Root high-affinity K+ and Cs+ uptake and plant fertility in tomato plants are dependent on the activity of the high-affinity K+ transporter SlHAK5

SlCIPK9 regulates pollen tube elongation in tomato plants via a K+-independent mechanism

Potassium (K+) is an essential macronutrient which contributes to osmotic- and turgor-related processes in plants. Calcineurin-B like Interacting Protein Kinases (CIPKs) play crucial roles in plants under low-K+ supply since they activate root K+ uptake transport systems such as AKT1 and AtHAK5. In Arabidopsis, AtCIPK9 is important for low-K+ tolerance since atcipk9 plants exhibited poor growth and leaf chlorosis when K+ was scarce. Part of these phenotypes could be ascribed to the activation of AtHAK5 by AtCIPK9. It has been reported that important differences exist between Arabidopsis and other plant species such as tomato with respect to the regulation of K+ uptake systems. Thus, our aim was to evaluate the contribution of SlCIPK9, the homologous protein of AtCIPK9 in tomato, to K+ nutrition. Unexpectedly, phenotyping experiments carried out with slcipk9 loss-of-function mutants revealed that SlCIPK9 did not play a clear role in tomato K+ homeostasis. By contrast, it was found that SlCIPK9 contributed to pollen tube elongation, but not to pollen germination, via a K+-independent mechanism. Therefore, our results highlight the remarkable differences that exist in Ca2+ signaling pathways between plant species and encourage the realization of more comparative studies as the one presented here.

Increased tolerance to low K+, and to cationic stress of Arabidopsis plants by expressing the F130S mutant version of the K+ transporter AtHAK5

Potassium (K+) selectivity of high-affinity K+ uptake systems is crucial for plant growth under low K+ and in the presence of inhibitors of K+ uptake that are toxic to plants such as Na+ or Cs+. Here, we express a mutated version of the Arabidopsis AtHAK5 high-affinity K+ transporter consisting on a change of phenylalanine 130 to serine (F130S) in athak5 akt1 double mutant plants. F130S-expressing plants show better growth, increased K+ uptake from low external concentrations and higher K+ contents when grown at low K+ (10 μM) and when grown at low K+ in the presence of Na+ (15 mM) or Cs+ (1 μM). In addition, these plants accumulate less Na+ and Cs+, resulting in lower Na+/K+ and Cs+/K+ ratios, which are important determinants of plant tolerance to salt stress and to Cs+-polluted soils. Structure analysis of AtHAK5 suggest that the F130 residue approaches the intracellular gate of the K+ tunnel of AtHAK5, affecting somehow its ionic selectivity. Modification of transport systems has a large potential to face challenges of future agriculture such as sustainable production under abiotic stress conditions imposed by climate change.

A B-box transcription factor OsBBX17 regulates saline-alkaline tolerance through the MAPK cascade pathway in rice

Rice OsBBX17 encodes a B-box zinc finger transcription factor in which the N-terminal B-box structural domain interacts with OsMPK1. In addition, it directly binds to the G-box of OsHAK2 and OsHAK7 promoters and represses their transcription. Under saline-alkaline conditions, the expression of OsBBX17 was inhibited. Meanwhile, activation of the OsMPK1-mediated mitogen-activated protein kinase cascade pathway caused OsMPK1 to interact with OsBBX17 and phosphorylate OsBBX17 at the Thr-95 site. It reduced OsBBX17 DNA-binding activity and enhanced saline-alkaline tolerance by deregulating transcriptional repression of OsHAK2 and OsHAK7. Genetic assays showed that the osbbx17-KO had an excellent saline-alkaline tolerance, whereas the opposite was in OsBBX17-OE. In addition, overexpression of OsMPK1 significantly improved saline-alkaline tolerance, but knockout of OsMPK1 caused an increased sensitivity. Further overexpression of OsBBX17 in the osmpk1-KO caused extreme saline-alkaline sensitivity, even a quick death. OsBBX17 was validated in saline-alkaline tolerance from two independent aspects, transcriptional level and post-translational protein modification, unveiling a mechanistic framework by which OsMPK1-mediated phosphorylation of OsBBX17 regulates the transcription of OsHAK2 and OsHAK7 to enhance the Na+ /K+ homeostasis, which partially explains light on the molecular mechanisms of rice responds to saline-alkaline stress via B-box transcription factors for the genetic engineering of saline-alkaline tolerant crops.

Relevance of the SlCIPK23 kinase in Na+ uptake and root morphology in K+-starved tomato plants

The beneficial effects of Na+ as a substitute for K+ have been well-documented at the physiological level. However, the transport systems and regulatory mechanisms that allow Na+ acquisition under K+ deficiency remain poorly understood in the majority of land plants. In tomato, SlCIPK23 kinase was involved in Na+ accumulation in K+-starved plants, in addition to activating the LKT1 K+ channel and the K+ transporter SlHAK5. We used the central role of SlCIPK23 in K+ and Na+ acquisition to study which molecular entities mediate Na+ uptake with knockout tomato mutants and expression in heterologous systems. Two main pathways for Na+ uptake were deduced in tomato plants: an NH4+-sensitive pathway dependent on SlCIPK23, and a second one sensitive to Ba2+, Ca2+, La3+, and Li+. The addition of Na+ (10 mM) to lkt1, slhak5, or slcipk23 mutant KO lines produced interesting changes in root morphology. In particular, the roots of slcipk23 plants were longer and lighter than those of the WT under K+-deficient conditions and this effect was reversed by the addition of 10 mM Na+. These results provide a stimulating perspective for the study of the beneficial effects of Na+ in crops.

A novel function of the tomato CALCINEURIN-B LIKE 10 gene as a root-located negative regulator of salt stress

Climate change exacerbates abiotic stresses like salinization, negatively impacting crop yield, so development of strategies, like using salt-tolerant rootstocks, is crucial. The CALCINEURIN B-LIKE 10 (SlCBL10) gene has been previously identified as a positive regulator of salt tolerance in the tomato shoot. Here, we report a different function of SlCBL10 in tomato shoot and root, as disruption of SlCBL10 only induced salt sensitivity when it was used in the scion but not in the rootstock. The use of SlCBL10 silencing rootstocks (Slcbl10 mutant and RNAi line) improved salt tolerance on the basis of fruit yield. These changes were associated with improved Na+ and K+ homoeostasis, as SlCBL10 silencing reduced the Na+ content and increased the K+ content under salinity, not only in the rootstock but also in the shoot. Improvement of Na+ homoeostasis in Slcbl10 rootstock seems to be mainly due to induction of SlSOS1 expression, while the higher K+ accumulation in roots seems to be mainly determined by expression of LKT1 transporter and SlSKOR channel. These findings demonstrate that SlCBL10 is a negative regulator of salt tolerance in the root, so the use of downregulated SlCBL10 rootstocks may provide a suitable strategy to increase tomato fruit production under salinity.

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.

Inhibition of SlSKOR by SlCIPK23-SlCBL1/9 uncovers CIPK-CBL-target network rewiring in land plants

Transport of K⁺ to the xylem is a key process in the mineral nutrition of the shoots. Although CIPK-CBL complexes have been widely shown to regulate K⁺ uptake transport systems, no information is available about the xylem ones. Here, we studied the physiological roles of the voltage-gated K⁺ channel SlSKOR and its regulation by the SlCIPK23-SlCBL1/9 complexes in tomato plants. We phenotyped gene-edited slskor and slcipk23 tomato knockout mutants and carried out two-electrode voltage-clamp (TEVC) and BiFC assays in Xenopus oocytes as key approaches. SlSKOR was preferentially expressed in the root stele and was important not only for K⁺ transport to shoots but also, indirectly, for that of Ca²⁺ , Mg²⁺ , Na⁺ , NO₃ ^- , and Cl^- . Surprisingly, the SlCIPK23-SlCBL1/9 complexes turned out to be negative regulators of SlSKOR. Inhibition of SlSKOR by SlCIPK23-SlCBL1/9 was observed in Xenopus oocytes and tomato plants. Regulation of SKOR-like channels by CIPK23-CBL1 complexes was also present in Medicago, grapevine, and lettuce but not in Arabidopsis and saltwater cress. Our results provide a molecular framework for coordinating root K⁺ uptake and its translocation to the shoot by SlCIPK23-SlCBL1/9 in tomato plants. Moreover, they evidenced that CIPK-CBL-target networks have evolved differently in land plants.

Genic male and female sterility in vegetable crops

Vegetable crops are greatly appreciated for their beneficial nutritional and health components. Hybrid seeds are widely used in vegetable crops for advantages such as high yield and improved resistance, which require the participation of male (stamen) and female (pistil) reproductive organs. Male- or female-sterile plants are commonly used for production of hybrid seeds or seedless fruits in vegetables. In this review we will focus on the types of genic male sterility and factors affecting female fertility, summarize typical gene function and research progress related to reproductive organ identity and sporophyte and gametophyte development in vegetable crops [mainly tomato (Solanum lycopersicum) and cucumber (Cucumis sativus)], and discuss the research trends and application perspectives of the sterile trait in vegetable breeding and hybrid production, in order to provide a reference for fertility-related germplasm innovation.

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.

Cesium transfer to millet and mustard as a function of Cs availability in soils

¹³⁷Cs is one of the most persistent radioactive contaminants in soil after a nuclear accident. It can be taken up by plants and enter the human food chain generating a potential human health hazard. Although a large amount of literature has highlighted the role of the different processes involved in Cs uptake by plants, there is still no simple way to predict its transfer for a specific plant from a particular soil. Based on the assumption that the concentration ratio (CR) of Cs can be predicted from one plant taxon if the CR of another taxon is known and taken as reference, whatever the supporting soils, a series of plant/soil Cs transfer experiments were performed on Rhizotest during 21 days using three soils with different textures, clay and organic matter contents and two plants (millet and mustard) with potentially contrasting Cs uptake capacity based on their phylogeny. CRs of each plant varied by 2-3 orders of magnitude depending on the soil and contrary to expectations, the CRs of mustard were either higher (for clay soil), equal (for clay-loam soil) or lower (for sandy soil) than the one of millet. Considering Cs availability in soils and defining a new CR based on the amount of Cs available in the soil (CR_avail) decreased the range of variation in CR between the different soil types for a given plant by one order of magnitude. Differences in Cs (and K) translocation to shoots, possibly specific to millet within Poales, could partly explain the relative CRs of millet and mustard as a function of soils.

The protein kinase SlCIPK23 boosts K+ and Na+ uptake in tomato plants

Regulation of root transport systems is essential under fluctuating nutrient supply. In the case of potassium (K⁺ ), HAK/KUP/KT K⁺ transporters and voltage-gated K⁺ channels ensure root K⁺ uptake in a wide range of K⁺ concentrations. In Arabidopsis, the CIPK23/CBL1-9 complex regulates both transporter- and channel-mediated root K⁺ uptake. However, research about K⁺ homeostasis in crops is in demand due to species-specific mechanisms. In the present manuscript, we studied the contribution of the voltage-gated K⁺ channel LKT1 and the protein kinase SlCIPK23 to K⁺ uptake in tomato plants by analysing gene-edited knockout tomato mutant lines, together with two-electrode voltage-clamp experiments in Xenopus oocytes and protein-protein interaction analyses. It is shown that LKT1 is a crucial player in tomato K⁺ nutrition by contributing approximately 50% to root K⁺ uptake under K⁺ -sufficient conditions. Moreover, SlCIPK23 was responsible for approximately 100% of LKT1 and approximately 40% of the SlHAK5 K⁺ transporter activity in planta. Mg⁺² and Na⁺ compensated for K⁺ deficit in tomato roots to a large extent, and the accumulation of Na⁺ was strongly dependent on SlCIPK23 function. The role of CIPK23 in Na⁺ accumulation in tomato roots was not conserved in Arabidopsis, which expands the current set of CIPK23-like protein functions in plants.

A 2.09 Mb fragment translocation on chromosome 6 causes abnormalities during meiosis and leads to less seed watermelon

Seedlessness is a valuable agronomic trait in watermelon (Citrullus lanatus) breeding. Conventional less seed watermelons are mainly triploid, which has many disadvantages due to unbalanced genome content. Less seed watermelon can be achieved at the diploid level when certain reproductive genes are mutated or by chromosome translocation, which leads to defects during meiosis. However, the formation mechanism of diploid less seed watermelons remains largely unknown. Here, we identified a spontaneous mutant line, watermelon line "148", which can set seeds normally when self-pollinated. A total of 148 × JM F₁ hybrid plants exhibited seed number reductions to 50.3% and 47.3% of those of the two parental lines, respectively, which are considered to be less seed. Examination of pollen viability and hybridization experiments revealed that F₁ hybrids produce semisterile pollen and ovules. Further cytological observations indicated that semisterility was a result of a reciprocal translocation of chromosomes, which exhibited one quadrivalent ring of four chromosomes at prometaphase I during meiosis. RT-qPCR analysis indirectly confirmed that the semisterile phenotype is caused by chromosome translocation rather than disruption of specific meiotic gene expression. F₂ population genetic analysis indicated that the "148" watermelon line is a homozygous translocation and that the less seed phenotype of the F₁ hybrid is prompted by one chromosome fragment translocation. The translocated fragment was further fine mapped to a 2.09 Mb region on chromosome 6 by whole-genome resequencing and genetic map cloning procedures. Our work revealed that a 2.09 Mb chromosome fragment translocation on chromosome 6, causing meiotic defects at metaphase I during meiosis, leads to diploid less seed watermelon. Our findings provide a new promising method for less seed watermelon breeding at the diploid level, as well as a fragment size reference for breeding less seed watermelon through artificially induced chromosome translocation.

Disruption of AtHAK/KT/KUP9 enhances plant cesium accumulation under low potassium supply

Understanding the molecular mechanisms that underlie cesium (Cs⁺ ) transport in plants is important to limit the entry of its radioisotopes from contaminated areas into the food chain. The potentially toxic element Cs⁺ , which is not involved in any biological process, is chemically closed to the macronutrient potassium (K⁺ ). Among the multiple K⁺ carriers, the high-affinity K⁺ transporters family HAK/KT/KUP is thought to be relevant in mediating opportunistic Cs⁺ transport. Of the 13 KUP identified in A. thaliana, only HAK5, the major contributor to root K⁺ acquisition under low K⁺ supply, has been functionally demonstrated to be involved in Cs⁺ uptake in planta. In the present study, we showed that accumulation of Cs⁺ increased by up to 30% in two A. thaliana mutant lines lacking KUP9 and grown under low K⁺ supply. Since further experiments revealed that Cs⁺ release from contaminated plants to the external medium is proportionally lower in the two kup9 mutant alleles, we proposed that KUP9 disruption could impair Cs⁺ efflux. By contrast, K⁺ status in kup9 mutants is not affected, suggesting that KUP9 disruption does not alter substantially K⁺ transport in experimental conditions used. The putative primary role of KUP9 in plants is further discussed.

Insights into the mechanisms of transport and regulation of the arabidopsis high-affinity K+ transporter HAK51

The high-affinity K+ transporter HAK5 from Arabidopsis (Arabidopsis thaliana) is essential for K+ acquisition and plant growth at low micromolar K+ concentrations. Despite its functional relevance in plant nutrition, information about functional domains of HAK5 is scarce. Its activity is enhanced by phosphorylation via the AtCIPK23/AtCBL1-9 complex. Based on the recently published three-dimensionalstructure of the bacterial ortholog KimA from Bacillus subtilis, we have modeled AtHAK5 and, by a mutational approach, identified residues G67, Y70, G71, D72, D201, and E312 as essential for transporter function. According to the structural model, residues D72, D201, and E312 may bind K+, whereas residues G67, Y70, and G71 may shape the selective filter for K+, which resembles that of K+shaker-like channels. In addition, we show that phosphorylation of residue S35 by AtCIPK23 is required for reaching maximal transport activity. Serial deletions of the AtHAK5 C-terminus disclosed the presence of an autoinhibitory domain located between residues 571 and 633 together with an AtCIPK23-dependent activation domain downstream of position 633. Presumably, autoinhibition of AtHAK5 is counteracted by phosphorylation of S35 by AtCIPK23. Our results provide a molecular model for K+ transport and describe CIPK-CBL-mediated regulation of plant HAK transporters.