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.

More than growth: Phytohormone-regulated transcription factors controlling plant immunity, plant development and plant architecture.

Activation of immunity by exogenous signals or mutations leading to autoimmunity has long been associated with decreased plant growth, known as the growth-defense tradeoff. Originally thought to be a redirection of metabolic resources towards defense and away from growth, recent studies have demonstrated that growth and defense can be uncoupled, indicating that metabolic regulation is not solely responsible for the growth-defense tradeoff. Immunity activation has effects on plant development beyond the reduction of plant biomass, including changes in plant architecture.

Phytohormone Profile of in Response to Mycorrhizal Fungi, Aphids, and Gibberellic Acid.

Although gibberellic acid (GA) is widely used in agriculture, it is unclear whether exogenous GA makes aphid-infested, mycorrhizal plants more susceptible to herbivory. This study investigates the role of GA in modulating defenses in barrel medic plants () that are infested with pea aphids () and colonized by the beneficial symbiont . Mock- and -inoculated potted plants were grown in a topsoil: sand mix for 42 days and were treated with GA or solvent.

Cytokinin Regulation of Source-Sink Relationships in Plant-Pathogen Interactions.

Cytokinins are plant hormones known for their role in mediating plant growth. First discovered for their ability to promote cell division, this class of hormones is now associated with many other cellular and physiological functions. One of these functions is the regulation of source-sink relationships, a tightly controlled process that is essential for proper plant growth and development.

A genome-wide association study reveals cytokinin as a major component in the root defense responses against Ralstonia solanacearum.

Bacterial wilt caused by the soil-borne pathogen Ralstonia solancearum is economically devastating, with no effective methods to fight the disease. This pathogen invades plants through their roots and colonizes their xylem, clogging the vasculature and causing rapid wilting. Key to preventing colonization are the early defense responses triggered in the host's root upon infection, which remain mostly unknown.

Directions for research and training in plant omics: Big Questions and Big Data.

A key remit of the NSF-funded "Arabidopsis Research and Training for the 21 Century" (ART-21) Research Coordination Network has been to convene a series of workshops with community members to explore issues concerning research and training in plant biology, including the role that research using can play in addressing those issues. A first workshop focused on training needs for bioinformatic and computational approaches in plant biology was held in 2016, and recommendations from that workshop have been published (Friesner et al., , 175, 2017, 1499).

RNA-Seq analysis reveals insight into enhanced rice Xa7-mediated bacterial blight resistance at high temperature.

Plant disease is a major challenge to agriculture worldwide, and it is exacerbated by abiotic environmental factors. During some plant-pathogen interactions, heat stress allows pathogens to overcome host resistance, a phenomenon which could severely impact crop productivity considering the global warming trends associated with climate change. Despite the importance of this phenomenon, little is known about the underlying molecular mechanisms.

Evolution of Hormone Signaling Networks in Plant Defense.

Studies with model plants such as Arabidopsis thaliana have revealed that phytohormones are central regulators of plant defense. The intricate network of phytohormone signaling pathways enables plants to activate appropriate and effective defense responses against pathogens as well as to balance defense and growth. The timing of the evolution of most phytohormone signaling pathways seems to coincide with the colonization of land, a likely requirement for plant adaptations to the more variable terrestrial environments, which included the presence of pathogens.

Towards engineering of hormonal crosstalk in plant immunity.

Plant hormones regulate physiological responses in plants, including responses to pathogens and beneficial microbes. The last decades have provided a vast amount of evidence about the contribution of different plant hormones to plant immunity, and also of how they cooperate to orchestrate immunity activation, in a process known as hormone crosstalk. In this review we highlight the complexity of hormonal crosstalk in immunity and approaches currently being used to further understand this process, as well as perspectives to engineer hormone crosstalk for enhanced pathogen resistance and overall plant fitness.

Communication in the Phytobiome.

The phytobiome is composed of plants, their environment, and diverse interacting microscopic and macroscopic organisms, which together influence plant health and productivity. These organisms form complex networks that are established and regulated through nutrient cycling, competition, antagonism, and chemical communication mediated by a diverse array of signaling molecules. Integration of knowledge of signaling mechanisms with that of phytobiome members and their networks will lead to a new understanding of the fate and significance of these signals at the ecosystem level.

Assessment of Cytokinin-Induced Immunity Through Quantification of Hyaloperonospora arabidopsidis Infection in Arabidopsis thaliana.

Cytokinins have been shown to regulate plant immunity. Application of high levels of cytokinin to plants leads to decreased susceptibility to pathogens. In this chapter, we describe a fast and accurate protocol for assessment of cytokinin-induced immunity in Arabidopsis plants against an oomycete plant pathogen.

Should I fight or should I grow now? The role of cytokinins in plant growth and immunity and in the growth-defence trade-off.

Background: Perception and activation of plant immunity require a remarkable level of signalling plasticity and control. In Arabidopsis and other plant species, constitutive defence activation leads to resistance to a broad spectrum of biotrophic pathogens, but also frequently to stunted growth and reduced seed set. Plant hormones are important integrators of the physiological responses that influence the outcome of plant-pathogen interactions.

Pathogen Tactics to Manipulate Plant Cell Death.

Cell death is a vital process for multicellular organisms. Programmed cell death (PCD) functions in a variety of processes including growth, development, and immune responses for homeostasis maintenance. In particular, plants and animals utilize PCD to control pathogen invasion and infected cell populations.

No hormone to rule them all: Interactions of plant hormones during the responses of plants to pathogens.

Plant hormones are essential regulators of plant growth and immunity. In the last few decades, a vast amount of information has been obtained detailing the role of different plant hormones in immunity, and how they work together to ultimately shape the outcomes of plant pathogen interactions. Here we provide an overview on the roles of the main classes of plant hormones in the regulation of plant immunity, highlighting their metabolic and signaling pathways and how plants and pathogens utilize these pathways to activate or suppress defence.

The cytokinin response factors modulate root and shoot growth and promote leaf senescence in Arabidopsis.

The cytokinin response factors (CRFs) are a group of related AP2/ERF transcription factors that are transcriptionally induced by cytokinin. Here we explore the role of the CRFs in Arabidopsis thaliana growth and development by analyzing lines with decreased and increased CRF function. While single crf mutations have no appreciable phenotypes, disruption of multiple CRFs results in larger rosettes, delayed leaf senescence, a smaller root apical meristem (RAM), reduced primary and lateral root growth, and, in etiolated seedlings, shorter hypocotyls.

Two-component elements mediate interactions between cytokinin and salicylic acid in plant immunity.

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant-pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment.

Cytokinin signaling and transcriptional networks.

The cytokinin signaling pathway consists of a phosphorelay mechanism that is initiated by binding of cytokinin to histidine kinase receptors and culminates with the transcription of cytokinin-responsive genes in the nucleus. Type-B response regulators (ARR) encode transcription factors that act as major players in the transcriptional activation of cytokinin-responsive genes, among which are many transcription factors. In this review, we highlight the transcriptional networks regulated by cytokinin that have been identified and their roles in the regulation of a subset of the many developmental and physiological processes regulated by this plant hormone.

Environmental perception avenues: the interaction of cytokinin and environmental response pathways.

Cytokinins were discovered in the 1950s by their ability to promote cell division in cultured plant cells. Recently, there have been significant breakthroughs in our understanding of the biosynthesis, metabolism, perception and signal transduction of this phytohormone. These advances, coupled with physiological and other approaches, have enabled remarkable progress to be made in our understanding of the interactions between cytokinin function and environmental inputs.

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

Arabidopsis thaliana histidine phosphotransfer proteins (AHPs) are similar to bacterial and yeast histidine phosphotransfer proteins (HPts), which act in multistep phosphorelay signaling pathways. A phosphorelay pathway is the current model for cytokinin signaling. To assess the role of AHPs in cytokinin signaling, we isolated T-DNA insertions in the five AHP genes that are predicted to encode functional HPts and constructed multiple insertion mutants, including an ahp1,2,3,4,5 quintuple mutant.

RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling.

The roots curl in naphthylphthalamic acid1 (rcn1) mutant of Arabidopsis (Arabidopsis thaliana) has altered auxin transport, gravitropism, and ethylene response, providing an opportunity to analyze the interplay between ethylene and auxin in control of seedling growth. Roots of rcn1 seedlings were previously shown to have altered auxin transport, growth, and gravitropism, while rcn1 hypocotyl elongation exhibited enhanced ethylene response. We have characterized auxin transport and gravitropism phenotypes of rcn1 hypocotyls and have explored the roles of auxin and ethylene in controlling these phenotypes.