Cytokinin activity increases stomatal density and transpiration rate in tomato

6-Benzylaminopurine mediated augmentation of cadmium phytostabilization potential in Strobilanthes alternata

This study unveiled the cadmium phytoremediation potential and its augmentation using 6-Benzylaminopurine in Strobilanthes alternata. Cadmium stress was provided by applying 250 mg/kg cadmium chloride in soil and 25 ppm of 6-BAP (25 ml) was administered to the plants as foliar spray. The results revealed high bioconcentration factor (BCF) (18.82 ± 0.54) and low translocation factor (TF) values (0.055 ± 0.002) for the plant based on which we strongly recommend S. alternata as a promising candidate for Cd phytoremediation. The phytostabilization potential of the plant was further enhanced by applying 6-BAP, which augmented its BCF to 22.09 ± 0.64 and reduced the TF to 0.038 ± 0.001. Cd toxicity caused a reduction of plant growth parameters, root volume, adaxial-abaxial stomatal indices, relative water content, tolerance index, moisture content, membrane stability index, and xylem vessel diameter in S. alternata. However, Cd + 6-BAP treated plants exhibited an increase of the same compared to Cd-treated plants. FTIR analysis of Cd + 6-BAP treated plants revealed increased deposition of hemicellulose, causing enhanced retention of Cd in the root xylem walls, which is largely responsible for increased phytostabilization of Cd. Therefore, 6-BAP application in S. alternata can be exploited to restore Cd-contaminated areas effectively.

TOR coordinates cytokinin and gibberellin signals mediating development and defense

Plants constantly perceive and process environmental signals and balance between the energetic demands of growth and defense. Growth arrest upon pathogen attack was previously suggested to result from a redirection of the plants' metabolic resources towards the activation of plant defense. The energy sensor Target of Rapamycin (TOR) kinase is a conserved master coordinator of growth and development in all eukaryotes. Although TOR is positioned at the interface between development and defense, little is known about the mechanisms by which TOR may potentially regulate the relationship between these two modalities. The plant hormones cytokinin (CK) and gibberellin (GA) execute various aspects of plant development and defense. The ratio between CK and GA was reported to determine the outcome of developmental programmes. Here, investigating the interplay between TOR-mediated development and TOR-mediated defense in tomato, we found that TOR silencing resulted in rescue of several different aberrant developmental phenotypes, demonstrating that TOR is required for the execution of developmental cues. In parallel, TOR inhibition enhanced immunity in genotypes with a low CK/GA ratio but not in genotypes with a high CK/GA ratio. TOR-inhibition mediated disease resistance was found to depend on developmental status, and was abolished in strongly morphogenetic leaves, while being strongest in mature, differentiated leaves. CK repressed TOR activity, suggesting that CK-mediated immunity may rely on TOR downregulation. At the same time, TOR activity was promoted by GA, and TOR silencing reduced GA sensitivity, indicating that GA signalling requires normal TOR activity. Our results demonstrate that TOR likely acts in concert with CK and GA signalling, executing signalling cues in both defense and development. Thus, differential regulation of TOR or TOR-mediated processes could regulate the required outcome of development-defense prioritisation.

Characteristics of photosynthetic rates in different vegetation types at high-altitude in mountainous regions

Mountains play an important role in the carbon cycle of the terrestrial ecosystem and are one of the most sensitive ecosystems to climate change. However, our current knowledge regarding the physiological responses of alpine plants to environmental changes remains limited due to the severe climatic conditions prevailing in these high-altitude regions. Therefore, this study quantified the variations in photosynthetic rates (An) and identified their driving factors of herbaceous plants, shrubs, and trees along an elevation gradient (2200 m asl to 3200 m asl) on Mount Gongga. Elevation emerged as a significant determinant of An, with a general increase observed, albeit followed by a decline above 3000 m asl. In high-altitude regions, trees displayed more significant fluctuations in An compared to herbaceous plants and shrubs. The lower levels of atmospheric carbon dioxide concentration (eCO2) and temperature in high-altitude regions resulted in a 16 % increase in An for herbaceous plants, 60 % increase for shrubs, and 43 % increase for trees compared to the low-altitude areas. Structural equation modeling (SEM) analyses underscored the considerable impact of environmental factors on An. Notably, photosynthetically active radiation, eCO2, and stomatal conductance were identified as positive influencers, while other factors exerted negative effects. Our results further highlighted that trees were subject to greater constraints from multiple factors compared to herbs and shrubs, aligning with the outcomes of our variance analysis. In summary, our study presents a comprehensive assessment of vegetation responses to environmental factors along elevational gradients. The significance of An in plants at high altitude to external factors suggests the potential adaptability of alpine plants, and also indicates that changes in photosynthetic physiological functions at high altitude should be paid more attention to in the study of climate change.

Biostimulant red seaweed (Gracilaria tenuistipitata var. liui) extracts spray improves yield and drought tolerance in soybean

Drought has a deleterious impact on the growth, physiology, and yield of various plants, including soybean. Seaweed extracts are rich in various bioactive compounds, including antioxidants, and can be used as biostimulants for improving yield and alleviating the adverse effect of drought stress. The purpose of this study was to evaluate the effect of soybean growth and yield with different concentrations (0.0%, 5.0%, and 10.0% v/v) of water extracts of the red seaweed Gracilaria tenuistipitata var. liui under well-watered (80% of field capacity (FC) and drought (40% of FC)) conditions. Drought stress decreased soybean grain yield by 45.58% compared to well-watered circumstances but increased the water saturation deficit by 37.87%. It also decreased leaf water, chlorophyll content, plant height, and the fresh weight of the leaf, stem, and petiole. Drought stress decreased soybean grain yield by 45.58% compared to well-watered circumstances but increased the water saturation deficit by 37.87%. It also decreased leaf water, chlorophyll content, plant height, and the fresh weight of the leaf, stem, and petiole. Under both drought and well-watered situations, foliar application of seaweed extracts dramatically improved soybean growth and production. Under drought and well-watered situations, 10.0% seaweed extract increased grain yield by 54.87% and 23.97%, respectively in comparison to untreated plants. The results of this study suggest that red seaweed extracts from Gracilaria tenuistipitata var. liui may be used as a biostimulant to improve soybean yield and drought tolerance in the presence of insufficient water. However, the actual mechanisms behind these improvements need to be further investigated in field conditions.

Transcriptomic and co-expression network analyses on diverse wheat landraces identifies candidate master regulators of the response to early drought

Introduction

Over four billion people around the world rely on bread wheat (Triticum aestivum L.) as a major constituent of their diet. The changing climate, however, threatens the food security of these people, with periods of intense drought stress already causing widespread wheat yield losses. Much of the research into the wheat drought response has centred on the response to drought events later in development, during anthesis or grain filling. But as the timing of periods of drought stress become increasingly unpredictable, a more complete understanding of the response to drought during early development is also needed.

Methods

Here, we utilized the YoGI landrace panel to identify 10,199 genes which were differentially expressed under early drought stress, before weighted gene co-expression network analysis (WGCNA) was used to construct a co-expression network and identify hub genes in modules particularly associated with the early drought response.

Results

Of these hub genes, two stood out as novel candidate master regulators of the early drought response - one as an activator (TaDHN4-D1; TraesCS5D02G379200) and the other as a repressor (uncharacterised gene; TraesCS3D02G361500).

Discussion

As well as appearing to coordinate the transcriptional early drought response, we propose that these hub genes may be able to regulate the physiological early drought response due to potential control over the expression of members of gene families well-known for their involvement in the drought response in many plant species, namely dehydrins and aquaporins, as well as other genes seemingly involved in key processes such as, stomatal opening, stomatal closing, stomatal morphogenesis and stress hormone signalling.

Influence of Plant Growth-Promoting Rhizobacteria on the Formation of Apoplastic Barriers and Uptake of Water and Potassium by Wheat Plants

The formation of apoplastic barriers is important for controlling the uptake of water and ions by plants, thereby influencing plant growth. However, the effects of plant growth-promoting bacteria on the formation of apoplastic barriers, and the relationship between these effects and the ability of bacteria to influence the content of hormones in plants, have not been sufficiently studied. The content of cytokinins, auxins and potassium, characteristics of water relations, deposition of lignin and suberin and the formation of Casparian bands in the root endodermis of durum wheat (Triticum durum Desf.) plants were evaluated after the introduction of the cytokinin-producing bacterium Bacillus subtilis IB-22 or the auxin-producing bacterium Pseudomonas mandelii IB-Ki14 into their rhizosphere. The experiments were carried out in laboratory conditions in pots with agrochernozem at an optimal level of illumination and watering. Both strains increased shoot biomass, leaf area and chlorophyll content in leaves. Bacteria enhanced the formation of apoplastic barriers, which were most pronounced when plants were treated with P. mandelii IB-Ki14. At the same time, P. mandelii IB-Ki14 caused no decrease in the hydraulic conductivity, while inoculation with B. subtilis IB-22, increased hydraulic conductivity. Cell wall lignification reduced the potassium content in the roots, but did not affect its content in the shoots of plants inoculated with P. mandelii IB-Ki14. Inoculation with B. subtilis IB-22 did not change the potassium content in the roots, but increased it in the shoots.

Stomatal properties of Arabidopsis cauline and rice flag leaves and their contributions to seed production and grain yield

Cauline leaves on the inflorescence stem of Arabidopsis thaliana may play important roles in supplying photosynthetic products to sinks, such as floral organs. Flag leaves in rice (Oryza sativa) have a higher photosynthetic capacity than other leaves, and are crucial for increasing grain yield. However, the detailed properties of stomata in cauline and flag leaves have not been investigated. In Arabidopsis, stomatal conductance and CO2 assimilation rate were higher in cauline leaves under white light than in rosette leaves, consistent with higher levels of plasma membrane (PM) H+-ATPase, a key enzyme for stomatal opening, in guard cells. Moreover, removal of cauline leaves significantly reduced the shoot biomass by approximately 20% and seed production by approximately 46%. In rice, higher stomatal density, stomatal conductance, and CO2 assimilation rate were observed in flag leaves than in fully expanded second leaves. Removal of the flag leaves significantly reduced grain yield by approximately 49%. Taken together, these results show that cauline and flag leaves have important roles in seed production and grain yield through enhanced stomatal conductance and CO2 assimilation rate.

Abscisic Acid and Cytokinins Are Not Involved in the Regulation of Stomatal Conductance of Scots Pine Saplings during Post-Drought Recovery

Delayed or incomplete recovery of gas exchange after water stress relief limits assimilation in the post-drought period and can thus negatively affect the processes of post-drought recovery. Abscisic acid (ABA) accumulation and antagonistic action between ABA and cytokinins (CKs) play an important role in regulation of stomatal conductance under water deficit. Specifically, in pine species, sustained ABA accumulation is thought to be the main cause of delayed post-drought gas exchange recovery, although the role of CKs is not yet known. Therefore, we aimed to study the effects of ABA and CKs on recovery of stomatal conductance in greenhouse-grown 3-year-old Scots pine saplings recovering from water stress. We analysed both changes in endogenous ABA and CK contents and the effects of treatment with exogenous CK on stomatal conductance. Drought stress suppressed stomatal conductance, and post-drought stomatal conductance remained suppressed for 2 weeks after plant rewatering. ABA accumulated during water stress, but ABA levels decreased rapidly after rewatering. Additionally, trans-zeatin/ABA and isopentenyladenine/ABA ratios, which were decreased in water-stressed plants, recovered rapidly in rewatered plants. Spraying plants with 6-benzylaminopurine (0.1–100 µM) did not influence recovery of either stomatal conductance or needle water status. It can be concluded that the delayed recovery of stomatal conductance in Scots pine needles was not due to sustained ABA accumulation or a sustained decrease in the CK/ABA ratio, and CK supplementation was unable to overcome this delayed recovery.

Molecular Regulation and Evolution of Cytokinin Signaling in Plant Abiotic Stresses

The sustainable production of crops faces increasing challenges from global climate change and human activities, which leads to increasing instances of many abiotic stressors to plants. Among the abiotic stressors, drought, salinity and excessive levels of toxic metals cause reductions in global agricultural productivity and serious health risks for humans. Cytokinins (CKs) are key phytohormones functioning in both normal development and stress responses in plants. Here, we summarize the molecular mechanisms on the biosynthesis, metabolism, transport and signaling transduction pathways of CKs. CKs act as negative regulators of both root system architecture plasticity and root sodium exclusion in response to salt stress. The functions of CKs in mineral-toxicity tolerance and their detoxification in plants are reviewed. Comparative genomic analyses were performed to trace the origin, evolution and diversification of the critical regulatory networks linking CK signaling and abiotic stress. We found that the production of CKs and their derivatives, pathways of signal transduction and drought-response root growth regulation are evolutionarily conserved in land plants. In addition, the mechanisms of CK-mediated sodium exclusion under salt stress are suggested for further investigations. In summary, we propose that the manipulation of CK levels and their signaling pathways is important for plant abiotic stress and is, therefore, a potential strategy for meeting the increasing demand for global food production under changing climatic conditions.

Plant hormone cytokinin at the crossroads of stress priming and control of photosynthesis

To cope with biotic and abiotic stress conditions, land plants have evolved several levels of protection, including delicate defense mechanisms to respond to changes in the environment. The benefits of inducible defense responses can be further augmented by defense priming, which allows plants to respond to a mild stimulus faster and more robustly than plants in the naïve (non-primed) state. Priming provides a low-cost protection of agriculturally important plants in a relatively safe and effective manner. Many different organic and inorganic compounds have been successfully tested to induce resistance in plants. Among the plethora of commonly used physicochemical techniques, priming by plant growth regulators (phytohormones and their derivatives) appears to be a viable approach with a wide range of applications. While several classes of plant hormones have been exploited in agriculture with promising results, much less attention has been paid to cytokinin, a major plant hormone involved in many biological processes including the regulation of photosynthesis. Cytokinins have been long known to be involved in the regulation of chlorophyll metabolism, among other functions, and are responsible for delaying the onset of senescence. A comprehensive overview of the possible mechanisms of the cytokinin-primed defense or stress-related responses, especially those related to photosynthesis, should provide better insight into some of the less understood aspects of this important group of plant growth regulators.

Genome-wide identification of the soybean cytokinin oxidase/dehydrogenase gene family and its diverse roles in response to multiple abiotic stress

Cytokinin oxidase/dehydrogenase (CKX) irreversibly degrades cytokinin, regulates growth and development, and helps plants to respond to environmental stress. Although the CKX gene has been well characterized in various plants, its role in soybean remains elusive. Therefore, in this study, the evolutionary relationship, chromosomal location, gene structure, motifs, cis-regulatory elements, collinearity, and gene expression patterns of GmCKXs were analyzed using RNA-seq, quantitative real-time PCR (qRT-PCR), and bioinformatics. We identified 18 GmCKX genes from the soybean genome and grouped them into five clades, each comprising members with similar gene structures and motifs. Cis-acting elements involved in hormones, resistance, and physiological metabolism were detected in the promoter regions of GmCKXs. Synteny analysis indicated that segmental duplication events contributed to the expansion of the soybean CKX family. The expression profiling of the GmCKXs genes using qRT-PCR showed tissue-specific expression patterns. The RNA-seq analysis also indicated that GmCKXs play an important role in response to salt and drought stresses at the seedling stage. The responses of the genes to salt, drought, synthetic cytokinin 6-benzyl aminopurine (6-BA), and the auxin indole-3-acetic acid (IAA) at the germination stage were further evaluated by qRT-PCR. Specifically, the GmCKX14 gene was downregulated in the roots and the radicles at the germination stage. The hormones 6-BA and IAA repressed the expression levels of GmCKX1, GmCKX6, and GmCKX9 genes but upregulated the expression levels of GmCKX10 and GmCKX18 genes. The three abiotic stresses also decreased the zeatin content in soybean radicle but enhanced the activity of the CKX enzymes. Conversely, the 6-BA and IAA treatments enhanced the CKX enzymes' activity but reduced the zeatin content in the radicles. This study, therefore, provides a reference for the functional analysis of GmCKXs in soybean in response to abiotic stresses.

Modifications of Phytohormone Metabolism Aimed at Stimulation of Plant Growth, Improving Their Productivity and Tolerance to Abiotic and Biotic Stress Factors

Due to the growing human population, the increase in crop yield is an important challenge for modern agriculture. As abiotic and biotic stresses cause severe losses in agriculture, it is also crucial to obtain varieties that are more tolerant to these factors. In the past, traditional breeding methods were used to obtain new varieties displaying demanded traits. Nowadays, genetic engineering is another available tool. An important direction of the research on genetically modified plants concerns the modification of phytohormone metabolism. This review summarizes the state-of-the-art research concerning the modulation of phytohormone content aimed at the stimulation of plant growth and the improvement of stress tolerance. It aims to provide a useful basis for developing new strategies for crop yield improvement by genetic engineering of phytohormone metabolism.

Plastid phylogenomics sheds light on divergence time and ecological adaptations of the tribe Persicarieae (Polygonaceae)

Southwestern China, adjacent to the Qinghai-Tibetan Plateau (QTP), is known as a hotspot for plant diversity and endemism, and it is the origin and diversification center of Persicarieae. As one of the major lineages in Polygonaceae, Persicarieae represents a diverse adaptation to various habitats. As a result of morphological plasticity and poorly resolving molecular markers, phylogenetic relationships and infrageneric classification within Persicarieae have long been controversial. In addition, neither plastome phylogenomic studies nor divergence time estimates on a larger sample of Persicarieae species have been made thus far. We sequenced and assembled 74 complete plastomes, including all of the recognized genera within Persicarieae and their relatives. We conducted a comprehensive phylogenetic study of the major clades within Persicarieae and, based on the thus obtained robust phylogeny, also estimated divergence time and the evolution of diagnostic morphological traits. Major relationships found in previous phylogenetic studies were confirmed, including those of the backbone of the tree, which had been a major problem in previous phylogenies of the tribe. Phylogenetic analysis revealed strong support for Koenigia as sister to Bistorta, and together they were sister to the robustly supported Persicaria. Based on the phylogenetic and morphological evidence, we recognize five sections in Persicaria: Persicaria, Amphibia, Tovara, Echinocaulon, and Cephalophilon. It is estimated that the divergence of the Persicarieae began around the late Paleocene, with diversification concentrated in the Eocene and Miocene. In addition, it is suggested that the increasing westerly and monsoon winds in conjunction with the uplift of the QTP may be the driving force for origin and diversification of Persicarieae species. These results provide a valuable evolutionary framework for the study of adaptation in Polygonaceae and insights into plant diversification on the QTP and adjacent areas.

Assessment of the Hormetic Effect of Arsenic on Growth and Physiology of Two Cultivars of Maize (Zea mays L.)

Although growth stimulation at low arsenic doses was observed in several plants, few studies have focused on this phenomenon in more detail. The effects of different concentrations of arsenic (0-50 mg kg-1 of soil: As0-As50) on the growth and selected physiological parameters of two maize cultivars (Zea mays L. cvs. Chapalu and MvNK 333) were tested. Cultivar MvNK 333 manifested a generally higher tolerance to As than cv. Chapalu, which may be related to the lower content of As in the tissues. The highest stimulatory effect of As was recorded at doses of As1 and As2 (cv. Chapalu), and at the As5 dose (MvNK 333), there was an increase in shoot elongation, biomass, and relative water content (RWC), as well as the content of photosynthetic pigments. The stimulatory effect of lower doses of As apparently represents an adaptation mechanism that is associated with water content regulation in the given conditions. The stomata of the studied cultivars were involved in this regulation in different ways. While cv. Chapalu exhibited increased numbers of stomata on both sides of leaves, cv. MvNK 333 instead responded to the given conditions with decreased stomata size. Although hormetic manifestations closely related to changes in stomatal number and size were observed, a typical stomatal hormetic response was not observed in the given range of As doses.

Insight into Recent Progress and Perspectives in Improvement of Antioxidant Machinery upon PGPR Augmentation in Plants under Drought Stress: A Review

Agriculture has a lot of responsibility as the rise in the world’s population demands more food requirements. However, more than one type of biotic and abiotic stress continually impacts agricultural productivity. Drought stress is a major abiotic stress that significantly affects agricultural productivity every year as the plants undergo several morphological, biochemical, and physiological modifications, such as repressed root and shoot growth, reduced photosynthesis and transpiration rate, excessive production of reactive oxygen species (ROS), osmotic adjustments, and modified leaf senescence regulating and stress signaling pathways. Such modifications may permanently damage the plants; therefore, mitigation strategies must be developed. The use of drought resistant crop cultivars is more expensive and labor-intensive with few advantages. However, exploiting plant growth promoting rhizobacteria (PGPR) is a proven alternative with numerous direct and indirect advantages. The PGPR confers induced systemic tolerance (IST) mechanisms in plants in response to drought stress via multiple mechanisms, including the alteration of root architecture, maintenance of high relative water content, improvement of photosynthesis rate, production of phytohormones, exopolysaccharides, ACC deaminase, carotenoids and volatiles, induction of antioxidant defense system, and alteration in stress-responsive gene expression. The commercial application of PGPR as bioinoculants or biostimulants will remain contingent on more robust strain selection and performance under unfavorable environmental conditions. This review highlights the possible mechanisms of PGPR by activating the plant adaptive defense systems for enhancing drought tolerance and improving overall growth and yield.

Assessing the Usefulness of Moringa oleifera Leaf Extract as a Biostimulant to Supplement Synthetic Fertilizers: A Review

The extensive use of synthetic chemical fertilizers is associated with environmental pollution and soil degradation. In addition, the high costs of these fertilizers necessitate the search for alternative, eco-friendly and safe natural sources of phytonutrients. The liquid extracted from moringa (Moringa oleifera Lam.) leaves has been used in agriculture to improve the growth and productivity of several crops. The efficacy of moringa leaf extract (MLE) is attributed to its high content of mineral nutrients, protein, vitamins, sugars, fiber, phenolics and free proline. In addition, MLE contains significant amounts of phytohormones, such as auxins, cytokinins and gibberellins. Furthermore, MLE is a valuable product promoting seed germination, plant growth and deeper root development, delaying fruit senescence and increasing the yield and quality of crops grown under normal or stressful conditions. Here, we review the research on MLE as a biostimulant to enhance crop growth and productivity. Moreover, we emphasize its possible introduction to smallholder farming systems to provide phytonutrients, and we further highlight research gaps in the existing knowledge regarding MLE application. Generally, MLE is an inexpensive, sustainable, eco-friendly and natural biostimulant that can be used to improve the growth and productivity attributes of various crops under non-stressful and stressful conditions.

Combining Physio-Biochemical Characterization and Transcriptome Analysis Reveal the Responses to Varying Degrees of Drought Stress in Brassica napus L

Brassica napus L. has become one of the most important oil-bearing crops, and drought stress severely influences its yield and quality. By combining physio-biochemical characterization and transcriptome analysis, we studied the response of B. napus plants to different degrees of drought stress. Some physio-biochemical traits, such as fresh weight (FW), dry weight (DW), abscisic acid (ABA) content, net photosynthetic rate (Pn), stomatal conductance (g_s), and transpiration rate (Tr), were measured, and the total content of the epidermal wax/cutin, as well as their compositions, was determined. The results suggest that both stomatal transpiration and cuticular transpiration are affected when B. napus plants are subjected to varying degrees of drought stress. A total of 795 up-regulated genes and 1050 down-regulated genes were identified under severe drought stress by transcriptome analysis. Gene ontology (GO) enrichment analysis of differentially expressed genes (DEGs) revealed that the up-regulated genes were mainly enriched in the stress response processes, such as response to water deprivation and abscisic acid, while the down-regulated genes were mainly enriched in the chloroplast-related parts affecting photosynthesis. Moreover, overexpression of BnaA01.CIPK6, an up-regulated DEG, was found to confer drought tolerance in B. napus. Our study lays a foundation for a better understanding of the molecular mechanisms underlying drought tolerance in B. napus.

Killing two birds with a single stone-genetic manipulation of cytokinin oxidase/dehydrogenase (CKX) genes for enhancing crop productivity and amelioration of drought stress response

The development of high-yielding, bio-fortified, stress-tolerant crop cultivars is the need of the hour in the wake of increasing global food insecurity, abrupt climate change, and continuous shrinking of resources and landmass suitable for agriculture. The cytokinin group of phytohormones positively regulates seed yield by simultaneous regulation of source capacity (leaf senescence) and sink strength (grain number and size). Cytokinins also regulate root-shoot architecture by promoting shoot growth and inhibiting root growth. Cytokinin oxidase/dehydrogenase (CKX) are the only enzymes that catalyze the irreversible degradation of active cytokinins and thus negatively regulate the endogenous cytokinin levels. Genetic manipulation of CKX genes is the key to improve seed yield and root-shoot architecture through direct manipulation of endogenous cytokinin levels. Downregulation of CKX genes expressed in sink tissues such as inflorescence meristem and developing seeds, through reverse genetics approaches such as RNAi and CRISPR/Cas9 resulted in increased yield marked by increased number and size of grains. On the other hand, root-specific expression of CKX genes resulted in decreased endogenous cytokinin levels in roots which in turn resulted in increased root growth indicated by increased root branching, root biomass, and root-shoot biomass ratio. Enhanced root growth provided enhanced tolerance to drought stress and improved micronutrient uptake efficiency. In this review, we have emphasized the role of CKX as a genetic factor determining yield, micronutrient uptake efficiency, and response to drought stress. We have summarised the efforts made to increase crop productivity and drought stress tolerance in different crop species through genetic manipulation of CKX family genes.

Stomata-mediated interactions between plants, herbivores, and the environment

Stomata play a central role in plant responses to abiotic and biotic stresses. Existing knowledge regarding the roles of stomata in plant stress is centered on abiotic stresses and plant-pathogen interactions, but how stomata influence plant-herbivore interactions remains largely unclear. Here, we summarize the functions of stomata in plant-insect interactions and highlight recent discoveries of how herbivores manipulate plant stomata. Because stomata are linked to interrelated physiological processes in plants, herbivory-induced changes in stomatal dynamics might have cellular, organismic, and/or even community-level impacts. We summarize our current understanding of how stomata mediate plant responses to herbivory and environmental stimuli, propose how herbivores may influence these responses, and identify key knowledge gaps in plant-herbivore interactions.

Auxin/Cytokinin Antagonistic Control of the Shoot/Root Growth Ratio and Its Relevance for Adaptation to Drought and Nutrient Deficiency Stresses

The hormones auxin and cytokinin regulate numerous aspects of plant development and often act as an antagonistic hormone pair. One of the more striking examples of the auxin/cytokinin antagonism involves regulation of the shoot/root growth ratio in which cytokinin promotes shoot and inhibits root growth, whereas auxin does the opposite. Control of the shoot/root growth ratio is essential for the survival of terrestrial plants because it allows growth adaptations to water and mineral nutrient availability in the soil. Because a decrease in shoot growth combined with an increase in root growth leads to survival under drought stress and nutrient limiting conditions, it was not surprising to find that auxin promotes, while cytokinin reduces, drought stress tolerance and nutrient uptake. Recent data show that drought stress and nutrient availability also alter the cytokinin and auxin signaling and biosynthesis pathways and that this stress-induced regulation affects cytokinin and auxin in the opposite manner. These antagonistic effects of cytokinin and auxin suggested that each hormone directly and negatively regulates biosynthesis or signaling of the other. However, a growing body of evidence supports unidirectional regulation, with auxin emerging as the primary regulatory component. This master regulatory role of auxin may not come as a surprise when viewed from an evolutionary perspective.

Salt tolerance mechanisms in the Lycopersicon clade and their trade-offs

Salt stress impairs growth and yield in tomato, which is mostly cultivated in arid and semi-arid areas of the world. A number of wild tomato relatives (Solanum pimpinellifolium, S. pennellii, S. cheesmaniae and S. peruvianum) are endemic to arid coastal areas and able to withstand higher concentration of soil salt concentrations, making them a good genetic resource for breeding efforts aimed at improving salt tolerance and overall crop improvement. However, the complexity of salt stress response makes it difficult to introgress tolerance traits from wild relatives that could effectively increase tomato productivity under high soil salt concentrations. Under commercial production, biomass accumulation is key for high fruit yields, and salt tolerance management strategies should aim to maintain a favourable plant water and nutrient status. In this review, we first compare the effects of salt stress on the physiology of the domesticated tomato and its wild relatives. We then discuss physiological and energetic trade-offs for the different salt tolerance mechanisms found within the Lycopersicon clade, with a focus on the importance of root traits to sustain crop productivity.

Cytokinin drives assembly of the phyllosphere microbiome and promotes disease resistance through structural and chemical cues

The plant hormone cytokinin (CK) is an important developmental regulator, promoting morphogenesis and delaying differentiation and senescence. From developmental processes, to growth, to stress tolerance, CKs are central in plant life. CKs are also known to mediate plant immunity and disease resistance, and several classes of microbes can also produce CKs, affecting the interaction with their plant hosts. While host species and genotype can be a driving force in shaping the plant microbiome, how plant developmental hormones such as CK can shape the microbiome is largely uninvestigated. Here, we examined the relationship between CK and the phyllosphere microbiome, finding that CK acts as a selective force in microbiome assembly, increasing richness, and promoting the presence of Firmicutes. CK-mediated immunity was found to partially depend on the microbial community, and bacilli isolated from previously described CK-rich plant genotypes, which overexpress a CK biosynthesis gene or have increased CK sensitivity, induced plant immunity, and promoted disease resistance. Using a biomimetic system, we investigated the relationship between the leaf microstructure, which is differentially patterned upon changes in CK content or signaling, and the growth of different phyllosphere microbes. We found that leaf structures derived from CK-rich plant genotypes support bacilli in the biomimetic system. CK was able to promote the growth, swarming, and biofilm formation of immunity inducing bacillus isolates in vitro. Overall, our results indicate that host genotype and hormonal profiles can act as a strong selective force in microbiome assembly, underlying differential immunity profiles, and pathogen resistance as a result.

Insight into the cellular and physiological regulatory modulations of Class-I TCP9 to enhance drought and salinity stress tolerance in cowpea

The Teosinte branched 1/Cycloidea/Proliferating cell factor (TCP) transcription factors are potent growth and developmental regulators in plants, also responsive to various hormonal and environmental stimuli. In this study, we primarily focused on the functional role of TCP9, a nuclear-localised Class-I TCP transcription factor in a drought and heat-tolerant legume crop, cowpea (Vigna unguiculata). Under drought stress, a higher protein expression level of TCP9 was observed in the leaves of the drought-tolerant cowpea cultivar Pusa Komal as compared to the drought-sensitive cultivar TVu-7778. Further, overexpression of VuTCP9 resulted in reduced cell and stomata size, aperture length and width while cell and overall stomatal density in the 35S::VuTCP9 transgenic cowpea lines increased. Phenotypic alterations, such as reduced leaf size and vigour, altered seed coats displaying extension pattern similar to the 'Watson pattern' and delayed senescence were prominent in the transgenic lines. Under normal conditions, the gas exchange and fluorescence measurements indicated reduction in transpiration rate (E), stomatal conductance (g_s ) and photosynthetic efficiency (Φ PSII). However, water usage efficiency (WUE) remained unaltered in the transgenic lines as compared to the wild-type (WT) plants. Furthermore, the transgenic lines displayed higher tolerance to oxidative, drought and salinity stress, maintained relatively higher relative water content and lower occurrence of H₂ O₂ , as compared to the WT plants. Genes related to the jasmonic acid biosynthesis, stomatal development and abiotic stress responsiveness, such as TTG1, NAC25, SPCH and GRP1, increased and LOX2 decreased significantly in the transgenic lines.

Guard-Cell-Specific Expression of Phototropin2 C-Terminal Fragment Enhances Leaf Transpiration

Phototropins (phot1 and phot2) are plant-specific blue light receptors that mediate chloroplast movement, stomatal opening, and phototropism. Phototropin is composed of the N-terminus LOV1 and LOV2 domains and the C-terminus Ser/Thr kinase domain. In previous studies, 35-P2CG transgenic plants expressing the phot2 C-terminal fragment–GFP fusion protein (P2CG) under the control of 35S promoter showed constitutive phot2 responses, including chloroplast avoidance response, stomatal opening, and reduced hypocotyl phototropism regardless of blue light, and some detrimental growth phenotypes. In this study, to exclude the detrimental growth phenotypes caused by the ectopic expression of P2C and to improve leaf transpiration, we used the PHOT2 promoter for the endogenous expression of GFP-fused P2C (GP2C) (P2-GP2C) and the BLUS1 promoter for the guard-cell-specific expression of GP2C (B1-GP2C), respectively. In P2-GP2C plants, GP2C expression induced constitutive phototropin responses and a relatively dwarf phenotype as in 35-P2CG plants. In contrast, B1-GP2C plants showed the guard-cell-specific P2C expression that induced constitutive stomatal opening with normal phototropism, chloroplast movement, and growth phenotype. Interestingly, leaf transpiration was significantly improved in B1-GP2C plants compared to that in P2-GP2C plants and WT. Taken together, this transgenic approach could be applied to improve leaf transpiration in indoor plants.

A simple method for the application of exogenous phytohormones to the grass leaf base protodermal zone to improve grass leaf epidermis development research

BACKGROUND

The leaf epidermis functions to prevent the loss of water and reduce gas exchange. As an interface between the plant and its external environment, it helps prevent damage, making it an attractive system for studying cell fate and development. In monocotyledons, the leaf epidermis grows from the basal meristem that contains protodermal cells. Leaf protoderm zone is covered by the leaf sheath or coleoptile in maize and wheat, preventing traditional exogenous phytohormone application methods, such as directly spraying on the leaf surface or indirectly via culture media, from reaching the protoderm areas directly. The lack of a suitable application method limits research on the effect of phytohormone on the development of grass epidermis.

RESULTS

Here, we describe a direct and straightforward method to apply exogenous phytohormones to the leaf protoderms of maize and wheat. We used the auxin analogs 2,4-D and cytokinin analogs 6-BA to test the system. After 2,4-D treatment, the asymmetrical division events and initial stomata development were decreased, and the subsidiary cells were induced in maize, the number of GMC (guard mother cell), SMC (subsidiary mother cell) and young stomata were increased in wheat, and the size of the epidermal cells increased after 6-BA treatment in maize. Thus, the method is suitable for the application of phytohormone to the grass leaf protodermal areas.

CONCLUSIONS

The method to apply hormones to the mesocotyls of maize and wheat seedlings is simple and direct. Only a small amount of externally applied substances are needed to complete the procedure in this method. The entire experimental process lasts for ten days generally, and it is easy to evaluate the phytohormones' effect on the epidermis development.

Gibberellin in tomato: metabolism, signaling and role in drought responses

The growth-promoting hormone gibberellin (GA) regulates numerous developmental processes throughout the plant life cycle. It also affects plant response to biotic and abiotic stresses. GA metabolism and signaling in tomato (Solanum lycopersicum) have been studied in the last three decades and major components of the pathways were characterized. These include major biosynthesis and catabolism enzymes and signaling components, such as the three GA receptors GIBBERELLIN INSENSITIVE DWARF 1 (GID1) and DELLA protein PROCERA (PRO), the central response suppressor. The role of these components in tomato plant development and response to the environment have been investigated. Cultivated tomato, similar to many other crop plants, are susceptible to water deficiency. Numerous studies on tomato response to drought have been conducted, including the possible role of GA in tomato drought resistance. Most studies showed that reduced levels or activity of GA improves drought tolerance and drought avoidance. This review aims to provide an overview on GA biosynthesis and signaling in tomato, how drought affects these pathways and how changes in GA activity affect tomato plant response to water deficiency. It also presents the potential of using the GA pathway to generate drought-tolerant tomato plants with improved performance under both irrigation and water-limited conditions.

Association of Barley Root Elongation with ABA-Dependent Transport of Cytokinins from Roots and Shoots under Supra-Optimal Concentrations of Nitrates and Phosphates

Changes in root elongation are important for the acquisition of mineral nutrients by plants. Plant hormones, cytokinins, and abscisic acid (ABA) and their interaction are important for the control of root elongation under changes in the availability of ions. However, their role in growth responses to supra-optimal concentrations of nitrates and phosphates has not been sufficiently studied and was addressed in the present research. Effects of supra-optimal concentrations of these ions on root elongation and distribution of cytokinins between roots and shoots were studied in ABA-deficient barley mutant Az34 and its parental variety, Steptoe. Cytokinin concentration in the cells of the growing root tips was analyzed with the help of an immunohistochemical technique. Increased concentrations of nitrates and phosphates led to the accumulation of ABA and cytokinins in the root tips, accompanied by a decline in shoot cytokinin content and inhibition of root elongation in Steptoe. Neither of the effects were detected in Az34, suggesting the importance of the ability of plants to accumulate ABA for the control of these responses. Since cytokinins are known to inhibit root elongation, the effect of supra-optimal concentration of nitrates and phosphates on root growth is likely to be due to the accumulation of cytokinins brought about by ABA-induced inhibition of cytokinin transport from roots to shoots.

Is Plant Life-History of Biseasonal Germination Consistent in Response to Extreme Precipitation?

Future climate is projected to increase in the intensity and frequency of extreme precipitation events, and the resulting ecological consequences are often more serious than those of normal precipitation events. In particular, in desert ecosystems, due to the low frequency and strong fluctuation of extreme precipitation, the destructive consequences for desert plants caused by extreme precipitation have not received enough attention for some time. Based on statistics of extreme precipitation events (1965–2018) in the Gurbantunggut Desert, we investigated the effects of extreme precipitation (+0%, CK; +50%, W1; +100%, W2; +200%, W3; maintenance of field capacity, W4) on the plant life-history of the spring-germinated (SG) and autumn-germinated (AG) ephemeral plant Erodium oxyrhynchum by monitoring seedling emergence, survival, phenology, organ size, biomass accumulation, and allocation. The results showed that extreme precipitation caused about 2.5% seedling emergence of E. oxyrhynchum in autumn 2018 and 3.0% seedling emergence in early spring 2019, which means that most seeds may be stored in the soil or have died. Meanwhile, extreme precipitation significantly improved the survival, organ size, and biomass accumulation of SG and AG plants, and W3 was close to the precipitation threshold of SG (326.70 mm) and AG (560.10 mm) plants corresponding to the maximum individual biomass; thus, AG plants with a longer life cycle need more water for growth. Conversely, W4 caused AG plants to enter the leaf stage in advance and led to death in winter, which indicates that extreme precipitation may not be good for AG plants. Root and reproduction biomass allocation of SG and AG plants showed a significantly opposite trend under extreme precipitation treatments, which might be related to their different life-history strategies. Therefore, when only taking into account the changing trend of extreme precipitation from the Coupled Model Intercomparison Project 6 (CMIP6) climate projections data, we speculate that extreme precipitation may promote the growth of SG and AG plants from the beginning to the middle of this century, but extreme precipitation in autumn exceeding a certain threshold may adversely affect the survival of AG plants at the end of the century.

Plant aquaporins: A frontward to make crop plants drought resistant

Drought stress alters gene expression and causes cellular damage in crop plants. Drought inhibits photosynthesis by reducing the content and the activity of the photosynthetic carbon reduction cycle, ultimately decreasing the crop yield. The role of aquaporins (AQP) in improving the growth and adaptation of crop plants under drought stress is of importance. AQP form channels and control water transport in and out of the cells and are associated with drought tolerance mechanisms. The current review addresses: (1) the evolution of AQPs in plants, (2) the classification of plant AQPs, (3) the role of AQPs in drought alleviation in crop plants, and (4) the phytohormone crosstalk with AQPs in crops exposed to drought stress.

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.

Contrasting Rootstock-Mediated Growth and Yield Responses in Salinized Pepper Plants (Capsicum annuum L.) Are Associated with Changes in the Hormonal Balance

Salinity provokes an imbalance of vegetative to generative growth, thus impairing crop productivity. Unlike breeding strategies, grafting is a direct and quick alternative to improve salinity tolerance in horticultural crops, through rebalancing plant development. Providing that hormones play a key role in plant growth and development and stress responses, we hypothesized that rootstock-mediated reallocation of vegetative growth and yield under salinity was associated with changes in the hormonal balance. To test this hypothesis, the hybrid pepper variety (Capsicum annuum L. “Gacela F1”) was either non-grafted or grafted onto three commercial rootstocks (Creonte, Atlante, and Terrano) and plants were grown in a greenhouse under control (0 mM NaCl) and moderate salinity (35 mM NaCl) conditions. Differential vegetative growth versus fruit yield responses were induced by rootstock and salinity. Atlante strongly increased shoot and root fresh weight with respect to the non-grafted Gacela plants associated with improved photosynthetic rate and K⁺ homeostasis under salinity. The invigorating effect of Atlante can be explained by an efficient balance between cytokinins (CKs) and abscisic acid (ABA). Creonte improved fruit yield and maintained the reproductive to vegetative ratio under salinity as a consequence of its capacity to induce biomass reallocation and to avoid Na⁺ accumulation in the shoot. The physiological responses associated with yield stability in Creonte were mediated by the inverse regulation of CKs and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. Finally, Terrano limited the accumulation of gibberellins in the shoot thus reducing plant height. Despite scion compactness induced by Terrano, both vegetative and reproductive biomass were maintained under salinity through ABA-mediated control of water relations and K⁺ homeostasis. Our data demonstrate that the contrasting developmental and physiological responses induced by the rootstock genotype in salinized pepper plants were critically mediated by hormones. This will be particularly important for rootstock breeding programs to improve salinity tolerance by focusing on hormonal traits.

Biosensors: A Sneak Peek into Plant Cell's Immunity

Biosensors are indispensable tools to understand a plant’s immunity as its spatiotemporal dimension is key in withstanding complex plant immune signaling. The diversity of genetically encoded biosensors in plants is expanding, covering new analytes with ever higher sensitivity and robustness, but their assortment is limited in some respects, such as their use in following biotic stress response, employing more than one biosensor in the same chassis, and their implementation into crops. In this review, we focused on the available biosensors that encompass these aspects. We show that in vivo imaging of calcium and reactive oxygen species is satisfactorily covered with the available genetically encoded biosensors, while on the other hand they are still underrepresented when it comes to imaging of the main three hormonal players in the immune response: salicylic acid, ethylene and jasmonic acid. Following more than one analyte in the same chassis, upon one or more conditions, has so far been possible by using the most advanced genetically encoded biosensors in plants which allow the monitoring of calcium and the two main hormonal pathways involved in plant development, auxin and cytokinin. These kinds of biosensor are also the most evolved in crops. In the last section, we examine the challenges in the use of biosensors and demonstrate some strategies to overcome them.

Comparative proteomics analysis of Tibetan hull-less barley under osmotic stress via data-independent acquisition mass spectrometry

Background

Tibetan hull-less barley (Hordeum vulgare L. var. nudum) is one of the primary crops cultivated in the mountains of Tibet and encounters low temperature, high salinity, and drought. Specifically, drought is one of the major abiotic stresses that affect and limit Tibetan barley growth. Osmotic stress is often simultaneously accompanied by drought conditions. Thus, to improve crop yield, it is critical to explore the molecular mechanism governing the responses of hull-less barley to osmotic/drought stress conditions.

Findings

In this study, we used quantitative proteomics by data-independent acquisition mass spectrometry to investigate protein abundance changes in tolerant (XL) and sensitive (DQ) cultivars. A total of 6,921 proteins were identified and quantified in all samples. Two distinct strategies based on pairwise and time-course comparisons were utilized in the comprehensive analysis of differentially abundant proteins. Further functional analysis of differentially abundant proteins revealed that some hormone metabolism–associated and phytohormone abscisic acid–induced genes are primarily affected by osmotic stress. Enhanced regulation of reactive oxygen species (may promote the tolerance of hull-less barley under osmotic stress. Moreover, we found that some regulators, such as GRF, PR10, MAPK, and AMPK, were centrally positioned in the gene regulatory network, suggesting that they may have a dominant role in the osmotic stress response of Tibetan barley.

Conclusions

Our findings highlight a subset of proteins and processes that are involved in the alleviation of osmotic stress. In addition, this study provides a large-scale and multidimensional proteomic data resource for the further investigation and improvement of osmotic/drought stress tolerance in hull-less barley or other plant species.

Characterization of the cytokinin sensor TCSv2 in arabidopsis and tomato

BACKGROUND

Hormones are crucial to plant life and development. Being able to follow the plants hormonal response to various stimuli and throughout developmental processes is an important and increasingly widespread tool. The phytohormone cytokinin (CK) has crucial roles in the regulation of plant growth and development.

RESULTS

Here we describe a version of the CK sensor Two Component signaling Sensor (TCS), referred to as TCSv2. TCSv2 has a different arrangement of binding motifs when compared to previous TCS versions, resulting in increased sensitivity in some examined tissues. Here, we examine the CK responsiveness and distribution pattern of TCSv2 in arabidopsis and tomato.

CONCLUSIONS

The increased sensitivity and reported expression pattern of TCSv2 make it an ideal TCS version to study CK response in particular hosts, such as tomato, and particular tissues, such as leaves and flowers.

Cytokinin response induces immunity and fungal pathogen resistance, and modulates trafficking of the PRR LeEIX2 in tomato

Plant immunity is often defined by the immunity hormones: salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). These hormones are well known for differentially regulating defence responses against pathogens. In recent years, the involvement of other plant growth hormones such as auxin, gibberellic acid, abscisic acid, and cytokinins (CKs) in biotic stresses has been recognized. Previous reports have indicated that endogenous and exogenous CK treatment can result in pathogen resistance. We show here that CK induces systemic immunity in tomato (Solanum lycopersicum), modulating cellular trafficking of the pattern recognition receptor (PRR) LeEIX2, which mediates immune responses to Xyn11 family xylanases, and promoting resistance to Botrytis cinerea and Oidium neolycopersici in an SA- and ET-dependent mechanism. CK perception within the host underlies its protective effect. Our results support the notion that CK promotes pathogen resistance by inducing immunity in the host.

Basal stomatal aperture is regulated by GA-DELLAs in Arabidopsis

Stomatal aperture is tightly regulated in order to achieve the best compromise between gas exchange and water conservation. Steady-state (basal) stomatal aperture is therefore understandably a key component in plant fitness. It has been shown previously in tomato that DELLA proteins act as positive regulators of closure of stomata, and their action is enhanced by the hormone ABA, which is itself important in mediating drought stress tolerance. DELLAs are regulated by a variety of signals which promote plant growth, most notably the hormones gibberellins, which have been shown to promote stomatal opening. We have found that DELLA proteins are also used in Arabidopsis for regulating basal stomatal aperture. We also discovered that the perception of endogenous gibberellins via the GID1 receptors is necessary for optimal basal stomatal aperture.

Establishment of an Agrobacterium mediated transformation protocol for the detection of cytokinin in the heterophyllous plant Hygrophila difformis (Acanthaceae)

This is the first report of a highly efficient Agrobacterium tumefaciens-mediated transformation protocol for Acanthaceae and its utilization in revealing important roles of cytokinin in regulating heterophylly in Hygrophila difformis. Plants show amazing morphological differences in leaf form in response to changes in the surrounding environment, which is a phenomenon called heterophylly. Previous studies have shown that the aquatic plant Hygrophila difformis (Acanthaceae) is an ideal model for heterophylly study. However, low efficiency and poor reproducibility of genetic transformation restricted H. difformis as a model plant. In this study, we reported successful induction of callus, shoots and the establishment of an efficient stable transformation protocol as mediated by Agrobacterium tumefaciens LBA4404. We found that the highest callus induction efficiency was achieved with 1 mg/L 1-Naphthaleneacetic acid (NAA) and 2 mg/L 6-benzyladenine (6-BA), that efficient shoot induction required 0.1 mg/L NAA and 0.1 mg/L 6-BA and that high transformation efficiency required 100 µM acetosyringone. Due to the importance of phytohormones in the regulation of heterophylly and the inadequate knowledge about the function of cytokinin (CK) in this process, we analyzed the function of CK in the regulation of heterophylly by exogenous CK application and endogenous CK detection. By using our newly developed transformation system to detect CK signals, contents and distribution in H. difformis, we revealed an important role of CK in environmental mediated heterophylly.

Heterologous Expression of a Soybean Gene RR34 Conferred Improved Drought Resistance of Transgenic Arabidopsis

Two-component systems (TCSs) have been identified as participants in mediating plant response to water deficit. Nevertheless, insights of their contribution to plant drought responses and associated regulatory mechanisms remain limited. Herein, a soybean response regulator (RR) gene RR34, which is the potential drought-responsive downstream member of a TCS, was ectopically expressed in the model plant Arabidopsis for the analysis of its biological roles in drought stress response. Results from the survival test revealed outstanding recovery ratios of 52%-53% in the examined transgenic lines compared with 28% of the wild-type plants. Additionally, remarkedly lower water loss rates in detached leaves as well as enhanced antioxidant enzyme activities of catalase and superoxide dismutase were observed in the transgenic group. Further transcriptional analysis of a subset of drought-responsive genes demonstrated higher expression in GmRR34-transgenic plants upon exposure to drought, including abscisic acid (ABA)-related genes NCED3, OST1, ABI5, and RAB18. These ectopic expression lines also displayed hypersensitivity to ABA treatment at germination and post-germination stages. Collectively, these findings indicated the ABA-associated mode of action of GmRR34 in conferring better plant performance under the adverse drought conditions.

Role and Regulation of Cytokinins in Plant Response to Drought Stress

Cytokinins (CKs) are key phytohormones that not only regulate plant growth and development but also mediate plant tolerance to drought stress. Recent advances in genome-wide association studies coupled with in planta characterization have opened new avenues to investigate the drought-responsive expression of CK metabolic and signaling genes, as well as their functions in plant adaptation to drought. Under water deficit, CK signaling has evolved as an inter-cellular communication network which is essential to crosstalk with other types of phytohormones and their regulating pathways in mediating plant stress response. In this review, we revise the current understanding of CK involvement in drought stress tolerance. Particularly, a genetic framework for CK signaling and CK crosstalk with abscisic acid (ABA) in the precise monitoring of drought responses is proposed. In addition, the potential of endogenous CK alteration in crops towards developing drought-tolerant crops is also discussed.

Vascular bundle sheath and mesophyll cells modulate leaf water balance in response to chitin

Plants can detect pathogen invasion by sensing microbe-associated molecular patterns (MAMPs). This sensing process leads to the induction of defense responses. Numerous MAMP mechanisms of action have been described in and outside the guard cells. Here, we describe the effects of chitin, a MAMP found in fungal cell walls and insects, on the cellular osmotic water permeability (P_f ) of the leaf vascular bundle-sheath (BS) and mesophyll cells (MCs), and its subsequent effect on leaf hydraulic conductance (K_leaf ). BS is a parenchymatic tissue that tightly encases the vascular system. BS cells (BSCs) have been shown to influence K_leaf through changes in their P_f , for example, after sensing the abiotic stress response-regulating hormone abscisic acid. It was recently reported that, in Arabidopsis, the chitin receptors-like kinases, chitin elicitor receptor kinase 1 (CERK1) and LYSINE MOTIF RECEPTOR KINASE 5 (LYK5) are highly expressed in the BS as well as the neighboring mesophyll. Therefore, we studied the possible impact of chitin on these cells. Our results revealed that BSCs and MCs exhibit a sharp decrease in P_f in response to chitin treatment. In addition, xylem-fed chitin decreased K_leaf and led to stomatal closure. However, Atlyk5 mutant showed none of these responses. Complementing AtLYK5 in the BSCs (using the SCARECROW promoter) resulted in the response to chitin that was similar to that observed in the wild-type. These results suggest that BS play a role in the perception of apoplastic chitin and in initiating chitin-triggered immunity.

Potassium Alleviates Post-anthesis Photosynthetic Reductions in Winter Wheat Caused by Waterlogging at the Stem Elongation Stage

Waterlogging occurs frequently at the stem elongation stage of wheat in southern China, decreasing post-anthesis photosynthetic rates and constraining grain filling. This phenomenon, and the mitigating effect of nutrient application, should be investigated as it could lead to improved agronomic guidelines. We exposed pot-cultured wheat plants at the stem elongation stage to waterlogging treatment in combination with two rates of potassium (K) application. Waterlogging treatment resulted in grain yield losses, which we attributed to a reduction in the 1,000-grain weight caused by an early decline in the net photosynthetic rate (Pn) post-anthesis. These decreases were offset by increasing K application. Stomatal conductance (G _s) and the intercellular CO₂ concentration (C _i) decreased in the period 7-21 days after anthesis (DAA), and these reductions were exacerbated by waterlogging. However, in the period 21-28 DAA, G _s and C _i increased, while Pn decreased continuously, suggesting that non-stomatal factors constrained photosynthesis. On DAA 21, Pn was reduced by waterlogging, but photochemical efficiency (Φ _PSII ) remained unchanged, indicating a reduction in the dissipation of energy captured by photosystem II (PSII) through the CO₂ assimilation pathway. This reduction in energy dissipation increased the risk of photodamage, as shown by early reductions in Φ _PSII in waterlogged plants on DAA 28. However, increased K application promoted root growth and nutrient status under waterlogging, thereby improving photosynthesis post-anthesis. In conclusion, the decrease in Pn caused by waterlogging was attributable to stomatal closure during early senescence; during later senescence, a reduction in CO₂ assimilation accounted for the reduced Pn and elevated the risk of photodamage. However, K application mitigated waterlogging-accelerated photosynthetic reductions and reduced yield losses.

Ectopic Expression of Glycine maxGmNAC109 Enhances Drought Tolerance and ABA Sensitivity in Arabidopsis

The NAC (NAM, ATAF1/2, CUC2) transcription factors are widely known for their various functions in plant development and stress tolerance. Previous studies have demonstrated that genetic engineering can be applied to enhance drought tolerance via overexpression/ectopic expression of NAC genes. In the present study, the dehydration- and drought-inducible GmNAC109 from Glycine max was ectopically expressed in Arabidopsis (GmNAC109-EX) plants to study its biological functions in mediating plant adaptation to water deficit conditions. Results revealed an improved drought tolerance in the transgenic plants, which displayed greater recovery rates by 20% to 54% than did the wild-type plants. In support of this finding, GmNAC109-EX plants exhibited lower water loss rates and decreased endogenous hydrogen peroxide production in leaf tissues under drought, as well as higher sensitivity to exogenous abscisic acid (ABA) treatment at germination and early seedling development stages. In addition, analyses of antioxidant enzymes indicated that GmNAC109-EX plants possessed stronger activities of superoxide dismutase and catalase under drought stress. These results together demonstrated that GmNAC109 acts as a positive transcriptional regulator in the ABA-signaling pathway, enabling plants to cope with adverse water deficit conditions.

Do Cytokinins and Strigolactones Crosstalk during Drought Adaptation?

Recent reports have demonstrated that cytokinins (CKs) and strigolactones (SLs) act as negative and positive regulators, respectively, in plant drought responses. These reports have also suggested potential crosstalk between CK and SL signaling pathways in several mechanisms underlying plant drought acclimation. We discuss these reports with a view to potentially exploiting this crosstalk in the design of drought-tolerant crops.

Distinctive phytohormonal and metabolic profiles of Arabidopsis thaliana and Eutrema salsugineum under similar soil drying

Arabidopsis and Eutrema show similar stomatal sensitivity to drying soil. In Arabidopsis, larger metabolic adjustments than in Eutrema occurred, with considerable differences in the phytohormonal responses of the two species. Although plants respond to soil drying via a series of concurrent physiological and molecular events, drought tolerance differs greatly within the plant kingdom. While Eutrema salsugineum (formerly Thellungiella salsuginea) is regarded as more stress tolerant than its close relative Arabidopsis thaliana, their responses to soil water deficit have not previously been directly compared. To ensure a similar rate of soil drying for the two species, daily soil water depletion was controlled to 5-10% of the soil water content. While partial stomatal closure occurred earlier in Arabidopsis (Day 4) than Eutrema (from Day 6 onwards), thereafter both species showed similar stomatal sensitivity to drying soil. However, both targeted and untargeted metabolite analysis revealed greater response to drought in Arabidopsis than Eutrema. Early peaks in foliar phytohormone concentrations and different sugar profiles between species were accompanied by opposing patterns in the bioactive cytokinin profiles. Untargeted analysis showed greater metabolic adjustment in Arabidopsis with more statistically significant changes in both early and severe drought stress. The distinct metabolic responses of each species during early drought, which occurred prior to leaf water status declining, seemed independent of later stomatal closure in response to drought. The two species also showed distinct water usage, with earlier reduction in water consumption in Eutrema (Day 3) than Arabidopsis (Day 6), likely reflecting temporal differences in growth responses. We propose Arabidopsis as a promising model to evaluate the mechanisms responsible for stress-induced growth inhibition under the mild/moderate soil drying that crop plants are typically exposed to.

Multifaceted activity of cytokinin in leaf development shapes its size and structure in Arabidopsis

The phytohormone cytokinin has been shown to affect many aspects of plant development ranging from the regulation of the shoot apical meristem to leaf senescence. However, some studies have reported contradictory effects of cytokinin on leaf physiology. Therefore cytokinin treatments cause both chlorosis and increased greening and both lead to decrease or increase in cell size. To elucidate this multifaceted role of cytokinin in leaf development, we have employed a system of temporal controls over the cytokinin pool and investigated the consequences of modulated cytokinin levels in the third leaf of Arabidopsis. We show that, at the cell proliferation phase, cytokinin is needed to maintain cell proliferation by blocking the transition to cell expansion and the onset of photosynthesis. Transcriptome profiling revealed regulation by cytokinin of a gene suite previously shown to affect cell proliferation and expansion and thereby a molecular mechanism by which cytokinin modulates a molecular network underlying the cellular responses. During the cell expansion phase, cytokinin stimulates cell expansion and differentiation. Consequently, a cytokinin excess at the cell expansion phase results in an increased leaf and rosette size fueled by higher cell expansion rate, yielding higher shoot biomass. Proteome profiling revealed the stimulation of primary metabolism by cytokinin, in line with an increased sugar content that is expected to increase turgor pressure, representing the driving force of cell expansion. Therefore, the developmental timing of cytokinin content fluctuations, together with a tight control of primary metabolism, is a key factor mediating transitions from cell proliferation to cell expansion in leaves.

Bioactive molecules derived from smoke and seaweed Ecklonia maxima showing phytohormone-like activity in Spinacia oleracea L

Bioactive compounds such as karrikinolide (KAR1 from smoke) and eckol (from the seaweed Ecklonia maxima) show promising effects on several important crop plants. These plant growth-stimulating organic biomolecules, along with crude extracts (smoke-water and Kelpak® product prepared from Ecklonia maxima), were tested on spinach plants. Eckol sprayed at 10-6 M significantly increased all the growth and biochemical parameters examined compared to control spinach plants. All tested plant growth biostimulants significantly increased total chlorophyll, carotenoids and protein content of spinach leaves. The cytokinin profile of spinach plants was also determined. Cis-zeatin, dihydrozeatin and isopentenyladenine types of cytokinins were promoted by both smoke- and seaweed-based biostimulants. In comparison to the control plants, the level of free sinapic acid was greater in all spinach plants treated with these biostimulants. The application of these biostimulants can help spinach crop by improving growth, yield and nutritional quality; moreover, they are organic and cost-effective.

The ratio of trichomes to stomata is associated with water use efficiency in Solanum lycopersicum (tomato)

Trichomes are specialised structures that originate from the aerial epidermis of plants, and play key roles in the interaction between the plant and the environment. In this study we investigated the trichome phenotypes of four lines selected from the Solanum lycopersicum × Solanum pennellii introgression line (IL) population for differences in trichome density, and their impact on plant performance under water-deficit conditions. We performed comparative analyses at morphological and photosynthetic levels of plants grown under well-watered (WW) and also under water-deficit (WD) conditions in the field. Under WD conditions, we observed higher trichome density in ILs 11-3 and 4-1, and lower stomatal size in IL 4-1 compared with plants grown under WW conditions. The intrinsic water use efficiency (WUEi ) was higher under WD conditions in IL 11-3, and the plant-level water use efficiency (WUEb ) was also higher in IL 11-3 and in M82 for WD plants. The ratio of trichomes to stomata (T/S) was positively correlated with WUEi and WUEb , indicating an important role for both trichomes and stomata in drought tolerance in tomato, and offering a promising way to select for improved water use efficiency of major crops.

Modulation of Asymmetric Division Diversity through Cytokinin and SPEECHLESS Regulatory Interactions in the Arabidopsis Stomatal Lineage

Coordinated growth of organs requires communication among cells within and between tissues. In plants, leaf growth is largely dictated by the epidermis; here, asymmetric and self-renewing divisions of the stomatal lineage create two essential cell types-pavement cells and guard cells-in proportions reflecting inputs from local, systemic, and environmental cues. The transcription factor SPEECHLESS (SPCH) is the prime regulator of divisions, but whether and how it is influenced by external cues to provide flexible development is enigmatic. Here, we show that the phytohormone cytokinin (CK) can act as an endogenous signal to affect the extent and types of stomatal lineage divisions and forms a regulatory circuit with SPCH. Local domains of low CK signaling are created by SPCH-dependent cell-type-specific activity of two repressive type-A ARABIDOPSIS RESPONSE REGULATORs (ARRs), ARR16 and ARR17, and two secreted peptides, CLE9 and CLE10, which, together with SPCH, can customize epidermal cell-type composition.

Hormonal and environmental signals guiding stomatal development

Stomata are pores on plant epidermis that facilitate gas exchange and water evaporation between plants and the environment. Given the central role of stomata in photosynthesis and water-use efficiency, two vital events for plant growth, stomatal development is tightly controlled by a diverse range of signals. A family of peptide hormones regulates stomatal patterning and differentiation. In addition, plant hormones as well as numerous environmental cues influence the decision of whether to make stomata or not in distinct and complex manners. In this review, we summarize recent findings that reveal the mechanism of these three groups of signals in controlling stomatal formation, and discuss how these signals are integrated into the core stomatal development pathway.

Uncovering the contribution of epigenetics to plant phenotypic variation in Mediterranean ecosystems

Epigenetic signals can affect plant phenotype and fitness and be stably inherited across multiple generations. Epigenetic regulation plays a key role in the mechanisms of plant response to the environment, without altering DNA sequence. As plants cannot adapt behaviourally or migrate instantly, such dynamic epigenetic responses may be particularly crucial for survival of plants within changing and challenging environments, such as the Mediterranean-Type Ecosystems (MTEs). These ecosystems suffer recurrent stressful events (warm and dry summers with associated fire regimes) that have selected for plants with similar phenotypic complex traits, resulting in similar vegetation growth forms. However, the potential role of epigenetics in plant adaptation to recurrent stressful environments such as the MTEs has generally been ignored. To understand the full spectrum of adaptive processes in such contexts, it is imperative to prompt study of the causes and consequences of epigenetic variation in natural populations. With this purpose, we review here current knowledge on epigenetic variation in natural populations and the genetic and epigenetic basis of some key traits for plants in the MTEs, namely those traits involved in adaptation to drought, fire and oligotrophic soils. We conclude there is still much to be learned about 'plant epigenetics in the wild' and, thus, we propose future research steps in the study of natural epigenetic variation of key traits in the MTEs at different scales.