Potassium deficiency induces the biosynthesis of oxylipins and glucosinolates in Arabidopsis thaliana

Comparative Transcriptomics of Multi-Stress Responses in Pachycladon cheesemanii and Arabidopsis thaliana

The environment is seldom optimal for plant growth and changes in abiotic and biotic signals, including temperature, water availability, radiation and pests, induce plant responses to optimise survival. The New Zealand native plant species and close relative to Arabidopsis thaliana, Pachycladon cheesemanii, grows under environmental conditions that are unsustainable for many plant species. Here, we compare the responses of both species to different stressors (low temperature, salt and UV-B radiation) to help understand how P. cheesemanii can grow in such harsh environments. The stress transcriptomes were determined and comparative transcriptome and network analyses discovered similar and unique responses within species, and between the two plant species. A number of widely studied plant stress processes were highly conserved in A. thaliana and P. cheesemanii. However, in response to cold stress, Gene Ontology terms related to glycosinolate metabolism were only enriched in P. cheesemanii. Salt stress was associated with alteration of the cuticle and proline biosynthesis in A. thaliana and P. cheesemanii, respectively. Anthocyanin production may be a more important strategy to contribute to the UV-B radiation tolerance in P. cheesemanii. These results allowed us to define broad stress response pathways in A. thaliana and P. cheesemanii and suggested that regulation of glycosinolate, proline and anthocyanin metabolism are strategies that help mitigate environmental stress.

Comparative transcriptome analyses under individual and combined nutrient starvations provide insights into N/P/K interactions in rice

Crops often suffer from simultaneous limitations of multiple nutrients in soils, including nitrogen (N), phosphorus (P) and potassium (K), which are three major macronutrients essential for ensuring growth and yield. Although plant responses to individual N, P, and K deficiency have been well documented, our understanding of the responses to combined nutrient deficiencies and the crosstalk between nutrient starvation responses is still limited. Here, we compared the physiological responses in rice under seven kinds of single and multiple low nutrient stress of N, P and K, and used RNA sequencing approaches to compare their transcriptome changes. A total of 13,000 genes were found to be differentially expressed under all these single and multiple low N/P/K stresses, and 66 and 174 of them were shared by all these stresses in roots and shoots, respectively. Functional enrichment analyses of the DEGs showed that a group of biological and metabolic processes were shared by these low N/P/K stresses. Comparative analyses indicated that DEGs under multiple low nutrient stress was not the simple summation of single nutrient stress. N was found to be the predominant factor affecting the transcriptome under combined nutrient stress. N, P, or K availability exhibited massive influences on the transcriptomic responses to starvation of other nutrients. Many genes involved in nutrient transport, hormone signaling, and transcriptional regulation were commonly responsive to low N/P/K stresses. Some transcription factors were predicted to regulate the expression of genes that are commonly responsive to N, P, and K starvations. These results revealed the interactions between N, P, and K starvation responses, and will be helpful for further elucidation of the molecular mechanisms underlying nutrient interactions.

Soil variation among natural habitats alters glucosinolate content in a wild perennial mustard

Baseline levels of glucosinolates-important defensive phytochemicals in brassicaceous plants-are determined by both genotype and environment. However, the ecological causes of glucosinolate plasticity are not well characterized. Fertilization is known to alter glucosinolate content of Brassica crops, but the effect of naturally occurring soil variation on glucosinolate content of wild plants is unknown. Here, we conducted greenhouse experiments using Boechera stricta to ask (i) whether soil variation among natural habitats shapes leaf and root glucosinolate profiles; (ii) whether such changes are caused by abiotic soil properties, soil microbes, or both; and (iii) whether soil-induced glucosinolate plasticity is genetically variable. Total glucosinolate quantity differed up to 2-fold between soils from different natural habitats, while the relative amounts of different compounds were less responsive. This effect was due to physico-chemical soil properties rather than microbial communities. We detected modest genetic variation for glucosinolate plasticity in response to soil. In addition, glucosinolate composition, but not quantity, of field-grown plants could be accurately predicted from measurements from greenhouse-grown plants. In summary, soil alone is sufficient to cause plasticity of baseline glucosinolate levels in natural plant populations, which may have implications for the evolution of this important trait across complex landscapes.

Glycine max (L.) Merr. (Soybean) metabolome responses to potassium availability

Potassium (K⁺) has vital physiological and metabolic functions in plants and its availability can impact tolerance to biotic and abiotic stress conditions. Limited studies have investigated the effect of K⁺ fertilization on soybean metabolism. Using integrated omics, ionomics and metabolomics, we investigated the field-grown Glycine max (soybean) response, after four K⁺ soil fertilization rates. Soybean leaf and pod tissue (valves and immature seeds) extracts were analysed by ultra-performance liquid chromatography coupled to high-resolution mass spectrometry (UPLC-HRMS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Multivariate analyses (PCA-X&Y e O2PLS-DA) showed that 51 compounds of 19 metabolic pathways were regulated in response to K⁺ availability. Under very low potassium availability, soybean plants accumulated of Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺, and B in young and old leaves. Potassium fertilization upregulated carbohydrate, galactolipid, and flavonol glycoside biosynthesis in leaves and pod valves, while K⁺ deficient pod tissues showed increasing amino acids, oligosaccharides, benzoic acid derivatives, and isoflavones contents. Severely K⁺ deficient soils elicited isoflavones, coumestans, pterocarpans, and soyasaponins in trifoliate leaves, likely associated to oxidative and photodynamic stress status. Additionally, results demonstrate that L-asparagine content is higher in potassium deficient tissues, suggesting this compound as a biomarker of K⁺ deficiency in soybean plants. These results demonstrate that potassium soil fertilization did not linearly contribute to changes in specialised constitutive metabolites of soybean. Altogether, this work provides a reference for improving the understanding of soybean metabolism as dependent on K⁺ availability.

Does Potassium (K+) Contribute to High-Nitrate (NO3-) Weakening of a Plant's Defense System against Necrotrophic Fungi?

In this opinion article, we have analyzed the relevancy of a hypothesis which is based on the idea that in Arabidopsis thaliana jasmonic acid, a (JA)-mediated defense system against necrotrophic fungi is weakened when NO₃^- supply is high. Such a hypothesis is based on the fact that when NO₃^- supply is high, it induces an increase in the amount of bioactive ABA which induces the sequestration of the phosphatase ABI2 (PP2C) into the PYR/PYL/RCAR receptor. Consequently, the Ca sensors CBL1/9-CIPK23 are not dephosphorylated by ABI2, thus remaining able to phosphorylate targets such as AtNPF6.3 and AtKAT1, which are NO₃^- and K⁺ transporters, respectively. Therefore, the impact of phosphorylation on the regulation of these two transporters, could (1) reduce NO₃^- influx as in its phosphorylated state AtNPF6.3 shifts to low capacity state and (2) increase K⁺ influx, as in its phosphorylated state KAT1 becomes more active. It is also well known that in roots, K⁺ loading in the xylem and its transport to the shoot is activated in the presence of NO₃^-. As such, the enrichment of plant tissues in K⁺ can impair a jasmonic acid (JA) regulatory pathway and the induction of the corresponding biomarkers. The latter are known to be up-regulated under K⁺ deficiency and inhibited when K⁺ is resupplied. We therefore suggest that increased K⁺ uptake and tissue content induced by high NO₃^- supply modifies the JA regulatory pathway, resulting in a weakened JA-mediated plant's defense system against necrotrophic fungi.

Glutamic Acid and Poly-γ-glutamic Acid Enhanced the Heat Resistance of Chinese Cabbage (Brassica rapa L. ssp. pekinensis) by Improving Carotenoid Biosynthesis, Photosynthesis, and ROS Signaling

Heat stress is one of the most common agrometeorological risks in crop production in the middle and lower reaches of the Yangtze River in China. This study aimed to investigate whether glutamic acid (Glu) or poly-γ-glutamic acid (γ-PGA) biostimulants can improve the thermotolerance of a cool-season Chinese cabbage (Brassica rapa L. ssp. pekinensis) crop. Priming with Glu (2.0 mM) or γ-PGA (20 mg·L⁻¹) was conducted at the third leaf stage by applying as daily foliar sprays for 5 days before 5 days of heat stress (45 °C in 16-h light/35 °C in 8-h dark). Coupled with morpho-physiological and biochemical analyses, transcriptomes of Glu or γ-PGA-primed Chinese cabbage under heat stress were examined by RNA-seq analysis. The results showed that the thermotolerance conferred by Glu and γ-PGA priming was associated with the increased parameters of vegetative growth, gas exchange, and chlorophyll fluorescence. Compared with the control, the dry weights of plants treated with Glu and γ-PGA increased by 51.52% and 39.39%, respectively. Glu and γ-PGA application also significantly increased the contents of total chlorophyll by 42.21% and 23.12%, and carotenoid by 32.00% and 24.00%, respectively. In addition, Glu- and γ-PGA-primed plants markedly inhibited the levels of malondialdehyde, electrolyte leakage, and super-oxide anion radical, which was accompanied by enhanced activity levels of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD). Enrichment analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) categories within the differentially expressed genes (DEGs) functional clusters of RNA-seq data indicated that the expression levels of the genes for DNA replication, DNA repair system, linoleic acid metabolism, cysteine and methionine metabolism, glutathione metabolism, purine and pyrimidine metabolism, carotenoid biosynthesis, and plant–pathogen interaction were commonly up-regulated by both Glu and γ-PGA priming. Glu treatment enhanced the expression levels of the genes involved in aliphatic glucosinolate and 2-oxocarboxylic acid, while γ-PGA treatment activated carotenoid cleavage reaction to synthesize abscisic acid. Taken together, both Glu and γ-PGA have great potential for the preadaptation of Chinese cabbage seedlings to heat stress, with Glu being more effective than γ-PGA.

Potassium Deficiency in Rice Aggravates Sarocladium oryzae Infection and Ultimately Leads to Alterations in Endophyte Communities and Suppression of Nutrient Uptake

Sheath rot disease is an emerging fungal disease in rice, whose infection causes severe yield loss. Sarocladium oryzae (S. oryzae) is the major causal agent. Previous study has demonstrated that rice deficiency in potassium (K) aggravates S. oryzae infection. However, the effects of S. oryzae infection on the nutrient-uptake process, endophyte communities, and hormone level of host plant under K-deficiency condition remain unclear, the mechanism of K mediated S. oryzae infection needs to be further study. The present study analyzed alterations in the endophytic community and nutrient-uptake process of host plants through an exogenous inoculation of S. oryzae in pot and hydroponics experiments. S. oryzae infection sharply increased the relative abundance of Ascomycota and decreased the Shannon and Simpson index of the endophytic community. Compared with the K-sufficient rice infected with S. oryzae, K-starved rice infected with S. oryzae (-K + I) increased the relative abundance of Ascomycota in leaf sheaths by 52.3%. Likewise, the -K + I treatment significantly decreased the Shannon and Simpson indexes by 27.7 and 25.0%, respectively. Sufficient K supply increased the relative abundance of Pseudomonas spp. in the host plant. S. oryzae infection profoundly inhibited the nutrient uptake of the host plant. The accumulation of oleic acid and linoleic acid in diseased rice decreased the biosynthesis of jasmonic acid (JA), and the content of JA was lowest in the -K + I treatment, which suppressed K⁺ uptake. These results emphasize the importance of K in resistance to S. oryzae infection by modulating endophyte community diversity and enhancing the nutrient-uptake capacity of the host plant.

Regulation and integration of plant jasmonate signaling: a comparative view of monocot and dicot

The phytohormone jasmonate plays a pivotal role in various aspects of plant life, including developmental programs and defense against pests and pathogens. A large body of knowledge on jasmonate biosynthesis, signal transduction as well as its functions in diverse plant processes has been gained in the past two decades. In addition, there exists extensive crosstalk between jasmonate pathway and other phytohormone pathways, such as salicylic acid (SA) and gibberellin (GA), in co-regulation of plant immune status, fine-tuning the balance of plant growth and defense, and so on, which were mostly learned from studies in the dicotyledonous model plants Arabidopsis thaliana and tomato but much less in monocot. Interestingly, existing evidence suggests both conservation and functional divergence in terms of core components of jasmonate pathway, its biological functions and signal integration with other phytohormones, between monocot and dicot. In this review, we summarize the current understanding on JA signal initiation, perception and regulation, and highlight the distinctive characteristics in different lineages of plants.

Integrated Physiological and Metabolomic Analyses of the Effect of Potassium Fertilizer on Citrus Fruit Splitting

Potassium (K) nutrition plays a key role in alleviating a variety of peel disorders in tree fruit, but the effect of this nutrient on the physiological and metabolic profiles involved in the fruit splitting of citrus remains unclear. Three levels of K were used to treat citrus 'Ehime Kashi 34' (Citrus Nishinoka × C. Shiranui), a hybrid cultivar with fruit that easily split. The results showed that the roots of the treatment with K fertilizer increased the contents of calcium (Ca²⁺), nitrogen (N), and K in the skin and flesh, the fruit firmness ratio of the peel to the flesh, photosynthetic rate, stomatal conductance, and concentration of intercellular CO₂. In contrast, it decreases the relative chlorophyll index and content of Ca²⁺ in the leaves. Simultaneously, 59 and 13 differentially expressed metabolites (DEMs) were detected in the peel and flesh, respectively, after treatment with K. Of them, five compounds were upregulated, including the synthesis of various amino acids in the peel and the accumulation of various glycoside metabolites in the flesh which were upregulated. The accumulation of levels of gibberellin and glycoside were downregulated. That could be the main reason why potassium alleviates fruit splitting.

Biofertilizers: An ecofriendly technology for nutrient recycling and environmental sustainability

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Agriculture plays an important role in a country's economy. In modern intensive agricultural practices, chemical fertilizers and pesticides are applied on large scale to increase crop production in order to meet the nutritional requirements of the ever-increasing world population. However, rapid urbanization with shrinking agricultural lands, dramatic change in climatic conditions and extensive use of agrochemicals in agricultural practices has been found to cause environmental disturbances and public health hazards affecting food security and sustainability in agriculture. Besides this, agriculture soils are continuously losing their quality and physical properties as well as their chemical (imbalance of nutrients) and biological health due to indiscriminate use of agrochemicals. Plant-associated microbes with their plant growth- promoting traits have enormous potential to solve these challenges and play a crucial role in enhancing plant biomass and crop yield under greenhouse and field conditions. The beneficial mechanisms of plant growth improvement include enhanced availability of nutrients (i.e., N, P, K, Zn and S), phytohormone modulation, biocontrol of phytopathogens and amelioration of biotic and abiotic stresses. This plant-microbe interplay is indispensable for sustainable agriculture and these microbes may perform essential role as an ecological engineer to reduce the use of chemical fertilizers. Various steps involved for production of solid-based or liquid biofertilizer formulation include inoculum preparation, addition of cell protectants such as glycerol, lactose, starch, a good carrier material, proper packaging and best delivery methods. In addition, recent developments of formulation include entrapment/microencapsulation, nano-immobilization of microbial bioinoculants and biofilm-based biofertilizers. Thus, inoculation with beneficial microbes has emerged as an innovative eco-friendly technology to feed global population with available resources. This review critically examines the current state-of-art on use of microbial strains as biofertilizers in different crop systems for sustainable agriculture and in maintaining soil fertility and enhancing crop productivity. It is believed that acquisition of advanced knowledge of plant-PGPR interactions, bioengineering of microbial communities to improve the performance of biofertilizers under field conditions, will help in devising strategies for sustainable, environment-friendly and climate smart agricultural technologies to deliver short and long terms solutions for improving crop productivity to feed the world in a more sustainable manner.

Modern intensive agricultural practices face numerous challenges that pose major threats to global food security. In order to address the nutritional requirements of the ever-increasing world population, chemical fertilizers and pesticides are applied on large scale to increase crop production. However, the injudicious use of agrochemicals has resulted in environmental pollution leading to public health hazards. Moreover, agriculture soils are continuously losing their quality and physical properties as well as their chemical (imbalance of nutrients) and biological health. Plant-associated microbes with their plant growth- promoting traits have enormous potential to solve these challenges and play a crucial role in enhancing plant biomass and crop yield. The beneficial mechanisms of plant growth improvement include enhanced nutrient availability, phytohormone modulation, biocontrol of phytopathogens and amelioration of biotic and abiotic stresses. Solid-based or liquid bioinoculant formulation comprises inoculum preparation, addition of cell protectants such as glycerol, lactose, starch, a good carrier material, proper packaging and best delivery methods. Recent developments of formulation include entrapment/microencapsulation, nano-immobilization of microbial bioinoculants and biofilm-based biofertilizers. This review critically examines the current state-of-art on use of microbial strains as biofertilizers and the important roles performed by these beneficial microbes in maintaining soil fertility and enhancing crop productivity.

Utilization of Microbial Consortia as Biofertilizers and Biopesticides for the Production of Feasible Agricultural Product

Farmers are now facing a reduction in agricultural crop yield, due to the infertility of soils and poor farming. The application of chemical fertilizers distresses soil fertility and also human health. Inappropriate use of chemical fertilizer leads to the rapid decline in production levels in most parts of the world, and hence requires the necessary standards of good cultivation practice. Biofertilizers and biopesticides have been used in recent years by farmers worldwide to preserve natural soil conditions. Biofertilizer, a replacement for chemical fertilizer, is cost-effective and prevents environmental contamination to the atmosphere, and is a source of renewable energy. In contrast to chemical fertilizers, biofertilizers are cost-effective and a source of renewable energy that preserves long-term soil fertility. The use of biofertilizers is, therefore, inevitable to increase the earth's productivity. A low-input scheme is feasible to achieve farm sustainability through the use of biological and organic fertilizers. This study investigates the use of microbial inoculants as biofertilizers to increase crop production.

Metabolomic and Transcriptomic Changes Induced by Potassium Deficiency During Sarocladium oryzae Infection Reveal Insights into Rice Sheath Rot Disease Resistance

Rice sheath rot disease caused by Sarocladium oryzae (S. oryzae) infection is an emerging disease, and infection can cause yield losses of 20-85%. Adequate potassium (K) application is a feasible strategy for rice tolerance to S. oryzae infection. However, little is known about the metabolic mechanisms regulated by K that allow rice to cope better with S. oryzae infection. The present study performed a comparative metabolome and transcriptome analysis of rice with different K nutrition statuses before and upon S. oryzae infection. Sarocladium oryzae infection triggered a hydrogen peroxide (H₂O₂) burst, and K starvation aggravated the accumulation of H₂O₂ in the flag leaf sheath (FLS), which resulted in lipid peroxidation. Likewise, K deficiency altered the lipid homeostasis of the host plants by hyperaccumulation of 1-alkyl-2-acylglycerophosphoethanolamine. K starvation decreased the content of glycoglycerolipids including monogalactosyldiacyglycerol and digalactosyldoacylglycerol during S. oryzae infection, which destroyed the stability of bilayer membranes. In contrast, sufficient K supply increased antioxidant-related transcript expression (for example, the genes related to glutathione-S-transferase biosynthesis were upregulated), which activated the antioxidant systems. Additionally, upon S. oryzae infection, K starvation amplified the negative impacts of S. oryzae infection on flag leaf photosynthetic potential. These results provide new insight into the role of K in alleviating S. oryzae infection. Adequate K supply decreased the negative impacts of sheath rot disease on rice growth by alleviating lipid peroxidation and maintaining lipid homeostasis.

Investigation of glucosinolates in the desert plant Ochradenus baccatus (Brassicales: Resedaceae). Unveiling glucoochradenin, a new arabinosylated glucosinolate

Here we describe the structure elucidation and quantification of six glucosinolates (GSLs) from the roots of the desert plant Ochradenus baccatus, Delile 1813 (family Resedaceae; order Brassicales). The structure elucidation was established on the corresponding enzymatically desulfated derivatives of the native GSLs of the plant. Among these GSLs we describe the previously undescribed 2″-O-(α-L-arabinopyranosyloxy)benzylglucosinolate (1a), for which we propose the name glucoochradenin. The other five glucosinolates (2a-6a) were (2S)-2-hydroxy-2-phenylethylglucosinolate (2a; glucobarbarin), 2″-O-(α-L-rhamnopyranosyloxy)benzylglucosinolate (3a), benzylglucosinolate (4a; glucotropaeolin), indol-3-ylmethylglucosinolate (5a; glucobrassicin) and phenethylglucosinolate (6a; gluconasturtiin), all elucidated as their desulfo-derivatives, 2b-6b respectively). Structures were elucidated by MS and 1D and 2D-NMR techniques, the identity of the arabinose verified by ion chromatography, and the absolute configuration of the sugar units determined by hydrolysis, coupling to cysteine methyl-ester and phenyl isothiocyanate followed by HPLC-MS analysis of the resulted diastereomers. Response factors were generated for desulfo-2″-O-(α-L-arabinopyranosyloxy)benzylglucosinolate and for desulfo-2″-O-(α-L-rhamnopyranosyloxy)benzylglucosinolate and all six GSLs were quantified, indicating that the root of O. baccatus is rich in GSLs (Avg. 61.3 ± 10.0 μmol/g DW and up to 337.2 μmol/g DW).

Sulfur Deprivation Modulates Salicylic Acid Responses via Nonexpressor of Pathogenesis-Related Gene 1 in Arabidopsis thaliana

Mineral nutrients are essential for plant growth and reproduction, yet only a few studies connect the nutritional status to plant innate immunity. The backbone of plant defense response is mainly controlled by two major hormones: salicylic acid (SA) and jasmonic acid (JA). This study investigated changes in the macronutrient concentration (deficiency/excess of nitrogen, phosphorus, potassium, magnesium, and sulfur) on the expression of PR1, a well-characterized marker in the SA-pathway, and PDF1.2 and LOX2 for the JA-pathway, analyzing plants carrying the promoter of each gene fused to GUS as a reporter. After histochemical GUS assays, we determined that PR1 gene was strongly activated in response to sulfur (S) deficiency. Using RT-PCR, we observed that the induction of PR1 depended on the function of Non-expressor of Pathogenesis-Related gene 1 (NPR1) and SA accumulation, as PR1 was not expressed in npr1-1 mutant and NahG plants under S-deprived conditions. Plants treated with different S-concentrations showed that total S-deprivation was required to induce SA-mediated defense responses. Additionally, bioassays revealed that S-deprived plants, induced resistance to the hemibiotrophic pathogen Pseudomonas syringae pv. DC3000 and increase susceptibility to the necrotrophic Botrytis cinerea. In conclusion, we observed a relationship between S and SA/JA-dependent defense mechanisms in Arabidopsis.

A Degeneration Gradient of Poplar Trees Contributes to the Taxonomic, Functional, and Resistome Diversity of Bacterial Communities in Rhizosphere Soils

Bacterial communities associated with roots influence the health and nutrition of the host plant. However, the microbiome discrepancy are not well understood under different healthy conditions. Here, we tested the hypothesis that rhizosphere soil microbial diversity and function varies along a degeneration gradient of poplar, with a focus on plant growth promoting bacteria (PGPB) and antibiotic resistance genes. Comprehensive metagenomic analysis including taxonomic investigation, functional detection, and ARG (antibiotics resistance genes) annotation revealed that available potassium (AK) was correlated with microbial diversity and function. We proposed several microbes, Bradyrhizobium, Sphingomonas, Mesorhizobium, Nocardioides, Variovorax, Gemmatimonadetes, Rhizobacter, Pedosphaera, Candidatus Solibacter, Acidobacterium, and Phenylobacterium, as candidates to reflect the soil fertility and the plant health. The highest abundance of multidrug resistance genes and the four mainly microbial resistance mechanisms (antibiotic efflux, antibiotic target protection, antibiotic target alteration, and antibiotic target replacement) in healthy poplar rhizosphere, corroborated the relationship between soil fertility and microbial activity. This result suggested that healthy rhizosphere soil harbored microbes with a higher capacity and had more complex microbial interaction network to promote plant growing and reduce intracellular levels of antibiotics. Our findings suggested a correlation between the plant degeneration gradient and bacterial communities, and provided insight into the role of high-turnover microbial communities as well as potential PGPB as real-time indicators of forestry soil quality, and demonstrated the inner interaction contributed by the bacterial communities.

Regulation of glucosinolate biosynthesis

Glucosinolates are secondary defense metabolites produced by plants of the order Brassicales, which includes the model species Arabidopsis and many crop species. In the past 13 years, the regulation of glucosinolate synthesis in plants has been intensively studied, with recent research revealing complex molecular mechanisms that connect glucosinolate production with responses to other central pathways. In this review, we discuss how the regulation of glucosinolate biosynthesis is ecologically relevant for plants, how it is controlled by transcription factors, and how this transcriptional machinery interacts with hormonal, environmental, and epigenetic mechanisms. We present the central players in glucosinolate regulation, MYB and basic helix-loop-helix transcription factors, as well as the plant hormone jasmonate, which together with other hormones and environmental signals allow the coordinated and rapid regulation of glucosinolate genes. Furthermore, we highlight the regulatory connections between glucosinolates, auxin, and sulfur metabolism and discuss emerging insights and open questions on the regulation of glucosinolate biosynthesis.

Syringic Acid Alleviates Cesium-Induced Growth Defect in Arabidopsis

Syringic acid, a phenolic compound, serves a variety of beneficial functions in cells. Syringic acid increases in plants in response to cesium, and exogenous application of syringic acid resulted in a significant attenuation of cesium-induced growth defects in Arabidopsis. In addition, cesium or syringic acid application to plants also resulted in increased lignin deposition in interfascicular fibers. To better understand the role of lignin and syringic acid in attenuating cesium-induced growth defects, two mutants for Arabidopsis REDUCED EPIDERMAL FLUORESCENE 4 (REF4) and fourteen laccase mutants, some of which have lower levels of lignin, were evaluated for their response to cesium. These mutants responded differently to cesium stress, compared to control plants, and the application of syringic acid alleviated cesium-induced growth defects in the laccase mutants but not in the ref4 mutants. These findings imply that lignin plays a role in cesium signaling but the attenuation of cesium stress defects by syringic acid is mediated by regulatory components of lignin biosynthesis and not lignin biosynthesis itself. In contrast, syringic acid did not alleviate any low potassium-induced growth defects. Collectively, our findings provide the first established link between lignin and cesium stress via syringic acid in plants.

OsJAZ9 overexpression modulates jasmonic acid biosynthesis and potassium deficiency responses in rice

Enhanced bioactive JA (JA-Ile) accumulation in OsJAZ9 overexpressing rice helps plants tolerate K deficiency. Potassium (K) represents up to 10% of the plant's total dry biomass, and its deficiency makes plants highly susceptible to both abiotic and biotic stresses. K shortage results in the inhibition of root and shoots growth, but the underlying mechanism of this response is unclear. Our RNA-Seq and qPCR analysis suggested leading roles for JA pathway genes under K deficiency in rice. Notably, K deficiency and JA application produced similar phenotypic and transcriptional responses. Here, we integrated molecular, physiological and morphological studies to analyze the role of OsJAZ9 in JA homeostasis and K deficiency responses. We raised OsJAZ9 over-expression, knockdown, transcriptional reporter, translational reporter and C-terminal deleted translational reporter lines in rice to establish the role of JA signaling in K ion homeostasis. JA profiling revealed significantly increased JA-Ile levels in OsJAZ9 OE lines under K deficiency. Furthermore, we established that OsJAZ9 overexpression and knockdown result in K deficiency tolerance and sensitivity, respectively, by modulating various K transporters and root system architecture. Our data provide evidence on the crucial roles of OsJAZ9 for improving K deficiency tolerance in rice by altering JA levels and JA responses.

Potassium deficiency reconfigures sugar export and induces catecholamine accumulation in oil palm leaves

Metabolic effects of potassium (K) deficiency have been described for nearly 70 years but specific effects of low K availability on sugar composition, sugar export rate and its relationship with other leaf metabolites are not very well documented. Having such pieces of information is nevertheless essential to identify metabolic signatures to monitor K fertilization. This is particularly true in oil-producing crop species such as oil palm (Elaeis guineensis), which is strongly K-demanding and involves high sugar dependence for fruit formation because of low carbon use efficiency in lipid synthesis. Here, we used metabolic analyses, measured sugar export rates with ¹³C isotopic labeling and examined the effects of K availability on both leaflet and rachis sugar metabolism in oil palm seedlings. We show that low K leads to a modification of sugar composition mostly in rachis and decreased sucrose and hexose export rates from leaflets. As a result, leaflets contained more starch and induced alternative pathways such as raffinose synthesis, although metabolites of the raffinose pathway remained quantitatively minor. The alteration of glycolysis by low K was compensated for by an increase in alternative sugar phosphate utilization by tyrosine metabolism, resulting in considerable amounts of tyramine and dopamine.

Vitamin E Is Superior to Vitamin C in Delaying Seedling Senescence and Improving Resistance in Arabidopsis Deficient in Macro-Elements

Nitrogen (N), phosphorus (P), and potassium (K) are three essential macro-elements for plant growth and development. Used to improve yield in agricultural production, the excessive use of chemical fertilizers often leads to increased production costs and ecological environmental pollution. Vitamins C and E are antioxidants that play an important role in alleviating abiotic stress. However, there are few studies on alleviating oxidative stress caused by macro-element deficiency. Here, we used Arabidopsis vitamin E synthesis-deficient mutant vte4 and vitamin C synthesis-deficient mutant vtc1 on which exogenous vitamin E and vitamin C, respectively, were applied at the bolting stage. In the deficiency of macro-elements, the Arabidopsis chlorophyll content decreased, malondialdehyde (MDA) content and relative electric conductivity increased, and reactive oxygen species (ROS) accumulated. The mutants vtc1 and vte4 are more severely stressed than the wild-type plants. Adding exogenous vitamin E was found to better alleviate stress than adding vitamin C. Vitamin C barely affected and vitamin E significantly inhibited the synthesis of ethylene (ETH) and jasmonic acid (JA) genes, thereby reducing the accumulation of ETH and JA that alleviated the senescence caused by macro-element deficiency at the later stage of bolting in Arabidopsis. A deficiency of macro-elements also reduced the yield and germination rate of the seeds, which were more apparent in vtc1 and vte4, and adding exogenous vitamin C and vitamin E, respectively, could restore them. This study reported, for the first time, that vitamin E is better than vitamin C in delaying seedling senescence caused by macro-element deficiency in Arabidopsis.

Glucosinolates: Natural Occurrence, Biosynthesis, Accessibility, Isolation, Structures, and Biological Activities

Glucosinolates (GSLs) are secondary plant metabolites abundantly found in plant order Brassicales. GSLs are constituted by an S-β-d-glucopyrano unit anomerically connected to O-sulfated (Z)-thiohydroximate moiety. The side-chain of the O-sulfate thiohydroximate moiety, which is derived from a different amino acid, contributes to the diversity of natural GSL, with more than 130 structures identified and validated to this day. Both the structural diversity of GSL and their biological implication in plants have been biochemically studied. Although chemical syntheses of GSL have been devised to give access to these secondary metabolites, direct extraction from biomass remains the conventional method to isolate natural GSL. While intact GSLs are biologically inactive, various products, including isothiocyanates, nitriles, epithionitriles, and cyanides obtained through their hydrolysis of GSLs, exhibit many different biological activities, among which several therapeutic benefits have been suggested. This article reviews natural occurrence, accessibility via chemical, synthetic biochemical pathways of GSL, and the current methodology of extraction, purification, and characterization. Structural information, including the most recent classification of GSL, and their stability and storage conditions will also be discussed. The biological perspective will also be explored to demonstrate the importance of these prominent metabolites.

Production of low potassium kale with increased glucosinolate content from vertical farming as a novel dietary option for renal dysfunction patients

The production of low potassium vegetables arose out of the dietary needs of patients with renal dysfunction. Attempts have been made to reduce potassium content in vegetables and fruits; however, induced potassium deficiency has often resulted in decreased yields. Here, we investigated a new method of producing low potassium kale and present the characteristics of the resulting produce. By substituting potassium nitrate with calcium nitrate in the nutrient solution 2 weeks before harvesting, the potassium content of kale was reduced by 70% without a deterioration in yield and semblance qualities. Despite no relationships being detected between potassium deficiency and anti-oxidative properties, the total glucosinolate content, an indicator of the anti-cancer effect of cruciferous vegetables, was significantly increased by potassium deficiency in kale. This study demonstrates a novel method of producing low potassium kale for patients with renal failure, without a reduction in yield but with beneficial increase in glucosinolates.

Potassium in Root Growth and Development

Potassium is an essential macronutrient that has been partly overshadowed in root science by nitrogen and phosphorus. The current boom in potassium-related studies coincides with an emerging awareness of its importance in plant growth, metabolic functions, stress tolerance, and efficient agriculture. In this review, we summarized recent progress in understanding the role of K+ in root growth, development of root system architecture, cellular functions, and specific plant responses to K+ shortage. K+ transport is crucial for its physiological role. A wide range of K+ transport proteins has developed during evolution and acquired specific functions in plants. There is evidence linking K+ transport with cell expansion, membrane trafficking, auxin homeostasis, cell signaling, and phloem transport. This places K+ among important general regulatory factors of root growth. K+ is a rather mobile element in soil, so the absence of systemic and localized root growth response has been accepted. However, recent research confirms both systemic and localized growth response in Arabidopsis thaliana and highlights K+ uptake as a crucial mechanism for plant stress response. K+-related regulatory mechanisms, K+ transporters, K+ acquisition efficiency, and phenotyping for selection of K+ efficient plants/cultivars are highlighted in this review.

Roles of Plant-Derived Secondary Metabolites during Interactions with Pathogenic and Beneficial Microbes under Conditions of Environmental Stress

Under natural conditions, plants generate a vast array of secondary metabolites. Several of these accumulate at widely varying levels in the same plant species and are reportedly critical for plant adaptation to abiotic and/or biotic stresses. Some secondary metabolite pathways are required for beneficial interactions with bacterial and fungal microbes and are also regulated by host nutrient availability so that beneficial interactions are enforced. These observations suggest an interplay between host nutrient pathways and the regulation of secondary metabolites that establish beneficial interactions with microbes. In this review, I introduce the roles of tryptophan-derived and phenylpropanoid secondary-metabolite pathways during plant interactions with pathogenic and beneficial microbes and describe how these pathways are regulated by nutrient availability.

The combination of K+ deficiency with other environmental stresses: What is the outcome?

Potassium (K⁺ ) is a macronutrient known for its high mobility and positive charge, which allows efficient and fast control of the electrical balance and osmotic potential in plant cells. Such features allow K⁺ to remarkably contribute to plant stress adaptation. Some agricultural lands are deficient in K⁺ , imposing a stress that reduces crop yield and makes fertilization a common practice. However, individual stress conditions in the field are rare, and crops usually face a combination of different stresses. As plant response to a stress combination cannot always be deduced from individual stress action, it is necessary to gain insights into the specific mechanisms that connect K⁺ homeostasis with other stress effects to improve plant performance in the context of climate change. Surprisingly, plant responses to environmental stresses under a K⁺ -limiting scenario are poorly understood. In the present review, we summarize current knowledge and find substantial gaps regarding specific outcomes of K⁺ deficiency in addition to other environmental stresses. In this regard, combined nutrient deficiencies of K⁺ and other macronutrients are covered in the first part of the review and interactions arising from K⁺ deficiency with salinity, drought and biotic factors in the second part. Information available so far suggests a prominent role of potassium and nitrate transport systems and their regulatory proteins in the response of plants to several stress combinations. Thus, such molecular pathways, which are located at the crossroad between K⁺ homeostasis and environmental stresses, could be considered biotechnological targets in future studies.

Evaluation of potassium solubilizing rhizobacteria (KSR): enhancing K-bioavailability and optimizing K-fertilization of maize plants under Indo-Gangetic Plains of India

Imbalanced potassium (K) fertilization in agricultural fields has led to considerable negative impacts and remains to be the foremost challenge for maize production in India-Gangetic region. Plant growth-promoting rhizobacteria, particularly potassium solubilizing rhizobacteria (KSR), could serve as inoculants and a promising strategy for enhancement of plant absorption of K hence reducing dependency on chemical fertilizers. Maize seeds were microbiolized for 30 min with KSR suspensions. In the present study, the use of chemical fertilizers along with Agrobacterium tumefaciens strain OPVS10 showed pronounced beneficial effect on growth and yield attributes in maize. There was a significant difference among different parameters studied when varying doses of K and KSR strains were applied. Results showed that the combined application of KSR strain OPVS10 with 100% RDK (recommended dose of K) was most effective in modulating growth, physio-biochemical, and yield attributes in maize thus could be regarded as a promising alternative to mineral K-fertilization. Principal component analysis (PCA) revealed that 100-grain weight and grain yield were the most important properties to improve the sustainable growth of maize. Therefore, these KSR strains have different mechanisms for modulating various activities in maize plants. Results suggested that the synergistic application of KSR strain OPVS10 with 100% RDK can be used for optimized breeding, screening, and nutrient assimilation in maize crop. Hence, this eco-friendly approach may be one of the efficient methods for reducing dependency on chemicals, which pose adverse effects on human health directly and indirectly.

Contrasting nutrient-disease relationships: Potassium gradients in barley leaves have opposite effects on two fungal pathogens with different sensitivities to jasmonic acid

Understanding the interactions between mineral nutrition and disease is essential for crop management. Our previous studies with Arabidopsis thaliana demonstrated that potassium (K) deprivation induced the biosynthesis of jasmonic acid (JA) and increased the plant's resistance to herbivorous insects. Here, we addressed the question of how tissue K affects the development of fungal pathogens and whether sensitivity of the pathogens to JA could play a role for the K-disease relationship in barley (Hordeum vulgare cv. Optic). We report that K-deprived barley plants showed increased leaf concentrations of JA and other oxylipins. Furthermore, a natural tip-to-base K-concentration gradient within leaves of K-sufficient plants was quantitatively mirrored by the transcript levels of JA-responsive genes. The local leaf tissue K concentrations affected the development of two economically important fungi in opposite ways, showing a positive correlation with powdery mildew (Blumeria graminis) and a negative correlation with leaf scald (Rhynchosporium commune) disease symptoms. B. graminis induced a JA response in the plant and was sensitive to methyl-JA treatment whereas R. commune initiated no JA response and was JA insensitive. Our study challenges the view that high K generally improves plant health and suggests that JA sensitivity of pathogens could be an important factor in determining the exact K-disease relationship.

Potassium: A Vital Regulator of Plant Responses and Tolerance to Abiotic Stresses

Among the plant nutrients, potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of the plant structure but it also has a regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, and enzyme activation. Several physiological processes depend on K, such as stomatal regulation and photosynthesis. In recent decades, K was found to provide abiotic stress tolerance. Under salt stress, K helps to maintain ion homeostasis and to regulate the osmotic balance. Under drought stress conditions, K regulates stomatal opening and helps plants adapt to water deficits. Many reports support the notion that K enhances antioxidant defense in plants and therefore protects them from oxidative stress under various environmental adversities. In addition, this element provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although considerable progress has been made in understanding K-induced abiotic stress tolerance in plants, the exact molecular mechanisms of these protections are still under investigation. In this review, we summarized the recent literature on the biological functions of K, its uptake, its translocation, and its role in plant abiotic stress tolerance.

Interaction between calcium and potassium modulates elongation rate in cotton fiber cells

Calcium (Ca2+) is necessary for fiber cell development in cotton (Gossypium hirsutum), both as a cell wall structural component and for environmental signaling responses. It is also known that potassium (K+) plays a critical role in cotton fiber cell elongation. However, it is unclear whether Ca2+ integrates its activities with K+ to regulate fiber elongation. Here, we report the novel discovery that Ca2+ deficiency, when integrated with K+ signaling, promotes fiber elongation. Using inductively coupled plasma-mass spectrometry (ICP-MS), we determined dynamic profiles of the ionome in ovules and fibers at different developmental stages, and found that a high accumulation of macro-elements, but not Ca2+, was associated with longer fibers. Using an in vitro ovule culture system, we found that under Ca2+-deficient conditions, sufficient K+ (52 mM) rapidly induced ovule and fiber browning, while reduced K+ (2 or 27 mM) not only suppressed tissue browning but also altered fiber elongation. Reduced K+ also enhanced reactive oxygen species scavenging ability and maintained abscisic acid and jasmonic acid levels, which in turn compensated for Ca2+ deficiency. Ca2+ deficiency combined with reduced K+ (0 mM Ca2+ and 27 mM K+) produced longer fibers in cultured ovules, due to cell wall loosening by phytosulfokine (PSK), expansin (EXP), and xyloglucan endotransglycosylase/hydrolase (XTH), and an increase of the K+ content of fiber cells. Using transgenic cotton, we showed that the CBL-INTERACTING PROTEIN KINASE 6 (GhCIPK6) gene mediates the uptake of K+ under Ca2+-deficient conditions. This study establishes a new link between Ca2+, K+, and fiber elongation.

Increasing carbon availability stimulates growth and secondary metabolites via modulation of phytohormones in winter wheat

Phytohormones play important roles in plant acclimation to changes in environmental conditions. However, their role in whole-plant regulation of growth and secondary metabolite production under increasing atmospheric CO2 concentrations ([CO2]) is uncertain but crucially important for understanding plant responses to abiotic stresses. We grew winter wheat (Triticum aestivum) under three [CO2] (170, 390, and 680 ppm) over 10 weeks, and measured gas exchange, relative growth rate (RGR), soluble sugars, secondary metabolites, and phytohormones including abscisic acid (ABA), auxin (IAA), jasmonic acid (JA), and salicylic acid (SA) at the whole-plant level. Our results show that, at the whole-plant level, RGR positively correlated with IAA but not ABA, and secondary metabolites positively correlated with JA and JA-Ile but not SA. Moreover, soluble sugars positively correlated with IAA and JA but not ABA and SA. We conclude that increasing carbon availability stimulates growth and production of secondary metabolites via up-regulation of auxin and jasmonate levels, probably in response to sugar-mediated signalling. Future low [CO2] studies should address the role of reactive oxygen species (ROS) in leaf ABA and SA biosynthesis, and at the transcriptional level should focus on biosynthetic and, in particular, on responsive genes involved in [CO2]-induced hormonal signalling pathways.

Roles and Transport of Sodium and Potassium in Plants

The two alkali cations Na(+) and K(+) have similar relative abundances in the earth crust but display very different distributions in the biosphere. In all living organisms, K(+) is the major inorganic cation in the cytoplasm, where its concentration (ca. 0.1 M) is usually several times higher than that of Na(+). Accumulation of Na(+) at high concentrations in the cytoplasm results in deleterious effects on cell metabolism, e.g., on photosynthetic activity in plants. Thus, Na(+) is compartmentalized outside the cytoplasm. In plants, it can be accumulated at high concentrations in vacuoles, where it is used as osmoticum. Na(+) is not an essential element in most plants, except in some halophytes. On the other hand, it can be a beneficial element, by replacing K(+) as vacuolar osmoticum for instance. In contrast, K(+) is an essential element. It is involved in electrical neutralization of inorganic and organic anions and macromolecules, pH homeostasis, control of membrane electrical potential, and the regulation of cell osmotic pressure. Through the latter function in plants, it plays a role in turgor-driven cell and organ movements. It is also involved in the activation of enzymes, protein synthesis, cell metabolism, and photosynthesis. Thus, plant growth requires large quantities of K(+) ions that are taken up by roots from the soil solution, and then distributed throughout the plant. The availability of K(+) ions in the soil solution, slowly released by soil particles and clays, is often limiting for optimal growth in most natural ecosystems. In contrast, due to natural salinity or irrigation with poor quality water, detrimental Na(+) concentrations, toxic for all crop species, are present in many soils, representing 6 % to 10 % of the earth's land area. Three families of ion channels (Shaker, TPK/KCO, and TPC) and 3 families of transporters (HAK, HKT, and CPA) have been identified so far as contributing to K(+) and Na(+) transport across the plasmalemma and internal membranes, with high or low ionic selectivity. In the model plant Arabidopsis thaliana, these families gather at least 70 members. Coordination of the activities of these systems, at the cell and whole plant levels, ensures plant K(+) nutrition, use of Na(+) as a beneficial element, and adaptation to saline conditions.

The Arabidopsis nitrate transporter NPF7.3/NRT1.5 is involved in lateral root development under potassium deprivation

Plants have evolved a large array of transporters and channels that are responsible for uptake, source-to-sink distribution, homeostasis and signaling of nitrate (NO3(-)), which is for most plants the primary nitrogen source and a growth-limiting macronutrient. To optimize NO3(-) uptake in response to changing NO3(-) concentrations in the soil, plants are able to modify their root architecture. Potassium is another macronutrient that influences the root architecture. We recently demonstrated that the Arabidopsis NO3(-) transporter NPF7.3/NRT1.5, which drives root-to-shoot transport of NO3(-), is also involved in root-to-shoot translocation of K(+) under low NO3(-) nutrition. Here, we show that K(+) shortage, but not limiting NO3(-) supply, causes in nrt1.5 mutant plants an altered root architecture with conspicuously reduced lateral root density. Since lateral root development is influenced by auxin, we discuss a possible involvement of NPF7.3/NRT1.5 in auxin homeostasis in roots under K(+) deprivation.

Phosphate Deficiency Induces the Jasmonate Pathway and Enhances Resistance to Insect Herbivory

During their life cycle, plants are typically confronted by simultaneous biotic and abiotic stresses. Low inorganic phosphate (Pi) is one of the most common nutrient deficiencies limiting plant growth in natural and agricultural ecosystems, while insect herbivory accounts for major losses in plant productivity and impacts ecological and evolutionary changes in plant populations. Here, we report that plants experiencing Pi deficiency induce the jasmonic acid (JA) pathway and enhance their defense against insect herbivory. Pi-deficient Arabidopsis (Arabidopsis thaliana) showed enhanced synthesis of JA and the bioactive conjugate JA-isoleucine, as well as activation of the JA signaling pathway, in both shoots and roots of wild-type plants and in shoots of the Pi-deficient mutant pho1 The kinetics of the induction of the JA signaling pathway by Pi deficiency was influenced by PHOSPHATE STARVATION RESPONSE1, the main transcription factor regulating the expression of Pi starvation-induced genes. Phenotypes of the pho1 mutant typically associated with Pi deficiency, such as high shoot anthocyanin levels and poor shoot growth, were significantly attenuated by blocking the JA biosynthesis or signaling pathway. Wounded pho1 leaves hyperaccumulated JA/JA-isoleucine in comparison with the wild type. The pho1 mutant also showed an increased resistance against the generalist herbivore Spodoptera littoralis that was attenuated in JA biosynthesis and signaling mutants. Pi deficiency also triggered increased resistance to S. littoralis in wild-type Arabidopsis as well as tomato (Solanum lycopersicum) and Nicotiana benthamiana, revealing that the link between Pi deficiency and enhanced herbivory resistance is conserved in a diversity of plants, including crops.

Diverse roles of jasmonates and ethylene in abiotic stress tolerance

Jasmonates (JAs) and ethylene (ET), often acting cooperatively, play essential roles in regulating plant defense against pests and pathogens. Recent research reviewed here has revealed mechanistic new insights into the mode of action of these hormones in plant abiotic stress tolerance. During cold stress, JAs and ET differentially regulate the C-repeat binding factor (CBF) pathway. Major JA and ET signaling hubs such as JAZ proteins, CTR1, MYC2, components of the mediator complex, EIN2, EIN3, and several members of the AP2/ERF transcription factor gene family all have complex regulatory roles during abiotic stress adaptation. Better understanding the roles of these phytohormones in plant abiotic stress tolerance will contribute to the development of crop plants tolerant to a wide range of stressful environments.

JAZ Repressors: Potential Involvement in Nutrients Deficiency Response in Rice and Chickpea

Jasmonates (JA) are well-known phytohormones which play important roles in plant development and defense against pathogens. Jasmonate ZIM domain (JAZ) proteins are plant-specific proteins and act as transcriptional repressors of JA-responsive genes. JA regulates both biotic and abiotic stress responses in plants; however, its role in nutrient deficiency responses is very elusive. Although, JA is well-known for root growth inhibition, little is known about behavior of JAZ genes in response to nutrient deficiencies, under which root architectural alteration is an important adaptation. Using protein sequence homology and a conserved-domains approach, here we identify 10 novel JAZ genes from the recently sequenced Chickpea genome, which is one of the most nutrient efficient crops. Both rice and chickpea JAZ genes express in tissue- and stimuli-specific manners. Many of which are preferentially expressed in root. Our analysis further showed differential expression of JAZ genes under macro (NPK) and micronutrients (Zn, Fe) deficiency in rice and chickpea roots. While both rice and chickpea JAZ genes showed a certain level of specificity toward type of nutrient deficiency, generally majority of them showed induction under K deficiency. Generally, JAZ genes showed an induction at early stages of stress and expression declined at later stages of macro-nutrient deficiency. Our results suggest that JAZ genes might play a role in early nutrient deficiency response both in monocot and dicot roots, and information generated here can be further used for understanding the possible roles of JA in root architectural alterations for nutrient deficiency adaptations.

K₂p channels in plants and animals

Two-pore domain potassium (K2P) channels are membrane proteins widely identified in mammals, plants, and other organisms. A functional channel is a dimer with each subunit comprising two pore-forming loops and four transmembrane domains. The genome of the model plant Arabidopsis thaliana harbors five genes coding for K2P channels. Homologs of Arabidopsis K2P channels have been found in all higher plants sequenced so far. As with the K2P channels in mammals, plant K2P channels are targets of external and internal stimuli, which fine-tune the electrical properties of the membrane for specialized transport and/or signaling tasks. Plant K2P channels are modulated by signaling molecules such as intracellular H(+) and calcium and physical factors like temperature and pressure. In this review, we ask the following: What are the similarities and differences between K2P channels in plants and animals in terms of their physiology? What is the nature of the last common ancestor (LCA) of these two groups of proteins? To answer these questions, we present physiological, structural, and phylogenetic evidence that discards the hypothesis proposing that the duplication and fusion that gave rise to the K2P channels occurred in a prokaryote LCA. Conversely, we argue that the K2P LCA was most likely a eukaryote organism. Consideration of plant and animal K2P channels in the same study is novel and likely to stimulate further exchange of ideas between students of these fields.

Will chemical defenses become more effective against specialist herbivores under elevated CO2?

Elevated atmospheric CO2 is known to affect plant-insect herbivore interactions. Elevated CO2 causes leaf nitrogen to decrease, the ostensible cause of herbivore compensatory feeding. CO2 may also affect herbivore consumption by altering chemical defenses via changes in plant hormones. We considered the effects of elevated CO2, in conjunction with soil fertility and damage (simulated herbivory), on glucosinolate concentrations of mustard (Brassica nigra) and collard (B. oleracea var. acephala) and the effects of leaf nitrogen and glucosinolate groups on specialist Pieris rapae consumption. Elevated CO2 affected B. oleracea but not B. nigra glucosinolates; responses to soil fertility and damage were also species-specific. Soil fertility and damage also affected B. oleracea glucosinolates differently under elevated CO2. Glucosinolates did not affect P. rapae consumption at either CO2 concentration in B. nigra, but had CO2-specific effects on consumption in B. oleracea. At ambient CO2, leaf nitrogen had strong effects on glucosinolate concentrations and P. rapae consumption but only gluconasturtiin was a feeding stimulant. At elevated CO2, direct effects of leaf nitrogen were weaker, but glucosinolates had stronger effects on consumption. Gluconasturtiin and aliphatic glucosinolates were feeding stimulants and indole glucosinolates were feeding deterrents. These results do not support the compensatory feeding hypothesis as the sole driver of changes in P. rapae consumption under elevated CO2. Support for hormone-mediated CO2 response (HMCR) was mixed; it explained few treatment effects on constitutive or induced glucosinolates, but did explain patterns in SEMs. Further, the novel feeding deterrent effect of indole glucosinolates under elevated CO2 in B. oleracae underscores the importance of defensive chemistry in CO2 response. We speculate that P. rapae indole glucosinolate detoxification mechanisms may have been overwhelmed under elevated CO2 forcing slowed consumption. Specialists may have to contend with hosts with poorer nutritional quality and more effective chemical defenses under elevated CO2.

Potassium in agriculture--status and perspectives

In this review we summarize factors determining the plant availability of soil potassium (K), the role of K in crop yield formation and product quality, and the dependence of crop stress resistance on K nutrition. Average soil reserves of K are generally large, but most of it is not plant-available. Therefore, crops need to be supplied with soluble K fertilizers, the demand of which is expected to increase significantly, particularly in developing regions of the world. Recent investigations have shown that organic exudates of some bacteria and plant roots play a key role in releasing otherwise unavailable K from K-bearing minerals. Thus, breeding for genotypes that have improved mechanisms to gain access to this fixed K will contribute toward more sustainable agriculture, particularly in cropping systems that do not have access to fertilizer K. In K-deficient crops, the supply of sink organs with photosynthates is impaired, and sugars accumulate in source leaves. This not only affects yield formation, but also quality parameters, for example in wheat, potato and grape. As K has beneficial effects on human health, its concentration in the harvest product is a quality parameter in itself. Owing to its fundamental roles in turgor generation, primary metabolism, and long-distance transport, K plays a prominent role in crop resistance to drought, salinity, high light, or cold as well as resistance to pests and pathogens. Despite the abundance of vital roles of K in crop production, an improvement of K uptake and use efficiency has not been a major focus of conventional or transgenic breeding in the past. In addition, current soil analysis methods for K are insufficient for some common soils, posing the risk of imbalanced fertilization. A stronger prioritization of these areas of research is needed to counter declines in soil fertility and to improve food security.

Going beyond nutrition: regulation of potassium homoeostasis as a common denominator of plant adaptive responses to environment

Partially and fully completed plant genome sequencing projects in both lower and higher plants allow drawing a comprehensive picture of the molecular and structural diversities of plant potassium transporter genes and their encoded proteins. While the early focus of the research in this field was aimed on the structure-function studies and understanding of the molecular mechanisms underlying K(+) transport, availability of Arabidopsis thaliana mutant collections in combination with micro-array techniques have significantly advanced our understanding of K(+) channel physiology, providing novel insights into the transcriptional regulation of potassium homeostasis in plants. More recently, posttranslational regulation of potassium transport systems has moved into the center stage of potassium transport research. The current review is focused on the most exciting developments in this field. By summarizing recent work on potassium transporter regulation we show that potassium transport in general, and potassium channels in particular, represent important targets and are mediators of the cellular responses during different developmental stages in a plant's life cycle. We show that regulation of intracellular K(+) homeostasis is essential to mediate plant adaptive responses to a broad range of abiotic and biotic stresses including drought, salinity, and oxidative stress. We further link post-translational regulation of K(+) channels with programmed cell death and show that K(+) plays a critical role in controlling the latter process. Thus, is appears that K(+) is not just the essential nutrient required to support optimal plant growth and yield but is also an important signaling agent mediating a wide range of plant adaptive responses to environment.

Transport, signaling, and homeostasis of potassium and sodium in plants

Potassium (K⁺) is an essential macronutrient in plants and a lack of K⁺ significantly reduces the potential for plant growth and development. By contrast, sodium (Na⁺), while beneficial to some extent, at high concentrations it disturbs and inhibits various physiological processes and plant growth. Due to their chemical similarities, some functions of K⁺ can be undertaken by Na⁺ but K⁺ homeostasis is severely affected by salt stress, on the other hand. Recent advances have highlighted the fascinating regulatory mechanisms of K⁺ and Na⁺ transport and signaling in plants. This review summarizes three major topics: (i) the transport mechanisms of K⁺ and Na⁺ from the soil to the shoot and to the cellular compartments; (ii) the mechanisms through which plants sense and respond to K⁺ and Na⁺ availability; and (iii) the components involved in maintenance of K⁺/Na⁺ homeostasis in plants under salt stress.

Comparative transcriptome profiling of potassium starvation responsiveness in two contrasting watermelon genotypes

Potassium (K) is one of the essential nutrients for crops, and K⁺ deficiency highly restricts crop yield and quality. Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is an economically important crop that often suffers from K⁺ deficiency. To elucidate the underlying tolerance mechanism of watermelon to K⁺ deficiency and to improve K efficiency of watermelon and other crops in the future, two watermelon genotypes, namely, YS and 8424, that exhibit contrasting K efficiencies were studied to compare their response mechanisms to K⁺ deficiency. YS was more tolerant of K⁺ deficiency and displayed less inhibited root growth than 8424. Roots of YS and 8424 seedlings with or without K⁺ supply were harvested at 6 and 120 h after treatment (HAT), and their transcriptomes were analyzed by Illumina RNA sequencing. Different regulation mechanisms of the root K⁺-uptake genes for short- and long-term stress were observed. Genes involved in jasmonic acid and reactive oxygen species production; Ca²⁺ and receptor-like kinase signaling; lignin biosynthesis; and other stress-related genes were repressed in YS, whereas a large number of such stress-related genes were induced in 8424 at 120 HAT. These results suggested that repressed defense and stress response can save energy for better root growth in YS, which can facilitate K⁺ uptake and increase K efficiency and tolerance to K⁺ deficiency. This study presents the first global root transcriptome in watermelon and provides new insights into the molecular mechanisms underlying tolerance to K⁺ deficiency of K-efficient watermelon genotypes.

Calcium and magnesium elimination enhances accumulation of cardenolides in callus cultures of endemic Digitalis species of Turkey

Elimination of calcium (Ca), magnesium (Mg) or both from the medium of callus cultures of Digitalis davisiana Heywood, Digitalis lamarckii Ivanina, Digitalis trojana Ivanina and Digitalis cariensis Boiss. ex Jaub. et Spach increased cardenolides production. Callus was induced from hypocotyl segments from one-month old seedlings were cultured on MS medium containing 0.5 μg ml(-1) thidiazuron (TDZ) and 0.25 μg ml(-1) indole acetic acid (IAA). After 30 days of culture, callus was transferred in hormone-free MS medium (MSO) as well as Ca or Mg or both were completely eliminated from same medium. The amount of five cardenolides from D. davisiana Heywood, D. lamarckii Ivanina, D. trojana Ivanina and D. cariensis Boiss. ex Jaub. et Spach were compared. Higher amounts of five cardenolides and total cardenolides were obtained when callus of four Digitalis species were incubated on MS medium lacking both Ca and Mg. The mean contents of total cardenolides obtained were in the order of D. lamarckii (2017.97 μg g(-1))>D. trojana (1385.75 μg g(-1))>D. cariensis (1038.65 μg g(-1))>D. davisiana (899.86 μg g(-1)) when both Ca and Mg were eliminated from the medium, respectively. This protocol is useful for development of new strategies for the large-scale production of cardenolides.

The physiological importance of glucosinolates on plant response to abiotic stress in Brassica

Glucosinolates, a class of secondary metabolites, mainly found in Brassicaceae, are affected by the changing environment. This review is focusing on the physiological significance of glucosinolates and their hydrolysis products in the plant response to different abiotic stresses. Special attention is paid to the crosstalk between some of the physiological processes involved in stress response and glucosinolate metabolism, with the resulting connection between both pathways in which signaling mechanisms glucosinolate may act as signals themselves. The function of glucosinolates, further than in defense switching, is discussed in terms of alleviating pathogen attack under abiotic stress. The fact that the exogenous addition of glucosinolate hydrolysis products may alleviate certain stress conditions through its effect on specific proteins is described in light of the recent reports, but the molecular mechanisms involved in this response merit further research. Finally, the transient allocation and re-distribution of glucosinolates as a response to environmental changes is summarized.