Nicotiana sylvestris calcineurin B-like protein NsylCBL10 enhances salt tolerance in transgenic Arabidopsis

Determination of optimal NH4+/K + concentration and corresponding ratio critical for growth of tobacco seedlings in a hydroponic system

Inherently, ammonium (NH₄ ⁺) is critical for plant growth; however, its toxicity suppresses potassium (K⁺) uptake and vice-versa. Hence, attaining a nutritional balance between these two ions (NH₄ ⁺ and K⁺) becomes imperative for the growth of tobacco seedlings. Therefore, we conducted a 15-day experimental study on tobacco seedlings exposed to different concentrations (47 treatments) of NH₄ ⁺/K⁺ at different corresponding 12 ratios simultaneously in a hydroponic system. Our study aimed at establishing the optimal NH₄ ⁺-K⁺ concentration and the corresponding ratio required for optimal growth of different tobacco plant organs during the seedling stage. The controls were the baseline for comparison in this study. Plants with low or excessive NH₄ ⁺-K⁺ concentration had leaf chlorosis or dark greenish colouration, stunted whole plant part biomass, and thin roots. We found that adequate K⁺ supply is a pragmatic way to mitigate NH₄ ⁺-induced toxicity in tobacco plants. The optimal growth for tobacco leaf and root was attained at NH₄ ⁺-K⁺ concentrations 2-2 mM (ratio 1:1), whereas stem growth was optimal at NH₄ ⁺-K⁺ 1-2 mM (1:2). The study provided an insight into the right combination of NH₄ ⁺/K⁺ that could mitigate or prevent NH₄ ⁺ or K⁺ stress in the tobacco seedlings.

Tobacco leaf tissue rapidly detoxifies direct salt loads without activation of calcium and SOS signaling

Salt stress is a major abiotic stress, responsible for declining agricultural productivity. Roots are regarded as hubs for salt detoxification, however, leaf salt concentrations may exceed those of roots. How mature leaves manage acute sodium chloride (NaCl) stress is mostly unknown. To analyze the mechanisms for NaCl redistribution in leaves, salt was infiltrated into intact tobacco leaves. It initiated pronounced osmotically-driven leaf movements. Leaf downward movement caused by hydro-passive turgor loss reached a maximum within 2 h. Salt-driven cellular water release was accompanied by a transient change in membrane depolarization but not an increase in cytosolic calcium ion (Ca²⁺ ) level. Nonetheless, only half an hour later, the leaves had completely regained turgor. This recovery phase was characterized by an increase in mesophyll cell plasma membrane hydrogen ion (H⁺ ) pumping, a salt uptake-dependent cytosolic alkalization, and a return of the apoplast osmolality to pre-stress levels. Although, transcript numbers of abscisic acid- and Salt Overly Sensitive pathway elements remained unchanged, salt adaptation depended on the vacuolar H⁺ /Na⁺ -exchanger NHX1. Altogether, tobacco leaves can detoxify sodium ions (Na⁺ ) rapidly even under massive salt loads, based on pre-established posttranslational settings and NHX1 cation/H⁺ antiport activity. Unlike roots, signaling and processing of salt stress in tobacco leaves does not depend on Ca²⁺ signaling.

Differential Effects of Ammonium (NH4+) and Potassium (K+) Nutrition on Photoassimilate Partitioning and Growth of Tobacco Seedlings

Plants utilize carbohydrates as the main energy source, but much focus has been on the impact of N and K on plant growth. Less is known about the combined impact of NH₄⁺ and K⁺ nutrition on photoassimilate distribution among plant organs, and the resultant effect of such distribution on growth of tobacco seedlings, hence this study. Here, we investigated the synergetic effect of NH₄⁺ and K⁺ nutrition on photoassimilate distribution, and their resultant effect on growth of tobacco seedlings. Soluble sugar and starch content peaks under moderate NH₄⁺ and moderate K⁺ (2-2 mM), leading to improved plant growth, as evidenced by the increase in tobacco weight and root activity. Whereas, a drastic reduction in the above indicators was observed in plants under high NH₄⁺ and low K⁺ (20-0.2 mM), due to low carbohydrate synthesis and poor photoassimilate distribution. A strong positive linear relationship also exists between carbohydrate (soluble sugar and starch) and the activities of these enzymes but not for invertase. Our findings demonstrated that NH₄⁺ and K⁺-induced ion imbalance influences plant growth and is critical for photoassimilate distribution among organs of tobacco seedlings.

Overexpression of NtCBL5A Leads to Necrotic Lesions by Enhancing Na+ Sensitivity of Tobacco Leaves Under Salt Stress

Many tobacco (Nicotiana tabacum) cultivars are salt-tolerant and thus are potential model plants to study the mechanisms of salt stress tolerance. The CALCINEURIN B-LIKE PROTEIN (CBL) is a vital family of plant calcium sensor proteins that can transmit Ca²⁺ signals triggered by environmental stimuli including salt stress. Therefore, assessing the potential of NtCBL for genetic improvement of salt stress is valuable. In our studies on NtCBL members, constitutive overexpression of NtCBL5A was found to cause salt supersensitivity with necrotic lesions on leaves. NtCBL5A-overexpressing (OE) leaves tended to curl and accumulated high levels of reactive oxygen species (ROS) under salt stress. The supersensitivity of NtCBL5A-OE leaves was specifically induced by Na⁺, but not by Cl^-, osmotic stress, or drought stress. Ion content measurements indicated that NtCBL5A-OE leaves showed sensitivity to the Na⁺ accumulation levels that wild-type leaves could tolerate. Furthermore, transcriptome profiling showed that many immune response-related genes are significantly upregulated and photosynthetic machinery-related genes are significantly downregulated in salt-stressed NtCBL5A-OE leaves. In addition, the expression of several cation homeostasis-related genes was also affected in salt-stressed NtCBL5A-OE leaves. In conclusion, the constitutive overexpression of NtCBL5A interferes with the normal salt stress response of tobacco plants and leads to Na⁺-dependent leaf necrosis by enhancing the sensitivity of transgenic leaves to Na⁺. This Na⁺ sensitivity of NtCBL5A-OE leaves might result from the abnormal Na⁺ compartmentalization, plant photosynthesis, and plant immune response triggered by the constitutive overexpression of NtCBL5A. Identifying genes and pathways involved in this unusual salt stress response can provide new insights into the salt stress response of tobacco plants.

The CBL-Interacting Protein Kinase NtCIPK23 Positively Regulates Seed Germination and Early Seedling Development in Tobacco (Nicotiana tabacum L.)

CBL-interacting protein kinase (CIPK) family is a unique group of serine/threonine protein kinase family identified in plants. Among this family, AtCIPK23 and its homologs in some plants are taken as a notable group for their importance in ions transport and stress responses. However, there are limited reports on their roles in seedling growth and development, especially in Solanaceae plants. In this study, NtCIPK23, a homolog of AtCIPK23 was cloned from Nicotiana tabacum. Expression analysis showed that NtCIPK23 is mainly expressed in the radicle, hypocotyl, and cotyledons of young tobacco seedlings. The transcriptional level of NtCIPK23 changes rapidly and spatiotemporally during seed germination and early seedling growth. To study the biological function of NtCIPK23 at these stages, the overexpressing and CRISPR/Cas9-mediated knock-out (ntcipk23) tobacco lines were generated. Phenotype analysis indicated that knock-out of NtCIPK23 significantly delays seed germination and the appearance of green cotyledon of young tobacco seedling. Overexpression of NtCIPK23 promotes cotyledon expansion and hypocotyl elongation of young tobacco seedlings. The expression of NtCIPK23 in hypocotyl is strongly upregulated by darkness and inhibited under light, suggesting that a regulatory mechanism of light might underlie. Consistently, a more obvious difference in hypocotyl length among different tobacco materials was observed in the dark, compared to that under the light, indicating that the upregulation of NtCIPK23 contributes greatly to the hypocotyl elongation. Taken together, NtCIPK23 not only enhances tobacco seed germination, but also accelerate early seedling growth by promoting cotyledon greening rate, cotyledon expansion and hypocotyl elongation of young tobacco seedlings.

Alfalfa MsCBL4 enhances calcium metabolism but not sodium transport in transgenic tobacco under salt and saline-alkali stress

KEY MESSAGE

MsCBL4 expression in tobacco enhanced its salt and saline-alkali stress tolerance by regulating calcium accumulation in roots, indicating the important role of calcium metabolism in plant saline-alkali stress tolerance The calcineurin B-like (CBL) family of proteins play important roles in plant abiotic stress tolerance and signal transduction. CBL4 is known to participate in the Salt Overly Sensitive pathway; however, little is currently known regarding the mechanisms underlying the response of CBL4 to saline-alkali stress. In this study, we cloned and characterized the alfalfa MsCBL4 gene. We found that MsCBL4 showed the highest expression in root tissues and was induced by salt and saline-alkali stress, with the latter causing higher induction. Overexpression of MsCBL4 in tobacco enhanced salt and saline-alkali stress tolerance and reduced the Na+/K+ ratio in roots of transgenic lines. Salt (30 and 300 mM NaCl) and saline-alkali (30 mM NaHCO3) stress assays performed for MsCBL4 transgenic tobacco lines revealed a substantial influx of sodium ions in roots under saline-alkali stress and indicated that the expression of MsCBL4 had little influence on sodium ion content reduction. In contrast, in roots subjected to saline-alkali stress, calcium accumulation occurred and was significantly enhanced by the overexpression of MsCBL4. Physiological and biochemical analyses indicated that MsCBL4 plays an important role in saline-alkali stress tolerance via its influence on the regulation of calcium transport and accumulation. These results provide novel insights into the saline-alkali stress tolerance mechanisms of plants.

NH4+ Toxicity, Which Is Mainly Determined by the High NH4+/K+ Ratio, Is Alleviated by CIPK23 in Arabidopsis

Ammonium (NH₄⁺) toxicity is always accompanied by ion imbalances, and NH₄⁺ and potassium (K⁺) exhibit a competitive correlation in their uptake and transport processes. In Arabidopsis thaliana, the typical leaf chlorosis phenotype in the knockout mutant of calcineurin B-like interacting protein kinase 23 (CIPK23) is high-NH₄⁺-dependent under low-K⁺ condition. However, the correlation of K⁺ and NH₄⁺ in the occurrence of leaf chlorosis in the cipk23 mutant has not been deeply elucidated. Here, a modified hydroponic experimental system with different gradients of NH₄⁺ and K⁺ was applied. Comparative treatments showed that NH₄⁺ toxicity, which is triggered mainly by the high ratio of NH₄⁺ to K⁺ (NH₄⁺/K⁺ ≥ 10:1 for cipk23) but not by the absolute concentrations of the ions, results in leaf chlorosis. Under high NH₄⁺/K⁺ ratios, CIPK23 is upregulated abundantly in leaves and roots, which efficiently reduces the leaf chlorosis by regulating the contents of NH₄⁺ and K⁺ in plant shoots, while promoting the elongation of primary and lateral roots. Physiological data were obtained to further confirm the role CIPK23 in alleviating NH₄⁺ toxicity. Taken all together, CIPK23 might function in different tissues to reduce stress-induced NH₄⁺ toxicity associated with high NH₄⁺/K⁺ ratios by regulating the NH₄⁺-K⁺ balance in Arabidopsis.

The Ca2+ Sensor Calcineurin B-Like Protein 10 in Plants: Emerging New Crucial Roles for Plant Abiotic Stress Tolerance

Ca²⁺ is a second messenger that mediates plant responses to abiotic stress; Ca²⁺ signals need to be decoded by Ca²⁺ sensors that translate the signal into physiological, metabolic, and molecular responses. Recent research regarding the Ca²⁺ sensor CALCINEURIN B-LIKE PROTEIN 10 (CBL10) has resulted in important advances in understanding the function of this signaling component during abiotic stress tolerance. Under saline conditions, CBL10 function was initially understood to be linked to regulation of Na⁺ homeostasis, protecting plant shoots from salt stress. During this process, CBL10 interacts with the CBL-interacting protein kinase 24 (CIPK24, SOS2), this interaction being localized at both the plasma and vacuolar (tonoplast) membranes. Interestingly, recent studies have exposed that CBL10 is a regulator not only of Na⁺ homeostasis but also of Ca²⁺ under salt stress, regulating Ca²⁺ fluxes in vacuoles, and also at the plasma membrane. This review summarizes new research regarding functions of CBL10 in plant stress tolerance, predominantly salt stress, as this is the most commonly studied abiotic stress associated with the function of this regulator. Special focus has been placed on some aspects that are still unclear. We also pay particular attention on the proven versatility of CBL10 to activate (in a CIPK-dependent manner) or repress (by direct interaction) downstream targets, in different subcellular locations. These in turn appear to be the link through which CBL10 could be a key master regulator of stress signaling in plants and also a crucial participant in fruit development and quality, as disruption of CBL10 results in inadequate Ca²⁺ partitioning in plants and fruit. New emerging roles associated with other abiotic stresses in addition to salt stress, such as drought, flooding, and K⁺ deficiency, are also addressed in this review. Finally, we provide an outline of recent advances in identification of potential targets of CBL10, as CBL10/CIPKs complexes and as CBL10 direct interactions. The aim is to showcase new research regarding this master regulator of abiotic stress tolerance that may be essential to the maintenance of crop productivity under abiotic stress. This is particularly pertinent when considering the scenario of a projected increase in extreme environmental conditions due to climate change.

Comparative expression analysis of Calcineurin B-like family gene CBL10A between salt-tolerant and salt-sensitive cultivars in B. oleracea

Calcineurin B-like proteins (CBLs) are plant calcium sensors that play a critical role in the regulation of plant growth and response to stress. Many CBLs have been identified in the calcium signaling pathway in both Arabidopsis and rice. However, information about BoCBLs genes from Brassica oleracea has not been reported. In the present study, we identified 13 candidate CBL genes in the B. oleracea genome based on the conserved domain of the Calcineurin B-like family, and we carried out a phylogenetic analysis of CBLs among Arabidopsis, rice, maize, cabbage and B. oleracea. For B. oleracea, the distribution of the predicted BoCBL genes was uneven among the five chromosomes. Sequence analysis showed that the nucleotide sequences and corresponding protein structure of BoCBLs were highly conserved, i.e., all of the putative BoCBLs contained 6-8 introns, and most of the exons of those genes contained the same number of nucleotides and had high sequence identities. All BoCBLs consisted of four EF-Hand functional domains, and the distance between the EF-hand motifs was conserved. Evolutionary analysis revealed that the CBLs were classified into two subgroups. Additionally, the CBL10A gene was cloned from salt-tolerant (CB6) and salt-sensitive (CB3) cultivars using RT-PCR. The results indicated that the cloned gene had a substantial difference in length (741bp in CB3 and 829bp in CB6) between these two cultivars. The deduced CBL10A protein in CB6 had four EF-hand structural domains, which have an irreplaceable role in calcium-binding and have calcineurin A subunit binding sites, while the BoCBL10A protein in CB3 had only two EF-hand structural domains and lacked calcineurin A subunit binding sites. The expression level of the BoCBL10A gene between salt tolerance (CB6)and sensitive varieties(CB3) under salt stress was significantly different (P<0.01 and P<0.05). The expression of BoCBL10A gene was relatively higher in salt-tolerant (CB6) cultivar under salt stress, with a longer period of up-regulation expression and a shorter time responding to salt, compared with the salt-sensitive (CB3) cultivar. We speculate that these differences in the coding region of BoCBL10A may lead to the different salt responses between these two cultivars.

Characterization of Salinity Tolerance of Transgenic Rice Lines Harboring HsCBL8 of Wild Barley (Hordeum spontanum) Line from Qinghai-Tibet Plateau

Rice is more sensitive to salinity, particularly at its early vegetative and later productive stages. Wild plants growing in harsh environments such as wild barley from Qinghai-Tibet Plateau adapt to the adverse environment with allelic variations at the loci responsible for stressful environment, which could be used for rice genetic improvement. In this study, we overexpressed HsCBL8 encoding a calcium-sensor calcineurin B-like (CBL) protein in rice. The gene was isolated from XZ166, a wild-barley (Hordeum spontanum) line originated from Qinghai-Tibet Plateau. We found that XZ166 responded to high NaCl concentration (200 mM) with more HsCBL8 transcripts than CM72, a cultivated barley line known for salinity tolerance. XZ166 is significantly different from CM72 with nucleotide sequences at HsCBL8. The overexpression of HsCBL8 in rice resulted in significant improvement of water protection in vivo and plasma membrane, more proline accumulation, and a reduction of overall Na+ uptake but little change in K+ concentration in the plant tissues. Notably, HsCBL8 did not act on some genes downstream of the rice CBL family genes, suggesting an interesting interaction between HsCBL8 and unknown factors to be further investigated.

Overexpression of SpCBL6, a calcineurin B-like protein of Stipa purpurea, enhanced cold tolerance and reduced drought tolerance in transgenic Arabidopsis

The purpose of the present study was to characterize SpCBL6 (GenBank accession number: KT780442) from Stipa purpurea and elucidate the function of this protein in abiotic stress. The full-length cDNA of SpCBL6 was isolated from S. purpurea by rapid amplification of cDNA ends methods. Laser confocal microscopy was used to analyze the subcellular localization of SpCBL6. The constructs of 35S:GFP-SpCBL6 was used to transform wild-type (WT) Arabidopsis plants (ecotype Columbia-0) with the floral dip method. Quantitative reverse-transcription PCR (qRT-PCR), water potential, photosynthetic efficiency (F v/F m), and ion leakage was performed to investigate the role of SpCBL6 in abiotic stress. The open reading frame of SpCBL6 contains 681 bp nucleotides and encodes a 227-amino acid polypeptide. Phylogenetic analysis indicated that SpCBL6 showed the highest similarity with rice OsCBL6. SpCBL6 transcripts were induced by freezing and drought treatments. Subcellular localization analysis showed that SpCBL6 was located in membrane of protoplast. Overexpression of SpCBL6 in Arabidopsis thaliana demonstrated that the transgenic plants were more tolerant to cold treatment, but less tolerant to drought, compared with the plants. qRT-PCR analysis showed that the drought stress marker genes were inhibited in transgenic plants, whereas the cold stress marker genes were enhanced. Further analysis showed that SpCBL6-overexpressing plants showed enhanced water potential, photosynthetic efficiency (F v/F m), and reduced ion leakage compared with the wild-type after cold treatment. Collectively, these results indicate that SpCBL6, a new member of the CBL gene family isolated from S. purpurea, enhances cold tolerance and reduces drought tolerance in plants.