Agricultural production continues to be constrained by a number of biotic and abiotic factors that can reduce crop yield quantity and quality. Potassium (K) is an essential nutrient that affects most of the biochemical and physiological processes that influence plant growth and metabolism. It also contributes to the survival of plants exposed to various biotic and abiotic stresses. The following review focuses on the emerging role of K in defending against a number of biotic and abiotic stresses, including diseases, pests, drought, salinity, cold and frost and waterlogging. The availability of K and its effects on plant growth, anatomy, morphology and plant metabolism are discussed. The physiological and molecular mechanisms of K function in plant stress resistance are reviewed. This article also evaluates the potential for improving plant stress resistance by modifying K fertilizer inputs and highlights the future needs for research about the role of K in agriculture.

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The critical role of potassium in plant stress response.

Potassium in agriculture - status and perspectives

Among the plant nutrients, potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of the plant structure but it also has a regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, and enzyme activation. Several physiological processes depend on K, such as stomatal regulation and photosynthesis. In recent decades, K was found to provide abiotic stress tolerance. Under salt stress, K helps to maintain ion homeostasis and to regulate the osmotic balance. Under drought stress conditions, K regulates stomatal opening and helps plants adapt to water deficits. Many reports support the notion that K enhances antioxidant defense in plants and therefore protects them from oxidative stress under various environmental adversities. In addition, this element provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although considerable progress has been made in understanding K-induced abiotic stress tolerance in plants, the exact molecular mechanisms of these protections are still under investigation. In this review, we summarized the recent literature on the biological functions of K, its uptake, its translocation, and its role in plant abiotic stress tolerance.

Potassium: A Vital Regulator of Plant Responses and Tolerance to Abiotic Stresses

Agricultural growing practices have been evolving towards organic, sustainable or environmental friendly systems The aim of modern agriculture is to reduce inputs without reducing the yield and quality These goals can be achieved by breeding programmes but would be species specific and time consuming The identification of organic molecules able to activate plant metabolism may allow an improvement in plant performance in a short period of time and in a cheaper way Biostimulants are plant extracts and contain a wide range of bioactive compounds that are mostly still unknown These products are usually able to improve the nutrient use efficiency of the plant and enhance tolerance to biotic and abiotic stresses In this review, the state of the art and future prospects for biostimulants are reported and discussed Moreover, particular attention has been paid to intensive agricultural systems such as horticultural and floricultural crops In vegetables, the application of biostimulants allowed a reduction

Biostimulants and crop responses: a review

Aim

Potassium (K) is the second most abundant nutrient in plant photosynthetic tissues after nitrogen (N). Thousands of physiological and metabolic studies in recent decades have established the fundamental role of K in plant function, especially in water-use efficiency and economy, and yet macroecological studies have mostly overlooked this nutrient.



Methods

We have reviewed available studies on the content, stoichiometry and roles of K in the soil–plant system and in terrestrial ecosystems. We have also reviewed the impacts of global change drivers on K content, stoichiometry and roles.



Conclusions

The current literature indicates that K, at a global level, is as limiting as N and phosphorus (P) for plant productivity in terrestrial ecosystems. Some degree of K limitation has been seen in up to 70% of all studied terrestrial ecosystems. However, in some areas atmospheric K deposition from human activities is greater than that from natural sources. We are far from understanding the K fluxes between the atmosphere and land, and the role of anthropogenic activities in these fluxes. The increasing aridity expected in wide areas of the world makes K more critical through its role in water-use efficiency. N deposition exerts a strong impact on the ecosystem K cycle, decreasing K availability and increasing K limitation. Plant invasive success is enhanced by higher soil K availability, especially in environments without strong abiotic stresses. The impacts of other drivers of global change, such as increasing atmospheric CO2 or changes in land use, remain to be elucidated. Current models of the responses of ecosystems and carbon storage to projected global climatic and atmospheric changes are now starting to consider N and P, but they should also consider K, mostly in arid and semi-arid ecosystems.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/geb.12259

Potassium: a neglected nutrient in global change.

Salinity is a major abiotic stress factor limiting plant growth and productivity. One possible method to enhance plant salt-resistance is to compartmentalize sodium ions away from the cytosol. In the present work, a vacuolar Na+/H+ antiporter gene AtNHX1 from Arabidopsis thaliana, was transferred into Populus × euramericana ‘Neva’ by Agrobacterium tumefaciens in order to enhance poplar salt-resistance. The results showed that the transgenic poplar were more resistant to NaCl than the wild-type (WT) in greenhouse condition. Compared with the WT, plant growth and photosynthetic capacity of the transgenic plants were enhanced, and the transgenic plants accumulated more Na+ and K+ in roots and leaves under the same NaCl condition, whereas malondialdehyde and relative electrical conductivity were lower. All of these properties of the transgenic poplar were likely to be a consequence of the overexpression of AtNHX1 caused Na+ sequestration in the vacuoles and improved K+ absorption, thus reducing their toxic effects. These results indicated overexpression of the AtNHX1 enhanced salt-resistance of poplar, and AtNHX1 played an important role in the compartmentation of Na+ into the vacuoles. Therefore, this study provides an effective way for improving salt resistance in trees.

Overexpression of Arabidopsis thaliana Na + /H + antiporter gene enhanced salt resistance in transgenic poplar ( Populus × euramericana ‘Neva’)

Among different mechanisms of salt resistance, regulation of ion distribution among various tissues and intracellular compartmentation are of great importance. In this study, we investigated the effects of salt stress on growth, photosynthesis, and Na(+) accumulation and distribution in leaf apoplast and symplast of two canola (Brassica napus L.) cultivars (NYY 1 and BZY 1). The results showed that the declines in shoot dry mass, leaf water potential and net photosynthetic rate of BZY 1 (salt sensitive) were higher than those of NYY 1 (salt resistant) in response to salt stress. Stomatal limitation to photosynthesis was mainly affected under moderate salinity, whereas the reduction in assimilation rate under severe salt stress was due to both stomatal and non-stomatal limitations. We also found that more Na(+) was distributed to leaf veins in NYY 1 than in BZY 1; simultaneously, less Na(+) accumulated in the leaf blade in NYY 1 than in BZY 1. The percentage of Na(+) in the leaf symplast in NYY 1 was markedly lower than that in BZY 1. Also, Na(+) diffusion in leaves through apoplastic and symplastic pathways of BZY 1 was stronger than that in NYY 1, and the transpiration rate in BZY 1, especially at the leaf edges, decreased more than in NYY 1. Our results showed that NYY 1 accumulated less Na(+) in the shoot, especially in leaf blades, and confined Na(+) to the apoplast to avoid leaf salt toxicity, which could be one reason for the higher resistance of NYY 1 than BZY 1 plants to salt stress.

Difference in sodium spatial distribution in the shoot of two canola cultivars under saline stress.

we show that the knockout of a sugar transporter gene OsSWEET15 led to a significant drop in rice fertility because near half of the knockout mutant spikelets bore blighted or empty grains. The rest spikelets bore fertile grains with slightly reduced weight. Notably, the ovaries in the blighted grains of the ossweet15 mutants expanded after flowering but terminated development before the endosperm cellularization stage and aborted subsequently. GUS and GFP representing OsSWEET15 expression showed that the protein was strongly expressed in the embryo surrounding region (ESR) which was supposed to supply nutrients for the embryo development. These results joined with the protein’s sucrose transport capacity and plasma membrane localization suggest that OsSWEET15 plays a prominent role during the caryopsis formation stage probably by releasing sucrose from the ESR to support the embryo development. By contrast, the empty grains were probably caused by the reduced pollen viability of the ossweet15 mutants. Investigation of the makeup of ossweet11 mutant grains revealed similar phenotypes that were observed in the ossweet15 mutants. These results indicate that both OsSWETT15 and OsSWEET11 play important and similar roles during rice pollen development, caryopsis formation, and seed-setting in addition to their function in seed-filling that was demonstrated previously.

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Qingsong Zheng

Papers

The critical role of potassium in plant stress response.

Overexpression of Arabidopsis thaliana Na + /H + antiporter gene enhanced salt resistance in transgenic poplar ( Populus × euramericana ‘Neva’)

Difference in sodium spatial distribution in the shoot of two canola cultivars under saline stress.

Knockout of OsSWEET15 impaired rice embryo formation and seed-setting