Auxin-induced K+ channel expression represents an essential step in coleoptile growth and gravitropism

Research Progress on Plant Shaker K+ Channels

Plant growth and development are driven by intricate processes, with the cell membrane serving as a crucial interface between cells and their external environment. Maintaining balance and signal transduction across the cell membrane is essential for cellular stability and a host of life processes. Ion channels play a critical role in regulating intracellular ion concentrations and potentials. Among these, K+ channels on plant cell membranes are of paramount importance. The research of Shaker K+ channels has become a paradigm in the study of plant ion channels. This study offers a comprehensive overview of advancements in Shaker K+ channels, including insights into protein structure, function, regulatory mechanisms, and research techniques. Investigating Shaker K+ channels has enhanced our understanding of the regulatory mechanisms governing ion absorption and transport in plant cells. This knowledge offers invaluable guidance for enhancing crop yields and improving resistance to environmental stressors. Moreover, an extensive review of research methodologies in Shaker K+ channel studies provides essential reference solutions for researchers, promoting further advancements in ion channel research.

Speedy stomata of a C4 plant correlate with enhanced K+ channel gating

Stomata are microscopic pores at the surface of plant leaves that facilitate gaseous diffusion to support photosynthesis. The guard cells around each stoma regulate the pore aperture. Plants that carry out C4 photosynthesis are usually more resilient than C3 plants to stress, and their stomata operate over a lower dynamic range of CO2 within the leaf. What makes guard cells of C4 plants more responsive than those of C3 plants? We used gas exchange and electrophysiology, comparing stomatal kinetics of the C4 plant Gynandropsis gynandra and the phylogenetically related C3 plant Arabidopsis thaliana. We found, with varying CO2 and light, that Gynandropsis showed faster changes in stomata conductance and greater water use efficiency when compared with Arabidopsis. Electrophysiological analysis of the dominant K+ channels showed that the outward-rectifying channels, responsible for K+ loss during stomatal closing, were characterised by a greater maximum conductance and substantial negative shift in the voltage dependence of gating, indicating a reduced inhibition by extracellular K+ and enhanced capacity for K+ flux. These differences correlated with the accelerated stomata kinetics of Gynandropsis, suggesting that subtle changes in the biophysical properties of a key transporter may prove a target for future efforts to engineer C4 stomatal kinetics.

Identification of a novel and plant height-independent QTL for coleoptile length in barley and validation of its effect using near isogenic lines

KEY MESSAGE

This study reported the identification and validation of novel QTL conferring coleoptile length in barley and predicted candidate genes underlying the largest effect QTL based on orthologous analysis and comparison of the whole genome assemblies for both parental genotypes of the mapping population. Coleoptile length (CL) is one of the most important agronomic traits in cereal crops due to its direct influence on the optimal depth for seed sowing which facilitates better seedling establishment. Varieties with longer coleoptiles are preferred in drought-prone areas where less moisture maintains at the top layer of the soil. Compared to wheat, genetic study on coleoptile length is limited in barley. Here, we reported a study on detecting the genomic regions associated with CL in barley by assessing a population consisting of 201 recombinant inbred lines. Four putative QTL conferring CL were consistently identified on chromosomes 1H, 5H, 6H, and 7H in each of the trials conducted. Of these QTL, the two located on chromosomes 5H and 6H (designated as Qcl.caf-5H and Qcl.caf-6H) are likely novel and Qcl.caf-5H showed the most significant effect explaining up to 30.9% of phenotypic variance with a LOD value of 15.1. To further validate the effect of this putative QTL, five pairs of near isogenic lines (NILs) were then developed and assessed. Analysis of the NILs showed an average difference of 21.0% in CL between the two isolines. Notably, none of the other assessed morphological characteristics showed consistent differences between the two isolines for each pair of the NILs. Candidate genes underlying the Qcl.caf-5H locus were also predicted by employing orthologous analysis and comparing the genome assemblies for both parental genotypes of the mapping population in the present study. Taken together, these findings expand our understanding on genetic basis of CL and will be indicative for further gene cloning and functional analysis underly this locus in barley.

Transcriptome analysis and identification of the low potassium stress-responsive gene SiSnRK2.6 in foxtail millet (Setaria italica L.)

KEY MESSAGE

The transcriptome is beneficial for dissecting the mechanism of millet in response to low potassium stress and SiSnRK2.6 was identified as a potential target for improving low potassium stress tolerance. Foxtail millet (Setaria italica L.), which originated in China, has high nutrient utilization character. Nevertheless, the molecular mechanism of its tolerance to low potassium stress is largely unclear. In this research, the low potassium tolerant variety "Yugu28" was screened out by low potassium stress treatment, and the transcriptome of "Yugu28" under low potassium stress was comprehensively analyzed. A total of 4254 differentially expressed genes (DEGs) were identified, including 1618 up-regulated and 2636 down-regulated genes, respectively. In addition, there were 302 transcription factor (TF) genes in the DEGs and MYB TFs accounted for the highest proportion, which was 14.9%. After functional analysis of all DEGs, a total of 7 genes involved in potassium transport and potassium ion channels and 50 genes corresponding to hormones were screened. The expression levels of randomly selected 17 DEGs were verified by qRT-PCR and the results coincided well with the RNA-seq analysis, indicating the reliability of our transcriptome data. Moreover, one of the ABA signaling pathway genes, SiSnRK2.6, was identified and selected for further functional verification. Compared with the wild type, transgenic rice with ecotopic expression of SiSnRK2.6 showed remarkably increased root length and root number, indicating that overexpression of SiSnRK2.6 can enhance the resistance of transgenic plants to low potassium stress.

Overexpression of KvCHX Enhances Salt Tolerance in Arabidopsis thaliana Seedlings

The CHX (cation/H+ exchanger) family plays an important role in the transmembrane transport of cation/H+ in plants. The aim of this study was to identify and functionally analyze the KvCHX gene in the halophyte Kosteletzkya virginica to investigate its role in regulating the K+/Na+ ratio under salinity tolerance. Based on a partial gene sequence of EST from K. virginica, the full-length DNA sequence of the KvCHX gene was obtained using genome walking technology. Structural analysis and phylogenetic relationship analysis showed that the KvCHX gene was closely related to the AtCHX17 gene. The KvCHX overexpression vector was successfully constructed and transformed into Arabidopsis via floral dipping. Arabidopsis seedlings overexpressing KvCHX showed an enhanced tolerance to salt stress compared with wild-type plants. Transgenic Arabidopsis seedlings grew better under K+ deficiency than WT. The results showed that KvCHX could promote the uptake of K+, increase the ratio of K+/Na+, and promote the growth of plants under K+ deficiency and treatment with NaCl solution. KvCHX is involved in K+ transport and improves plant salt tolerance by coordinating K+ acquisition and homeostasis.

A matter of time: auxin signaling dynamics and the regulation of auxin responses during plant development

As auxin is a major regulator of plant development, studying the signaling mechanisms by which auxin influences cellular activities is of primary importance. In this review, we describe the current knowledge on the different modalities of signaling, from the well-characterized canonical nuclear auxin pathway, to the more recently discovered or re-discovered non-canonical modes of auxin signaling. In particular, we discuss how both the modularity of the nuclear auxin pathway and the dynamic regulation of its core components allow to trigger specific transcriptomic responses. We highlight the fact that the diversity of modes of auxin signaling allows for a wide range of timescales of auxin responses, from second-scale cytoplasmic responses to minute/hour-scale modifications of gene expression. Finally, we question the extent to which the temporality of auxin signaling and responses contributes to development in both the shoot and the root meristems. We conclude by stressing the fact that future investigations should allow to build an integrative view not only of the spatial control, but also of the temporality of auxin-mediated regulation of plant development, from the cell to the whole organism.

Multifractal Analysis of the Influence of Indole-3-Acetic Acid on Fast-Activating Vacuolar (FV) Channels of Beta vulgaris L. Taproot Cells

In this paper, the multifractal properties of the ion current time series in the fast-activating vacuolar (FV) channels of Beta vulgaris L. taproot cells were investigated. These channels are permeable for only monovalent cations and mediate K⁺ at very low concentrations of cytosolic Ca²⁺ and large voltages of either polarity. Using the patch clamp technique, the currents of the FV channels in red beet taproot vacuoles were recorded and analysed by using the multifractal detrended fluctuation analysis (MFDFA) method. The activity of the FV channels depended on the external potential and was sensitive to the auxin. It was also shown that the singularity spectrum of the ion current in the FV channels is non-singular, and the multifractal parameters, i.e., the generalised Hurst exponent and the singularity spectrum, were modified in the presence of IAA. Taking into account the obtained results, it can be suggested that the multifractal properties of fast-activating vacuolar (FV) K⁺ channels, indicating the existence of long-term memory, should be taken into account in the molecular mechanism of the auxin-induced growth of plant cells.

MeCIPK10 regulates the transition of the K+ transport activity of MeAKT2 between low- and high-affinity molds in cassava

AKT1 is an inward-rectifying K⁺ channel that was originally thought to function only within a low-affinity K⁺ concentration range. However, the growth of an akt1 mutant of Arabidopsis was shown to be severely inhibited within a high-affinity range. This suggested that AKT1 may also be a high-affinity K⁺ transporter, but it remains unclear how the two modes of AKT1 coordinate to uptake K⁺. One gene (MeAKT2) encodes for a putatively inward-rectifying K⁺ channel and was isolated from cassava. Relative to other tissues, the MeAKT2 gene was expressed mainly in roots, and its transcriptional level was observed to be significantly increased under low-K⁺ conditions. Functional analyses were performed using a yeast expression system. When MeAKT2 was expressed alone in yeast cells, transgenic yeast could grow only in nutrient media supplied with >0.5 mM potassium. A yeast two-hybrid assay showed that both MeCIPK10 and MeCIPK12 clearly interacted with MeAKT2. Additionally, 0.05 mM K⁺ was sufficient for the growth of yeast cells co-expressing MeAKT2 with MeCIPK10, but also their co-expression significantly enhanced the growth capacity of yeast cells in the low range of K⁺ concentrations. Change in K⁺ uptake rate in co-transgenic yeast cells grown across a wide range of K⁺ concentrations showed that MeAKT2-mediated K⁺ uptake displayed a biphasic pattern, but also the switching from low-to high-affinity K⁺ uptake was regulated by CIPK10. This indicated that MeAKT2 functioned as a dual-affinity transporter to uptake K⁺ under both low- and high-affinity K⁺ conditions.

Structural basis for the activity regulation of a potassium channel AKT1 from Arabidopsis

The voltage-gated potassium channel AKT1 is responsible for primary K⁺ uptake in Arabidopsis roots. AKT1 is functionally activated through phosphorylation and negatively regulated by a potassium channel α-subunit AtKC1. However, the molecular basis for the modulation mechanism remains unclear. Here we report the structures of AKT1, phosphorylated-AKT1, a constitutively-active variant, and AKT1-AtKC1 complex. AKT1 is assembled in 2-fold symmetry at the cytoplasmic domain. Such organization appears to sterically hinder the reorientation of C-linkers during ion permeation. Phosphorylated-AKT1 adopts an alternate 4-fold symmetric conformation at cytoplasmic domain, which indicates conformational changes associated with symmetry switch during channel activation. To corroborate this finding, we perform structure-guided mutagenesis to disrupt the dimeric interface and identify a constitutively-active variant Asp379Ala mediates K⁺ permeation independently of phosphorylation. This variant predominantly adopts a 4-fold symmetric conformation. Furthermore, the AKT1-AtKC1 complex assembles in 2-fold symmetry. Together, our work reveals structural insight into the regulatory mechanism for AKT1.

Arabidopsis thaliana potassium channel AKT1 is responsible for primary K + uptake from soil, which is functionally activated through phosphorylation and negatively regulated by an α-subunit AtKC1. Here, the authors report the structures of AKT1 at different states, revealing a 2- fold to 4-fold symmetry switch at cytoplasmic domain associated with AKT1 activity regulation.

Potassium ion regulates hormone, Ca2+ and H2O2 signal transduction and antioxidant activities to improve salt stress resistance in tobacco

Although improvement of plant salt tolerance by potassium ions (K+) has been widely studied, whether the tolerance is mediated via hormone signaling or antioxidant systems remains to be explored. This study combined plant physiology with transcriptomic techniques to study how K+ interacts with hormones and antioxidant enzymes to improve plant salt tolerance. Tobacco was used as the test material to study the effects of exogenous potassium application on photosynthetic function, hormone signal transduction, and reactive oxygen species (ROS) production under NaCl stress. The study also evaluated the function of the Ca2+ signaling pathway in salt stress tolerance. Transcriptome data showed that 4413 up-regulated genes and 3743 down-regulated genes were found in tobacco leaves treated with NaCl compared with the control. Compared with NaCl, the down-regulated genes in tobacco leaves were significantly reduced under NaCl + KCL treatment. The results showed that NaCl stress caused oxidative damage to tobacco leaves due to increased superoxide anion (O2-) content, superoxide dismutase (SOD) dismutates superoxide anion to produce hydrogen peroxide and the accumulation of H2O2 caused by reduced ascorbate peroxidase (APX) and peroxidase (POD) activities. NaCl stress also increased abscisic acid (ABA) content in tobacco leaves, resulting in stomatal closure and reduced photosynthetic capacity. Transcriptome data showed that 5 SOD, 1 POD, 1 CAT, 5 APX, and 3 GPX genes were significantly down-regulated by the NaCl treatment. Contrarily, NaCl + KCl treatment reduced the accumulation of O2-and SOD activity but increased POD activity, thereby reducing the accumulation of H2O2 and alleviating oxidative damage. The expression of 2 SOD and 3 APX and 2 GPX genes was significantly higher in NaCl + KCl treatment than that in NaCl treatment. Sufficient K+ also increased indole acetic acid (IAA) levels in tobacco leaves under NaCl stress but reduced ABA content, promoting stomatal opening and improving the photosynthetic capacity. In conclusion, K+ can improve plant salt tolerance by alleviating oxidative damage and regulating hormone signal transduction.

The effect of DC electric field on the elongation growth, proton extrusion and membrane potential of Zea mays L. coleoptile cells; a laboratory study

BACKGROUND

In this study, we investigated the effect of an electric field, with an intensity similar to that of the Earth's field, on plant cells growth. The molecular mechanism underlying this effect remains unclear.

RESULTS

It was found that the electric field, depending on the applied voltage, its duration and the polarization of the maize seedlings, stimulated or inhibited the growth of the seedling organs (root, mesocotyl and coleoptile). Moreover, it was also noticed that the gravitropic response of maize seedlings was inhibited at all voltages studied. Simultaneous measurements of growth and external medium pH show that auxin(IAA, indole-3-acetic acid)- and fusicoccin(FC)-induced elongation growth and proton extrusion of maize coleoptile segments were significantly inhibited at higher voltages. The ionic current flowing through the single coleoptile segment during voltage application was 1.7-fold lower in segments treated with cation channel blocker tetraethylammonium chloride (TEA-Cl) and 1.4-fold higher with IAA compared to the control. The electrophysiological experiments show that the electric field caused the depolarization of the membrane potential of parenchymal coleoptile cells, which was not reversible over 120 min.

CONCLUSION

It is suggested that a DC electric field inhibits the plasma membrane H⁺ pump activity and K⁺ uptake through voltage-dependent, inwardly rectifying ZMK1 channels (Zea mays K⁺ channel 1). The data presented here are discussed, taking into account the "acid growth hypothesis" of the auxin action and the mechanism of gravitropic response induction.

Identification and Characterization of Shaker K+ Channel Gene Family in Foxtail Millet (Setaria italica) and Their Role in Stress Response

Potassium (K⁺) is one of the indispensable elements in plant growth and development. The Shaker K⁺ channel protein family is involved in plant K⁺ uptake and distribution. Foxtail millet (Setaria italica), as an important crop, has strong tolerance and adaptability to abiotic stresses. However, no systematic study focused on the Shaker K⁺ channel family in foxtail millet. Here, ten Shaker K⁺ channel genes in foxtail millet were identified and divided into five groups through phylogenetic analysis. Gene structures, chromosome locations, cis-acting regulatory elements in promoter, and post-translation modification sites of Shaker K⁺ channels were analyzed. In silico analysis of transcript level demonstrated that the expression of Shaker K⁺ channel genes was tissue or developmental stage specific. The transcription levels of Shaker K⁺ channel genes in foxtail millet under different abiotic stresses (cold, heat, NaCl, and PEG) and phytohormones (6-BA, BR, MJ, IAA, NAA, GA3, SA, and ABA) treatments at 0, 12, and 24 h were detected by qRT-PCR. The results showed that SiAKT1, SiKAT3, SiGORK, and SiSKOR were worth further research due to their significant responses after most treatments. The yeast complementation assay verified the inward K⁺ transport activities of detectable Shaker K⁺ channels. Finally, we found interactions between SiKAT2 and SiSNARE proteins. Compared to research in Arabidopsis, our results showed a difference in SYP121 related Shaker K⁺ channel regulation mechanism in foxtail millet. Our results indicate that Shaker K⁺ channels play important roles in foxtail millet and provide theoretical support for further exploring the K⁺ absorption mechanism of foxtail millet under abiotic stress.

The molecular mechanism of plasma membrane H+-ATPases in plant responses to abiotic stress

Plasma membrane H⁺-ATPases (PM H⁺-ATPases) are critical proton pumps that export protons from the cytoplasm to the apoplast. The resulting proton gradient and difference in electrical potential energize various secondary active transport events. PM H⁺-ATPases play essential roles in plant growth, development, and stress responses. In this review, we focus on recent studies of the mechanism of PM H⁺-ATPases in response to abiotic stresses in plants, such as salt and high pH, temperature, drought, light, macronutrient deficiency, acidic soil and aluminum stress, as well as heavy metal toxicity. Moreover, we discuss remaining outstanding questions about how PM H⁺-ATPases contribute to abiotic stress responses.

The rectification control and physiological relevance of potassium channel OsAKT2

AKT2 potassium (K+) channels are members of the plant Shaker family which mediate dual-directional K+ transport with weak voltage-dependency. Here we show that OsAKT2 of rice (Oryza sativa) functions mainly as an inward rectifier with strong voltage-dependency and acutely suppressed outward activity. This is attributed to the presence of a unique K191 residue in the S4 domain. The typical bi-directional leak-like property was restored by a single K191R mutation, indicating that this functional distinction is an intrinsic characteristic of OsAKT2. Furthermore, the opposite R195K mutation of AtAKT2 changed the channel to an inward-rectifier similar to OsAKT2. OsAKT2 was modulated by OsCBL1/OsCIPK23, evoking the outward activity and diminishing the inward current. The physiological relevance in relation to the rectification diversity of OsAKT2 was addressed by functional assembly in the Arabidopsis (Arabidopsis thaliana) akt2 mutant. Overexpression (OE) of OsAKT2 complemented the K+ deficiency in the phloem sap and leaves of the mutant plants but did not significantly contribute to the transport of sugars. However, the expression of OsAKT2-K191R overcame both the shortage of phloem K+ and sucrose of the akt2 mutant, which was comparable to the effects of the OE of AtAKT2, while the expression of the inward mutation AtAKT2-R195K resembled the effects of OsAKT2. Additionally, OE of OsAKT2 ameliorated the salt tolerance of Arabidopsis.

Recent Advances in Genome-wide Analyses of Plant Potassium Transporter Families

Plants require potassium (K⁺) as a macronutrient to support numerous physiological processes. Understanding how this nutrient is transported, stored, and utilized within plants is crucial for breeding crops with high K⁺ use efficiency. As K⁺ is not metabolized, cross-membrane transport becomes a rate-limiting step for efficient distribution and utilization in plants. Several K⁺ transporter families, such as KUP/HAK/KT and KEA transporters and Shaker-like and TPK channels, play dominant roles in plant K⁺ transport processes. In this review, we provide a comprehensive contemporary overview of our knowledge about these K⁺ transporter families in angiosperms, with a major focus on the genome-wide identification of K⁺ transporter families, subcellular localization, spatial expression, function and regulation. We also expanded the genome-wide search for the K⁺ transporter genes and examined their tissue-specific expression in Camelina sativa, a polyploid oil-seed crop with a potential to adapt to marginal lands for biofuel purposes and contribution to sustainable agriculture. In addition, we present new insights and emphasis on the study of K⁺ transporters in polyploids in an effort to generate crops with high K⁺ Utilization Efficiency (KUE).

Potassium: A track to develop salinity tolerant plants

Salinity is one of the major constraints to plant growth and development across the globe that leads to the huge crop productivity loss. Salinity stress causes impairment in plant's metabolic and cellular processes including disruption in ionic homeostasis due to excess of sodium (Na⁺) ion influx and potassium (K⁺) efflux. This condition subsequently results in a significant reduction of the cytosolic K⁺ levels, eventually inhibiting plant growth attributes. K⁺ plays a crucial role in alleviating salinity stress by recasting key processes of plants. In addition, K⁺ acquisition and retention also serve as the perquisite trait to establish salt tolerant mechanism. In addition, an intricate network of genes and their regulatory elements are involved in coordinating salinity stress responses. Furthermore, plant growth regulators (PGRs) and other signalling molecules influence K⁺-mediated salinity tolerance in plants. Recently, nanoparticles (NPs) have also been found several implications in plants with respect to their roles in mediating K⁺ homoeostasis during salinity stress in plants. The present review describes salinity-induced adversities in plants and role of K⁺ in mitigating salinity-induced damages. The review also highlights the efficacy of PGRs and other signalling molecules in regulating K⁺ mediated salinity tolerance along with nano-technological perspective for improving K⁺ mediated salinity tolerance in plants.

Fusicoccin (FC)-Induced Rapid Growth, Proton Extrusion and Membrane Potential Changes in Maize (Zea mays L.) Coleoptile Cells: Comparison to Auxin Responses

The fungal toxin fusicoccin (FC) induces rapid cell elongation, proton extrusion and plasma membrane hyperpolarization in maize coleoptile cells. Here, these three parameters were simultaneously measured using non-abraded and non-peeled segments with the incubation medium having access to their lumen. The dose–response curve for the FC-induced growth was sigmoidal shaped with the maximum at 10⁻⁶ M over 10 h. The amplitudes of the rapid growth and proton extrusion were significantly higher for FC than those for indole-3-acetic acid (IAA). The differences between the membrane potential changes that were observed in the presence of FC and IAA relate to the permanent membrane hyperpolarization for FC and transient hyperpolarization for IAA. It was also found that the lag times of the rapid growth, proton extrusion and membrane hyperpolarization were shorter for FC compared to IAA. At 30 °C, the biphasic kinetics of the IAA-induced growth rate could be changed into a monophasic (parabolic) one, which is characteristic for FC-induced rapid growth. It has been suggested that the rates of the initial phase of the FC- and IAA-induced growth involve two common mechanisms that consist of the proton pumps and potassium channels whose contribution to the action of both effectors on the rapid growth is different.

ZMK1 Is Involved in K+ Uptake and Regulated by Protein Kinase ZmCIPK23 in Zea mays

Potassium (K+) is one of essential mineral elements for plant growth and development. K+ channels, especially AKT1-like channels, play crucial roles in K+ uptake in plant roots. Maize is one of important crops; however, the K+ uptake mechanism in maize is little known. Here, we report the physiological functions of K+ channel ZMK1 in K+ uptake and homeostasis in maize. ZMK1 is a homolog of Arabidopsis AKT1 channel in maize, and mainly expressed in maize root. Yeast complementation experiments and electrophysiological characterization in Xenopus oocytes indicated that ZMK1 could mediate K+ uptake. ZMK1 rescued the low-K+-sensitive phenotype of akt1 mutant and enhanced K+ uptake in Arabidopsis. Overexpression of ZMK1 also significantly increased K+ uptake activity in maize, but led to an oversensitive phenotype. Similar to AKT1 regulation, the protein kinase ZmCIPK23 interacted with ZMK1 and phosphorylated the cytosolic region of ZMK1, activating ZMK1-mediated K+ uptake. ZmCIPK23 could also complement the low-K+-sensitive phenotype of Arabidopsis cipk23/lks1 mutant. These findings demonstrate that ZMK1 together with ZmCIPK23 plays important roles in K+ uptake and homeostasis in maize.

Some New Methodological and Conceptual Aspects of the "Acid Growth Theory" for the Auxin Action in Maize (Zea mays L.) Coleoptile Segments: Do Acid- and Auxin-Induced Rapid Growth Differ in Their Mechanisms?

Two arguments against the “acid growth theory” of auxin-induced growth were re-examined. First, the lack of a correlation between the IAA-induced growth and medium acidification, which is mainly due to the cuticle, which is a barrier for proton diffusion. Second, acid- and the IAA-induced growth are additive processes, which means that acid and the IAA act via different mechanisms. Here, growth, medium pH, and membrane potential (in some experiments) were simultaneously measured using non-abraded and non-peeled segments but with the incubation medium having access to their lumen. Using such an approach significantly enhances both the IAA-induced growth and proton extrusion (similar to that of abraded segments). Staining the cuticle on the outer and inner epidermis of the coleoptile segments showed that the cuticle architecture differs on both sides of the segments. The dose-response curves for the IAA-induced growth and proton extrusion were bell-shaped with the maximum at 10⁻⁴ M over 10 h. The kinetics of the IAA-induced hyperpolarisation was similar to that of the rapid phase of the IAA-induced growth. It is also proposed that the K⁺/H⁺ co-transporters are involved in acid-induced growth and that the combined effect of the K⁺ channels and K⁺/ H⁺ co-transporters is responsible for the IAA-induced growth. These findings support the “acid growth theory” of auxin action.

Potassium Control of Plant Functions: Ecological and Agricultural Implications

Potassium, mostly as a cation (K+), together with calcium (Ca2+) are the most abundant inorganic chemicals in plant cellular media, but they are rarely discussed. K+ is not a component of molecular or macromolecular plant structures, thus it is more difficult to link it to concrete metabolic pathways than nitrogen or phosphorus. Over the last two decades, many studies have reported on the role of K+ in several physiological functions, including controlling cellular growth and wood formation, xylem-phloem water content and movement, nutrient and metabolite transport, and stress responses. In this paper, we present an overview of contemporary findings associating K+ with various plant functions, emphasizing plant-mediated responses to environmental abiotic and biotic shifts and stresses by controlling transmembrane potentials and water, nutrient, and metabolite transport. These essential roles of K+ account for its high concentrations in the most active plant organs, such as leaves, and are consistent with the increasing number of ecological and agricultural studies that report K+ as a key element in the function and structure of terrestrial ecosystems, crop production, and global food security. We synthesized these roles from an integrated perspective, considering the metabolic and physiological functions of individual plants and their complex roles in terrestrial ecosystem functions and food security within the current context of ongoing global change. Thus, we provide a bridge between studies of K+ at the plant and ecological levels to ultimately claim that K+ should be considered at least at a level similar to N and P in terrestrial ecological studies.

Regulation of Sixth Seminal Root Formation by Jasmonate in Triticum aestivum L

A well-developed root system is an important characteristic of crop plants, which largely determines their productivity, especially under conditions of water and nutrients deficiency. Being Poaceous, wheat has more than one seminal root. The number of grown seminal roots varies in different wheat accessions and is regulated by environmental factors. Currently, the molecular mechanisms determining the number of germinated seminal roots remain poorly understood. The analysis of the root system development in germinating seeds of genetically modified hexaploid wheat plants with altered activity of jasmonate biosynthesis pathway and seeds exogenously treated with methyl jasmonate revealed the role of jasmonates in the regulation of sixth seminal root development. This regulatory effect strongly depends on the jasmonate concentration and the duration of the exposure to this hormone. The maximum stimulatory effect of exogenously applied methyl jasmonate on the formation of the sixth seminal root was achieved at 200 μM concentration after 48 h of treatment. Further increase in concentration and exposure time does not increase the stimulating effect. While 95% of non-transgenic plants under non-stress conditions possess five or fewer seminal roots, the number of plants with developed sixth seminal root reaches up to 100% when selected transgenic lines are treated with methyl jasmonate.

Potassium and phosphorus transport and signaling in plants

Nitrogen (N), potassium (K), and phosphorus (P) are essential macronutrients for plant growth and development, and their availability affects crop yield. Compared with N, the relatively low availability of K and P in soils limits crop production and thus threatens food security and agricultural sustainability. Improvement of plant nutrient utilization efficiency provides a potential route to overcome the effects of K and P deficiencies. Investigation of the molecular mechanisms underlying how plants sense, absorb, transport, and use K and P is an important prerequisite to improve crop nutrient utilization efficiency. In this review, we summarize current understanding of K and P transport and signaling in plants, mainly taking Arabidopsis thaliana and rice (Oryza sativa) as examples. We also discuss the mechanisms coordinating transport of N and K, as well as P and N.

How to Grow a Tree: Plant Voltage-Dependent Cation Channels in the Spotlight of Evolution

Phylogenetic analysis can be a powerful tool for generating hypotheses regarding the evolution of physiological processes. Here, we provide an updated view of the evolution of the main cation channels in plant electrical signalling: the Shaker family of voltage-gated potassium channels and the two-pore cation (K⁺) channel (TPC1) family. Strikingly, the TPC1 family followed the same conservative evolutionary path as one particular subfamily of Shaker channels (K_out) and remained highly invariant after terrestrialisation, suggesting that electrical signalling was, and remains, key to survival on land. We note that phylogenetic analyses can have pitfalls, which may lead to erroneous conclusions. To avoid these in the future, we suggest guidelines for analyses of ion channel evolution in plants.

The Venus flytrap trigger hair-specific potassium channel KDM1 can reestablish the K+ gradient required for hapto-electric signaling

The carnivorous plant Dionaea muscipula harbors multicellular trigger hairs designed to sense mechanical stimuli upon contact with animal prey. At the base of the trigger hair, mechanosensation is transduced into an all-or-nothing action potential (AP) that spreads all over the trap, ultimately leading to trap closure and prey capture. To reveal the molecular basis for the unique functional repertoire of this mechanoresponsive plant structure, we determined the transcriptome of D. muscipula’s trigger hair. Among the genes that were found to be highly specific to the trigger hair, the Shaker-type channel KDM1 was electrophysiologically characterized as a hyperpolarization- and acid-activated K⁺-selective channel, thus allowing the reuptake of K⁺ ions into the trigger hair’s sensory cells during the hyperpolarization phase of the AP. During trap development, the increased electrical excitability of the trigger hair is associated with the transcriptional induction of KDM1. Conversely, when KDM1 is blocked by Cs⁺ in adult traps, the initiation of APs in response to trigger hair deflection is reduced, and trap closure is suppressed. KDM1 thus plays a dominant role in K⁺ homeostasis in the context of AP and turgor formation underlying the mechanosensation of trigger hair cells and thus D. muscipula’s hapto-electric signaling.

Transcriptomic and electrophysiological studies of the carnivorous Venus flytrap reveal that potassium uptake via a trigger hair-specific potassium channel builds the basis for mechanosensation of likely prey and generation of an action potential that triggers closure of the trap.

Combined Allosteric Responses Explain the Bifurcation in Non-Linear Dynamics of 15N Root Fluxes Under Nutritional Steady-State Conditions for Nitrate

With regard to thermodynamics out of equilibrium, seedlings are open systems that dissipate energy towards their environment. Accordingly, under nutritional steady-state conditions, changes in external concentrations of one single ion provokes instability and reorganization in the metabolic and structure/architecture of the seedling that is more favorable to the fluxes of energy and matter. This reorganization is called a bifurcation and is described in mathematics as a non-linear dynamic system. In this study, we investigate the non-linear dynamics of ¹⁵N fluxes among cellular compartments of B. napus seedlings in response to a wide range of external NO 3 - 15 concentrations (from 0.05 to 20 mM): this allows to determine whether any stationary states and bifurcations could be found. The biphasic behavior of the root NO 3 - 15 uptake rate (v_in ) was explained by the combined cooperative properties between the v_app (N uptake, storage and assimilation rate) and v_out (N translocation rate) ¹⁵N fluxes that revealed a unique and stable stationary state around 0.28 mM nitrate. The disappearance of this stationary state around 0.5 mM external nitrate concentrations provokes a dramatic bifurcation in ¹⁵N flux pattern. This bifurcation in the v_in and v_out ¹⁵N fluxes fits better with the increase of BnNPF6.3/NRT1.1 expression than BnNRT2.1 nitrate transporter genes, confirming the allosteric property of the BnNPF6/NRT1.1 transporter, as reported in the literature between low and high nitrate concentrations. Moreover, several statistically significant power-law equations were found between variations in the shoots tryptophan concentrations (i.e., IAA precursor) with changes in the v_app and v_out ¹⁵N fluxes as well as a synthetic parameter of plant N status estimated from the root/shoot ratio of total free amino acids concentrations. These relationships designate IAA as one of the major biological parameters related to metabolic and structural-morphological reorganization coupled with the N and water fluxes induced by nitrate. The results seriously challenge the scientific grounds of the concept of high- and low-affinity of nitrate transporters and are therefore discussed in terms of the ecological significance and physiological implications on the basis of recent agronomic, physiological and molecular data of the literature.

Genome-wide association mapping reveals novel genes associated with coleoptile length in a worldwide collection of barley

BACKGROUND

Drought is projected to become more frequent and severe in a changing climate, which requires deep sowing of crop seeds to reach soil moisture. Coleoptile length is a key agronomic trait in cereal crops such as barley, as long coleoptiles are linked to drought tolerance and improved seedling establishment under early water-limited growing conditions.

RESULTS

In this study, we detected large genetic variation in a panel of 328 diverse barley (Hordeum vulgare L.) accessions. To understand the overall genetic basis of barley coleoptile length, all accessions were germinated in the dark and phenotyped for coleoptile length after 2 weeks. The investigated barleys had significant variation for coleoptile length. We then conducted genome-wide association studies (GWASs) with more than 30,000 molecular markers and identified 8 genes and 12 intergenic loci significantly associated with coleoptile length in our barley panel. The Squamosa promoter-binding-like protein 3 gene (SPL3) on chromosome 6H was identified as a major candidate gene. The missense variant on the second exon changed serine to alanine in the conserved SBP domain, which likely impacted its DNA-binding activity.

CONCLUSION

This study provides genetic loci for seedling coleoptile length along with candidate genes for future potential incorporation in breeding programmes to enhance early vigour and yield potential in water-limited environments.

Effect of Auxin (IAA) on the Fast Vacuolar (FV) Channels in Red Beet (Beta vulgaris L.) Taproot Vacuoles

In contrast to the well-studied effect of auxin on the plasma membrane K⁺ channel activity, little is known about the role of this hormone in regulating the vacuolar K⁺ channels. Here, the patch-clamp technique was used to investigate the effect of auxin (IAA) on the fast-activating vacuolar (FV) channels. It was found that the macroscopic currents displayed instantaneous currents, which at the positive potentials were about three-fold greater compared to the one at the negative potentials. When auxin was added to the bath solution at a final concentration of 1 µM, it increased the outward currents by about 60%, but did not change the inward currents. The imposition of a ten-fold vacuole-to-cytosol KCl gradient stimulated the efflux of K⁺ from the vacuole into the cytosol and reduced the K⁺ current in the opposite direction. The addition of IAA to the bath solution with the 10/100 KCl gradient decreased the outward current and increased the inward current. Luminal auxin reduced both the outward and inward current by approximately 25% compared to the control. The single channel recordings demonstrated that cytosolic auxin changed the open probability of the FV channels at the positive voltages to a moderate extent, while it significantly increased the amplitudes of the single channel outward currents and the number of open channels. At the positive voltages, auxin did not change the unitary conductance of the single channels. We suggest that auxin regulates the activity of the fast-activating vacuolar (FV) channels, thereby causing changes of the K⁺ fluxes across the vacuolar membrane. This mechanism might serve to tightly adjust the volume of the vacuole during plant cell expansion.

H2O2, Ca2+, and K+ in subsidiary cells of maize leaves are involved in regulatory signaling of stomatal movement

The stomata of maize (Zea mays) contain a pair of guard cells and a pair of subsidiary cells. To determine whether H₂O₂, Ca²⁺, and K⁺ in subsidiary cells were involved in stomatal movement, we treated four-week-old maize (Zhengdan 958) leaves with H₂O₂, diphenylene iodonium (DPI), CaCl₂, and LaCl₃. Changes in content and distribution of H₂O₂, Ca²⁺, and K⁺ during stomatal movement were observed. When exogenous H₂O₂ was applied, Ca²⁺ increased and K⁺ decreased in guard cells, while both ions increased in subsidiary cells, leading to stomatal closure. After DPI treatment, Ca²⁺ decreased and K⁺ increased in guard cells, but both Ca²⁺ and K⁺ decreased in subsidiary cells, resulting in open stomata. Exogenous CaCl₂ increased H₂O₂ and reduced K⁺ in guard cells, while significantly increasing them in subsidiary cells and causing stomatal closure. After LaCl₃ treatment, H₂O₂ decreased and K⁺ increased in guard cells, whereas both decreased in subsidiary cells and stomata became open. Results indicate that H₂O₂ and Ca²⁺ correlate positively with each other and with K⁺ in subsidiary cells during stomatal movement. Both H₂O₂ and Ca²⁺ in subsidiary cells promote an inflow of K⁺, indirectly regulating stomatal closure.

The unexpected concentration-dependent response of periphytic biofilm during indole acetic acid removal

Due to its extensive application in agriculture as a germinating agent and growth promoter, indole acetic acid (IAA) is present in a variety of aquatic ecosystems. To explore the response of microbial aggregates to exogenous IAA in aquatic ecosystems, periphytic biofilm, a typical microbial aggregate, was exposed to IAA at different concentrations. Results reveal an unexpected concentration-dependent effect of IAA on periphytic biofilm. Concentrations of IAA less than 10 mg/L inhibit periphytic growth, but stimulate growth when the IAA concentration exceeds 50 mg/L. Periphytic biofilm adapts to different IAA concentrations by antioxidant enzyme activation, community structure optimization and carbon-metabolism pattern change, and promotes bioremediation of IAA contaminated water in the process. The removal rates of IAA reached up to 95%-100%. This study reveals the capacity of periphytic biofilm for IAA removal in practice.

Phosphatidic acid directly binds with rice potassium channel OsAKT2 to inhibit its activity

The plant Shaker K⁺ channel AtAKT2 has been identified as a weakly rectifying channel that can stabilize membrane potentials to promote photoassimilate phloem loading and translocation. Thus, studies on functional characterization and regulatory mechanisms of AtAKT2-like channels in crops are highly important for improving crop production. Here, we identified the rice OsAKT2 as the ortholog of Arabidopsis AtAKT2, which is primarily expressed in the shoot phloem and localized at the plasma membrane. Using an electrophysiological assay, we found that OsAKT2 operated as a weakly rectifying K⁺ channel, preventing H⁺ /sucrose-symport-induced membrane depolarization. Three critical amino acid residues (K193, N206, and S326) are essential to the phosphorylation-mediated gating change of OsAKT2, consistent with the roles of the corresponding sites in AtAKT2. Disruption of OsAKT2 results in delayed growth of rice seedlings under short-day conditions. Interestingly, the lipid second messenger phosphatidic acid (PA) inhibits OsAKT2-mediated currents (both instantaneous and time-dependent components). Lipid dot-blot assay and liposome-protein binding analysis revealed that PA directly bound with two adjacent arginine residues in the ANK domain of OsAKT2, which is essential to PA-mediated inhibition of OsAKT2. Electrophysiological and phenotypic analyses also showed the PA-mediated inhibition of AtAKT2 and the negative correlation between intrinsic PA level and Arabidopsis growth, suggesting that PA may inhibit AKT2 function to affect plant growth and development. Our results functionally characterize the Shaker K⁺ channel OsAKT2 and reveal a direct link between phospholipid signaling and plant K⁺ channel modulation.

Rapid Auxin-Mediated Cell Expansion

The promotive effect of auxin on shoot cell expansion provided the bioassay used to isolate this central plant hormone nearly a century ago. While the mechanisms underlying auxin perception and signaling to regulate transcription have largely been elucidated, how auxin controls cell expansion is only now attaining molecular-level definition. The good news is that the decades-old acid growth theory invoking plasma membrane H⁺-ATPase activation is still useful. The better news is that a mechanistic framework has emerged, wherein Small Auxin Up RNA (SAUR) proteins regulate protein phosphatases to control H⁺-ATPase activity. In this review, we focus on rapid auxin effects, their relationship to H⁺-ATPase activation and other transporters, and dependence on TIR1/AFB signaling. We also discuss how some observations, such as near-instantaneous effects on ion transport and root growth, do not fit into a single, comprehensive explanation of how auxin controls cell expansion, and where more research is warranted.

A mathematical model explores the contributions of bending and stretching forces to shoot gravitropism in Arabidopsis

Plant shoot gravitropism is a complex phenomenon resulting from gravity sensing, curvature sensing (proprioception), the ability to uphold self-weight and growth. Although recent data analysis and modelling have revealed the detailed morphology of shoot bending, the relative contribution of bending force (derived from the gravi-proprioceptive response) and stretching force (derived from shoot axial growth) behind gravitropism remains poorly understood. To address this gap, we combined morphological data with a theoretical model to analyze shoot bending in wild-type and lazy1-like 1 mutant Arabidopsis thaliana. Using data from actual bending events, we searched for model parameters that minimized discrepancies between the data and mathematical model. The resulting model suggests that both the bending force and the stretching force differ significantly between the wild type and mutant. We discuss the implications of the mechanical forces associated with differential cell growth and present a plausible mechanical explanation of shoot gravitropism.

Posture control in land plants: growth, position sensing, proprioception, balance, and elasticity

The colonization of the atmosphere by land plants was a major evolutionary step. The mechanisms that allow for vertical growth through air and the establishment and control of a stable erect habit are just starting to be understood. A key mechanism was found to be continuous posture control to counterbalance the mechanical and developmental challenges of maintaining a growing upright structure. An interdisciplinary systems biology approach was invaluable in understanding the underlying principles and in designing pertinent experiments. Since this discovery previously held views of gravitropic perception had to be reexamined and this has led to the description of proprioception in plants. In this review, we take a purposefully pedagogical approach to present the dynamics involved from the cellular to whole-plant level. We show how the textbook model of how plants sense gravitational force has been replaced by a model of position sensing, a clinometer mechanism that involves both passive avalanches and active motion of statoliths, granular starch-filled plastids, in statocytes. Moreover, there is a transmission of information between statocytes and other specialized cells that sense the degree of organ curvature and reset asymmetric growth to straighten and realign the structure. We give an overview of how plants have used the interplay of active posture control and elastic sagging to generate a whole range of spatial displays during their life cycles. Finally, a position-integrating mechanism has been discovered that prevents directional plant growth from being disrupted by wind-induced oscillations.

Transcriptomic and ionomic analysis provides new insight into the beneficial effect of Al on tea roots' growth and nutrient uptake

Transcriptome profiling of roots indicated that genes involved in cell wall modification, cytoskeleton, H⁺ exchange and K⁺ influx played important roles in tea root growth under Al addition. Tea (Camellia sinensis) is considered as an Al accumulator species. It can accumulate a high concentration of Al in mature leaves without any symptom of toxicity, even improve roots' growth and nutrient uptake. However, the molecular mechanisms underlying this tolerance remain unclear. Here, we investigated the accumulation of elements and transcriptional profiles in tea roots treated with various Al doses. The results showed that the growth of tea plants was improved by a low dose of Al (0.2, 0.4, 0.6, 1 mM); however, this beneficial effect disappeared when higher concentrations of Al were supplied (2, 4, 10 mM). Ionomic analysis suggested that accumulation of P and K increased under a low Al supply (< 1 mM), while Ca and Mg contents were negatively correlated with external Al doses. The RNA seq obtained 523,391 unigenes, among which 20,448 were annotated in all databases. In total, 1876 unigenes were expressed significantly different in any Al treatment. A large number of DEGs involved in cell growth and division, such as those linked to cell wall-modifying enzymes, actin cytoskeleton, cyclin and H⁺-ATPase were identified, suggesting that these pathways were involved in root growth under different Al supply. Furthermore, expression of transporters significantly changed in roots supplied with Al. Among them, HAK5, which is involved in K uptake by plants, had a significant positive correlation with the K content.

Saccharomyces cerevisiae as a Tool to Investigate Plant Potassium and Sodium Transporters

Sodium and potassium are two alkali cations abundant in the biosphere. Potassium is essential for plants and its concentration must be maintained at approximately 150 mM in the plant cell cytoplasm including under circumstances where its concentration is much lower in soil. On the other hand, sodium must be extruded from the plant or accumulated either in the vacuole or in specific plant structures. Maintaining a high intracellular K+/Na+ ratio under adverse environmental conditions or in the presence of salt is essential to maintain cellular homeostasis and to avoid toxicity. The baker's yeast, Saccharomyces cerevisiae, has been used to identify and characterize participants in potassium and sodium homeostasis in plants for many years. Its utility resides in the fact that the electric gradient across the membrane and the vacuoles is similar to plants. Most plant proteins can be expressed in yeast and are functional in this unicellular model system, which allows for productive structure-function studies for ion transporting proteins. Moreover, yeast can also be used as a high-throughput platform for the identification of genes that confer stress tolerance and for the study of protein-protein interactions. In this review, we summarize advances regarding potassium and sodium transport that have been discovered using the yeast model system, the state-of-the-art of the available techniques and the future directions and opportunities in this field.

Effects of Naphthazarin (DHNQ) Combined with Lawsone (NQ-2-OH) or 1,4-Naphthoquinone (NQ) on the Auxin-Induced Growth of Zea mays L. Coleoptile Segments

Naphthoquinones, plants secondary metabolites are known for their antibacterial, antifungal, anti-inflammatory, anti-cancer and anti-parasitic properties. The biological activity of naphthoquinones is connected with their ability to generate reactive oxygen species and to modify biological molecules at their nucleophilic sites. In our research, the effect of naphthazarin (DHNQ) combined with 2-hydroxy-1,4-naphthoquinone (NQ-2-OH) or 1,4-naphthoquinone (1,4-NQ) on the elongation growth, pH changes of the incubation medium, oxidative stress and redox activity of maize coleoptile cells were investigated. This paper describes experiments performed with maize (Zea mays L.) coleoptile segments, which is a classical model system to study plant cell elongation growth. The data presented clearly demonstrate that lawsone and 1,4-naphthoquinone combined with naphthazarin, at low concentrations (1 and 10 nM), reduced the endogenous and IAA-induced (Indole-3-Acetic Acid) elongation growth of maize coleoptile segments. Those changes in growth correlated with the proton concentration in the incubation medium, which suggests that the changes in the growth of maize coleoptile segments observed in the presence of naphthoquinones are mediated through the activity of PM H⁺-ATPase. The presence of naphthoquinones induced oxidative stress in the maize coleoptile tissue by producing hydrogen peroxide and causing changes in the redox activity. Moreover, the incubation of maize segments with both naphthoquinones combined with naphthazarin resulted in lipid peroxidation and membrane damage. The regulation of PM H⁺-ATPase activity, especially its inhibition, may result from two major types of reaction: first, a direct interaction between an enzyme and naphthoquinone, which leads to the covalent modification of the protein thiols and the generation of thioethers, which have been found to alter the activity of the PM H⁺-ATPases; second, naphthoquinones induce reactive oxygen species (ROS) production, which inhibits PM H⁺-ATPases by increasing cytosolic Ca2+. This harmful effect was stronger when naphthazarin and 1,4-naphthoquinone were added together. Taking these results into account, it can be suggested that by combining naphthoquinones in small quantities, an alternative to synthetic pesticides could be developed.

Electrophysiological Identification and Activity Analyses of Plasma Membrane K+ Channels in Maize Guard Cells

Stomatal movement, which plays an essential role in plant transpiration and photosynthesis, is controlled by ion channels that mediate K+ and anion fluxes across the plasma membrane (PM) of guard cells. These channels in dicots are accurately regulated by various physiological factors, such as pH, abscisic acid (ABA) and Ca2+; however, the data in monocots are limited. Here the whole-cell patch-clamping technique was applied to analyze the properties and regulations of PM K+ channels in maize guard cells. The results indicated that the hyperpolarization-activated inward-rectifying channels were highly K+-selective. These inward K+ (Kin) channels were sensitive to extracellular K+. Their slope factor (S) decreased when the apoplastic K+ concentration decline, causing a positive shift of the half-activation potential (V1/2). Their activities were promoted by apoplastic acidification but inhibited by apoplastic and cytosolic alkalization. Nevertheless, the outward K+ (Kout) channel activities were uniquely promoted by cytosolic alkalization. Both apoplastic and cytosolic ABA inhibited Kin channels independent of cytosolic Ca2+ ([Ca2+]cyt). And two Ca2+-dependent mechanisms with different Ca2+ affinities may mediate resting- and high-[Ca2+]cyt-induced inhibition on Kin channels, respectively. However, resting [Ca2+]cyt impaired the inhibition of Kin channels induced by apoplastic ABA, not cytosolic ABA. Furthermore, the result that high [Ca2+]cyt attenuated ABA-induced inhibition highlighted the importance of [Ca2+]cyt for Kin channel regulation. There may exist a Ca2+-dependent regulation of the Ca2+-independent ABA signaling pathways for Kin channel inhibition. These results provided an electrophysiological view of the multiple level regulations of PM K+ channel activities and kinetics in maize guard cells.

Characterization of the grapevine Shaker K+ channel VvK3.1 supports its function in massive potassium fluxes necessary for berry potassium loading and pulvinus-actuated leaf movements

In grapevine, climate changes lead to increased berry potassium (K⁺ ) contents that result in must with low acidity. Consequently, wines are becoming 'flat' to the taste, with poor organoleptic properties and low potential aging, resulting in significant economic loss. Precise investigation into the molecular determinants controlling berry K⁺ accumulation during its development are only now emerging. Here, we report functional characterization by electrophysiology of a new grapevine Shaker-type K⁺ channel, VvK3.1. The analysis of VvK3.1 expression patterns was performed by qPCR and in situ hybridization. We found that VvK3.1 belongs to the AKT2 channel phylogenetic branch and is a weakly rectifying channel, mediating both inward and outward K⁺ currents. We showed that VvK3.1 is highly expressed in the phloem and in a unique structure located at the two ends of the petiole, identified as a pulvinus. From the onset of fruit ripening, all data support the role of the VvK3.1 channel in the massive K⁺ fluxes from the phloem cell cytosol to the berry apoplast during berry K⁺ loading. Moreover, the high amount of VvK3.1 transcripts detected in the pulvinus strongly suggests a role for this Shaker in the swelling and shrinking of motor cells involved in paraheliotropic leaf movements.

Revealing the hierarchy of processes and time-scales that control the tropic response of shoots to gravi-stimulations

Gravity is a major abiotic cue for plant growth. However, little is known about the responses of plants to various patterns of gravi-stimulation, with apparent contradictions being observed between the dose-like responses recorded under transient stimuli in microgravity environments and the responses under steady-state inclinations recorded on earth. Of particular importance is how the gravitropic response of an organ is affected by the temporal dynamics of downstream processes in the signalling pathway, such as statolith motion in statocytes or the redistribution of auxin transporters. Here, we used a combination of experiments on the whole-plant scale and live-cell imaging techniques on wheat coleoptiles in centrifuge devices to investigate both the kinematics of shoot-bending induced by transient inclination, and the motion of the statoliths in response to cell inclination. Unlike previous observations in microgravity, the response of shoots to transient inclinations appears to be independent of the level of gravity, with a response time much longer than the duration of statolith sedimentation. This reveals the existence of a memory process in the gravitropic signalling pathway, independent of statolith dynamics. By combining this memory process with statolith motion, a mathematical model is built that unifies the different laws found in the literature and that predicts the early bending response of shoots to arbitrary gravi-stimulations.

Systems, variation, individuality and plant hormones

Inter-individual variation in plants and particularly in hormone content, figures strongly in evolution and behaviour. Homo sapiens and Arabidopsis exhibit similar and substantial phenotypic and molecular variation. Whereas there is a very substantial degree of hormone variation in mankind, reports of inter-individual variation in plant hormone content are virtually absent but are likely to be as large if not larger than that in mankind. Reasons for this absence are discussed. Using an example of inter-individual variation in ethylene content in ripening, the article shows how biological time is compressed by hormones. It further resolves an old issue of very wide hormone dose response that result directly from negative regulation in hormone (and light) transduction. Negative regulation is used because of inter-individual variability in hormone synthesis, receptors and ancillary proteins, a consequence of substantial genomic and environmental variation. Somatic mosaics have been reported for several plant tissues and these too contribute to tissue variation and wide variation in hormone response. The article concludes by examining what variation exists in gravitropic responses. There are multiple sensing systems of gravity vectors and multiple routes towards curvature. These are an aspect of the need for reliability in both inter-individual variation and unpredictable environments. Plant hormone inter-individuality is a new area for research and is likely to change appreciation of the mechanisms that underpin individual behaviour.

Role of auxin (IAA) in the regulation of slow vacuolar (SV) channels and the volume of red beet taproot vacuoles

BACKGROUND

Auxin (IAA) is a central player in plant cell growth. In contrast to the well-established function of the plasma membrane in plant cell expansion, little is known about the role of the vacuolar membrane (tonoplast) in this process.

RESULTS

It was found that under symmetrical 100 mM K⁺ and 100 μM cytoplasmic Ca²⁺ the macroscopic currents showed a typical slow activation and a strong outward rectification of the steady-state currents. The addition of IAA at a final concentration of 1 μM to the bath medium stimulated the SV currents, whereas at 0.1 and 10 μM slight inhibition of SV currents was observed. The time constant, τ, decreased in the presence of this hormone. When single channels were analyzed, an increase in their activity was recorded with IAA compared to the control. The single-channel recordings that were obtained in the presence of IAA showed that auxin increased the amplitude of the single-channel currents. Interestingly, the addition of IAA to the bath medium with the same composition as the one that was used in the patch-clamp experiments showed that auxin decreased the volume of the vacuoles.

CONCLUSIONS

It is suggested that the SV channels and the volume of red beet taproot vacuoles are modulated by auxin (IAA).

Coupled ultradian growth and curvature oscillations during gravitropic movement in disturbed wheat coleoptiles

To grow straight and upright, plants need to regulate actively their posture. Gravitropic movement, which occurs when plants modify their growth and curvature to orient their aerial organ against the force of gravity, is a major feature of this postural control. A recent model has shown that graviception and proprioception are sufficient to account for the gravitropic movement and subsequent organ posture demonstrated by a range of species. However, some plants, including wheat coleoptiles, exhibit a stronger regulation of posture than predicted by the model. Here, we performed an extensive kinematics study on wheat coleoptiles during a gravitropic perturbation (tilting) experiment in order to better understand this unexpectedly strong regulation. Close temporal observations of the data revealed that both perturbed and unperturbed coleoptiles showed oscillatory pulses of elongation and curvature variation that propagated from the apex to the base of their aerial organs. In perturbed coleoptiles, we discovered a non-trivial coupling between the oscillatory dynamics of curvature and elongation. The relationship between those oscillations and the postural control of the organ remains unclear, but indicates the presence of a mechanism that is capable of affecting the relationship between elongation rate, differential growth, and curvature.

The Role of Auxin in Cell Wall Expansion

Plant cells are surrounded by cell walls, which are dynamic structures displaying a strictly regulated balance between rigidity and flexibility. Walls are fairly rigid to provide support and protection, but also extensible, to allow cell growth, which is triggered by a high intracellular turgor pressure. Wall properties regulate the differential growth of the cell, resulting in a diversity of cell sizes and shapes. The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility. Auxin participates in the regulation of cell wall properties by inducing wall loosening. Here, we review what is known on cell wall property regulation by auxin. We focus particularly on the auxin role during cell expansion linked directly to cell wall modifications. We also analyze downstream targets of transcriptional auxin signaling, which are related to the cell wall and could be linked to acid growth and the action of wall-loosening proteins. All together, this update elucidates the connection between hormonal signaling and cell wall synthesis and deposition.

Potassium channel AKT1 is involved in the auxin-mediated root growth inhibition in Arabidopsis response to low K+ stress

The changes in external K⁺ concentration affect plant root growth. However, the molecular mechanism for perceiving a K⁺ signal to modulate root growth remains unknown. It is hypothesized that the K⁺ channel AKT1 is involved in low K⁺ sensing in the Arabidopsis root and subsequent regulation of root growth. Along with the decline of external K⁺ concentration, the primary root growth of wild-type plants was gradually inhibited. However, the primary root of the akt1 mutant could still grow under low K⁺ (LK) conditions. Application of NAA inhibited akt1 root growth, but promoted wild-type root growth under LK conditions. By using the ProDR5:GFP and ProPIN1:PIN1-GFP lines, we found that LK treatment reduced auxin accumulation in wild-type root tips by degrading PIN1 proteins, which did not occur in the akt1 mutant. The LK-induced PIN1 degradation may be due to the inhibition of vesicle trafficking of PIN1 proteins. In conclusion, our findings indicate that AKT1 is required for an Arabidopsis response to changes in external K⁺ , and subsequent regulation of K⁺ -dependent root growth by modulating PIN1 degradation and auxin redistribution in the root.

The K+ channel KZM2 is involved in stomatal movement by modulating inward K+ currents in maize guard cells

Stomata are the major gates in plant leaf that allow water and gas exchange, which is essential for plant transpiration and photosynthesis. Stomatal movement is mainly controlled by the ion channels and transporters in guard cells. In Arabidopsis, the inward Shaker K⁺ channels, such as KAT1 and KAT2, are responsible for stomatal opening. However, the characterization of inward K⁺ channels in maize guard cells is limited. In the present study, we identified two KAT1-like Shaker K⁺ channels, KZM2 and KZM3, which were highly expressed in maize guard cells. Subcellular analysis indicated that KZM2 and KZM3 can localize at the plasma membrane. Electrophysiological characterization in HEK293 cells revealed that both KZM2 and KZM3 were inward K⁺ (K_in ) channels, but showing distinct channel kinetics. When expressed in Xenopus oocytes, only KZM3, but not KZM2, can mediate inward K⁺ currents. However, KZM2 can interact with KZM3 forming heteromeric K_in channel. In oocytes, KZM2 inhibited KZM3 channel conductance and negatively shifted the voltage dependence of KZM3. The activation of KZM2-KZM3 heteromeric channel became slower than the KZM3 channel. Patch-clamping results showed that the inward K⁺ currents of maize guard cells were significantly increased in the KZM2 RNAi lines. In addition, the RNAi lines exhibited faster stomatal opening after light exposure. In conclusion, the presented results demonstrate that KZM2 functions as a negative regulator to modulate the K_in channels in maize guard cells. KZM2 and KZM3 may form heteromeric K_in channel and control stomatal opening in maize.

Protoplast Swelling and Hypocotyl Growth Depend on Different Auxin Signaling Pathways

Members of the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEIN (TIR1/AFB) family are known auxin receptors. To analyze the possible receptor function of AUXIN BINDING PROTEIN1 (ABP1), an auxin receptor currently under debate, we performed different approaches. We performed a pharmacological approach using α-(2,4-dimethylphenylethyl-2-oxo)-indole-3-acetic acid (auxinole), α-(phenylethyl-2-oxo)-indole-3-acetic acid (PEO-IAA), and 5-fluoroindole-3-acetic acid (5-F-IAA) to discriminate between ABP1- and TIR1/AFB-mediated processes in Arabidopsis (Arabidopsis thaliana). We used a peptide of the carboxyl-terminal region of AtABP1 as a tool. We performed mutant analysis with the null alleles of ABP1, abp1-c1 and abp1-TD1, and the TILLING mutant abp1-5 We employed Coimbra, an accession that exhibits an amino acid exchange in the auxin-binding domain of ABP1. We measured either volume changes of single hypocotyl protoplasts or hypocotyl growth, both at high temporal resolution. 5-F-IAA selectively activated the TIR1/AFB pathway but did not induce protoplast swelling; instead, it showed auxin activity in the hypocotyl growth test. In contrast, PEO-IAA induced an auxin-like swelling response but no hypocotyl growth. The carboxyl-terminal peptide of AtABP1 induced an auxin-like swelling response. In the ABP1-related mutants and Coimbra, no auxin-induced protoplast swelling occurred. ABP1 seems to be involved in mediating rapid auxin-induced protoplast swelling, but it is not involved in the control of rapid auxin-induced growth.

Auxin transport and response requirements for root hydrotropism differ between plant species

The direction of auxin transport changes in gravistimulated roots, causing auxin accumulation in the lower side of horizontally reoriented roots. This study found that auxin was similarly involved in hydrotropism and gravitropism in rice and pea roots, but hydrotropism in Lotus japonicus roots was independent of both auxin transport and response. Application of either auxin transport inhibitors or an auxin response inhibitor decreased both hydrotropism and gravitropism in rice roots, and reduced hydrotropism in pea roots. However, Lotus roots treated with these inhibitors showed reduced gravitropism but an unaltered or an enhanced hydrotropic response. Inhibiting auxin biosynthesis substantially reduced both tropisms in rice and Lotus roots. Removing the final 0.2 mm (including the root cap) from the root tip inhibited gravitropism but not hydrotropism in rice seedling roots. These results suggested that modes of auxin involvement in hydrotropism differed between plant species. In rice roots, although auxin transport and responses were required for both gravitropism and hydrotropism, the root cap was involved in the auxin regulation of gravitropism but not hydrotropism. Hydrotropism in Lotus roots, however, may be regulated by a novel mechanism that is independent of both auxin transport and the TIR1/AFBs auxin response pathway.

Auxin, microtubules, and vesicle trafficking: conspirators behind the cell wall

Plant morphogenesis depends on the synchronized anisotropic expansion of individual cells in response to developmental and environmental cues. The magnitude of cell expansion depends on the biomechanical properties of the cell wall, which in turn depends on both its biosynthesis and extensibility. Although the control of cell expansion by the phytohormone auxin is well established, its regulation of cell wall composition, trafficking of H+-ATPases, and K+ influx that drives growth is still being elucidated. Furthermore, the maintenance of auxin fluxes via the interaction between the cytoskeleton and PIN protein recycling on the plasma membrane remains under investigation. This review proposes a model that describes how the cell wall, auxin, microtubule binding-protein CLASP and Kin7/separase complexes, and vesicle trafficking are co-ordinated on a cellular level to mediate cell wall loosening during cell expansion.

Osmolyte cooperation affects turgor dynamics in plants

Scientists have identified turgor-based actuation as a fundamental mechanism in plant movements. Plant cell turgor is generated by water influx due to the osmolyte concentration gradient through the cell wall and the plasma membrane behaving as an osmotic barrier. Previous studies have focused on turgor modulation with respect to potassium chloride (KCl) concentration changes, although KCl is not efficiently retained in the cell, and many other compounds, including L-glutamine (L-Gln) and D-glucose (D-Glc), are present in the cytosol. In fact, the contributions of other osmolytes to turgor dynamics remain to be elucidated. Here, we show the association of osmolytes and their consequent cooperative effects on the time-dependent turgor profile generated in a model cytosol consisting of KCl, D-Glc and L-Gln at experimentally measured plant motor/generic cell concentrations and at modified concentrations. We demonstrate the influence and association of the osmolytes using osmometry and NMR measurements. We also show, using a plant cell-inspired device we previously developed, that osmolyte complexes, rather than single osmolytes, permit to obtain higher turgor required by plant movements. We provide quantitative cues for deeper investigations of osmolyte transport for plant movement, and reveal the possibility of developing osmotic actuators exploiting a dynamically varying concentration of osmolytes.