Phosphorylation of ARF2 Relieves Its Repression of Transcription of the K+ Transporter Gene HAK5 in Response to Low Potassium Stress

WRKY6 transcription factor modulates root potassium acquisition through promoting expression of AKT1 in Arabidopsis

Potassium (K+), being an essential macronutrient in plants, plays a central role in many aspects. Root growth is highly plastic and is affected by many different abiotic stresses including nutrient deficiency. The Shaker-type K+ channel Arabidopsis (Arabidopsis thaliana) K+ Transporter 1 (AKT1) is responsible for K+ uptake under both low and high external K+ conditions. However, the upstream transcription factor of AKT1 is not clear. Here, we demonstrated that the WRKY6 transcription factor modulates root growth to low potassium (LK) stress in Arabidopsis. WRKY6 showed a quick response to LK stress and also to many other abiotic stress treatments. The two wrky6 T-DNA insertion mutants were highly sensitive to LK treatment, whose primary root lengths were much shorter, less biomass and lower K+ content in roots than those of wild-type plants, while WRKY6-overexpression lines showed opposite phenotypes. A further investigation showed that WRKY6 regulated the expression of the AKT1 gene via directly binding to the W-box elements in its promoter through EMSA and ChIP-qPCR assays. A dual luciferase reporter analysis further demonstrated that WRKY6 enhanced the transcription of AKT1. Genetic analysis further revealed that the overexpression of AKT1 greatly rescued the short root phenotype of the wrky6 mutant under LK stress, suggesting AKT1 is epistatic to WRKY6 in the control of LK response. Further transcriptome profiling suggested that WRKY6 modulates LK response through a complex regulatory network. Thus, this study unveils a transcription factor that modulates root growth under potassium deficiency conditions by affecting the potassium channel gene AKT1 expression.

NtARF11 positively regulates cadmium tolerance in tobacco by inhibiting expression of the nitrate transporter NtNRT1.1

Heavy metal cadmium (Cd) is widespread in contaminated soil and an important factor limiting plant growth. NO3- (nitrate) affects Cd uptake and thus changes Cd tolerance in plants; however, the underlying molecular regulatory mechanisms have not yet been elucidated. Here, we analyzed a novel gene, NtARF11 (auxin response factor), which regulates Cd tolerance in tobacco via the NO3- uptake pathway, through experiments with NtARF11-knockout and NtARF11-overexpression transgenic tobacco lines. NtARF11 was highly expressed under Cd stress in tobacco plants. Under Cd stress, overexpression of NtARF11 enhanced Cd tolerance in tobacco compared to that in wild-type tobacco, as shown by the low Cd concentration, high chlorophyll concentration, and low accumulation of reactive oxygen species in NtARF11-overexpressing tobacco. Moreover, low NO3- concentrations were observed in NtARF11-overexpressing tobacco plants. Further analyses revealed direct binding of NtARF11 to the promoter of the nitrate transporter NtNRT1.1, thereby negatively regulating its expression in tobacco. Notably, NtNRT1.1 knockout reduced NO3- uptake, which resulted in low Cd concentrations in tobacco. Altogether, these results demonstrate that the NtARF11-NtNRT1.1 module functions as a positive regulator of Cd tolerance by reducing the Cd uptake in tobacco, providing new insights for improving Cd tolerance of plants through genetic engineering.

A viral protein competitively bound to rice CIPK23 inhibits potassium absorption and facilitates virus systemic infection in rice

Potassium (K+) plays a crucial role as a macronutrient in the growth and development of plants. Studies have definitely determined the vital roles of K+ in response to pathogen invasion. Our previous investigations revealed that rice plants infected with rice grassy stunt virus (RGSV) displayed a reduction in K+ content, but the mechanism by which RGSV infection subverts K+ uptake remains unknown. In this study, we found that overexpression of RGSV P1, a specific viral protein encoded by viral RNA1, results in enhanced sensitivity to low K+ stress and exhibits a significantly lower rate of K+ influx compared to wild-type rice plants. Further investigation revealed that RGSV P1 interacts with OsCIPK23, an upstream regulator of Shaker K+ channel OsAKT1. Moreover, we found that the P1 protein recruits the OsCIPK23 to the Cajal bodies (CBs). In vivo assays demonstrated that the P1 protein competitively binds to OsCIPK23 with both OsCBL1 and OsAKT1. In the nucleus, the P1 protein enhances the binding of OsCIPK23 to OsCoilin, a homologue of the signature protein of CBs in Arabidopsis, and facilitates their trafficking through these CB structures. Genetic analysis indicates that mutant in oscipk23 suppresses RGSV systemic infection. Conversely, osakt1 mutants exhibited increased sensitivity to RGSV infection. These findings suggest that RGSV P1 hinders the absorption of K+ in rice plants by recruiting the OsCIPK23 to the CB structures. This process potentially promotes virus systemic infection but comes at the expense of inhibiting OsAKT1 activity.

The EIN3/EIL-ERF9-HAK5 transcriptional cascade positively regulates high-affinity K+ uptake in Gossypium hirsutum

High-affinity K+ (HAK) transporters play essential roles in facilitating root K+ uptake in higher plants. Our previous studies revealed that GhHAK5a, a member of the HAK family, is crucial for K+ uptake in upland cotton. Nevertheless, the precise regulatory mechanism governing the expression of GhHAK5a remains unclear. The yeast one-hybrid screening was performed to identify the transcription factors responsible for regulating GhHAK5a, and ethylene response factor 9 (GhERF9) was identified as a potential candidate. Subsequent dual-luciferase and electrophoretic mobility shift assays confirmed that GhERF9 binds directly to the GhHAK5a promoter, thereby activating its expression. Silencing of GhERF9 decreased the expression of GhHAK5a and exacerbated K+ deficiency symptoms in leaves, also decreased K+ uptake rate and K+ content in roots. Additionally, it was observed that the application of ethephon (an ethylene-releasing reagent) resulted in a significant upregulation of GhERF9 and GhHAK5a, accompanied by an increased rate of K+ uptake. Expectedly, GhEIN3b and GhEIL3c, the two key components involved in ethylene signaling, bind directly to the GhERF9 promoter. These findings provide valuable insights into the molecular mechanisms underlying the expression of GhHAK5a and ethylene-mediated K+ uptake and suggest a potential strategy to genetically enhance cotton K+ uptake by exploiting the EIN3/EILs-ERF9-HAK5 module.

Evolution and function analysis of auxin response factors reveal the molecular basis of the developed root system of Zygophyllum xanthoxylum

BACKGROUND

As a xerophytic shrub, forming developed root system dominated with lateral roots is one of the effective strategies for Zygophyllum xanthoxylum to adapt to desert habitat. However, the molecular mechanism of lateral root formation in Z. xanthoxylum is still unclear. Auxin response factors (ARFs) are a master family of transcription factors (TFs) in auxin-mediated biological processes including root growth and development.

RESULTS

Here, to determine the relationship between ARFs and root system formation in Z. xanthoxylum, a total of 30 potential ZxARF genes were first identified, and their classifications, evolutionary relationships, duplication events and conserved domains were characterized. 107 ARF protein sequences from alga to higher plant species including Z. xanthoxylum are split into A, B, and C 3 Clades, consisting with previous studies. The comparative analysis of ARFs between xerophytes and mesophytes showed that A-ARFs of xerophytes expanded considerably more than that of mesophytes. Furthermore, in this Clade, ZxARF5b and ZxARF8b have lost the important B3 DNA-binding domain partly and completely, suggesting both two proteins may be more functional in activating transcription by dimerization with AUX/IAA repressors. qRT-PCR results showed that all A-ZxARFs are high expressed in the roots of Z. xanthoxylum, and they were significantly induced by drought stress. Among these A-ZxARFs, the over-expression assay showed that ZxARF7c and ZxARF7d play positive roles in lateral root formation.

CONCLUSION

This study provided the first comprehensive overview of ZxARFs and highlighted the importance of A-ZxARFs in the lateral root development.

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.

The Molecular Mechanism of Potassium Absorption, Transport, and Utilization in Rice

Potassium is essential for plant growth and development and stress adaptation. The maintenance of potassium homeostasis involves a series of potassium channels and transporters, which promote the movement of potassium ions (K+) across cell membranes and exhibit complex expression patterns and regulatory mechanisms. Rice is a major food crop in China. The low utilization rate of potassium fertilizer limits the yield and quality of rice. Elucidating the molecular mechanisms of potassium absorption, transport, and utilization is critical in improving potassium utilization efficiency in rice. Although some K+ transporter genes have been identified from rice, research on the regulatory network is still in its infancy. Therefore, this review summarizes the relevant information on K+ channels and transporters in rice, covering the absorption of K+ in the roots, transport to the shoots, the regulation pathways, the relationship between K+ and the salt tolerance of rice, and the synergistic regulation of potassium, nitrogen, and phosphorus signals. The related research on rice potassium nutrition has been comprehensively reviewed, the existing research foundation and the bottleneck problems to be solved in this field have been clarified, and the follow-up key research directions have been pointed out to provide a theoretical framework for the cultivation of potassium-efficient rice.

TORC pathway intersects with a calcium sensor kinase network to regulate potassium sensing in Arabidopsis

Potassium (K) is an essential macronutrient for plant growth, and its availability in the soil varies widely, requiring plants to respond and adapt to the changing K nutrient status. We show here that plant growth rate is closely correlated with K status in the medium, and this K-dependent growth is mediated by the highly conserved nutrient sensor, target of rapamycin (TOR). Further study connected the TOR complex (TORC) pathway with a low-K response signaling network consisting of calcineurin B-like proteins (CBL) and CBL-interacting kinases (CIPK). Under high K conditions, TORC is rapidly activated and shut down the CBL-CIPK low-K response pathway through regulatory-associated protein of TOR (RAPTOR)-CIPK interaction. In contrast, low-K status activates CBL-CIPK modules that in turn inhibit TORC by phosphorylating RAPTOR, leading to dissociation and thus inactivation of the TORC. The reciprocal regulation of the TORC and CBL-CIPK modules orchestrates plant response and adaptation to K nutrient status in the environment.

AUXIN RESPONSE FACTOR protein accumulation and function

Auxin is a key regulator of plant developmental processes. Its effects on transcription are mediated by the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. ARFs tightly control specific auxin responses necessary for proper plant growth and development. Recent research has revealed that regulated ARF protein accumulation and ARF nucleo-cytoplasmic partitioning can determine auxin transcriptional outputs. In this review, we explore these recent findings and consider the potential for regulated ARF accumulation in driving auxin responses in plants.

Dynamic transcriptome analysis unravels key regulatory genes of maize root growth and development in response to potassium deficiency

MAIN CONCLUSION

Integrated root phenotypes and transcriptome analysis have revealed key candidate genes responsible for maize root growth and development in potassium deficiency. Potassium (K) is a vital macronutrient for plant growth, but our understanding of its regulatory mechanisms in maize root system architecture (RSA) and K+ uptake remains limited. To address this, we conducted hydroponic and field trials at different growth stages. K+ deficiency significantly inhibited maize root growth, with metrics like total root length, primary root length, width and maximum root number reduced by 50% to 80% during early seedling stages. In the field, RSA traits exhibited maximum values at the silking stage but continued to decline thereafter. Furthermore, K deprivation had a pronounced negative impact on root morphology and RSA growth and grain yield. RNA-Seq analysis identified 5972 differentially expressed genes (DEGs), including 17 associated with K+ signaling, transcription factors, and transporters. Weighted gene co-expression network analysis revealed 23 co-expressed modules, with enrichment of transcription factors at different developmental stages under K deficiency. Several DEGs and transcription factors were predicted as potential candidate genes responsible for maize root growth and development. Interestingly, some of these genes exhibited homology to well-known regulators of root architecture or development in Arabidopsis, such as Zm00001d014467 (AtRCI3), Zm00001d011237 (AtWRKY9), and Zm00001d030862 (AtAP2/ERF). Identifying these key genes helps to provide a deeper understanding of the molecular mechanisms governing maize root growth and development under nutrient deficient conditions offering potential benefits for enhancing maize production and improving stress resistance through targeted manipulation of RSA traits in modern breeding efforts.

Modulation of potassium transport to increase abiotic stress tolerance in plants

Potassium is the major cation responsible for the maintenance of the ionic environment in plant cells. Stable potassium homeostasis is indispensable for virtually all cellular functions, and, concomitantly, viability. Plants must cope with environmental changes such as salt or drought that can alter ionic homeostasis. Potassium fluxes are required to regulate the essential process of transpiration, so a constraint on potassium transport may also affect the plant's response to heat, cold, or oxidative stress. Sequencing data and functional analyses have defined the potassium channels and transporters present in the genomes of different species, so we know most of the proteins directly participating in potassium homeostasis. The still unanswered questions are how these proteins are regulated and the nature of potential cross-talk with other signaling pathways controlling growth, development, and stress responses. As we gain knowledge regarding the molecular mechanisms underlying regulation of potassium homeostasis in plants, we can take advantage of this information to increase the efficiency of potassium transport and generate plants with enhanced tolerance to abiotic stress through genetic engineering or new breeding techniques. Here, we review current knowledge of how modifying genes related to potassium homeostasis in plants affect abiotic stress tolerance at the whole plant level.

Rice potassium transporter OsHAK18 mediates phloem K+ loading and redistribution

High-affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) are important pathways mediating K+ transport across cell membranes, which function in maintaining K+ homeostasis during plant growth and stress response. An increasing number of studies have shown that HAK/KUP/KT transporters play crucial roles in root K+ uptake and root-to-shoot translocation. However, whether HAK/KUP/KT transporters also function in phloem K+ translocation remain unclear. In this study, we revealed that a phloem-localized rice HAK/KUP/KT transporter, OsHAK18, mediated cell K+ uptake when expressed in yeast, Escherichia coli and Arabidopsis. It was localized at the plasma membrane. Disruption of OsHAK18 rendered rice seedlings insensitive to low-K+ (LK) stress. After LK stress, some WT leaves showed severe wilting and chlorosis, whereas the corresponding leaves of oshak18 mutant lines (a Tos17 insertion line and two CRISPR lines) remained green and unwilted. Compared with WT, the oshak18 mutants accumulated more K+ in shoots but less K+ in roots after LK stress, leading to a higher shoot/root ratio of K+ per plant. Disruption of OsHAK18 does not affect root K+ uptake and K+ level in xylem sap, but it significantly decreases phloem K+ concentration and inhibits root-to-shoot-to-root K+ (Rb+ ) translocation in split-root assay. These results reveal that OsHAK18 mediates phloem K+ loading and redistribution, whose disruption is in favor of shoot K+ retention under LK stress. Our findings expand the understanding of HAK/KUP/KT transporters' functions and provide a promising strategy for improving rice tolerance to K+ deficiency.

Xylem K+ loading modulates K+ and Cs+ absorption and distribution in Arabidopsis under K+-limited conditions

Potassium (K+) is an essential macronutrient for plant growth. The transcriptional regulation of K+ transporter genes is one of the key mechanisms by which plants respond to K+ deficiency. Among the HAK/KUP/KT transporter family, HAK5, a high-affinity K+ transporter, is essential for root K+ uptake under low external K+ conditions. HAK5 expression in the root is highly induced by low external K+ concentration. While the molecular mechanisms of HAK5 regulation have been extensively studied, it remains unclear how plants sense and coordinates K+ uptake and translocation in response to changing environmental conditions. Using skor mutants, which have a defect in root-to-shoot K+ translocation, we have been able to determine how the internal K+ status affects the expression of HAK5. In skor mutant roots, under K+ deficiency, HAK5 expression was lower than in wild-type although the K+ concentration in roots was not significantly different. These results reveal that HAK5 is not only regulated by external K+ conditions but it is also regulated by internal K+ levels, which is in agreement with recent findings. Additionally, HAK5 plays a major role in the uptake of Cs+ in roots. Therefore, studying Cs+ in roots and having more detailed information about its uptake and translocation in the plant would be valuable. Radioactive tracing experiments revealed not only a reduction in the uptake of 137Cs+ and 42K+in skor mutants compared to wild-type but also a different distribution of 137Cs+ and 42K+ in tissues. In order to gain insight into the translocation, accumulation, and repartitioning of both K+ and Cs+ in plants, long-term treatment and split root experiments were conducted with the stable isotopes 133Cs+ and 85Rb+. Finally, our findings show that the K+ distribution in plant tissues regulates root uptake of K+ and Cs+ similarly, depending on HAK5; however, the translocation and accumulation of the two elements are different.

Calcium signaling-mediated transcriptional reprogramming during abiotic stress response in plants

Calcium (Ca2+) is a second messenger in plants growth and development, as well as in stress responses. The transient elevation in cytosolic Ca2+ concentration have been reported to be involved in plants response to abiotic and biotic stresses. In plants, Ca2+-induced transcriptional changes trigger molecular mechanisms by which plants adapt and respond to environment stresses. The mechanism for transcription regulation by Ca2+ could be either rapid in which Ca2+ signals directly cause the related response through the gene transcript and protein activities, or involved amplification of Ca2+ signals by up-regulation the expression of Ca2+ responsive genes, and then increase the transmission of Ca2+ signals. Ca2+ regulates the expression of genes by directly binding to the transcription factors (TFs), or indirectly through its sensors like calmodulin, calcium-dependent protein kinases (CDPK) and calcineurin B-like protein (CBL). In recent years, significant progress has been made in understanding the role of Ca2+-mediated transcriptional regulation in different processes in plants. In this review, we have provided a comprehensive overview of Ca2+-mediated transcriptional regulation in plants in response to abiotic stresses including nutrition deficiency, temperature stresses (like heat and cold), dehydration stress, osmotic stress, hypoxic, salt stress, acid rain, and heavy metal stress.

The Knockdown of AUXIN RESPONSE FACTOR 2 Confers Enhanced Tolerance to Salt and Drought Stresses in Tomato (Solanum lycopersicum L.)

Auxin response factors (ARFs) act as key elements of the auxin-signaling pathway and play important roles in the process of a plant's growth, development, and response to environmental conditions. We studied the implication of the SlARF2 gene in the tomato response to salt (150 mM of NaCl) and drought (15% PEG 20000) stresses. The functional characterization of SlARF2 knockdown tomato mutants revealed that the downregulation of this gene enhanced primary root length and root branching and reduced plant wilting. At the physiological level, the arf2 mutant line displayed higher chlorophyll, soluble sugars, proline, and relative water contents as well as lower stomatal conductance and a decreased malondialdehyde content. Moreover, SlARF2 knockdown tomato mutants demonstrated higher activities of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) under salt and drought stresses than the wild type. Indeed, the stress tolerance of the arf2 mutant was also reflected by the upregulation of stress-related genes involved in ROS scavenging and plant defense, including SOD, CAT, dehydration-responsive element-binding protein, and early responsive to dehydration, which can ultimately result in a better resistance to salt and drought stresses. Furthermore, the transcriptional levels of the Δ1-pyrroline-5-carboxylate synthase (P5CS) gene were upregulated in the arf2 mutant after stress, in correlation with the higher levels of proline. Taken together, our findings reveal that SlARF2 is implicated in salt and drought tolerance in tomato and provides some considerable elements for improving the abiotic stress tolerance and increasing the crop yields of tomato.

GhbZIP30-GhCCCH17 module accelerates corm dormancy release by reducing endogenous ABA under cold storage in Gladiolus

Gladiolus hybridus is one of the most popular flowers worldwide. However, its corm dormancy characteristic largely limits its off-season production. Long-term cold treatment (LT), which increases sugar content and reduces abscisic acid (ABA), is an efficient approach to accelerate corm dormancy release (CDR). Here, we identified a GhbZIP30-GhCCCH17 module that mediates the antagonism between sugars and ABA during CDR. We showed that sugars promoted CDR by reducing ABA levels in Gladiolus. Our data demonstrated that GhbZIP30 transcription factor directly binds the GhCCCH17 zinc finger promoter and activates its transcription, confirmed by yeast one-hybrid, dual-luciferase (Dual-LUC), chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) and electrophoretic mobility shift assay (EMSA). GhCCCH17 is a transcriptional activator, and its nuclear localisation is altered by glucose and cytokinin treatments. Both GhbZIP30 and GhCCCH17 positively respond to LT, sugars, and cytokinin treatments. Silencing GhbZIP30 or GhCCCH17 resulted in delayed CDR by regulating ABA metabolic genes, while their overexpression promoted CDR. Taken together, we propose that the GhbZIP30-GhCCCH17 module is involved in cold- and glucose-induced CDR by regulating ABA metabolic genes.

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.

HY5 inhibits lateral root initiation in Arabidopsis through negative regulation of the microtubule-stabilizing protein TPXL5

Tight control of lateral root (LR) initiation is vital for root system architecture and function. Regulation of cortical microtubule reorganization is involved in the asymmetric radial expansion of founder cells during LR initiation in Arabidopsis (Arabidopsis thaliana). However, critical genetic evidence on the role of microtubules in LR initiation is lacking and the mechanisms underlying this regulation are poorly understood. Here, we found that the previously uncharacterized microtubule-stabilizing protein TPX2-LIKE5 (TPXL5) participates in LR initiation, which is finely regulated by the transcription factor ELONGATED HYPOCOTYL5 (HY5). In tpxl5 mutants, LR density was decreased and more LR primordia (LRPs) remained in stage I, indicating delayed LR initiation. In particular, the cell width in the peripheral domain of LR founder cells after the first asymmetric cell division was larger in tpxl5 mutants than in the wild-type. Consistently, ordered transverse cortical microtubule arrays were not well generated in tpxl5 mutants. In addition, HY5 directly targeted the promoter of TPXL5 and downregulated TPXL5 expression. The hy5 mutant exhibited higher LR density and fewer stage I LRPs, indicating accelerated LR initiation. Such phenotypes were partially suppressed by TPXL5 knockout. Taken together, our data provide genetic evidence supporting the notion that cortical microtubules are essential for LR initiation and unravel a molecular mechanism underlying HY5 regulation of TPXL5-mediated microtubule reorganization and cell remodeling during LR initiation.

Advances in structure and function of auxin response factor in plants

Auxin is a crucial phytohormone that has various effects on the regulators of plant growth and development. Auxin signal transduction is mainly controlled by two gene families: auxin response factor (ARF) and auxin/indole-3-acetic acid (Aux/IAA). ARFs are plant-specific transcription factors that bind directly to auxin response elements in the promoters of auxin-responsive genes. ARF proteins contain three conserved regions: a conserved N-terminal B3 DNA-binding domain, a variable intermediate middle region domain that functions in activation or repression, and a C-terminal domain including the Phox and Bem1p region for dimerization, similar to the III and IV elements of Aux/IAA, which facilitate protein-protein interaction through homodimerization of ARF proteins or heterodimerization of ARF and Aux/IAA proteins. In the two decades following the identification of the first ARF, 23 ARF members have been identified and characterized in Arabidopsis. Using whole-genome sequencing, 22, 25, 23, 25, and 36 ARF genes have been identified in tomato, rice, wheat, sorghum, and maize, respectively, in addition to which the related biofunctions of some ARFs have been reported. ARFs play crucial roles in regulating the growth and development of roots, leaves, flowers, fruits, seeds, responses to biotic and abiotic stresses, and phytohormone signal crosstalk. In this review, we summarize the research progress on the structures and functions of ARFs in Arabidopsis, tomato, and cereal crops, to provide clues for future basic research on phytohormone signaling and the molecular design breeding of crops.

Living with high potassium: Balance between nutrient acquisition and K-induced salt stress signaling

High potassium (K) in the growth medium induces salinity stress in plants. However, the molecular mechanisms underlying plant responses to K-induced salt stress are virtually unknown. We examined Arabidopsis (Arabidopsis thaliana) and its extremophyte relative Schrenkiella parvula using a comparative multiomics approach to identify cellular processes affected by excess K and understand which deterministic regulatory pathways are active to avoid tissue damages while sustaining growth. Arabidopsis showed limited capacity to curb excess K accumulation and prevent nutrient depletion, contrasting to S. parvula which could limit excess K accumulation without restricting nutrient uptake. A targeted transcriptomic response in S. parvula promoted nitrogen uptake along with other key nutrients followed by uninterrupted N assimilation into primary metabolites during excess K-stress. This resulted in larger antioxidant and osmolyte pools and corresponded with sustained growth in S. parvula. Antithetically, Arabidopsis showed increased reactive oxygen species levels, reduced photosynthesis, and transcriptional responses indicative of a poor balance between stress signaling, subsequently leading to growth limitations. Our results indicate that the ability to regulate independent nutrient uptake and a coordinated transcriptomic response to avoid nonspecific stress signaling are two main deterministic steps toward building stress resilience to excess K+-induced salt stress.

Phylogeny, gene structures, and expression patterns of the auxin response factor (GhARF2) in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Auxin response factors (ARFs) are a class of transcription factors that regulate the expression of auxin-responsive genes and play important functions in plant growth and development. To understand the biological functions of the auxin response factor GhARF2 gene in upland cotton, the coding sequence (CDS) of GhARF2 gene was cloned, and its protein sequence, evolutionary relationship, subcellular localization and expression pattern were analysed.

METHODS

The CDS sequence of GhARF2 gene was cloned from upland cotton variety Baimian No.1, and its protein sequence was analyzed by bioinformatics method. The subcellular localization of GhARF2 protein was detected by tobacco epidermal transient transformation system, and the tissue expression and stress expression pattern of GhARF2 were analyzed by quantitative Real‑Time PCR (qRT-PCR).

RESULTS

The full-length CDS of GhARF2 gene was 2583 bp, encoded 860 amino acids, and had a molecular weight and an isoelectric point of 95.46 KDa and 6.02, respectively. The GhARF2 protein had multiple phosphorylation sites, no transmembrane domain, and secondary structures dominated by random coils and alpha helix. The GhARF2 protein had 3 conserved typical domains of ARF gene family members, including the B3 DNA binding domain, the Auxin_resp domain, and the Aux/IAA domain. Phylogenetic analysis revealed that ARF2 proteins in different species were clustered in the Group A subgroup, in which GhARF2 was closely related to TcARF2 of Theobroma cacao L. (Malvaceae). The subcellular localization results showed that the GhARF2 protein was localized in the nucleus. Analysis of tissue expression pattern showed that the GhARF2 gene was expressed in all tested tissues, with the highest expression levels in sepal, followed by leaf, and the lowest expression levels in fiber. Further stress expression analysis showed that the GhARF2 gene was induced by drought, high-temperature, low-temperature and salt stress, and had different expression patterns under different stress conditions.

CONCLUSION

These results established a foundation for understanding the functions of GhARF2 and breeding varieties with high-stress tolerance in cotton.

Potassium nutrient status drives posttranslational regulation of a low-K response network in Arabidopsis

Under low-potassium (K+) stress, a Ca2+ signaling network consisting of calcineurin B-like proteins (CBLs) and CBL-interacting kinases (CIPKs) play essential roles. Specifically, the plasma membrane CBL1/9-CIPK pathway and the tonoplast CBL2/3-CIPK pathway promotes K+ uptake and remobilization, respectively, by activating a series of K+ channels. While the dual CBL-CIPK pathways enable plants to cope with low-K+ stress, little is known about the early events that link external K+ levels to the CBL-CIPK proteins. Here we show that K+ status regulates the protein abundance and phosphorylation of the CBL-CIPK-channel modules. Further analysis revealed low K+-induced activation of VM-CBL2/3 happened earlier and was required for full activation of PM-CBL1/9 pathway. Moreover, we identified CIPK9/23 kinases to be responsible for phosphorylation of CBL1/9/2/3 in plant response to low-K+ stress and the HAB1/ABI1/ABI2/PP2CA phosphatases to be responsible for CBL2/3-CIPK9 dephosphorylation upon K+-repletion. Further genetic analysis showed that HAB1/ABI1/ABI2/PP2CA phosphatases are negative regulators for plant growth under low-K+, countering the CBL-CIPK network in plant response and adaptation to low-K+ stress.

Transcriptomic and functional characterization reveals CsHAK5;3 as a key player in K+ homeostasis in grafted cucumbers under saline conditions

Grafting can improve the salt tolerance of many crops. However, critical genes in scions responsive to rootstock under salt stress remain a mystery. We found that pumpkin rootstock decreased the content of Na⁺ by 70.24 %, increased the content of K⁺ by 25.9 %, and increased the K⁺/Na⁺ ratio by 366.0 % in cucumber scion leaves. RNA-seq analysis showed that ion transport-related genes were the key genes involved in salt stress tolerance in grafted cucumber. The identification and analysis of the expression of K⁺ transporter proteins in cucumber and pumpkin revealed six and five HAK5 members, respectively. The expression of CsHAK5;3 in cucumber was elevated in different graft combinations under salt stress and most notably in cucumber scion/pumpkin rootstock. CsHAK5;3 was localized to the plasma membrane, and a yeast complementation assay revealed that it can transport K⁺. CsHAK5;3 knockout in hairy root mutants decreased the K⁺ content of leaves (45.6 %) and roots (50.3 %), increased the Na⁺ content of leaves (29.3 %) and roots (34.8 %), and decreased the K⁺/Na⁺ ratio of the leaves (57.9 %) and roots (62.9 %) in cucumber. However, CsHAK5;3 overexpression in hairy roots increased the K⁺ content of the leaves (31.2 %) and roots (38.3 %), decreased the Na⁺ content of leaves (17.2 %) and roots (14.3 %), and increased the K⁺/Na⁺ ratio of leaves (58.9 %) and roots (61.6 %) in cucumber. In conclusion, CsHAK5;3 in cucumber can mediate K⁺ transport and is one of the key target pumpkin genes that enhance salt tolerance of cucumber grafted.

Brassinolide improves the tolerance of Malus hupehensis to alkaline stress

Malus hupehensis is one of the most widely used apple rootstocks in china but is severely damaged by alkaline soil. Alkaline stress can cause more serious harmful effects on apple plants than salt stress because it also induces high pH stress except for ion toxicity, osmotic stress, and oxidative damage. Brassinolide (BL) plays important roles in plant responses to salt stress. However, its role and function mechanism in apple plants in response to alkaline stress has never been reported. This study showed that applying exogenous 0.2 mg/L BL significantly enhanced the resistance of M. hupehensis seedlings to alkaline stress. The main functional mechanisms were also explored. First, exogenous BL could decrease the rhizosphere pH and promote Ca²⁺ and Mg²⁺ absorption by regulating malic acid and citric acid contents and increasing H⁺ excretion. Second, exogenous BL could alleviate ion toxicity caused by alkaline stress through enhancing Na⁺ efflux and inhibiting K⁺ expel and vacuole compartmentalization. Last, exogenous BL could balance osmotic stress by accumulating proline and reduce oxidative damage through increasing the activities of antioxidant enzymes and antioxidants contents. This study provides an important theoretical basis for further analyzing the mechanism of exogenous BL in improving alkaline tolerance of apple plants.

Proteomic Analysis of Roots Response to Potassium Deficiency and the Effect of TaHAK1-4A on K+ Uptake in Wheat

Potassium (K+) is essential for plant growth and stress responses. A deficiency in soil K+ contents can result in decreased wheat quality and productivity. Thus, clarifying the molecular mechanism underlying wheat responses to low-K+ (LK) stress is critical. In this study, a tandem mass tag (TMT)-based quantitative proteomic analysis was performed to investigate the differentially abundant proteins (DAPs) in roots of the LK-tolerant wheat cultivar “KN9204” at the seedling stage after exposure to LK stress. A total of 104 DAPs were identified in the LK-treated roots. The DAPs related to carbohydrate and energy metabolism, transport, stress responses and defense, and post-translational modifications under LK conditions were highlighted. We identified a high-affinity potassium transporter (TaHAK1-4A) that was significantly up-regulated after the LK treatment. Additionally, TaHAK1-4A was mainly expressed in roots, and the encoded protein was localized in the plasma membrane. The complementation assay in yeast suggested that TaHAK1-4A mediates K+ uptake under extreme LK conditions. The overexpression of TaHAK1-4A increased the fresh weight and root length of Arabidopsis under LK conditions and improved the growth of Arabidopsis athak5 mutant seedlings, which grow poorly under LK conditions. Moreover, silencing of TaHAK1-4A in wheat roots treated with LK stress decreased the root length, dry weight, K+ concentration, and K+ influx. Accordingly, TaHAK1-4A is important for the uptake of K+ by roots exposed to LK stress. Our results reveal the protein metabolic changes in wheat induced by LK stress. Furthermore, we identified a candidate gene potentially relevant for developing wheat lines with increased K+ use efficiency.

FveARF2 negatively regulates fruit ripening and quality in strawberry

Auxin response factors (ARFs) are transcription factors that play important roles in plants. ARF2 is a member of the ARF family and participates in many plant growth and developmental processes. However, the role of ARF2 in strawberry fruit quality remains unclear. In this study, FveARF2 was isolated from the woodland strawberry 'Ruegen' using reverse transcription-polymerase chain reaction (RT-PCR), which showed that FveARF2 expression levels were higher in the stem than in other organs of the 'Ruegen' strawberry. Moreover, FaARF2 was higher in the white fruit stage of cultivated strawberry fruit than in other stage. Subcellular localization analysis showed that FveARF2 is located in the nucleus, while transcriptional activation assays showed that FveARF2 inhibited transcription in yeast. Silencing FveARF2 in cultivated strawberry fruit revealed earlier coloration and higher soluble solid, sugar, and anthocyanin content in the transgenic fruit than in the control fruit, overexpression of FveARF2 in strawberry fruit delayed ripening and lower soluble solid, sugar, and anthocyanin content compared to the control fruit. Gene expression analysis indicated that the transcription levels of the fruit ripening genes FaSUT1, FaOMT, and FaCHS increased in FveARF2-RNAi fruit and decreased in FveARF2-OE fruit, when compared with the control. Furthermore, yeast one-hybrid (Y1H) and GUS activity experiments showed that FveARF2 can directly bind to the AuxRE (TGTCTC) element in the FaSUT1, FaOMT, and FaCHS promoters in vitro and in vivo. Potassium ion supplementation improved the quality of strawberry fruit, while silencing FveARF2 increased potassium ion content in transgenic fruit. The Y1H and GUS activity experiments also confirmed that FveARF2 could directly bind to the promoter of FveKT12, a potassium transporter gene, and inhibited its expression. Taken together, we found that FveARF2 can negatively regulate strawberry fruit ripening and quality, which provides new insight for further study of the molecular mechanism of strawberry fruit ripening.

ARF2-PIF5 interaction controls transcriptional reprogramming in the ABS3-mediated plant senescence pathway

One of the hallmarks of plant senescence is the global transcriptional reprogramming coordinated by a plethora of transcription factors (TFs). However, mechanisms underlying the interactions between different TFs in modulating senescence remain obscure. Previously, we discovered that plant ABS3 subfamily MATE transporter genes regulate senescence and senescence-associated transcriptional changes. In a genetic screen for mutants suppressing the accelerated senescence phenotype of the gain-of-function mutant abs3-1D, AUXIN RESPONSE FACTOR 2 (ARF2) and PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) were identified as key TFs responsible for transcriptional regulation in the ABS3-mediated senescence pathway. ARF2 and PIF5 (as well as PIF4) interact directly and function interdependently to promote senescence, and they share common target genes such as key senescence promoting genes ORESARA 1 (ORE1) and STAY-GREEN 1 (SGR1) in the ABS3-mediated senescence pathway. In addition, we discovered reciprocal regulation between ABS3-subfamily MATEs and the ARF2 and PIF5/4 TFs. Taken together, our findings reveal a regulatory paradigm in which the ARF2-PIF5/4 functional module facilitates the transcriptional reprogramming in the ABS3-mediated senescence pathway.

Genome-wide identification, characterization and expression analysis of HAK genes and decoding their role in responding to potassium deficiency and abiotic stress in Medicago truncatula

Background

The HAK family is the largest potassium (K⁺) transporter family, vital in K⁺ uptake, plant growth, and both plant biotic and abiotic stress responses. Although HAK family members have been characterized and functionally investigated in many species, these genes are still not studied in detail in Medicago truncatula, a good model system for studying legume genetics.

Methods

In this study, we screened the M. truncatula HAK family members (MtHAKs). Furthermore, we also conducted the identification, phylogenetic analysis, and prediction of conserved motifs of MtHAKs. Moreover, we studied the expression levels of MtHAKs under K⁺ deficiency, drought, and salt stresses using quantitative real-time PCR (qRT-PCR).

Results

We identified 20 MtHAK family members and classified them into three clusters based on phylogenetic relationships. Conserved motif analyses showed that all MtHAK proteins besides MtHAK10 contained the highly conserved K⁺ transport domain (GVVYGDLGTSPLY). qRT-PCR analysis showed that several MtHAK genes in roots were induced by abiotic stress. In particular, MtHAK15, MtHAK17, and MtHAK18 were strongly up-regulated in the M. truncatula roots under K⁺ deficiency, drought, and salt stress conditions, thereby implying that these genes are good candidates for high-affinity K⁺ uptake and therefore have essential roles in drought and salt tolerance.

Discussions

Our results not only provided the first genetic description and evolutionary relationships of the K⁺ transporter family in M. truncatula, but also the potential information responding to K⁺ deficiency and abiotic stresses, thereby laying the foundation for molecular breeding of stress-resistant legume crops in the future.

Transcriptome analysis of sweet potato responses to potassium deficiency

Background

As one of three essential nutrients, potassium is regarded as a main limiting factor for growth and development in plant. Sweet potato (Ipomoea batatas L.) is one of seven major food crops grown worldwide, and is both a nutrient-rich food and a bioenergy crop. It is a typical ‘K-favoring’ crop, and the level of potassium ion (K⁺) supplementation directly influences its production. However, little is known about the transcriptional changes in sweet potato genes under low-K⁺ conditions. Here, we analyzed the transcriptomic profiles of sweet potato roots in response to K⁺ deficiency to determine the effect of low-K⁺ stress on this economically important crop.

Results

The roots of sweet potato seedlings with or without K⁺ treatment were harvested and used for transcriptome analyses. The results showed 559 differently expressed genes (DEGs) in low and high K⁺ groups. Among the DEGs, 336 were upregulated and 223 were downregulated. These DEGs were involved in transcriptional regulation, calcium binding, redox-signaling, biosynthesis, transport, and metabolic process. Further analysis revealed previously unknow genes involved in low-K⁺ stress, which could be investigated further to improve low K⁺ tolerance in plants. Confirmation of RNA-sequencing results using qRT-PCR displayed a high level of consistency between the two experiments. Analysis showed that many auxin-, ethylene- and jasmonic acid-related genes respond to K⁺ deficiency, suggesting that these hormones have important roles in K⁺ nutrient signaling in sweet potato.

Conclusions

According to the transcriptome data of sweet potato, various DEGs showed transcriptional changes in response to low-K⁺ stress. However, the expression level of some kinases, transporters, transcription factors (TFs), hormone-related genes, and plant defense-related genes changed significantly, suggesting that they have important roles during K⁺ deficiency. Thus, this study identifies potential genes for genetic improvement of responses to low-K⁺ stress and provides valuable insight into the molecular mechanisms regulating low K⁺ tolerance in sweet potato. Further research is required to clarify the function of these DEGs under low-K⁺ stress.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08870-5.

Overexpression of OsHAK5 potassium transporter enhances virus resistance in rice (Oryza sativa)

Intracellular potassium (K⁺ ) transported by plants under the action of a number of transport proteins is crucial for plant survival under distinct abiotic and biotic stresses. A correlation between K⁺ status and disease incidence has been found in many studies, but the roles of K⁺ in regulating disease resistance to viral diseases remain elusive. Here, we report that HIGH-AFFINITY K⁺ TRANSPORTER 5 (OsHAK5) regulates the infection of rice grassy stunt virus (RGSV), a negative-sense single-stranded bunyavirus, in rice (Oryza sativa). We found the K⁺ content in rice plants was significantly inhibited on RGSV infection. Meanwhile, a dramatic induction of OsHAK5 transcripts was observed in RGSV-infected rice plants and in rice plants with K⁺ deficiency. Genetic analysis indicated that disruption of OsHAK5 facilitated viral pathogenicity. In contrast, overexpression of OsHAK5 enhanced resistance to RGSV infection. Our analysis of reactive oxygen species (ROS) including H₂ O₂ and O^2- , by DAB and NBT staining, respectively, indicated that RGSV infection as well as OsHAK5 overexpression increased ROS accumulation in rice leaves. The accumulation of ROS is perhaps involved in the induction of host resistance against RGSV infection in OsHAK5 transgenic overexpression rice plants. Furthermore, RGSV-encoded P3 induced OsHAK5 promoter activity, suggesting that RGSV P3 is probably an elicitor for the induction of OsHAK5 transcripts during RGSV infection. These findings indicate the crucial role of OsHAK5 in host resistance to virus infection. Our results may be exploited in the future to increase crop yield as well as improve host resistance via genetic manipulations.

Genome Editing Targets for Improving Nutrient Use Efficiency and Nutrient Stress Adaptation

In recent years, the development of RNA-guided genome editing (CRISPR-Cas9 technology) has revolutionized plant genome editing. Under nutrient deficiency conditions, different transcription factors and regulatory gene networks work together to maintain nutrient homeostasis. Improvement in the use efficiency of nitrogen (N), phosphorus (P) and potassium (K) is essential to ensure sustainable yield with enhanced quality and tolerance to stresses. This review outlines potential targets suitable for genome editing for understanding and improving nutrient use (NtUE) efficiency and nutrient stress tolerance. The different genome editing strategies for employing crucial negative and positive regulators are also described. Negative regulators of nutrient signalling are the potential targets for genome editing, that may improve nutrient uptake and stress signalling under resource-poor conditions. The promoter engineering by CRISPR/dead (d) Cas9 (dCas9) cytosine and adenine base editing and prime editing is a successful strategy to generate precise changes. CRISPR/dCas9 system also offers the added advantage of exploiting transcriptional activators/repressors for overexpression of genes of interest in a targeted manner. CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) are variants of CRISPR in which a dCas9 dependent transcription activation or interference is achieved. dCas9-SunTag system can be employed to engineer targeted gene activation and DNA methylation in plants. The development of nutrient use efficient plants through CRISPR-Cas technology will enhance the pace of genetic improvement for nutrient stress tolerance of crops and improve the sustainability of agriculture.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

Arabidopsis nitrate-induced aspartate oxidase gene expression is necessary to maintain metabolic balance under nitrogen nutrient fluctuation

Nitrate is a nutrient signal that regulates growth and development through NLP transcription factors in plants. Here we identify the L-aspartate oxidase gene (AO) necessary for de novo NAD⁺ biosynthesis as an NLP target in Arabidopsis. We investigated the physiological significance of nitrate-induced AO expression by expressing AO under the control of the mutant AO promoter lacking the NLP-binding site in the ao mutant. Despite morphological changes and severe reductions in fresh weight, the loss of nitrate-induced AO expression resulted in minimum effects on NAD(H) and NADP(H) contents, suggesting compensation of decreased de novo NAD⁺ biosynthesis by reducing the growth rate. Furthermore, metabolite profiling and transcriptome analysis revealed that the loss of nitrate-induced AO expression causes pronounced impacts on contents of TCA cycle- and urea cycle-related metabolites, gene expression profile, and their modifications in response to changes in the nitrogen nutrient condition. These results suggest that proper maintenance of metabolic balance requires the coordinated regulation of multiple metabolic pathways by NLP-mediated nitrate signaling in plants.

NLP transcription factors directly regulate aspartate oxidase gene expression connected to multiple metabolic pathways in Arabidopsis in response to changes in the nitrogen nutrient condition.

Identification of ARF family in blueberry and its potential involvement of fruit development and pH stress response

Background

Auxin responsive factor (ARF) family is one of core components in auxin signalling pathway, which governs diverse developmental processes and stress responses. Blueberry is an economically important berry-bearing crop and prefers to acidic soil. However, the understandings of ARF family has not yet been reported in blueberry.

Results

In the present study, 60 ARF genes (VcARF) were identified in blueberry, and they showed diverse gene structures and motif compositions among the groups and similar within each group in the phylogenetic tree. Noticeably, 9 digenic, 5 trigenic and 6 tetragenic VcARF pairs exhibited more than 95% identity to each other. Computational analysis indicated that 23 VcARFs harbored the miRNA responsive element (MRE) of miR160 or miR167 like other plant ARF genes. Interestingly, the MRE of miR156d/h-3p was observed in the 5’UTR of 3 VcARFs, suggesting a potentially novel post-transcriptional control. Furthermore, the transcript accumulations of VcARFs were investigated during fruit development, and three categories of transcript profiles were observed, implying different functional roles. Meanwhile, the expressions of VcARFs to different pH conditions (pH4.5 and pH6.5) were surveyed in pH-sensitive and tolerant blueberry species, and a number of VcARFs showed different transcript accumulations. More importantly, distinct transcriptional response to pH stress (pH6.5) were observed for several VcARFs (such as VcARF6s and VcARF19-3/19–4) between pH-sensitive and tolerant species, suggesting their potential roles in adaption to pH stress.

Conclusions

Sixty VcARF genes were identified and characterized, and their transcript profiles were surveyed during fruit development and in response to pH stress. These findings will contribute to future research for eliciting the functional roles of VcARFs and regulatory mechanisms, especially fruit development and adaption to pH stress.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08556-y.

Insights to improve the plant nutrient transport by CRISPR/Cas system

We need to improve food production to feed the ever growing world population especially in a changing climate. Nutrient deficiency in soils is one of the primary bottlenecks affecting the crop production both in developed and developing countries. Farmers are forced to apply synthetic fertilizers to improve the crop production to meet the demand. Understanding the mechanism of nutrient transport is helpful to improve the nutrient-use efficiency of crops and promote the sustainable agriculture. Many transporters involved in the acquisition, export and redistribution of nutrients in plants are characterized. In these studies, heterologous systems like yeast and Xenopus were most frequently used to study the transport function of plant nutrient transporters. CRIPSR/Cas system introduced recently has taken central stage for efficient genome editing in diverse organisms including plants. In this review, we discuss the key nutrient transporters involved in the acquisition and redistribution of nutrients from soil. We draw insights on the possible application CRISPR/Cas system for improving the nutrient transport in plants by engineering key residues of nutrient transporters, transcriptional regulation of nutrient transport signals, engineering motifs in promoters and transcription factors. CRISPR-based engineering of plant nutrient transport not only helps to study the process in native plants with conserved regulatory system but also aid to develop non-transgenic crops with better nutrient use-efficiency. This will reduce the application of synthetic fertilizers and promote the sustainable agriculture strengthening the food and nutrient security.

Identification and stress function verification of the HAK/KUP/KT family in Gossypium hirsutum

The potassium transporter family HAK/KUP/KT is a large group of proteins that are important in plant potassium transport and play a crucial role in plant growth and development. The members of the family play an important role in the response of plants to abiotic stress by maintaining osmotic balance. However, the function of the family in cotton is unclear. In this study, whole genome identification and characterization of the HAK/KUP/KT family from upland cotton (Gossypium hirsutum) were carried out. Bioinformatics methods were used to identify HAK/KUP/KT family members from the G. hirsutum genome and to analyse the physical and chemical properties, basic characteristics, phylogeny, chromosome location and expression of HAK/KUP/KT family members. A total of 41 HAK/KUP/KT family members were identified in the G. hirsutum genome. Phylogenetic analysis grouped these genes into four clusters (I, II, III, IV), containing 6, 10, 3 and 22 genes, respectively. Chromosomal distribution, gene structure and conserved motif analyses of the 41 GhHAK genes were subsequently performed. The RNA-seq data and qRT-PCR results showed that the family had a wide range of tissue expression patterns, and they responded to certain drought stresses. Through expression analysis, seven HAK/KUP/KT genes involved in drought stress were screened, and four genes with obvious phenotypes under drought stress were obtained by VIGS verification, which laid a theoretical foundation for the function of the cotton HAK/KUP/KT family.

BcSOC1 Promotes Bolting and Stem Elongation in Flowering Chinese Cabbage

Flowering Chinese cabbage is one of the most economically important stalk vegetables. However, the molecular mechanisms underlying bolting, which is directly related to stalk quality and yield, in this species remain unknown. Previously, we examined five key stem development stages in flowering Chinese cabbage. Here, we identified a gene, BcSOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1), in flowering Chinese cabbage using transcriptome analysis, whose expression was positively correlated with bolting. Exogenous gibberellin (GA3) and low-temperature treatments significantly upregulated BcSOC1 and promoted early bolting and flowering. Additionally, BcSOC1 overexpression accelerated early flowering and stem elongation in both Arabidopsis and flowering Chinese cabbage, whereas its knockdown dramatically delayed bolting and flowering and inhibited stem elongation in the latter; the inhibition of stem elongation was more notable than delayed flowering. BcSOC1 overexpression also induced cell expansion by upregulating genes encoding cell wall structural proteins, such as BcEXPA11 (cell wall structural proteins and enzymes) and BcXTH3 (xyloglucan endotransglycosidase/hydrolase), upon exogenous GA3 and low-temperature treatments. Moreover, the length of pith cells was correlated with stem height, and BcSOC1 interacted with BcAGL6 (AGAMOUS-LIKE 6) and BcAGL24 (AGAMOUS-LIKE 24). Thus, BcSOC1 plays a vital role in bolting and stem elongation of flowering Chinese cabbage and may play a novel role in regulating stalk development, apart from the conserved function of Arabidopsis SOC1 in flowering alone.

Ethylene-induced potassium transporter AcKUP2 gene is involved in kiwifruit postharvest ripening

BACKGROUND

Potassium (K) is important in the regulation of plant growth and development. It is the most abundant mineral element in kiwifruit, and its content increases during fruit ripening. However, how K⁺ transporter works in kiwifruit postharvest maturation is not yet clear.

RESULTS

Here, 12 K⁺ transporter KT/HAK/KUP genes, AcKUP1 ~ AcKUP12, were isolated from kiwifruit, and their phylogeny, genomic structure, chromosomal location, protein properties, conserved motifs and cis-acting elements were analysed. Transcription analysis revealed that AcKUP2 expression increased rapidly and was maintained at a high level during postharvest maturation, consistent with the trend of K content; AcKUP2 expression was induced by ethylene, suggesting that AcKUP2 might play a role in ripening. Fluorescence microscopy showed that AcKUP2 is localised in the plasma membrane. Cis-elements, including DER or ethylene response element (ERE) responsive to ethylene, were found in the AcKUP2 promoter sequence, and ethylene significantly enhanced the AcKUP2 promoter activity. Furthermore, we verified that AcERF15, an ethylene response factor, directly binds to the AcKUP2 promoter to promote its expression. Thus, AcKUP2 may be an important potassium transporter gene which involved in ethylene-regulated kiwifruit postharvest ripening.

CONCLUSIONS

Therefore, our study establishes the first genome-wide analysis of the kiwifruit KT/HAK/KUP gene family and provides valuable information for understanding the function of the KT/HAK/KUP genes in kiwifruit postharvest ripening.

UNFERTILIZED EMBRYO SAC 12 phosphorylation plays a crucial role in conferring salt tolerance

Arabidopsis (Arabidopsis thaliana) UNFERTILIZED EMBRYO SAC 12 (AtUNE12) belongs to the basic helix-loop-helix DNA-binding superfamily of proteins. However, its function is not well known. Here, we found that AtUNE12 plays an important role in mediating salt tolerance. AtUNE12 is a transcriptional activator located in the nucleus whose expression is induced by NaCl, mannitol, and abscisic acid. In addition to binding to the G-box "CACGTG", AtUNE12 also binds to the low temperature responsive element 15 (LTRE15) "CCGAC". Furthermore, the serine residue at position 108 of AtUNE12 is phosphorylated during the salt stress response, enabling AtUNE12 to trigger gene expression by binding to G-box and/or LTRE15 motifs. Phosphorylated AtUNE12 regulates the expression of the genes involved in ion transport leading to reduced Na+ accumulation and K+ loss. At the same time, phosphorylation of AtUNE12 also induces the expression of AtMYB61 to decrease stomatal aperture, leading to a reduced transpiration rate. Overall, AtUNE12 serves as a transcriptional activator that is induced and phosphorylated upon salt stress, and the induction and phosphorylation of AtUNE12 in turn activate the salt-overly-sensitive pathway and decrease the stomatal aperture, enabling improved salt tolerance.

Morphophysiological and transcriptome analysis reveal that reprogramming of metabolism, phytohormones and root development pathways governs the potassium (K+) deficiency response in two contrasting chickpea cultivars

Potassium (K⁺) is an essential macronutrient for plant growth and development. K⁺ deficiency hampers important plant processes, such as enzyme activation, protein synthesis, photosynthesis and stomata movement. Molecular mechanism of K⁺ deficiency tolerance has been partly understood in model plants Arabidopsis, but its knowledge in legume crop chickpea is missing. Here, morphophysiological analysis revealed that among five high yielding desi chickpea cultivars, PUSA362 shows stunted plant growth, reduced primary root growth and low K⁺ content under K⁺ deficiency. In contrast, PUSA372 had negligible effect on these parameters suggesting that PUSA362 is K⁺ deficiency sensitive and PUSA372 is a K⁺ deficiency tolerant chickpea cultivar. RNA-seq based transcriptome analysis under K⁺ deficiency revealed a total of 820 differential expressed genes (DEG's) in PUSA362 and 682 DEGs in PUSA372. These DEGs belongs to different functional categories, such as plant metabolism, signal transduction components, transcription factors, ion/nutrient transporters, phytohormone biosynthesis and signalling, and root growth and development. RNA-seq expression of randomly selected 16 DEGs was validated by RT-qPCR. Out of 16 genes, 13 showed expression pattern similar to RNA-seq expression, that verified the RNA-seq expression data. Total 258 and 159 genes were exclusively up-regulated, and 386 and 347 genes were down-regulated, respectively in PUSA362 and PUSA372. 14 DEGs showed contrasting expression pattern as they were up-regulated in PUSA362 and down-regulated in PUSA372. These include somatic embryogenesis receptor-like kinase 1, thaumatin-like protein, ferric reduction oxidase 2 and transcription factor bHLH93. Nine genes which were down-regulated in PUSA362 found to be up-regulated in PUSA372, including glutathione S-transferase like, putative calmodulin-like 19, high affinity nitrate transporter 2.4 and ERF17-like protein. Some important carbohydrate metabolism related genes, like fructose-1,6-bisphosphatase and sucrose synthase, and root growth related Expansin gene were exclusively down-regulated, while an ethylene biosynthesis gene 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1) was up-regulated in PUSA362. Interplay of these and several other genes related to hormones (auxin, cytokinin, GA etc.), signal transduction components (like CBLs and CIPKs), ion transporters and transcription factors might underlie the contrasting response of two chickpea cultivars to K⁺ deficiency. In future, some of these key genes will be utilized in genetic engineering and breeding programs for developing chickpea cultivars with improved K⁺ use efficiency (KUE) and K⁺ deficiency tolerance traits.

Nitrogen form-mediated ethylene signal regulates root-to-shoot K+ translocation via NRT1.5

Nitrogen-potassium synergistic and antagonistic interactions are the typical case of nutrient interactions. However, the underlying mechanism for the integration of the external N form into K⁺ homeostasis remains unclear. Here, we show that opposite effects of NO₃^- and NH₄⁺ on root-shoot K⁺ translocation were due to differential regulation of an ethylene signalling pathway targeting the NRT1.5 transporter. NH₄⁺ upregulated the transcriptional activity of EIN3, but repressed the expression of NRT1.5. However, the addition of NO₃^- strongly suppressed the activity of EIN3, whereas its addition upregulated the expression of AtNRT1.5 and shoot K⁺ concentration. The 35S:EIN3/ein3eil1 plants, nrt1.5 mutants and nrt1.5/skor double mutants displayed a low K⁺ chlorosis phenotype, especially under NH₄⁺ conditions with low K⁺ supply. Ion content analyses indicate that root-to-shoot K⁺ translocation was significantly reduced in these mutants. A Y1H assay, an EMSA and a transient expression assay confirmed that AtEIN3 protein could directly bind to the promoter of NRT1.5 to repress its expression. Furthermore, grafted plants with the roots of 35S:EIN3 and ein3eil1/nrt1.5 mutants displayed marked leaf chlorosis with a low K⁺ concentration. Collectively, our findings reveal that the interaction between N form and K⁺ was achieved by modulating root-derived ethylene signals to regulate root-to-shoot K⁺ translocation via NRT1.5.

A spatial-temporal understanding of gene regulatory networks and NtARF-mediated regulation of potassium accumulation in tobacco

MAIN CONCLUSION

After tobacco topping, changes in the auxin content could affect K+ uptake by inhibiting the activity of K+ uptake-related genes through the NtARF genes, thus causing changes in K+ content. Tobacco (Nicotiana tabacum) is a valuable industrial and commercial crop, and the leaf is its primary product. Topping (removing apical buds) is a common agronomic practice that significantly improves the yield of tobacco leaves. Potassium (K+) plays an important physiological role in tobacco growth and leaf traits, including combustibility, aroma, and safety in cigarette products, and its levels are significantly decreased after topping. Here, to present global spatial-temporal gene expression profiles and gene regulatory networks of the core elements of K+ uptake, leaves and roots from topped and untopped plants at short- and long-term time points after topping were sampled for transcriptome analysis. We found that the wounding response was initiated in leaves in the early stages after topping. Then, in the long term, processes related to metabolism and transcription regulation, as well as ion binding and transport, were altered. The expression profiles showed that core elements of K+ uptake and xylem loading were drastically suppressed in roots after topping. Finally, transient expression experiments confirmed that changes in the auxin content could affect K+ uptake by inhibiting the activity of K+ uptake-related genes through the tobacco auxin response factor (NtARF) genes, thus causing changes in the K+ content. These results suggest that some ARFs could be selected as targets to enhance the expressions of K+ uptake transporters, leading to increment of K+ contents and improvement of leaf quality in tobacco breeding.

The Landscape of Alternative Splicing Regulating Potassium Use Efficiency in Nicotiana tabacum

Alternative splicing (AS) occurs extensively in eukaryotes as an essential mechanism for regulating transcriptome complexity and diversity, but the AS landscape regulating potassium (K) use efficiency in plants is unclear. In this study, we performed high-throughput transcriptome sequencing of roots and shoots from allopolyploid Nicotiana tabacum under K⁺ deficiency. Preliminary physiological analysis showed that root system architecture was dramatically changed due to potassium deficiency and that IAA content was significantly reduced in root and shoot. AS analysis showed that a total of 28,179 genes exhibited 54,457 AS events, and 1,510 and 1,732 differentially alternatively spliced (DAS) events were identified in shoots and roots under low K⁺ stress. Nevertheless, only 120 DAS events occurred in both shoots and roots, implying that most DAS events were tissue-specific. Both in shoot and the root, the proportion of DAS genes in differentially expressed (DE) genes equaled that in non-DE genes, which indicated that AS might play a unique regulatory role in response to low potassium. Gene ontology analysis further indicated that transcription regulation and AS modulation worked independently in response to low K⁺ stress in tobacco, as their target biological processes were different. Totally 45 DAS transcription factors (TFs) were found, which were involved in 18 TF families. Five Auxin response factor (ARF) TFs were significantly DAS in root, suggesting that response to auxin was probably subject to AS regulation in the tobacco root. Our study shows that AS variation occurs extensively and has a particular regulatory mechanism under K⁺ deficiency in tobacco. The study also links changes in root system architecture with the changes in AS of ARF TFs, which implied the functional significance of these AS events for root growth and architecture.

PpMYB36 Encodes a MYB-Type Transcription Factor That Is Involved in Russet Skin Coloration in Pear (Pyrus pyrifolia)

Fruit color is one of the most important external qualities of pear (Pyrus pyrifolia) fruits. However, the mechanisms that control russet skin coloration in pear have not been well characterized. Here, we explored the molecular mechanisms that determine the russet skin trait in pear using the F₁ population derived from a cross between russet skin ('Niitaka') and non-russet skin ('Dangshansu') cultivars. Pigment measurements indicated that the lignin content in the skin of the russet pear fruits was greater than that in the non-russet pear skin. Genetic analysis revealed that the phenotype of the russet skin pear is associated with an allele of the PpRus gene. Using bulked segregant analysis combined with the genome sequencing (BSA-seq), we identified two simple sequence repeat (SSR) marker loci linked with the russet-colored skin trait in pear. Linkage analysis showed that the PpRus locus maps to the scaffold NW_008988489.1: 53297-211921 on chromosome 8 in the pear genome. In the mapped region, the expression level of LOC103929640 was significantly increased in the russet skin pear and showed a correlation with the increase of lignin content during the ripening period. Genotyping results demonstrated that LOC103929640 encoding the transcription factor MYB36 is the causal gene for the russet skin trait in pear. Particularly, a W-box insertion at the PpMYB36 promoter of russet skin pears is essential for PpMYB36-mediated regulation of lignin accumulation and russet coloration in pear. Overall, these results show that PpMYB36 is involved in the regulation of russet skin trait in pear.

Casting the Net-Connecting Auxin Signaling to the Plant Genome

Auxin represents one of the most potent and most versatile hormonal signals in the plant kingdom. Built on a simple core of only a few dedicated components, the auxin signaling system plays important roles for diverse aspects of plant development, physiology, and defense. Key to the diversity of context-dependent functional outputs generated by cells in response to this small molecule are gene duplication events and sub-functionalization of signaling components on the one hand, and a deep embedding of the auxin signaling system into complex regulatory networks on the other hand. Together, these evolutionary innovations provide the mechanisms to allow each cell to display a highly specific auxin response that suits its individual requirements. In this review, we discuss the regulatory networks connecting auxin with a large number of diverse pathways at all relevant levels of the signaling system ranging from biosynthesis to transcriptional response.

MYB77 regulates high-affinity potassium uptake by promoting expression of HAK5

In Arabidopsis, the high-affinity K⁺ transporter HAK5 is the major pathway for root K⁺ uptake when below 100 µM; HAK5 responds to Low-K⁺ (LK) stress by strongly and rapidly increasing its expression during K⁺ -deficiency. Therefore, positive regulators of HAK5 expression have the potential to improve K⁺ uptake under LK. Here, we show that mutants of the transcription factor MYB77 share a LK-induced leaf chlorosis phenotype, lower K⁺ content, and lower Rb⁺ uptake of the hak5 mutant, but not the shorter root growth, and that overexpression of MYB77 enhanced K⁺ uptake and improved tolerance to LK stress. Furthermore, we demonstrated that MYB77 positively regulates the expression of HAK5, by binding to the HAK5 promoter and enhances high-affinity K⁺ uptake of roots. As such, our results reveal a novel pathway for enhancing HAK5 expression under LK stress, and provides a candidate for increasing the tolerance of plants to LK.

A potassium-sensing niche in Arabidopsis roots orchestrates signaling and adaptation responses to maintain nutrient homeostasis

Organismal homeostasis of the essential ion K⁺ requires sensing of its availability, efficient uptake, and defined distribution. Understanding plant K⁺ nutrition is essential to advance sustainable agriculture, but the mechanisms underlying K⁺ sensing and the orchestration of downstream responses have remained largely elusive. Here, we report where plants sense K⁺ deprivation and how this translates into spatially defined ROS signals to govern specific downstream responses. We define the organ-scale K⁺ pattern of roots and identify a postmeristematic K⁺-sensing niche (KSN) where rapid K⁺ decline and Ca²⁺ signals coincide. Moreover, we outline a bifurcating low-K⁺-signaling axis of CIF peptide-activated SGN3-LKS4/SGN1 receptor complexes that convey low-K⁺-triggered phosphorylation of the NADPH oxidases RBOHC, RBOHD, and RBOHF. The resulting ROS signals simultaneously convey HAK5 K⁺ uptake-transporter induction and accelerated Casparian strip maturation. Collectively, these mechanisms synchronize developmental differentiation and transcriptome reprogramming for maintaining K⁺ homeostasis and optimizing nutrient foraging by roots.

Whole-Genome Duplication and Purifying Selection Contributes to the Functional Redundancy of Auxin Response Factor (ARF) Genes in Foxtail Millet (Setaria italica L.)

Auxin response factors (ARFs) play crucial roles in auxin-mediated response, whereas molecular genetics of ARF genes was seldom investigated in Setaria italica, an important crop and C₄ model plant. In the present study, genome-wide evolutionary analysis of ARFs was performed in S. italica. Twenty-four SiARF genes were identified and unevenly distributed on eight of the nine chromosomes in S. italica. Duplication mode exploration implied that 13 SiARF proteins were originated from whole-genome duplication and suffered purifying selection. Phylogeny reconstruction of SiARFs by maximum likelihood and neighbor-joining trees revealed SiARFs could be divided into four clades. SiARFs clustered within the same clade shared similar gene structure and protein domain composition, implying functional redundancy. Moreover, amino acid composition of the middle regions was conserved in SiARFs belonged to the same clade. SiARFs were categorized into either activators or repressors according to the enrichment of specific amino acids. Intrinsic disorder was featured in the middle regions of ARF activators. Finally, expression profiles of SiARFs under hormone and abiotic stress treatment not only revealed their potential function in stress response but also indicate their functional redundancy. Overall, our results provide insights into evolutionary aspects of SiARFs and benefit for further functional characterization.

Transcriptome analysis reveals the promotive effect of potassium by hormones and sugar signaling pathways during adventitious roots formation in the apple rootstock

Apples are economically valuable and widely consumed fruits. The adventitious roots (ARs) formation is gridlock for apple trees mass propagation. The possible function of multiple hormones and sugar signaling pathways regulating ARs formation has not been completely understood in apple. In this study, B9 stem cuttings were treated with KCl treatment, where the highest root numbers (220) and maximum root length of 731.2 cm were noticed in KCl-treated cuttings, which were 98.2% and 215% higher than control cuttings. The content of endogenous hormones: IAA, ZR, JA, GA, and ABA were detected higher in response to KCl at most time-points. To figure out the molecular mechanisms underlying this effect, we investigated transcriptome analysis. In total, 4631 DEGs were determined, from which about 202 DEGs were considerably enriched in pathways associated with hormone signaling, sugar metabolism, root development, and cell cycle-related and were thereupon picked out on their potential involvements in ARs formation. Though, IAA accumulation and up-regulation of various genes contribute to induce AR formation. These results suggest that AR formation is a complex biological process in apple rootstocks, influenced mainly by the auxin signaling pathway and sugar metabolism.