additionally, PF treatment notably increased lignin content through enhanced phenylalanine ammonia lyase, cinnamate 3-hydroxylase, and peroxidase activity compared to CK and FO treatment. Moreover, PF treatment resulted in longer roots and a higher average diameter and surface area, potentially due to increased endogenous levels of auxin and zeatin riboside, coupled with decreased abscisic acid content. PF treatment significantly elevated chlorophyll content and the maximum photochemical efficiency of PSII in the light-adapted state, the actual photochemical efficiency and the proportion of PSII reaction centers open, leading to improved photosynthetic performance. Confrontation culture assays revealed PF's notable inhibitory effects on Fusarium oxysporum growth, subsequently reducing rot disease incidence in the field. Ultimately, PF treatment led to increased peanut yield by enhancing plant numbers and pod weight compared to FO treatment, indicating its potential in mitigating Fusarium oxysporum-induced root rot disease under continuous cropping systems.

Genomic insights into CKX genes: key players in cotton fibre development and abiotic stress responses

Cytokinin oxidase/dehydrogenase (CKX), responsible for irreversible cytokinin degradation, also controls plant growth and development and response to abiotic stress. While the CKX gene has been studied in other plants extensively, its function in cotton is still unknown. Therefore, a genome-wide study to identify the CKX gene family in the four cotton species was conducted using transcriptomics, quantitative real-time PCR (qRT-PCR) and bioinformatics. As a result, in G. hirsutum and G. barbadense (the tetraploid cotton species), 87 and 96 CKX genes respectively and 62 genes each in G. arboreum and G. raimondii, were identified. Based on the evolutionary studies, the cotton CKX gene family has been divided into five distinct subfamilies. It was observed that CKX genes in cotton have conserved sequence logos and gene family expansion was due to segmental duplication or whole genome duplication (WGD). Collinearity and multiple synteny studies showed an expansion of gene families during evolution and purifying selection pressure has been exerted. G. hirsutum CKX genes displayed multiple exons/introns, uneven chromosomal distribution, conserved protein motifs, and cis-elements related to growth and stress in their promoter regions. Cis-elements related to resistance, physiological metabolism and hormonal regulation were identified within the promoter regions of the CKX genes. Expression analysis under different stress conditions (cold, heat, drought and salt) revealed different expression patterns in the different tissues. Through virus-induced gene silencing (VIGS), the GhCKX34A gene was found to improve cold resistance by modulating antioxidant-related activity. Since GhCKX29A is highly expressed during fibre development, we hypothesize that the increased expression of GhCKX29A in fibres has significant effects on fibre elongation. Consequently, these results contribute to our understanding of the involvement of GhCKXs in both fibre development and response to abiotic stress.

Effects of long-term different-scale rice-duck farming on the growth and yield of paddy rice

BACKGROUND

To maintain rice production and increase revenue, rice-duck (RD) farming is a contemporary ecological cycle technology that has been widely used in Asia. However, due to the clustering activity of duck flocks, the consequences of long-term RD farming on rice growth at different scales are still unknown. Here, we studied RD farming using several different treatments (CK: conventional rice farming; RD1: 667 m2 ; RD2: 2000 m2 ; and RD3: 3333 m2 ).

RESULTS

The results demonstrated that the maximum tillers, effective spikes, dry matter accumulation, and lodging index of rice under RD farming were significantly decreased by 17.9%, 9.8%, 14.8%, and 17.8%, respectively, which ultimately caused a significant decrease in yield of 10.6%. However, RD farming significantly increased root oxidation activity and the ear-bearing tiller rate of rice by 25.5% and 11.1%, respectively, and improved yield stability. For different scales of RD farming, the lodging resistance index of RD1 was significantly lower than that of RD2 and RD3 by 10.0% and 15.2%, respectively, whereas the root oxidation activity and dry matter accumulation of RD2 were significantly higher than those of RD1 and RD3 by 11.1%, 4.7%, 8.6%, and 5.1%, respectively. For rice yield, there was no significant difference among the different scales.

CONCLUSION

This long-term experiment helped elucidate the complicated effects of RD farming at different scales on the growth and yield of rice. It is also critical to consider the economic advantages of different scales of RD farming to assess the impact of this system more thoroughly. © 2023 Society of Chemical Industry.

The LaCLE35 peptide modifies rootlet density and length in cluster roots of white lupin

White lupin (lupinus albus L.) forms special bottlebrush-like root structures called cluster roots (CR) when phosphorus is low, to remobilise sparingly soluble phosphates in the soil. The molecular mechanisms that control the CR formation remain unknown. Root development in other plants is regulated by CLE  (CLAVATA3/ EMBRYO SURROUNDING REGION (ESR)-RELATED) peptides, which provide more precise control mechanisms than common phytohormones. This makes these peptides interesting candidates to be involved in CR formation, where fine tuning to environmental factors is required. In this study we present an analysis of CLE peptides in white lupin. The peptides LaCLE35 (RGVHy PSGANPLHN) and LaCLE55 (RRVHy PSCHy PDPLHN) reduced root growth and altered CR in hydroponically cultured white lupins. We demonstrate that rootlet density and rootlet length were locally, but not systemically, impaired by exogenously applied CLE35. The peptide was identified in the xylem sap. The inhibitory effect of CLE35 on root growth was attributed to arrested cell elongation in root tips. Taken together, CLE peptides affect both rootlet density and rootlet length, which are two critical factors for CR formation, and may be involved in fine tuning this peculiar root structure that is present in a few crops and many Proteaceae species, under low phosphorus availability.

Modulating root system architecture: cross-talk between auxin and phytohormones

Root architecture is an important agronomic trait that plays an essential role in water uptake, soil compactions, nutrient recycling, plant-microbe interactions, and hormone-mediated signaling pathways. Recently, significant advancements have been made in understanding how the complex interactions of phytohormones regulate the dynamic organization of root architecture in crops. Moreover, phytohormones, particularly auxin, act as internal regulators of root development in soil, starting from the early organogenesis to the formation of root hair (RH) through diverse signaling mechanisms. However, a considerable gap remains in understanding the hormonal cross-talk during various developmental stages of roots. This review examines the dynamic aspects of phytohormone signaling, cross-talk mechanisms, and the activation of transcription factors (TFs) throughout various developmental stages of the root life cycle. Understanding these developmental processes, together with hormonal signaling and molecular engineering in crops, can improve our knowledge of root development under various environmental conditions.

MicroRNA 4407 modulates nodulation in soybean by repressing a root-specific ISOPENTENYLTRANSFERASE (GmIPT3)

MicroRNAs (miRNAs) are important regulators of plant biological processes, including soybean nodulation. One miRNA, miR4407, was identified in soybean roots and nodules. However, the function of miR4407 in soybean is still unknown. MiR4407, unique to soybean, positively regulates lateral root emergence and root structures and represses a root-specific ISOPENTENYLTRANSFERASE (GmIPT3). By altering the expression of miR4407 and GmIPT3, we investigated the role of miR4407 in lateral root and nodule development. Both miR4407 and GmIPT3 are expressed in the inner root cortex and nodule primordia. Upon rhizobial inoculation, miR4407 was downregulated while GmIPT3 was upregulated. Overexpressing miR4407 reduced the number of nodules in transgenic soybean hairy roots while overexpressing the wild-type GmIPT3 or a miR4407-resistant GmIPT3 mutant (mGmIPT3) significantly increased the nodule number. The mechanism of miR4407 and GmIPT3 functions was also linked to autoregulation of nodulation (AON), where miR4407 overexpression repressed miR172c and activated its target, GmNNC1, turning on AON. Exogenous CK mimicked the effects of GmIPT3 overexpression on miR172c, supporting the notion that GmIPT3 regulates nodulation by enhancing root-derived CK. Overall, our data revealed a new miRNA-mediated regulatory mechanism of nodulation in soybean. MiR4407 showed a dual role in lateral root and nodule development.

Arabidopsis TIE1 and TIE2 transcriptional repressors dampen cytokinin response during root development

Cytokinin plays critical roles in root development. Cytokinin signaling depends on activation of key transcription factors known as type B Arabidopsis response regulators (ARRs). However, the mechanisms underlying the finely tuned regulation of type B ARR activity remain unclear. In this study, we demonstrate that the ERF-associated amphiphilic repression (EAR) motif-containing protein TCP interactor containing ear motif protein2 (TIE2) forms a negative feedback loop to finely tune the activity of type B ARRs during root development. Disruption of TIE2 and its close homolog TIE1 causes severely shortened roots. TIE2 interacts with type B ARR1 and represses transcription of ARR1 targets. The cytokinin response is correspondingly enhanced in tie1-1 tie2-1. We further show that ARR1 positively regulates TIE1 and TIE2 by directly binding to their promoters. Our findings demonstrate that TIEs play key roles in controlling plant development and reveal an important negative feedback regulation mechanism for cytokinin signaling.

Spatio-temporal transcriptome profiling and subgenome analysis in Brassica napus

Brassica napus is an important oil crop and an allotetraploid species. However, the detailed analysis of gene function and homoeologous gene expression in all tissues at different developmental stages was not explored. In this study, we performed a global transcriptome analysis of 24 vegetative and reproductive tissues at six developmental stages (totally 111 tissues). These samples were clustered into eight groups. The gene functions of silique pericarp were similar to roots, stems and leaves. In particular, glucosinolate metabolic process was associated with root and silique pericarp. Genes involved in protein phosphorylation were often associated with stamen, anther and the early developmental stage of seeds. Transcription factor (TF) genes were more specific than structural genes. A total of 17 100 genes that were preferentially expressed in one tissue (tissue-preferred genes, TPGs), including 889 TFs (5.2%), were identified in the 24 tissues. Some TPGs were identified as hub genes in the co-expression network analysis, and some TPGs in different tissues were involved in different hormone pathways. About 67.0% of the homoeologs showed balanced expression, whereas biased expression of homoeologs was associated with structural divergence. In addition, the spatiotemporal expression of homoeologs was related to the presence of transposable elements (TEs) and regulatory elements (REs); more TEs and fewer REs in the promoters resulted in divergent expression in different tissues. This study provides a valuable transcriptional map for understanding the growth and development of B. napus, for identifying important genes for future crop improvement, and for exploring gene expression patterns in the B. napus.

Histological, Morpho-Physiological, and Biochemical Changes during Adventitious Rooting Induced by Exogenous Auxin in Magnolia wufengensis Cuttings

Magnolia wufengensis, a rare ornamental tree species, is now in a huge gap between market demand and actual supply of seedlings. As cutting propagation is one of the most important means to solve the shortage of seedling supply, this study developed an efficient cutting propagation procedure of M. wufengensis, revealed the morphological and histological changes of adventitious root formation, and explored the rhythm correlation between rooting process and physiological and biochemical changes. Cuttings pre-treated with NAA:IBA (2:1) exhibited the best rooting performance. Anatomical analysis demonstrated that adventitious root primordia of M. wufengensis were initiated from cambial and parenchyma cells of xylem, with no relationship to the callus formed on the epidermis. The rooting process of M. wufengenis can be divided into four periods: induction phase (0–8 dap) (dap means days after planting), initiation phase (8–13 dap), expression phase (13–18 dap), and extension phase (18–28 dap). NAA:IBA (2:1) induced the accumulations of 3-indoleacetic-acid and increased the contents of peroxidase and polyphenol-oxidase near the wounding at induction phase. The initiation phase, with the first histological modifications to the formation of meristemoids, correspond to the increase of peroxidase, polyphenol-oxidase, and soluble protein contents. The synergistic reaction of low 3-indoleacetic-acid and high levels of gibberellins and zeatin also stimulates the initiation phase. In the expression and extension phase, high activities of polyphenol-oxidase, IAA-oxidase, and increased contents of soluble protein co-stimulate the emergence and outgrowth of adventitious roots. The present study not only provides optimized protocol by application of auxin combination but also presents insights in the histological, morpho-physiological, and biochemical changes in stem cuttings of M. wufengensis.

It's Time for a Change: The Role of Gibberellin in Root Meristem Development

One of the most amazing characteristics of plants is their ability to grow and adapt their development to environmental changes. This fascinating feature is possible thanks to the activity of meristems, tissues that contain lasting self-renewal stem cells. Because of its simple and symmetric structure, the root meristem emerged as a potent system to uncover the developmental mechanisms behind the development of the meristems. The root meristem is formed during embryogenesis and sustains root growth for all the plant's lifetime. In the last decade, gibberellins have emerged as a key regulator for root meristem development. This phytohormone functions as a molecular clock for root development. This mini review discusses the latest advances in understanding the role of gibberellin in root development and highlights the central role of this hormone as developmental timer.

Phytohormones: plant switchers in developmental and growth stages in potato

BACKGROUND

Potato is one of the most important food crops worldwide, contributing key nutrients to the human diet. Plant hormones act as vital switchers in the regulation of various aspects of developmental and growth stages in potato. Due to the broad impacts of hormones on many developmental processes, their role in potato growth and developmental stages has been investigated. This review presents a description of hormonal basic pathways, various interconnections between hormonal network and reciprocal relationships, and clarification of molecular events underlying potato growth. In the last decade, new findings have emerged regarding their function during sprout development, vegetative growth, tuber initiation, tuber development, and maturation in potato. Hormones can control the regulation of various aspects of growth and development in potato, either individually or in combination with other hormones. The molecular characterization of interplay between cytokinins (CKs), abscisic acid (ABA), and auxin and/or gibberellins (GAs) during tuber formation requires further undertaking. Recently, new evidences regarding the relative functions of hormones during various stages and an intricate network of several hormones controlling potato tuber formation are emerging. Although some aspects of their functions are widely covered, remarkable breaks in our knowledge and insights yet exist in the regulation of hormonal networks and their interactions during different stages of growth and various aspects of tuber formation.

SHORT CONCLUSION

The present review focuses on the relative roles of hormones during various developmental stages with a view to recognize their mechanisms of function in potato tuber development. For better insight, relevant evidences available on hormonal interaction during tuber development in other species are also described. We predict that the present review highlights some of the conceptual developments in the interplay of hormones and their associated downstream events influencing tuber formation.

Biotechnological Resources to Increase Disease-Resistance by Improving Plant Immunity: A Sustainable Approach to Save Cereal Crop Production

Plant diseases are globally causing substantial losses in staple crop production, undermining the urgent goal of a 60% increase needed to meet the food demand, a task made more challenging by the climate changes. Main consequences concern the reduction of food amount and quality. Crop diseases also compromise food safety due to the presence of pesticides and/or toxins. Nowadays, biotechnology represents our best resource both for protecting crop yield and for a science-based increased sustainability in agriculture. Over the last decades, agricultural biotechnologies have made important progress based on the diffusion of new, fast and efficient technologies, offering a broad spectrum of options for understanding plant molecular mechanisms and breeding. This knowledge is accelerating the identification of key resistance traits to be rapidly and efficiently transferred and applied in crop breeding programs. This review gathers examples of how disease resistance may be implemented in cereals by exploiting a combination of basic research derived knowledge with fast and precise genetic engineering techniques. Priming and/or boosting the immune system in crops represent a sustainable, rapid and effective way to save part of the global harvest currently lost to diseases and to prevent food contamination.

Cytokinin-Controlled Gradient Distribution of Auxin in Arabidopsis Root Tip

The plant root is a dynamic system, which is able to respond promptly to external environmental stimuli by constantly adjusting its growth and development. A key component regulating this growth and development is the finely tuned cross-talk between the auxin and cytokinin phytohormones. The gradient distribution of auxin is not only important for the growth and development of roots, but also for root growth in various response. Recent studies have shed light on the molecular mechanisms of cytokinin-mediated regulation of local auxin biosynthesis/metabolism and redistribution in establishing active auxin gradients, resulting in cell division and differentiation in primary root tips. In this review, we focus our attention on the molecular mechanisms underlying the cytokinin-controlled auxin gradient in root tips.

The Arabidopsis NRT1/PTR FAMILY protein NPF7.3/NRT1.5 is an indole-3-butyric acid transporter involved in root gravitropism

Active membrane transport of plant hormones and their related compounds is an essential process that determines the distribution of the compounds within plant tissues and, hence, regulates various physiological events. Here, we report that the Arabidopsis NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY 7.3 (NPF7.3) protein functions as a transporter of indole-3-butyric acid (IBA), a precursor of the major endogenous auxin indole-3-acetic acid (IAA). When expressed in yeast, NPF7.3 mediated cellular IBA uptake. Loss-of-function npf7.3 mutants showed defective root gravitropism with reduced IBA levels and auxin responses. Nevertheless, the phenotype was restored by exogenous application of IAA but not by IBA treatment. NPF7.3 was expressed in pericycle cells and the root tip region including root cap cells of primary roots where the IBA-to-IAA conversion occurs. Our findings indicate that NPF7.3-mediated IBA uptake into specific cells is required for the generation of appropriate auxin gradients within root tissues.

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.

Topology of regulatory networks that guide plant meristem activity: similarities and differences

Plants adapt their morphology in response to variable environmental conditions such as nitrate availability, drought, and temperature shifts. Three crucial aspects to this developmental plasticity are the control of initiation, identity and activity of meristems. At the cellular level, the activity of meristems is controlled by balancing self-renewal in stem cells, amplifying divisions in their daughter cells, and cell differentiation. Recent studies in plants have uncovered transcription factors regulating meristem activity at cellular resolution, and regulatory networks that couple these factors with phytohormone signalling for global plant growth regulation. Here, we highlight selected recent advances in our understanding of the multidimensional transcriptional networks that regulate meristem activity and discuss emerging insights on how a selection of environmental cues impinges on these networks.

A Single-Cell RNA Sequencing Profiles the Developmental Landscape of Arabidopsis Root

Cells of eukaryotic multicellular organisms have inherent heterogeneity. Recent advances in single-cell gene expression studies enable us to explore transcriptional regulation in dynamic development processes and highly heterogeneous cell populations. In this study, using a high-throughput single-cell RNA-sequencing assay, we found that the cells in Arabidopsis root are highly heterogeneous in their transcriptomes. A total of 24 putative cell clusters and the cluster-specific marker genes were identified. The spatial distribution and temporal ordering of the individual cells at different developmental stages illustrate their hierarchical structures and enable the reconstruction of continuous differentiation trajectory of root development. Moreover, we found that each root cell cluster exhibits distinct patterns of ion assimilation and hormonal responses. Collectively, our study reveals a high degree of heterogeneity of root cells and identifies the expression signatures of intermediate states during root cell differentiation at single-cell resolution. We also established a web server (http://wanglab.sippe.ac.cn/rootatlas/) to facilitate the use of the datasets generated in this study.

Roles of nitrogen and cytokinin signals in root and shoot communications in maximizing of plant productivity and their agronomic applications

Nitrogen is an essential, often limiting, factor in plant growth and development. To regulate growth under limited nitrogen supply, plants sense the internal and external nitrogen status, and coordinate various metabolic processes and developmental programs accordingly. This coordination requires the transmission of various signaling molecules that move across the entire plant. Cytokinins, phytohormones derived from adenine and synthesized in various parts of the plant, are considered major local and long-distance messengers. Cytokinin metabolism and signaling are closely associated with nitrogen availability. They are systemically transported via the vasculature from plant roots to shoots, and vice versa, thereby coordinating shoot and root development. Tight linkage exists between the nitrogen signaling network and cytokinins during diverse developmental and physiological processes. However, the cytokinin-nitrogen interactions and the communication systems involved in sensing rhizospheric nitrogen status and in regulating canopy development remain obscure. We review current knowledge on cytokinin biosynthesis, transport and signaling, nitrogen acquisition, metabolism and signaling, and their interactive roles in regulating root-shoot morphological and physiological characteristics. We also discuss the role of spatio-temporal regulation of cytokinins in enhancing beneficial crop traits of yield and nitrogen use efficiency.

Commonalities and Differences in Controlling Multipartite Intracellular Infections of Legume Roots by Symbiotic Microbes

Legumes have the almost unique ability to establish symbiotic associations with rhizobia and arbuscular mycorrhizal fungi. Forward and reverse genetics have identified a large number of genes that are required for either or both interactions. However, and in sharp contrast to natural soils, these interactions have been almost exclusively investigated under laboratory conditions by using separate inoculation systems, whereas both symbionts are simultaneously present in the field. Considering our recent understanding of the individual symbioses, the community is now promisingly positioned to co-inoculate plants with two or more microbes in order to understand mechanistically how legumes efficiently balance, regulate and potentially separate these symbioses and other endophytic microbes within the same root. Here, we discuss a number of key control layers that should be considered when assessing tri- or multipartite beneficial interactions and that may contribute to colonization patterns in legume roots.

Crosstalk Complexities between Auxin, Cytokinin, and Ethylene in Arabidopsis Root Development: From Experiments to Systems Modeling, and Back Again

Understanding how hormones and genes interact to coordinate plant growth in a changing environment is a major challenge in plant developmental biology. Auxin, cytokinin, and ethylene are three important hormones that regulate many aspects of plant development. This review critically evaluates the crosstalk between the three hormones in Arabidopsis root development. We integrate a variety of experimental data into a crosstalk network, which reveals multiple layers of complexity in auxin, cytokinin, and ethylene crosstalk. In particular, data integration reveals an additional, largely overlooked link between the ethylene and cytokinin pathways, which acts through a phosphorelay mechanism. This proposed link addresses outstanding questions on whether ethylene application promotes or inhibits receptor kinase activity of the ethylene receptors. Elucidating the complexity in auxin, cytokinin, and ethylene crosstalk requires a combined experimental and systems modeling approach. We evaluate important modeling efforts for establishing how crosstalk between auxin, cytokinin, and ethylene regulates patterning in root development. We discuss how a novel methodology that iteratively combines experiments with systems modeling analysis is essential for elucidating the complexity in crosstalk of auxin, cytokinin, and ethylene in root development. Finally, we discuss the future challenges from a combined experimental and modeling perspective.

Plants under Stress: Involvement of Auxin and Cytokinin

Plant growth and development are critically influenced by unpredictable abiotic factors. To survive fluctuating changes in their environments, plants have had to develop robust adaptive mechanisms. The dynamic and complementary actions of the auxin and cytokinin pathways regulate a plethora of developmental processes, and their ability to crosstalk makes them ideal candidates for mediating stress-adaptation responses. Other crucial signaling molecules responsible for the tremendous plasticity observed in plant morphology and in response to abiotic stress are reactive oxygen species (ROS). Proper temporal and spatial distribution of ROS and hormone gradients is crucial for plant survival in response to unfavorable environments. In this regard, the convergence of ROS with phytohormone pathways acts as an integrator of external and developmental signals into systemic responses organized to adapt plants to their environments. Auxin and cytokinin signaling pathways have been studied extensively. Nevertheless, we do not yet understand the impact on plant stress tolerance of the sophisticated crosstalk between the two hormones. Here, we review current knowledge on the function of auxin and cytokinin in redirecting growth induced by abiotic stress in order to deduce their potential points of crosstalk.

Insights into the functional relationship between cytokinin-induced root system phenotypes and nitrate uptake in Brassica napus

Root system architecture is the spatial arrangement of roots that impacts the capacity of plants to access nutrients and water. We employed pharmacologically generated morphological and molecular phenotypes and used in situ 15N isotope labelling, to investigate whether contrasting root traits are of functional interest in relation to nitrate acquisition. Brassica napus L. were grown in solidified phytogel culture media containing 1mM KNO3 and treated with the cytokinin, 6-benzylaminopurine, the cytokinin antagonist, PI-55, or both in combination. The pharmacological treatments inhibited root elongation relative to the control. The contrasting root traits induced by PI-55 and 6-benzylaminopurine were strongly related to 15N uptake rate. Large root proliferation led to greater 15N cumulative uptake rather than greater 15N uptake efficiency per unit root length, due to a systemic response in the plant. This relationship was associated with changes in C and N resource distribution between the shoot and root, and in expression of BnNRT2.1, a nitrate transporter. The root:shoot biomass ratio was positively correlated with 15N cumulative uptake, suggesting the functional utility of root investment for nutrient acquisition. These results demonstrate that root proliferation in response to external nitrate is a behaviour which integrates local N availability and the systemic N status of the plant.

The Arabidopsis polyamine oxidase/dehydrogenase 5 interferes with cytokinin and auxin signaling pathways to control xylem differentiation

In plants, the polyamines putrescine, spermidine, spermine (Spm), and thermospermine (Therm-Spm) participate in several physiological processes. In particular, Therm-Spm is involved in the control of xylem differentiation, having an auxin antagonizing effect. Polyamine oxidases (PAOs) are FAD-dependent enzymes involved in polyamine catabolism. In Arabidopsis, five PAOs are present, among which AtPAO5 catalyzes the back-conversion of Spm, Therm-Spm, and N1-acetyl-Spm to spermidine. In the present study, it is shown that two loss-of-function atpao5 mutants and a 35S::AtPAO5 Arabidopsis transgenic line present phenotypical differences from the wild-type plants with regard to stem and root elongation, differences that are accompanied by changes in polyamine levels and the number of xylem vessels. It is additionally shown that cytokinin treatment, which up-regulates AtPAO5 expression in roots, differentially affects protoxylem differentiation in 35S::AtPAO5, atpao5, and wild-type roots. Together with these findings, Therm-Spm biosynthetic genes, as well as auxin-, xylem-, and cytokinin-related genes (such as ACL5, SAMDC4, PIN1, PIN6, VND6, VND7, ATHB8, PHB, CNA, PXY, XTH3, XCP1, and AHP6) are shown to be differentially expressed in the various genotypes. These data suggest that AtPAO5, being involved in the control of Therm-Spm homeostasis, participates in the tightly controlled interplay between auxin and cytokinins that is necessary for proper xylem differentiation.

Cytokinins on the Move

Cytokinins are important signals that participate in different plant processes, and are well known for their strong influence in plant development. With the years, knowledge has been built about their effects, chemical nature, metabolism, and signaling mechanisms. However, one aspect about cytokinins that has been lagging behind is cytokinin transport. Recent reports are providing more information about how cytokinins are transported and how their transport is connected to their effects in development. This review provides a general overview of what is known about cytokinin transport, with a focus on the latest reports.

Accumulation pattern of endogenous cytokinins and phenolics in different organs of 1-year-old cytokinin pre-incubated plants: implications for conservation

A better understanding of phytohormone physiology can provide an essential basis to coherently achieve a conservation drive/strategy for valuable plant species. We evaluated the distribution pattern of cytokinins (CKs) and phenolic compounds in different organs of 1-year-old greenhouse-grown Tulbaghia simmleri pre-treated (during micropropagation) with three aromatic CKs (benzyladenine = BA, meta-topolin = mT, meta-topolin riboside = mTR). The test species is highly valuable due to its medicinal and ornamental uses. Based on UHPLC-MS/MS quantification, mT and mTR pre-treated plants had the highest total CK, mostly resulting from the isoprenoid CK-type, which occurred at highest concentrations in the roots. Although occurring in much lower concentrations when compared to isoprenoid CKs, aromatic CKs were several-fold more abundant in the root of mT pre-treated plants than with other treatments. Possibly related to the enhanced aromatic CKs, free bases and ribonucleotides, plants pre-treated with mT generally displayed better morphology than the other treatments. A total of 12 bioactive phenolic compounds, including four hydroxybenzoic acids, five hydroxycinnamic acids and three flavonoids at varying concentrations, were quantified in T. simmleri. The occurrence, distribution and levels of these phenolic compounds were strongly influenced by the CK pre-treatments, thereby confirming the importance of CKs in phenolic biosynthesis pathways.

Insights into the multifaceted application of microscopic techniques in plant tissue culture systems

Microscopic techniques remain an integral tool which has allowed for the better understanding and manipulation of in vitro plant culture systems. The recent advancements will inevitably help to unlock the long-standing mysteries of fundamental biological mechanisms of plant cells. Beyond the classical applications in micropropagation aimed at the conservation of endangered and elite commercial genotypes, plant cell, tissue and organ cultures have become a platform for elucidating a myriad of fundamental physiological and developmental processes. In conjunction with microscopic techniques, in vitro culture technology has been at the centre of important breakthroughs in plant growth and development. Applications of microscopy and plant tissue culture have included elucidation of growth and development processes, detection of in vitro-induced physiological disorders as well as subcellular localization using fluorescent protein probes. Light and electron microscopy have been widely used in confirming the bipolarity of somatic embryos during somatic embryogenesis. The technique highlights basic anatomical, structural and histological evidence for in vitro-induced physiological disorders during plant growth and development. In this review, we discuss some significant biological insights in plant growth and development, breakthroughs and limitations of various microscopic applications and the exciting possibilities offered by emergent in vivo live imaging and fluorescent protein engineering technologies.

Spatiotemporal modelling of hormonal crosstalk explains the level and patterning of hormones and gene expression in Arabidopsis thaliana wild-type and mutant roots

Patterning in Arabidopsis root development is coordinated via a localized auxin concentration maximum in the root tip, requiring the regulated expression of specific genes. However, little is known about how hormone and gene expression patterning is generated. Using a variety of experimental data, we develop a spatiotemporal hormonal crosstalk model that describes the integrated action of auxin, ethylene and cytokinin signalling, the POLARIS protein, and the functions of PIN and AUX1 auxin transporters. We also conduct novel experiments to confirm our modelling predictions. The model reproduces auxin patterning and trends in wild-type and mutants; reveals that coordinated PIN and AUX1 activities are required to generate correct auxin patterning; correctly predicts shoot to root auxin flux, auxin patterning in the aux1 mutant, the amounts of cytokinin, ethylene and PIN protein, and PIN protein patterning in wild-type and mutant roots. Modelling analysis further reveals how PIN protein patterning is related to the POLARIS protein through ethylene signalling. Modelling prediction of the patterning of POLARIS expression is confirmed experimentally. Our combined modelling and experimental analysis reveals that a hormonal crosstalk network regulates the emergence of patterns and levels of hormones and gene expression in wild-type and mutants.

Rhizobium Lipo-chitooligosaccharide Signaling Triggers Accumulation of Cytokinins in Medicago truncatula Roots

Legume rhizobium symbiosis is initiated upon perception of bacterial secreted lipo-chitooligosaccharides (LCOs). Perception of these signals by the plant initiates a signaling cascade that leads to nodule formation. Several studies have implicated a function for cytokinin in this process. However, whether cytokinin accumulation and subsequent signaling are an integral part of rhizobium LCO signaling remains elusive. Here, we show that cytokinin signaling is required for the majority of transcriptional changes induced by rhizobium LCOs. In addition, we demonstrate that several cytokinins accumulate in the root susceptible zone 3 h after rhizobium LCO application, including the biologically most active cytokinins, trans-zeatin and isopentenyl adenine. These responses are dependent on calcium- and calmodulin-dependent protein kinase (CCaMK), a key protein in rhizobial LCO-induced signaling. Analysis of the ethylene-insensitive Mtein2/Mtsickle mutant showed that LCO-induced cytokinin accumulation is negatively regulated by ethylene. Together with transcriptional induction of ethylene biosynthesis genes, it suggests a feedback loop negatively regulating LCO signaling and subsequent cytokinin accumulation. We argue that cytokinin accumulation is a key step in the pathway leading to nodule organogenesis and that this is tightly controlled by feedback loops.

Plant hormone cross-talk: the pivot of root growth

Root indeterminate growth and its outstanding ability to produce new tissues continuously make this organ a highly dynamic structure able to respond promptly to external environmental stimuli. Developmental processes therefore need to be finely tuned, and hormonal cross-talk plays a pivotal role in the regulation of root growth. In contrast to what happens in animals, plant development is a post-embryonic process. A pool of stem cells, placed in a niche at the apex of the meristem, is a source of self-renewing cells that provides cells for tissue formation. During the first days post-germination, the meristem reaches its final size as a result of a balance between cell division and cell differentiation. A complex network of interactions between hormonal pathways co-ordinates such developmental inputs. In recent years, by means of molecular and computational approaches, many efforts have been made aiming to define the molecular components of these networks. In this review, we focus our attention on the molecular mechanisms at the basis of hormone cross-talk during root meristem size determination.

The CRE1 cytokinin pathway is differentially recruited depending on Medicago truncatula root environments and negatively regulates resistance to a pathogen

Cytokinins are phytohormones that regulate many developmental and environmental responses. The Medicago truncatula cytokinin receptor MtCRE1 (Cytokinin Response 1) is required for the nitrogen-fixing symbiosis with rhizobia. As several cytokinin signaling genes are modulated in roots depending on different biotic and abiotic conditions, we assessed potential involvement of this pathway in various root environmental responses. Phenotyping of cre1 mutant roots infected by the Gigaspora margarita arbuscular mycorrhizal (AM) symbiotic fungus, the Aphanomyces euteiches root oomycete, or subjected to an abiotic stress (salt), were carried out. Detailed histological analysis and quantification of cre1 mycorrhized roots did not reveal any detrimental phenotype, suggesting that MtCRE1 does not belong to the ancestral common symbiotic pathway shared by rhizobial and AM symbioses. cre1 mutants formed an increased number of emerged lateral roots compared to wild-type plants, a phenotype which was also observed under non-stressed conditions. In response to A. euteiches, cre1 mutants showed reduced disease symptoms and an increased plant survival rate, correlated to an enhanced formation of lateral roots, a feature previously linked to Aphanomyces resistance. Overall, we showed that the cytokinin CRE1 pathway is not only required for symbiotic nodule organogenesis but also affects both root development and resistance to abiotic and biotic environmental stresses.

CLASP: a microtubule-based integrator of the hormone-mediated transitions from cell division to elongation

Plants use robust mechanisms to optimize organ size to prevailing conditions. Modulating the transition from cell division to elongation dramatically affects morphology and size. Although it is well established that auxin, cytokinin and brassinosteroid mediate these transitions, recent works show that the cytoskeleton, which is normally thought to act downstream of these hormones, plays a key role in this regulatory process. In particular, the microtubule-associated protein CLASP has a dual role in meristem maintenance. CLASP modulates levels of the auxin efflux carrier PIN2 by tethering SNX1 endosomes to cortical microtubules, which in turn fine tunes auxin maxima in the root apical meristem. CLASP is also required for transfacial microtubule bundle formation at the sharp cell edges, a feature strongly associated with maintaining the capacity for further cell division.

Variation in virus symptom development and root architecture attributes at the onset of storage root initiation in 'beauregard' sweetpotato plants grown with or without nitrogen

It has been shown that virus infections, often symptomless, significantly limit sweetpotato productivity, especially in regions characterized by low input agricultural systems. In sweetpotatoes, the successful emergence and development of lateral roots (LRs), the main determinant of root architecture, determines the competency of adventitious roots to undergo storage root initiation. This study aimed to investigate the effect of some plant viruses on root architecture attributes during the onset of storage root initiation in 'Beauregard' sweetpotatoes that were grown with or without the presence of nitrogen. In two replicate experiments, virus-tested plants consistently failed to show visible symptoms at 20 days regardless of nitrogen treatment. In both experiments, the severity of symptom development among infected plants ranged from 25 to 118% when compared to the controls (virus tested plants grown in the presence of nitrogen). The presence of a complex of viruses (Sweet potato feathery mottle virus, Sweet potato virus G, Sweet potato virus C, and Sweet potato virus 2) was associated with 51% reduction in adventitious root number among plants grown without nitrogen. The effect of virus treatments on first order LR development depended on the presence or absence of nitrogen. In the presence of nitrogen, only plants infected with Sweet potato chlorotic stunt virus showed reductions in first order LR length, number, and density, which were decreased by 33%, 12%, and 11%, respectively, when compared to the controls. In the absence of nitrogen, virus tested and infected plants manifested significant reductions for all first order LR attributes. These results provide evidence that virus infection directly influences sweetpotato yield potential by reducing both the number of adventitious roots and LR development. These findings provide a framework for understanding how virus infection reduces sweetpotato yield and could lead to the development of novel strategies to mitigate virus effects on sweetpotato productivity.

Balanced activity of microRNA166/165 and its target transcripts from the class III homeodomain-leucine zipper family regulates root growth in Arabidopsis thaliana

Overexpression of miR166/165 down-regulates target HD - ZIP IIIs and promotes root growth by enhancing cell division and meristematic activity, whereas overexpression of HD - ZIP IIIs inhibits root growth in Arabidopsis thaliana. Post-embryonic growth of higher plants is maintained by active meristems harbouring undifferentiated cells. Shoot and root apical meristems (SAM and RAM) utilize both similar and distinct signalling mechanisms for their maintenance in Arabidopsis thaliana. An important regulatory role in this context has the interaction of microRNAs with their target mRNAs, mostly encoding transcription factors. One class of microRNA166/165 (miR166/165) has been implicated in the maintenance of SAM and vascular patterning. Here, we show that miR166/165 plays an important role in root growth also by negatively regulating its target transcripts, HD-ZIP IIIs, in the RAM. While overexpression of miR166 promotes RAM activity, overexpression of its targets reduces RAM activity. These results reveal a conserved role of miR166/165 in the maintenance of SAM and RAM activity in A. thaliana.

Role of LONELY GUY genes in indeterminate nodulation on Medicago truncatula

LONELY GUY (LOG) genes encode cytokinin riboside 5'-monophosphate phosphoribohydrolases and are directly involved in the activation of cytokinins. To assess whether LOG proteins affect the influence of cytokinin on nodulation, we studied two LOG genes of Medicago truncatula. Expression analysis showed that MtLOG1 and MtLOG2 were upregulated during nodulation in a CRE1-dependent manner. Expression was mainly localized in the dividing cells of the nodule primordium. In addition, RNA interference revealed that MtLOG1 is involved in nodule development and that the gene plays a negative role in lateral root development. Ectopic expression of MtLOG1 resulted in a change in cytokinin homeostasis, triggered cytokinin-inducible genes and produced roots with enlarged vascular tissues and shortened primary roots. In addition, those 35S:LOG1 roots also displayed fewer nodules than the wild-type. This inhibition in nodule formation was local, independent of the SUPER NUMERIC NODULES gene, but coincided with an upregulation of the MtCLE13 gene, encoding a CLAVATA3/EMBRYO SURROUNDING REGION peptide. In conclusion, we demonstrate that in M. truncatula LOG proteins might be implicated in nodule primordium development and lateral root formation.

Ovule development, a new model for lateral organ formation

In spermatophytes the ovules upon fertilization give rise to the seeds. It is essential to understand the mechanisms that control ovule number and development as they ultimately determine the final number of seeds and, thereby, the yield in crop plants. In Arabidopsis thaliana, ovules arise laterally from a meristematic tissue within the carpel referred to as placenta. For a correct determination of the number of ovules, a precise establishment of the positions where ovule primordia emerge is needed, and a tight definition of the boundaries between ovules is therefore also required. In the last decades, few factors have been identified to be involved in the determination of ovule number. Recently, plant hormones have also been revealed as fundamental players in the control of the initiation of ovule formation. In this review we summarize the current knowledge about both the molecular and hormonal mechanisms that control ovule formation in Arabidopsis thaliana.