Potassium transporter KUP9 participates in K+ distribution in roots and leaves under low K+ stress

Surrounded by luxury: The necessities of subsidiary cells

The evolution of stomata marks one of the key advances that enabled plants to colonise dry land while allowing gas exchange for photosynthesis. In large measure, stomata retain a common design across species that incorporates paired guard cells with little variation in structure. By contrast, the cells of the stomatal complex immediately surrounding the guard cells vary widely in shape, size and count. Their origins in development are similarly diverse. Thus, the surrounding cells are likely a luxury that the necessity of stomatal control cannot do without (with apologies to Oscar Wilde). Surrounding cells are thought to support stomatal movements as solute reservoirs and to shape stomatal kinetics through backpressure on the guard cells. Their variety may also reflect a substantial diversity in function. Certainly modelling, kinetic analysis and the few electrophysiological studies to date give hints of much more complex contributions in stomatal physiology. Even so, our knowledge of the cells surrounding the guard cells in the stomatal complex is far from complete.

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.

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.

Potassium transporter KUP9 regulates plant response to K+ deficiency and affects carbohydrate allocation in A.thaliana

Due to the essential roles of K+ in plants, its up to 10% share in plant dry matter, and its mostly low availability in soil, effective potassium management poses a significant challenge for the plant. To enable efficient uptake and allocation of K+, numerous transporters and channels have evolved. During the last two decades, efforts have been made to characterise these transport proteins in Arabidopsis thaliana using knock-out mutants. Several KT/HAK/KUP transporters have been assigned specific functions. In this work, we contribute to an understanding of the role of AtKUP9 in plant adaptation to low K+ availability. We found that in vitro, atkup9 has reduced lateral root growth under low-K conditions, and root apical meristem proliferation is reduced in lateral roots compared with the primary root. We also documented AtKUP9 expression in both roots and shoots and showed that AtKUP9 expression is modulated during plant ontogeny and as a result of K+ deprivation. Altered carbohydrate allocation was also documented in atkup9. Mutants exported more soluble saccharides from leaves under K+ rich conditions and, under K+ deficiency, atkup9 accumulated more soluble saccharides in the shoots. A possible role of AtKUP9 in these processes is discussed.

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.

Mapping QTL for Mineral Accumulation and Shoot Dry Biomass in Barley under Different Levels of Zinc Supply

Zinc (Zn) deficiency is a common limiting factor in agricultural soils, which leads to significant reduction in both the yield and nutritional quality of agricultural produce. Exploring the quantitative trait loci (QTL) for shoot and grain Zn accumulation would help to develop new barley cultivars with greater Zn accumulation efficiency. In this study, two glasshouse experiments were conducted by growing plants under adequate and low Zn supply. From the preliminary screening of ten barley cultivars, Sahara (0.05 mg/pot) and Yerong (0.06 mg/pot) showed the lowest change in shoot Zn accumulation, while Franklin (0.16 mg/pot) had the highest change due to changes in Zn supply for plant growth. Therefore, the double haploid (DH) population derived from Yerong × Franklin was selected to identify QTL for shoot mineral accumulation and biomass production. A major QTL hotspot was detected on chromosome 2H between 31.91 and 73.12 cM encoding genes for regulating shoot mineral accumulations of Zn, Fe, Ca, K and P, and the biomass. Further investigation demonstrated 16 potential candidate genes for mineral accumulation, in addition to a single candidate gene for shoot biomass in the identified QTL region. This study provides a useful resource for enhancing nutritional quality and yield potential in future barley breeding programs.