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

Virulence regulation in plant-pathogenic bacteria by host-secreted signals

Bacterial pathogens manipulate host signaling pathways and evade host defenses using effector molecules, coordinating their deployment to ensure successful infection. However, host-derived metabolites as signals, and their critical role in regulating bacterial virulence requires further insights. Effective regulation of virulence, which is essential for pathogenic bacteria, involves controlling factors that enable colonization, defense evasion, and tissue damage. This regulation is dynamic, influenced by environmental cues including signals from host plants like exudates. Plant exudates, comprising of diverse compounds released by roots and tissues, serve as rich chemical signals affecting the behavior and virulence of associated bacteria. Plant nutrients act as signaling molecules that are sensed through membrane-localized receptors and intracellular response mechanisms in bacteria. This review explains how different bacteria detect and answer to secreted chemical signals, regulating virulence gene expression. Our main emphasis is exploring the recognition process of host-originated signaling molecules through molecular sensors on cellular membranes and intracellular signaling pathways. This review encompasses insights into how bacterial strains individually coordinate their virulence in response to various distinct host-derived signals that can positively or negatively regulate their virulence. Furthermore, we explained the interruption of plant defense with the perception of host metabolites to dampen pathogen virulence. The intricate interplay between pathogens and plant signals, particularly in how pathogens recognize host metabolic signals to regulate virulence genes, portrays a crucial initial interaction leading to profound influences on infection outcomes. This work will greatly aid researchers in developing new strategies for preventing and treating infections.

CaIAA2-CaARF9 module mediates the trade-off between pepper growth and immunity

To challenge the invasion of various pathogens, plants re-direct their resources from plant growth to an innate immune defence system. However, the underlying mechanism that coordinates the induction of the host immune response and the suppression of plant growth remains unclear. Here we demonstrate that an auxin response factor, CaARF9, has dual roles in enhancing the immune resistance to Ralstonia solanacearum infection and in retarding plant growth by repressing the expression of its target genes as exemplified by Casmc4, CaLBD37, CaAPK1b and CaRROP1. The expression of these target genes not only stimulates plant growth but also negatively impacts pepper resistance to R. solanacearum. Under normal conditions, the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 is active when promoter-bound CaARF9 is complexed with CaIAA2. Under R. solanacearum infection, however, degradation of CaIAA2 is triggered by SA and JA-mediated signalling defence by the ubiquitin-proteasome system, which enables CaARF9 in the absence of CaIAA2 to repress the expression of Casmc4, CaLBD37, CaAPK1b and CaRROP1 and, in turn, impeding plant growth while facilitating plant defence to R. solanacearum infection. Our findings uncover an exquisite mechanism underlying the trade-off between plant growth and immunity mediated by the transcriptional repressor CaARF9 and its deactivation when complexed with CaIAA2.

Extracellular niche establishment by plant pathogens

The plant extracellular space, referred to as the apoplast, is inhabited by a variety of microorganisms. Reflecting the crucial nature of this compartment, both plants and microorganisms seek to control, exploit and respond to its composition. Upon sensing the apoplastic environment, pathogens activate virulence programmes, including the delivery of effectors with well-established roles in suppressing plant immunity. We posit that another key and foundational role of effectors is niche establishment - specifically, the manipulation of plant physiological processes to enrich the apoplast in water and nutritive metabolites. Facets of plant immunity counteract niche establishment by restricting water, nutrients and signals for virulence activation. The complex competition to control and, in the case of pathogens, exploit the apoplast provides remarkable insights into the nature of virulence, host susceptibility, host defence and, ultimately, the origin of phytopathogenesis. This novel framework focuses on the ecology of a microbial niche and highlights areas of future research on plant-microorganism interactions.

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.

Structure, evolution, and roles of SWEET proteins in growth and stress responses in plants

Carbohydrates are exported by the SWEET family of transporters, which is a novel class of carriers that can transport sugars across cell membranes and facilitate sugar's long-distance transport from source to sink organs in plants. SWEETs play crucial roles in a wide range of physiologically important processes by regulating apoplastic and symplastic sugar concentrations. These processes include host-pathogen interactions, abiotic stress responses, and plant growth and development. In the present review, we (i) describe the structure and organization of SWEETs in the cell membrane, (ii) discuss the roles of SWEETs in sugar loading and unloading processes, (iii) identify the distinct functions of SWEETs in regulating plant growth and development including flower, fruit, and seed development, (iv) shed light on the importance of SWEETs in modulating abiotic stress resistance, and (v) describe the role of SWEET genes during plant-pathogen interaction. Finally, several perspectives regarding future investigations for improving the understanding of sugar-mediated plant defenses are proposed.

Parasitic plant-host interaction between the holoparasite Cytinus hypocistis and the shrub Cistus albidus in their natural Mediterranean habitat: local and systemic hormonal effects

Mediterranean-type ecosystems provide a unique opportunity to study parasitic plant-host interactions, such as the relationship between the dominant shrub Cistus albidus L. and the root holoparasitic plant Cytinus hypocistis L. We examined this interaction (i) locally, by measuring the hormonal profiling of the interaction zone between the holoparasitic plant and the host, and (ii) systemically, by examining the hormonal profiling and physiological status of leaves from infested and uninfested plants. Furthermore, we explored how temporal variation (seasonal effects) and geographical location influenced the systemic hormonal and physiological response of leaves. Results shed light on tissue-related variations in hormones, suggesting the parasite exerted a sink effect, mainly influenced by cytokinins. Jasmonates triggered a defense response in leaves, far from the infestation point, and both jasmonates and abscisic acid (ABA) appeared to be involved in the tolerance to holoparasitism when plants were simultaneously challenged with summer drought. Parasitism did not have any major negative impact on the host, as indicated by physiological stress markers in leaves, thus indicating a high tolerance of the shrub C. albidus to the root holoparasitic plant C. hypocistis. Rather, parasitism seemed to exert a priming-like effect and some compensatory effects were observed (increased chlorophyll contents) in the host under mild climatic conditions. We conclude that (i) cytokinins, jasmonates and ABA play a role at the local and systemic levels in the response of C. albidus to the biotic stress caused by C. hypocistis, and that (ii) seasonal changes in environmental conditions and geographical location may impact holoparasitic plant-host interactions in the field, modulating the physiological response.

Cytokinin Translocation to, and Biosynthesis and Metabolism within, Cereal and Legume Seeds: Looking Back to Inform the Future

Early in the history of cytokinins, it was clear that Zea mays seeds contained not just trans-zeatin, but its nucleosides and nucleotides. Subsequently, both pods and seeds of legumes and cereal grains have been shown to contain a complex of cytokinin forms. Relative to the very high quantities of cytokinin detected in developing seeds, only a limited amount appears to have been translocated from the parent plant. Translocation experiments, and the detection of high levels of endogenous cytokinin in the maternal seed coat tissues of legumes, indicates that cytokinin does not readily cross the maternal/filial boundary, indicating that the filial tissues are autonomous for cytokinin biosynthesis. Within the seed, trans-zeatin plays a key role in sink establishment and it may also contribute to sink strength. The roles, if any, of the other biologically active forms of cytokinin (cis-zeatin, dihydrozeatin and isopentenyladenine) remain to be elucidated. The recent identification of genes coding for the enzyme that leads to the biosynthesis of trans-zeatin in rice (OsCYP735A3 and 4), and the identification of a gene coding for an enzyme (CPN1) that converts trans-zeatin riboside to trans-zeatin in the apoplast, further cements the key role played by trans-zeatin in plants.

The Inferior Grain Filling Initiation Promotes the Source Strength of Rice Leaves

Poor grain-filling initiation in inferior spikelets severely impedes rice yield improvement, while photo-assimilates from source leaves can greatly stimulate the initiation of inferior grain-filling (sink). To investigate the underlying mechanism of source-sink interaction, a two-year field experiment was conducted in 2019 and 2020 using two large-panicle rice cultivars (CJ03 and W1844). The treatments included intact panicles and partial spikelet removal. These two cultivars showed no significant difference in the number of spikelets per panicle. However, after removing spikelet, W1844 showed higher promotion on 1000-grain weight and seed-setting rate than CJ03, particularly for inferior spikelets. The reason was that the better sink activity of W1844 led to a more effective initiation of inferior grain-filling compared to CJ03. The inferior grain weight of CJ03 and W1844 did not show a significant increase until 8 days poster anthesis (DPA), which follows a similar pattern to the accumulation of photo-assimilates in leaves. After removing spikelets, the source leaves of W1844 exhibited lower photosynthetic inhibition compared to CJ03, as well as stronger metabolism and transport of photo-assimilates. Although T6P levels remained constant in both cultivars under same conditions, the source leaves of W1844 showed notable downregulation of SnRK1 activity and upregulation of phytohormones (such as abscisic acid, cytokinins, and auxin) after removing spikelets. Hence, the high sink strength of inferior spikelets plays a role in triggering the enhancement of source strength in rice leaves, thereby fulfilling grain-filling initiation demands.

Ill Communication: Host Metabolites as Virulence-Regulating Signals for Plant-Pathogenic Bacteria

Plant bacterial pathogens rely on host-derived signals to coordinate the deployment of virulence factors required for infection. In this review, I describe how diverse plant-pathogenic bacteria detect and respond to plant-derived metabolic signals for the purpose of virulence gene regulation. I highlight examples of how pathogens perceive host metabolites through membrane-localized receptors as well as intracellular response mechanisms. Furthermore, I describe how individual strains may coordinate their virulence using multiple distinct host metabolic signals, and how plant signals may positively or negatively regulate virulence responses. I also describe how plant defenses may interfere with the perception of host metabolites as a means to dampen pathogen virulence. The emerging picture is that recognition of host metabolic signals for the purpose of virulence gene regulation represents an important primary layer of interaction between pathogenic bacteria and host plants that shapes infection outcomes.

Transcriptional profiling reveals a critical role of GmFT2a in soybean staygreen syndrome caused by the pest Riptortus pedestris

Soybean staygreen syndrome, characterized by delayed leaf and stem senescence, abnormal pods, and aborted seeds, has recently become a serious and prominent problem in soybean production. Although the pest Riptortus pedestris has received increasing attention as the possible cause of staygreen syndrome, the mechanism remains unknown. Here, we clarify that direct feeding by R. pedestris, not transmission of a pathogen by this pest, is the primary cause of typical soybean staygreen syndrome and that critical feeding damage occurs at the early pod stage. Transcriptome profiling of soybean indicated that many signal transduction pathways, including photoperiod, hormone, defense response, and photosynthesis, respond to R. pedestris infestation. Importantly, we discovered that members of the FLOWERING LOCUS T (FT) gene family were suppressed by R. pedestris infestation, and overexpression of floral inducer GmFT2a attenuates staygreen symptoms by mediating soybean defense response and photosynthesis. Together, our findings systematically illustrate the association between pest infestation and soybean staygreen syndrome and provide the basis for establishing a targeted soybean pest prevention and control system.

Gain-of-function of the cytokinin response activator ARR1 increases heat shock tolerance in Arabidopsis thaliana

In addition to its well-established role in plant development, the hormone cytokinin regulates plant responses to biotic and abiotic stresses. It was previously shown that cytokinin signaling acts negatively upon drought and osmotic stress tolerance and that gain-of-function of the cytokinin response regulator ARR1 causes osmotic stress hypersensitivity. Here we show that increased ARR1 action increases tolerance to heat shock and that this is correlated with increased accumulation of the heat shock proteins Hsp17.6 and Hsp70. These results show that the heat shock tolerance of plants can be elevated by increasing the expression of a cytokinin response activator.

Climate change impedes plant immunity mechanisms

Rapid climate change caused by human activity is threatening global crop production and food security worldwide. In particular, the emergence of new infectious plant pathogens and the geographical expansion of plant disease incidence result in serious yield losses of major crops annually. Since climate change has accelerated recently and is expected to worsen in the future, we have reached an inflection point where comprehensive preparations to cope with the upcoming crisis can no longer be delayed. Development of new plant breeding technologies including site-directed nucleases offers the opportunity to mitigate the effects of the changing climate. Therefore, understanding the effects of climate change on plant innate immunity and identification of elite genes conferring disease resistance are crucial for the engineering of new crop cultivars and plant improvement strategies. Here, we summarize and discuss the effects of major environmental factors such as temperature, humidity, and carbon dioxide concentration on plant immunity systems. This review provides a strategy for securing crop-based nutrition against severe pathogen attacks in the era of climate change.

Role of cytokinins in seed development in pulses and oilseed crops: Current status and future perspective

Cytokinins constitutes a vital group of plant hormones regulating several developmental processes, including growth and cell division, and have a strong influence on grain yield. Chemically, they are the derivatives of adenine and are the most complex and diverse group of hormones affecting plant physiology. In this review, we have provided a molecular understanding of the role of cytokinins in developing seeds, with special emphasis on pulses and oilseed crops. The importance of cytokinin-responsive genes including cytokinin oxidases and dehydrogenases (CKX), isopentenyl transferase (IPT), and cytokinin-mediated genetic regulation of seed size are described in detail. In addition, cytokinin expression in germinating seeds, its biosynthesis, source-sink dynamics, cytokinin signaling, and spatial expression of cytokinin family genes in oilseeds and pulses have been discussed in context to its impact on increasing economy yields. Recently, it has been shown that manipulation of the cytokinin-responsive genes by mutation, RNA interference, or genome editing has a significant effect on seed number and/or weight in several crops. Nevertheless, the usage of cytokinins in improving crop quality and yield remains significantly underutilized. This is primarily due to the multigene control of cytokinin expression. The information summarized in this review will help the researchers in innovating newer and more efficient ways of manipulating cytokinin expression including CKX genes with the aim to improve crop production, specifically of pulses and oilseed crops.

Integrated transcriptome and plant growth substance profiles to identify the regulatory factors involved in floral sex differentiation in Zanthoxylum armatum DC

Zanthoxylum armatum is a prominent plant for food industries. Its male flowers often occur in gynogenesis plants; however, the potential mechanism remains poorly understood. Herein, a total of 26 floral sex differentiation stages were observed to select four vital phases to reveal key factors by using RNA-seq, phytohormones and carbohydrates investigation. The results showed that a selective abortion of stamen or pistil primordia could result in the floral sex differentiation in Z. armatum. Carbohydrates might collaborate with cytokinin to effect the male floral differentiation, whereas female floral differentiation was involved in SA, GA₁, and ABA biosynthesis and signal transduction pathways. Meanwhile, these endogenous regulators associated with reproductive growth might be integrated into ABCDE model to regulate the floral organ differentiation in Z. armatum. Furthermore, the 21 crucial candidates were identified in co-expression network, which would contribute to uncovering their roles in floral sex differentiation of Z. armatum in further studies. To the best of our knowledge, this study was the first comprehensive investigation to link floral sex differentiation with multi-level endogenous regulatory factors in Z. armatum. It also provided new insights to explore the regulatory mechanism of floral sex differentiation, which would be benefited to cultivate high-yield varieties in Z. armatum.

Increasing Sugar Content in Source for Biofuel Production Using Agrochemical and Genetic Approaches at the Stages of BioMass Preharvesting and Harvesting

In order to optimize biofuel (including bioethanol) production processes, various problems need to be solved, such as increasing the sugar content of raw materials/biomass to gain a higher yield of the product. This task can be solved in several ways, with their own advantages and disadvantages, and an integrated approach, such as using a combination of ripening agents and phytohormones or application of a superabsorbent polymer with at least one sugar-enhancing agent, can be applied as well. Here, we reviewed several methods, including pre- and postharvest factors (light, temperature, partial replacement of potassium with magnesium, etc.), genetic modifications (traditional breeding, phytohormones, etc.), chemical ripening methods (Ethephon, Moddus, etc.), and some alternative methods (DMSO treatment, ionic liquids, etc.). The aim of this review was to provide a comprehensive, up-to-date summary of methods of increasing the carbohydrate level in plants/biomass for bioethanol production.

Interplay between phytohormone signalling pathways in plant defence - other than salicylic acid and jasmonic acid

Phytohormones are essential for all aspects of plant growth, development, and immunity; however, it is the interplay between phytohormones, as they dynamically change during these processes, that is key to this regulation. Hormones have traditionally been split into two groups: growth-promoting and stress-related. Here, we will discuss and show that all hormones play a role in plant defence, regardless of current designation. We highlight recent advances in our understanding of the complex phytohormone networks with less focus on archetypal immunity-related pathways and discuss protein and transcription factor signalling hubs that mediate hormone interplay.

Infection by Moniliophthora perniciosa reprograms tomato Micro-Tom physiology, establishes a sink, and increases secondary cell wall synthesis

Witches' broom disease of cacao is caused by the pathogenic fungus Moniliophthora perniciosa. By using tomato (Solanum lycopersicum) cultivar Micro-Tom (MT) as a model system, we investigated the physiological and metabolic consequences of M. perniciosa infection to determine whether symptoms result from sink establishment during infection. Infection of MT by M. perniciosa caused reductions in root biomass and fruit yield, a decrease in leaf gas exchange, and down-regulation of photosynthesis-related genes. The total leaf area and water potential decreased, while ABA levels, water conductance/conductivity, and ABA-related gene expression increased. Genes related to sugar metabolism and those involved in secondary cell wall deposition were up-regulated upon infection, and the concentrations of sugars, fumarate, and amino acids increased. 14C-glucose was mobilized towards infected MT stems, but not in inoculated stems of the MT line overexpressing CYTOKININ OXIDASE-2 (35S::AtCKX2), suggesting a role for cytokinin in establishing a sugar sink. The up-regulation of genes involved in cell wall deposition and phenylpropanoid metabolism in infected MT, but not in 35S::AtCKX2 plants, suggests establishment of a cytokinin-mediated sink that promotes tissue overgrowth with an increase in lignin. Possibly, M. perniciosa could benefit from the accumulation of secondary cell walls during its saprotrophic phase of infection.