Potassium transporter OsHAK18 mediates potassium and sodium circulation and sugar translocation in rice.

High-affinity potassium (K+) transporter (HAK)/K+ uptake permease (KUP)/K+ transporter (KT) have been identified in all genome-sequenced terrestrial plants. They play an important role in K+ acquisition and translocation and in enhancing salt tolerance. Here, we report that plasma membrane-located OsHAK18 functions in K+ and sodium (Na+) circulation and sugar translocation in rice (Oryza sativa). OsHAK18 was expressed mainly, though not exclusively, in vascular tissues and particularly in the phloem. Knockout (KO) of OsHAK18 reduced K+ concentration in phloem sap and roots but increased K+ accumulation in the shoot of both 'Nipponbare' and 'Zhonghua11' cultivars, while overexpression (OX) of OsHAK18 driven by its endogenous promoter increased K+ concentration in phloem sap and roots and promoted Na+ retrieval from the shoot to the root under salt stress. Split-root experimental analysis of rubidium (Rb+) uptake and circulation indicated that OsHAK18-OX promoted Rb+ translocation from the shoot to the root. In addition, OsHAK18-KO increased while OsHAK18-OX reduced soluble sugar content in the shoot and oppositely affected the sugar concentration in the phloem and its content in the root. Moreover, OsHAK18-OX dramatically increased grain yield and physiological K+ utilization efficiency. Our results suggest that-unlike other OsHAKs analyzed heretofore-OsHAK18 is critical for K+ and Na+ recirculation from the shoot to the root and enhances the source-to-sink translocation of photo-assimilates.

Efficient potassium (K) recycling and root carbon (C) metabolism improve K use efficiency in pear rootstock genotypes.

To investigate K absorption and transport mechanisms by which pear rootstock genotypes respond to low-K stress, seedlings of a potassium-efficient pear rootstock, Pyrus ussuriensis, and a potassium-sensitive rootstock, Pyrus betulifolia, were supplied with different K concentrations in solution culture. Significant differences in the absorption rate, Vmax and Km between the genotypes indicate that P. ussuriensis acclimatizes more readily to low-K stress by regulating its absorption and internal cycling. We also found that the K content in the leaves of P. betulifolia was significantly lower than that of P. ussuriensis, and the proportion of K that was returned to root from shoot, relative to K that was transported from root to shoot, was greater in P. ussuriensis, which suggests that P. ussuriensis more efficiently recycles and reuses K. When the transcriptomes of the two genotypes were compared, we found that photosynthetic genes such as CABs (Chlorophyll a/b-binding proteins), Lhcbs (Photosystem II-related proteins), and Psas (Photosystem Ⅰ associated proteins) displayed lower expression in leaves of P. betulifolia under no-K conditions, but not in P. ussuriensis. However, in the root of P. ussuriensis, carbon metabolism-related genes SS (Sucrose Synthase), HK (HexoKinase) and SDH (Sorbitol Dehydrogenase) and components of the TCA cycle (Tricarboxylic Acid cycle) were differentially expressed, indicating that changes in C metabolism may provide energy for increased K+ cycling in these plants, thereby allowing it to better adapt to the low-K environment. In addition, exogenous supply of various sugars to the roots influenced K+ influx, supporting the conclusion that sugar metabolism in roots significantly affects K+ absorption in pear.

Plant DNA Methylation Responds to Nutrient Stress.

Nutrient stress as abiotic stress has become one of the important factors restricting crop yield and quality. DNA methylation is an essential epigenetic modification that can effectively regulate genome stability. Exploring DNA methylation responses to nutrient stress could lay the foundation for improving plant tolerance to nutrient stress. This article summarizes the plant DNA methylation patterns, the effects of nutrient stress, such as nitrogen, phosphorus, iron, zinc and sulfur stress, on plant DNA methylation and research techniques for plant DNA methylation, etc. Our discussion provides insight for further research on epigenetics response to nutrient stress in the future.

Transcriptome Analysis of Differentially Expressed Genes Induced by Low and High Potassium Levels Provides Insight into Fruit Sugar Metabolism of Pear.

Potassium (K) deficiency is a common abiotic stress that can inhibit the growth of fruit and thus reduce crop yields. Little research has been conducted on pear transcriptional changes under low and high K conditions. Here, we performed an experiment with 7-year-old pot-grown "Huangguan" pear trees treated with low, Control or high K levels (0, 0.4, or 0.8 g·K2O/kg soil, respectively) during fruit enlargement and mature stages. We identified 36,444 transcripts from leaves and fruit using transcriptome sequencing technology. From 105 days after full blooming (DAB) to 129 DAB, the number of differentially expressed genes (DEGs) in leaves and fruit in response to low K increased, while in response to high K, the number of DEGs in leaves and fruit decreased. We selected 17 of these DEGs for qRT-PCR analysis to confirm the RNA sequencing results. Based on GO enrichment and KEGG pathway analysis, we found that low-K treatment significantly reduced K nutrient and carbohydrate metabolism of the leaves and fruit compared with the Control treatment. During the fruit development stages, AKT1 (gene39320) played an important role on K+ transport of the leaves and fruit response to K stress. At maturity, sucrose and acid metabolic pathways were inhibited by low K. The up-regulation of the expression of three SDH and two S6PDH genes involved in sorbitol metabolism was induced by low K, promoting the fructose accumulation. Simultaneously, higher expression was found for genes encoding amylase under low K, promoting the decomposition of the starch and leading the glucose accumulation. High K could enhance leaf photosynthesis, and improve the distribution of the nutrient and carbohydrate from leaf to fruit. Sugar components of the leaves and fruit under low K were regulated by the expression of genes encoding 8 types of hormone signals and reactive oxygen species (ROS). Our data revealed the gene expression patterns of leaves and fruit in response to different K levels during the middle and late stages of fruit development as well as the molecular mechanism of improvement of fruit sugar levels by K and provided a scientific basis for improving fruit quality with supplemental K fertilizers.