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Potassium deficiency in plants: Effects and signaling cascades

May 2014
Acta Physiologiae Plantarum 36(5)

DOI:10.1007/s11738-014-1491-2

Authors:

Chokri Hafsi

Centre de Biotechnologie, Technopole Borj Cédria

Ahmed Debez

Centre de Biotechnologie, Technopole Borj Cédria

Chedly Abdelly

Centre de Biotechnologie, Technopole Borj Cédria

Potassium (K+) is an important macronutrient for plant growth and productivity. It fulfills important functions and it is widely included in fertilization management strategies to increase crop production. Although K+ is one of the most abundant elements of the earth crust, its availability to plants is usually limited leading to severe reduction in plant growth and yield. In plants, K+ shortage induces several responses at different levels: morphological, physiological, biochemical, and molecular. Activation of signaling cascades including reactive oxygen species, phytohormones (ethylene, auxin, and jasmonic acid), Ca2+, and phosphatidic acid is also triggered. In this review, we summarize the main of these adaptive responses evolved by plants to cope with K+ deficiency in the rhizosphere.

Schematic representation of possible consequences of K ? deficiency on various processes in plants. In K ? -deficient plants, the decline in photosynthetic activity and restricted phloem export of photoassimilates out of the leaves led to an oxidative stress, which has deleterious effects on plants. This resulted in growth and productivity

…

Generic pathway for plant sensing and signaling to K ? deficiency. K ? deficiency is first perceived by the membrane receptors and then activates complex intracellular signaling cascade. The external K ? deficiency activates ethylene and calcium signals to regulate ROS production and the CBL-CIPK23 network, respectively. CIPK23 phosphorylates and then activates the potassium uptake channel AKT1. AKT1 activity is also regulated by AtKC1 (a regulatory K ? channel subunit) and SYP121 (a SNARE protein) leading to the formation of a tripartite complex which plays a pivotal

…

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Content uploaded by Ahmed Debez

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REVIEW

Potassium deﬁciency in plants: effects and signaling cascades

Chokri Hafsi •Ahmed Debez •Chedly Abdelly

Received: 16 February 2013 / Revised: 3 January 2014 / Accepted: 8 January 2014 / Published online: 24 January 2014

ÓFranciszek Go

´rski Institute of Plant Physiology, Polish Academy of Sciences, Krako

´w 2014

Abstract Potassium (K

) is an important macronutrient

for plant growth and productivity. It fulﬁlls important

functions and it is widely included in fertilization man-

agement strategies to increase crop production. Although

is one of the most abundant elements of the earth crust,

its availability to plants is usually limited leading to severe

reduction in plant growth and yield. In plants, K

shortage

induces several responses at different levels: morphologi-

cal, physiological, biochemical, and molecular. Activation

of signaling cascades including reactive oxygen species,

phytohormones (ethylene, auxin, and jasmonic acid), Ca

and phosphatidic acid is also triggered. In this review, we

summarize the main of these adaptive responses evolved

by plants to cope with K

deﬁciency in the rhizosphere.

Keywords K

deﬁciency Plants Tolerance Signaling

Introduction

is one of the essential macronutrients playing an

important role in plant growth and development (Leigh and

Wyn Jones 1984; Marschner 1995; Schachtman and Shin

2007; Wang and Wu 2013). It is the most abundant cation

in plants and is associated or involved in several physio-

logical processes supporting crop growth and development

(Pettigrew 2008). The importance of K

in photosynthesis,

osmoregulation, enzyme activation, protein synthesis, ion

homeostasis, and the maintenance of anion–cation balances

in plants is well documented (Bhandal and Malik 1988;

Marschner 1986; Zhao et al. 2001; Kanai et al. 2007).

Uptake of this cation by plant roots has been found to be

accomplished by at least two distinct kinetic systems

(Epstein et al. 1963): (1) high-afﬁnity systems with

apparent afﬁnities (Km) for K

in the range of about

4–35 lM and operating at low external K

concentration

usually within 0.2 mM K

(Epstein et al. 1963; Maathuis

and Sanders 1994), showing saturation at about 300 lM

external K

and (2) constitutive low-afﬁnity components,

which have an average Km of about 3–19 mM and oper-

ating when the external K

concentration is higher than

1 mM. The high-afﬁnity K

uptake is mediated by K

transporters whereas K

absorption in the low-afﬁnity

range of concentrations is mediated by K

channels (Ma-

athuis and Sanders 1996). More recently, molecular, elec-

trophysiological, and reverse genetics approaches in plants

identiﬁed genes encoding K

transporters and K

channels

(Rodrı

´guez-Navarro 2000;Ve

´ry and Sentenac 2003; Gierth

and Ma

¨ser 2007). K

channels are classiﬁed into three

groups according to their structural features: shaker-type

channels, two-pore channels, and cyclic nucleotide-gated

channels (reviewed by Ashley et al. 2006). K

transporters

are present in three families of membrane proteins: the K

uptake permeases (KT/HAK/KUP), the K

transporter

(Trk/HKT) family, and the cation proton antiporters (CPA)

(Gierth and Ma

¨ser 2007).

Although K

is not assimilated into organic matter, it is

well documented that K

deﬁciency severely impacts plant

metabolism. In K

-deﬁcient rice roots, gene ontology

analysis identiﬁed the expression of several genes which

were classiﬁed into 13 functional categories including

metabolic process, membrane, cation binding, kinase

Communicated by P. Wojtaszek.

C. Hafsi (&)A. Debez C. Abdelly

Laboratoire des Plantes Extre

ˆmophiles, Centre de

Biotechnologie a

`la Technopole de Borj Ce

´dria,

BP 901, Hammam-Lif 2050, Tunisie

e-mail: hafsichokri@yahoo.fr

123

Acta Physiol Plant (2014) 36:1055–1070

DOI 10.1007/s11738-014-1491-2

activity, transport, protein modiﬁcation, and response to

stress (Ma et al. 2012). Although plant responses to K

deﬁciency are well documented at the physiological and

transcriptional levels, the regulatory mechanisms underly-

ing these changes are poorly understood (Ashley et al.

2006). To cope with K

deﬁciency, plants must sense

changes in its availability and transmit the signals to

modulate their metabolism. Recent works have clariﬁed

sensing and signaling pathway that involve components

like ROS, ethylene, Ca

, auxin, jasmonic acid, and

phosphatidic acid (Shin and Schachtman 2004; Armengaud

et al. 2004; Li et al. 2006; Jung et al. 2009; Hafsi et al.

2008,2009). This article discusses soil K availability and

dynamics, physiological roles of K

, plant’s responses to

low-K

conditions, and signaling cascades regulating

responses to K

deﬁciency.

Potassium in the rhizosphere

K is abundant in the earth crust but in the soil solution K

is present at relatively low concentrations (Grattan and

Grieve 1992). It represents the fourth most abundant ele-

ment of the lithosphere. However, only a low fraction

(1–4 %) on the surface of clay humus particles of the soil is

bioavailable. The major fraction is part of mineral com-

positions and is unavailable to plants (Mengel and Kirkby

1982). Sparks (1987) distinguished four principal forms of

potassium in the soil: potassium as ionic form K

(in soil

solution, 0.1–0.2 %), exchangeable K (1–2 %), not

exchangeable K (ﬁxed in 2:1 clays 1–10 %), and unavail-

able form of K (90–98 %). In the soil, the latter form is

found in primary aluminosilicate minerals, which include

muscovite micas, K-feldspars and biotite (Bertsch and

Thomas 1985). The ﬁrst two forms of soil K are considered

to be labile and meet the immediate requirements of

growing plants, while the last two forms are considered

non-labile and are responsible for the long-term supply of

K to plants (Askegaard et al. 2003). As shown in the K

cycle (Fig. 1), K is in perpetual transformation. The dif-

ferent forms of K in soils are dynamically interconverted

by ﬁxation and release processes (Yawson et al. 2011). A

balance exists between the different forms. The exchange is

deﬁned as a balance maintained between soil solution K

and exchangeable K through ﬁxation and release processes.

The amount of K that is made available depends on the

intensity of weathering, time of deposition, and the pro-

portion and the type of clay minerals that are present in the

soil (Sparks and Huang 1985). The release of K from the

exchangeable pool is often much lower than K

uptake by

plants (Sparks and Huang 1985) and consequently K

concentration in certain soils is a limiting factor for plant

growth (Johnston 2005).

Potassium roles and K

deﬁciency effects on plants

is a vital macronutrient for plant growth and develop-

ment (Schachtman and Shin 2007). It is the most abundant

cation in plants, with up to 6 % of the plant dry weight

(Raven et al. 1976). The physiological roles of K

in plants

are often divided in to biophysical and biochemical func-

tions (Fig. 2). As proposed by Leigh and Wyn Jones (1984)

these might be distinct in different cellular compartments.

Weathering

Unavailable K in primary

minerals (muscovite

micas, K-feldspars

Non-echangeable K+

Animals

Fertilizer

Soil solution K+

Plants and animals residues

Uptake

Release

Fixation

Leaching

Sorption

Desorption

Echangeable K+

Fig. 1 Potassium cycle in the

soil

1056 Acta Physiol Plant (2014) 36:1055–1070

123

In the cytoplasm, K

plays important biochemical func-

tions and it cannot be replaced by other solutes. In the

vacuole, it is mainly involved in the regulation of osmotic

potential and turgor. K

has several functions in plants. In

phanerogams, both pollen germination after reaching the

stigma and the development of pollen tube depend on the

transport of inorganic ions like Ca

and K

through

plasma membrane of the pollen and/or the pollen tube

(Feijo

´et al. 1995; Taylor and Hepler 1997). K

ﬂux into

non-germinated pollen can initiate an osmotic inﬂux nec-

essary for germination suggesting that K

can serve as an

osmoticum thus maintaining the turgescence pressure of

pollen cells (Obermeyer and Blatt 1995). It was demon-

strated that the inward-rectifying K

channel SPIK, a

Shaker-type channel, is involved in K

uptake into pollen

tubes across the plasma membrane and then regulates

pollen tube growth and development (Mouline et al. 2002).

is the most abundant cation in the cytoplasm of a

plant cell (as well as cells in other organisms), typically

reaching 100–200 mM (Walker et al. 1996). Considering

its high concentration in the cytoplasm and chloroplasts,

neutralizes soluble anions (e.g., acid organic anions,

inorganic anions) and the macromolecular insoluble anions

and stabilizes the pH between 7 and 8, which is the optimal

pH for most enzymatic reactions in the cytosol (Marschner

1986).

plays important role in the secretion of mugineic

acid family phytosiderophores (MAs). In fact, under iron

deﬁciency conditions, the secretion of these compounds

from the roots to the rhizosphere represents an important

step in the acquisition of sparingly soluble iron (Fe) in

strategy II plants (Ro

¨mheld 1987; Marschner and Ro

¨mheld

1994). Iron is then taken up in the form of a MAs-iron

complex. Sakaguchi et al. (1999) demonstrated that MAs

are secreted from iron-deﬁcient barley roots together with

in an almost equimolar ratio suggesting that MAs are

secreted in the form of monovalent anions via anion

channels using the K

gradient between the cytoplasm and

the extracellular medium.

activates more than 50 enzymes (Bhandal and Malik

1988). Protein synthesis needs high K

concentration

(Blaha et al. 2000) because of its intervention in the syn-

thesis of ribosomes and aminoacyl-tRNA, protein synthesis

from charged tRNA transfer and messenger RNA turnover

(Evans and Wildes 1971). K

stimulates the uptake and

transport of NO

within the plant, as K

serves as the

accompanying counter cation (Blevins et al. 1978). K

deﬁciency inhibits nitrogen assimilation. In two cultivars of

cotton (Liaomian 18 and NuCOTN99

) differing in their

tolerance to K

deﬁciency, Wang et al. (2012) observed a

reduction in nitrate reductase activity and protein content,

whereas the content of amino acids increased. In maize

plants exposed to K

deﬁciency for 5 weeks, Qu et al.

(2011) observed a reduction in the activities of important

nitrogen metabolism enzymes (nitrate reductase, glutamate

dehydrogenase, glutamate synthase, urease, glutamic–

pyruvic transaminase, and glutamic–oxaloacetic transami-

nase). In Arabidopsis, Armengaud et al. (2004) demon-

strated that the expression of three nitrate transporter genes

(AtNRT2;1,AtNRT2;3, and AtNRT2;6) was reduced by K

deﬁciency and increased after K

resupply. Similar results

have been observed by Ma et al. (2012). These authors

observed a decrease in the expression of two nitrate

transporter genes (Os02g0112600 and Os02g0689900)in

rice roots subjected to K

starvation for 5 days.

Furthermore, K

is important for the ﬁxation of

molecular nitrogen by root nodules. Mengel et al. (1974)

studied the effect of K

supply on Vicia faba growth and

ﬁxation based on experiments adding

gas to the

growth compartment. Control plants showed higher con-

tents of

N in the soluble amino fraction and in the protein

fraction of various plant organs as compared with K

deﬁcient plants. This was explained by a better supply of

nodules with carbohydrates in plants optimally supplied

with K

, resulting in a higher carbohydrate turnover in the

nodules. More recently, in Trifolium repens, Høgh-Jensen

(2003) observed that K

deprivation induced changes in

the relative growth of roots, nodules, and shoots rather than

changes in N and/or carbon uptake rates per unit mass or

area of these organs. In this context, it is important to

conduct experiments to clarify in more detail the effects of

deﬁciency on nitrogen uptake and metabolism.

is also implicated in carbohydrate accumulation

since its deﬁciency was found to result ﬁrst in an accu-

mulation followed by a strong decrease of these

K+

Osmoregulation

Enzymes

activation

Regulation of

stomatal movement

Photosynthesis

Protein synthesis

Maintenance of

cation-anion balances

Fig. 2 Major roles of K

: inside the plant, K

has a wide range of

functions/reactions including (1) the generation of turgor as it

constitutes the main cation in the vacuole, (2) photosynthesis, (3)

control of stomatal aperture, (4) protein synthesis, (5) maintenance of

cation–anion balances, and (6) enzymes activation by acting as a

cofactor in enzymatic reactions

Acta Physiol Plant (2014) 36:1055–1070 1057

123

compounds (Fageria et al. 1997; Cakmak 2005). It also

plays an important role in transport of solutes through the

phloem including sucrose movement from shoot to root

and to the sink tissues such as fruits (White and Karley

2010). The carbon ﬂow may be stopped because of the

inhibitory effect of K

-deﬁciency stress on sink activity by

disturbing the water status of the plant (Kanai et al. 2007).

More recently, Kanai et al. (2011) reported that K

deﬁ-

ciency may reduce aquaporin activity, consequently sup-

pressing root hydraulic conductance and water supply to

the growing stem for diameter expansion and leaf for

transpiration suggesting the existence of close coupling

between aquaporins and K

-channel transporters. K

plays

important role in maintenance of photosynthesis too.

Indeed, it inﬂuences this physiological process mainly

through ATP synthesis, the activation of the enzymes

involved in photosynthesis, CO

uptake, the balance of the

electric charges required for photophosphorylation in

chloroplasts, and acts as the counter ion to light-induced

ﬂux across the thylakoid membranes (Marschner 1986).

deﬁciency leads to a reduction of photosynthetic

activity in a number of ways. Zhao et al. (2001) reported

that K

deﬁciency modiﬁes the chloroplast ultrastructure

of cotton leaves. The control had more well-deﬁned grana

stacks and extensive stroma lamellae with very small

amounts of starch granules. In K

-deﬁcient plants, chlo-

roplasts were ﬁlled with large starch granules and con-

tained signiﬁcantly more plastoglobuli together with fewer

grana. It was demonstrated that guard-cell osmoregulation

is mediated by K

and its counter-ions malate and Cl

(Talbott and Zeiger 1996) in addition to sucrose which

plays important role in regulating stomatal opening (Tal-

bott and Zeiger 1996; Kang et al. 2007;Ni2012). The

negative impact of K

deﬁciency on photosynthesis, tran-

spiration, and stomatal conductance is well documented

(Degl’Innocenti et al. 2009; Kanai et al. 2011), whereas

photorespiration and respiration are rather stimulated

(Bottrill et al. 1970; Singh and Blanke 2000). Biochemical

factors may explain the reduced photosynthesis observed

especially under severe K

deﬁciency (Bednarz et al. 1998;

Tester and Blatt 1989) such as the need for K

in the

photosynthetic transfer of radiant energy into chemical

energy through the production of ATP (Havlin et al. 2005).

The energy that is derived from ATP is required to power

metabolic processes in plants that produce carbohydrates,

proteins, and other compounds essential for crop produc-

tivity and quality. In addition to reduced photosynthesis

under plant K

starvation, the transport of photoassimilates

away from the source tissues is also restricted (Cakmak

2005; Gerardeaux et al. 2010; Kanai et al. 2011). Zhao

et al. (2001) observed in cotton plants that reduction in leaf

photosynthesis by K

deﬁciency was mainly associated

with low chlorophyll content, poor chloroplast

ultrastructure, and restricted photoassimilates transport,

rather than limited stomata conductance.

In plants, K

deﬁciency-related symptoms include

brown scorching and curling of leaf tips as well as chlo-

rosis between leaf veins, which is directly bound up with

chlorophyll degradation (Xiao-Lei et al. 2012). Often, K

deﬁciency symptoms ﬁrst appear on the older leaves

(Fageria et al. 2001). This may represent a survival strategy

adopted by plants exposed to K

-shortage stress consisting

in the mobilization of K

from mature and senescing

organs to make it available for the youngest ones (Hewitt

1963; Cochrane and Cochrane 2009). Cakmak (2005)

suggested that those signs could be associated with the

oxidative degradation of chlorophyll by reactive oxygen

species (ROS), whose production is enhanced by K

deﬁciency. The reduction in photosynthetic activity and the

restriction in sucrose export from the source to the sink

organs result in a strong accumulation of carbohydrates in

the source leaves leading to an over-reduction of the pho-

tosynthetic electron transport chain that enhances the pro-

duction of ROS (Cakmak 2005) (Fig. 3).

Several authors have described a reduction in leaf area

under K

deﬁciency (Zhao et al. 2001; Degl’Innocenti

et al. 2009; Gerardeaux et al. 2010). Jordan-Meille and

Pellerin (2004) demonstrated in maize plants that the

limitation of leaf elongation by K

deﬁciency was not

related to restricted carbohydrates availability in leaf

growing zones, suggesting that this restriction was proba-

bly due to altered plant–water relations. More recently,

Gerardeaux et al. (2010) suggested that the occurrence of

organ size (leaves, internodes) reduction in cotton was not

directly caused by lack of photoassimilates and cell turgor

but can be due to sugar starvation of sink tissues probably

through sugar signaling.

Plant responses to K

deﬁciency

Morphological responses

The plant root system is characterized by a high level of

plasticity. This plasticity is possible due to a continuous

production of new meristems (Williamson et al. 2001). The

root morphology is inﬂuenced by the plant environment

and especially by the availability of nutrients (Thaler and

Pages 1998). Root morphological and physiological traits

are important for absorbing nutrients from the soil (Satt-

elmacher et al. 1994). Yang et al. (2003) showed that the

-efﬁcient rice genotypes had a longer root system than

the inefﬁcient ones under low-K

conditions. Chen and

Gabelman (2000) in tomato and Hafsi et al. (2011)in

Hordeum maritimum and Catapodium rigidum species

showed that K

deﬁciency increases root length, hence

1058 Acta Physiol Plant (2014) 36:1055–1070

123

contributing to the increase of exploring surface to enhance

the K

uptake. In cotton, Xiao-Li et al. (2008) indicated

that K

accumulation was positively correlated with two

features of the root architecture: the root length and the

root surface area. The genetic variation in root traits and

nutrient acquisition of 10 lentil genotypes was investigated

by Gahoonia et al. (2006). These authors suggested that the

ability of the most performing genotype (Barimasur-4) in

acquiring K

could be partly ascribed to its ability to

produce a longer root system covered with longer and

denser root hairs. Finally, in cotton, Brouder and Cassman

(1990) and Han-Bai et al. (2009) found that the K-efﬁcient

cultivar had more root surface area than the K-sensitive

cultivar.

The reduction in the number and the growth of lateral

roots was observed in barley (Drew 1975) and Arabidopsis

thaliana (Shin and Schachtman 2004) grown under low-K

concentrations. This was related to an increase in ethylene

concentration in response to K

deﬁciency (Shin and

Schachtman 2004). Yet, this response was found to be not

necessarily accompanied by gene activation related to

ethylene signalization and metabolism (Armengaud et al.

2004). Interestingly, lesser export of photoassimilates from

leaves under K

lacking conditions, thereby inhibiting root

growth has been also reported (Cakmak et al. 1994;

Degl’Innocenti et al. 2009; Gerardeaux et al. 2010). Nat-

ural variation of root traits such as primary root length,

lateral root length, and total root size within 26 Arabidopsis

accessions was observed by Kellermeier et al. (2013)

which reveals different strategies to adapt to K

deﬁciency.

Quantitative trait loci (QTLs) analysis using Col-

09Catania-1 populations revealed several loci determin-

ing speciﬁc subsets of root architectural traits (Kellermeier

et al. 2013).

Light

Reduced photosynthesis

Restricted transport

of sucrose into roots

Inhibition of phloem

export

Photoassimilates

accumulation Over-reduction of

electron transport

Increased ROS

production

Photooxidation

Chlorosis

Necrosis

Reduced leaf surface area

Increased uptake of

Mg, Na, and Ca

Regulation of root architecture and K+ uptake

systems leading to increased K+ uptake efficiency

Recirculation of K+ from older

leaves to younger ones

Increased K+ utilization efficiency

Fig. 3 Schematic representation of possible consequences of K

deﬁciency on various processes in plants. In K

-deﬁcient plants, the

decline in photosynthetic activity and restricted phloem export of

photoassimilates out of the leaves led to an oxidative stress, which has

deleterious effects on plants. This resulted in growth and productivity

reductions. To cope with K

deﬁciency, plants develop some

tolerance strategies including increased K

uptake and utilization,

increased uptake of other cation like Na

,Mg

, and Ca

to replace

in some physiological and biochemical functions, and recircula-

tion of K

from older leaves to youngest ones

Acta Physiol Plant (2014) 36:1055–1070 1059

123

One of the early reactions of K

deﬁciency stress in

plants is its impact on dry matter partitioning between roots

and shoots. The root/shoot DW ratio under K

deﬁciency

depends on the species and conditions of culture (Andrews

et al. 1999). An increase in this parameter was observed in

wheat (Andrews et al. 1999), mulberry (Tewari et al.

2007), Brassica oleracea (Singh and Blanke 2000), and H.

maritimum (Hafsi et al. 2011). In contrary, Cakmak et al.

(1994) and Marschner et al. (1996) reported a decrease in

root/shoot DW ratio in K

-deﬁcient plants whereas no

statistically signiﬁcant effect was found in maize (Tewari

et al. 2004), C. rigidum (Hafsi et al. 2011) and A. thaliana

(Gruber et al. 2013) plants. More works are needed to

explain this variation in dry matter partitioning between

roots and shoots under K

-deﬁcient conditions.

Physiological responses

It is well established that K

deﬁciency induces the acid-

iﬁcation of extracellular medium. Minjian et al. (2007)

demonstrated that root K

absorption depends on the

activity of the proton pumps (H

-ATPases) and the

occurrence of K

transporters on the cellular membrane.

The degree of H

expulsion can be used as a criterion of

tolerance to K

deﬁciency. Chen and Gabelman (2000)

observed in tomato strains that K

uptake efﬁciency is

associated with high net K

inﬂux coupled with low pH

around root surfaces. The proton-electrochemical gradient

may energize K

uptake, and indeed it is used by some KT/

KUP/HAK transporters, which co-transport K

and H

(Rodrı

´guez-Navarro 2000). Under K

-deﬁcient conditions,

Arend et al. (2004) observed in young stems of poplar

plants, an up-regulation of speciﬁc isoforms of the plasma

membrane (PM) H

-ATPase in the vessel-associated cells

of the wood ray parenchyma suggesting a key role for these

PM H

-ATPases in unloading K

from the xylem stream.

In addition, in response to mineral deﬁciency, a change

in membrane potential is a spectacular effect. For instance,

deﬁciency triggers a membrane hyperpolarization of

root cells; which appears within just a few minutes after the

decrease in external K

(Spalding et al. 1999; Nieves-

Cordones et al. 2008; Wang and Wu 2010). It is not clear

how plant cells detect the changes in membrane potential

(Schachtman and Shin 2007). The hyperpolarization of

plasma membrane following K

deﬁciency may activate

channels and enhance K

uptake (Sentenac et al. 1992;

Gambale and Uozumi 2006). Nieves-Cordones et al. (2008)

demonstrated that the expression of the gene (LeHAK5)

encoding the tomato high-afﬁnity K

transporter HAK5 in

tomato root cells correlated with the steady-state plasma

membrane potentials of plants but not with their cyto-

plasmic K

concentrations nor with the concentrations of

in the external medium.

Under a severe K

deﬁciency, cytosol K

concentration

is maintained constant at the expense of its vacuolar pool

(Leigh and Wyn Jones 1984; Walker et al. 1996; Ottow

et al. 2005). This can be due to a cytosol acidiﬁcation

(Walker et al. 1996) and a high increase of putrescine

concentration (up to 10 mM) (Bruggemann et al. 1998;

Dobrovinskaya et al. 1999), which inhibits the tonoplast

fast-activating vacuolar canal under K

deﬁciency pre-

venting its reabsorption from the cytosol to the vacuole

(Pottosin and Este

´vez 2003). K

mobilization from the

vacuole into the cytosol is much higher in K

-efﬁcient

barley genotypes than inefﬁcient ones (Memon et al. 1985).

Under moderate K

deﬁciency, Leigh and Wyn Jones

(1984) have suggested that plants maintain the K

con-

centration in the cytoplasm in the range of 100–200 mM to

ensure biochemical processes at the expense of the K

concentration in the vacuole. Recently, it has been reported

in Arabidopsis that a tonoplast-located calcineurin B-like

protein (CBL2 or CBL3) may form a complex with their

target CBL-interacting protein kinase (CIPK9) which

phosphorylates an unknown tonoplast-located K

trans-

porter or channel like the tandem-pore K

channel 1

(TPK1), leading to K

efﬂux from the vacuole into the

cytoplasm (Pandey et al. 2007; Liu et al. 2013). This

phenomenon is important to regulate K

homeostasis

between these two compartments under K

-deﬁcient

conditions.

The presence of Na

,Mg

, and Ca

is important in

alleviating the effects of K

deﬁciency. Increased con-

centrations of Na

,Mg

, and Ca

in response to K

deﬁciency and decreased concentrations of these cations

under high concentrations of K

have been reported (Diem

and Godbold 1993; Pujos and Morard 1997; Jordan-Meille

and Pellerin 2008; Gerardeaux et al. 2009; Hafsi et al.

2011; Hernandez et al. 2012). The occurrence of such

antagonisms may contribute to maintain constant the cation

balance for a given organ irrespective of K

external

changes. K

and Mg

have in part similar roles such as

osmoregulation, enzyme activation, and cellular pH control

(Marschner 1986). Na

could replace K

in non-speciﬁc

physiological and biochemical functions (Flowers and

¨uchli 1983) (Fig. 3).

Under K

-deﬁcient conditions, an early decrease of K

concentration in barley roots was detected (Hafsi et al.

2008,2009) which may be due to its translocation from

roots to shoots during K

deprivation period. Hafsi et al.

(2011) demonstrated that the lower K

requirement of H.

maritimum to express its optimal growth compared to C.

rigidum can be attributed to its higher efﬁciency to acquire

and transfer K

to shoots.

Species and genotypes tolerant to nutrient deﬁciency

have also developed speciﬁc morphological and physio-

logical mechanisms allowing them to acquire sufﬁcient

1060 Acta Physiol Plant (2014) 36:1055–1070

123

quantities of a speciﬁc nutrient (uptake efﬁciency) (Satt-

elmacher et al. 1994; Xiao-Lei et al. 2012). K

uptake

efﬁciency determines the ability of plants to take up K

under low soil K

availability (Samal et al. 2010). Chen

and Gabelman (1995) showed that the tomato genotype

576 responded morphologically to K

deﬁciency by pro-

liferating root length thereby increasing root surface areas

to capture K

whereas in the genotype 483 a physiological

response was demonstrated by high net K

inﬂux. Chen and

Gabelman (2000) concluded that the mechanism for strain

483 in acquisition of K

in comparison to 320 and 525

strains was due to a high absorption capacity.

Variation among plants in the capacity to take up K

from the external medium has been investigated in several

works (Guoping et al. 1999; Xiao-Li et al. 2008; Han-Bai

et al. 2009; Xiao-Lei et al. 2012). Under K

-deﬁcient

conditions, a potato K-efﬁcient genotype had about a

twofold higher K

uptake rate than a K-inefﬁcient one

(Trehan and Sharma 2002). Under K

starvation, plant

survival depends not only on its uptake efﬁciency, but also

on its utilization efﬁciency (KUE), i.e., the quantity of

biomass produced per unit of absorbed K

(Sattelmacher

et al. 1994; Rengel and Damon 2008). This parameter is

commonly measured in relation to vegetative growth,

particularly in forage crops (Woodend and Glass 1993).

Sauerbeck and Helal (1990) deﬁned nutrient use efﬁciency

as plant yield per unit of nutrient supply. In both H.

maritimum and C. rigidum, KUE for shoot biomass pro-

duction increased as K

concentration in the culture

medium decreased (Hafsi et al. 2011). Mahmood et al.

(2001) showed that wheat genotypes differed considerably

in terms of their growth response, K

uptake and utiliza-

tion efﬁciency under different K

treatments. Damon et al.

(2007) reported a genotypic variation between canola

genotypes in response to changes in K

availability in the

external medium which was due to differences due to

genotypic differences in both the uptake and the utilization

of K

Regulation of K

transport systems

Plants have a wide variety of transport systems that play

crucial roles in K

uptake, redistribution, and homeostasis

(see, e.g., the reviews by Ve

´ry and Sentenac 2003; Britto

and Kronzucker 2008; Alema

´n et al. 2011; Wang and Wu

2013) which may be related to the fact that most plants are

sessile organisms and must be able to adapt to the varia-

tions of K

concentration in the medium as suggested by

Fairbairn et al. (2000). In A. thaliana,71K

transporters

and channels were identiﬁed (Chen et al. 2008). The wheat

High-Afﬁnity K

transporter1 (HKT1), which is mainly

expressed in roots and permeable to both K

and Na

represents the ﬁrst plant K

transporter to be cloned

(Schachtman and Schroeder 1994). Functional character-

ization in oocytes demonstrated that EcHKT1 and EcH-

KT2, two K

transporters from Eucalyptus camaldulensis,

are implicated in K

and Na

uptake as has been shown for

HKT1 (Fairbairn et al. 2000). In Arabidopsis, the High-

Afﬁnity K

Transporter5 (AtHAK5) and the K

channel

Arabidopsis K

Transporter1 (AtAKT1) have been iden-

tiﬁed as the two major systems involved in K

uptake in

the high-afﬁnity range (Gierth et al. 2005; Rubio et al.

2008; Pyo et al. 2010; Kim et al. 2012). A lesion of either

AtHAK5 or AKT1 signiﬁcantly impairs the high-afﬁnity

) uptake (Hirsch et al. 1998; Gierth et al. 2005;

Pyo et al. 2010). Furthermore, a reduction in the ability of

akt1-1 Arabidopsis mutant to grow in K

-deﬁcient condi-

tions was observed by Hirsch et al. (1998). In addition to

mediating primary K

uptake, these transport systems play

important role in K

redistribution between different plant

organs. The outward-rectifying K

channel (SKOR), a

Shaker-type efﬂux channel found in stellar parenchyma,

mediates K

secretion from root cells into the xylem sap

for transport toward shoots (Gaymard et al. 1998). These

authors observed a 50 % reduction in shoot K

content in

SKOR-deﬁcient A. thaliana mutants, while root content

was unaffected (Gaymard et al. 1998). K

recirculation in

the phloem was conducted by another Shaker-type channel,

AKT2, which was identiﬁed in phloem cells using b-glu-

curonidase reporting and in situ hybridization (Lacombe

et al. 2000; Deeken et al. 2000; Che

´rel et al. 2002; Ga-

jdanowicz et al. 2011). Stomatal regulation is also con-

trolled by the function of some other channels. The inward-

rectifying channels, KAT1 and KAT2, mediate K

uptake

into guard cells during stomatal opening (Schachtman et al.

1992; Pilot et al. 2001), while the outward-rectifying K

channel GORK is responsible for K

release from guard

cells during stomatal closure (Hosy et al. 2003).

Several works demonstrated that K

deﬁciency induces

quantitative and qualitative changes of K

uptake systems

(Benlloch et al. 1989; Maathuis and Sanders 1995). The

high-afﬁnity uptake mechanism has been shown to be

inducible, whereas the low-afﬁnity system may be consti-

tutive (Glass and Dunlop 1978; Fernando et al. 1990).

Transcriptome analysis of rice roots during the response to

starvation, Ma et al. (2012) revealed that several genes

in ion transporter families were markedly up-regulated,

suggesting that they may play important roles in rice

responses to K

deﬁciency.

Regulation of K

transporters

Gene response to nutrient starvation has been characterized

as early responsive genes, responding rapidly and often

non-speciﬁcally, and late responsive genes, induced by

long-term stress conditions, which is considered to be more

Acta Physiol Plant (2014) 36:1055–1070 1061

123

speciﬁc for nutrient deprivation (Hammond et al. 2004;

Schachtman and Shin 2007). Shin and Schachtman (2004)

demonstrated that K

deﬁciency provokes an increase in its

absorption. This activation is generally associated with an

induction of high-afﬁnity transporters and is considered as

the main adaptation mechanism to K

deﬁciency (Ashley

et al. 2006). Wang et al. (1998) demonstrated, in a work on

barley and wheat roots, that K

deﬁciency is accompanied

by a fast up-regulation at transcriptional level of high-

afﬁnity transporters HKT1 which is suppressed following

resupply. Similar results were observed in the Arabi-

opsis AtHAK5 transporter (high-afﬁnity K

transporter5)

(Armengaud et al. 2004, Shin and Schachtman 2004;

Gierth et al. 2005) and in Oryza sativa OsHAK1 (Ban

˜uelos

et al. 2002) and Capsicum annuum CaHAK1 (Martı

´nez-

Cordero et al. 2004) transporters. According to Shin and

Schachtman (2004), AtHAK5 is rapidly and transiently

(after 6 h of K

deﬁciency) activated whereas Gierth et al.

(2005) indicate that this transporter is only activated after

48 h and this activation is maintained at least 5 days after

deﬁciency. In Nicotiana rustica roots, the expression

level of the gene NrHAK1 was higher in the root tip and

was up-regulated when exposed to K

starvation (Guo

et al. 2008). In Arabidopsis, Qi et al. (2008) demonstrated

that AtHAK5, a plasma membrane uptake mechanism,

plays a role during severe K

deprivation. In Arabidopsis,

Kim et al. (2012) identiﬁed a transcription factor RAP2.11

as a component in the response to low K

through regu-

lation of the high-afﬁnity K

uptake transporter AtHAK5

and other components of the low-K

signal transduction

pathway. More recently, Hong et al. (2013) demonstrated

that the expression of four transcription factors [dwarf and

delayed ﬂowering 2 (DDF2), jagged lateral organs (JLO),

basic helix-loop-helix 121 (bHLH121), and transcription

initiation factor II_A gamma chain (TFII_A)] was up-

regulated by K

deﬁciency suggesting that these factors

can bind to the HAK5 promoter and then activate HAK5

expression.

In K

-deﬁcient rice roots, Ma et al. (2012) observed an

up-regulation of three OsHAK genes. These authors sug-

gested that the increase in expression levels of genes

encoding K

transporters may be a rapid and direct strat-

egy for plants to increase K

-uptake and overcome K

deﬁciency.

Regulation of K

channels

Electrophysiological studies on Arabidopsis roots revealed

an activation of inwardly rectifying K

channels in

response to K

deﬁciency (Maathuis and Sanders 1995;

Hirsch et al. 1998). The Arabidopsis AtAKT1 channel,

which localized to the plasma membrane, is involved in K

uptake in the high-afﬁnity range of concentrations (Hirsch

et al. 1998; Gierth et al. 2005). In this species, no tran-

scriptional activation of AKT1 channel was observed (Pilot

et al. 2003; Hampton et al. 2004) suggesting a posttran-

scriptional activation of this channel (Li et al. 2006). In the

calcium section, the regulation of AKT1 by CBLs and

CIPK23 is described. Other works demonstrated a down-

regulation of transcription of SKOR and AKT2 in Arabi-

dopsis (Maathuis et al. 2003; Pilot et al. 2003). These

channels are involved in long distance transport of K

Therefore, it was suggested that the decrease in their

expression is important to limit its recirculation between

plant tissues and organs. Such alteration of long distance

transport can be also important to insure a communi-

cation between shoots and roots concerning K

status

(Pilot et al. 2003). In Triticum aestivum roots, an up-reg-

ulation of the mRNA level of the TaAKT1 gene was

observed in response to K

starvation (Buschmann et al.

2000). More recently, it was demonstrated that AKT1

channel is regulated by some other regulators, such as

AtKC1 and SNARE. AtKC1 is identiﬁed as a-subunit that

negatively regulates the Arabidopsis inward Shaker chan-

nel (AKT1) activity (Duby et al. 2008; Wang et al. 2010;

Jeanguenin et al. 2011). Alone, AtKC1 is unable to form a

functional homotetrameric K

channel (Reintanz et al.

2002) but its interaction with AKT1 leads to the formation

of the heterotetrameric AKT1-AtKC1 complex K

channel

whose activation results in inhibition of AKT1-mediated

currents by negatively shifting the voltage dependence

of the Shaker-type K

channel (Duby et al. 2008; Geiger

et al. 2009; Wang et al. 2010) (Fig. 4). As a consequence,

loss through AKT1 can be limited in Arabidopsis roots

under K

- conditions (Geiger et al. 2009; Wang et al.

2010). In addition, Honsbein et al. (2009) demonstrated in

Arabidopsis that AtKC1 selectively interacts with a

SNARE (soluble N-ethylmaleimide—sensitive factor pro-

tein attachment protein receptor) protein SYP121 leading

to the formation of a tripartite K

channel complex which

plays a pivotal role in plant K

nutrition under low-K

stress (Fig. 4).

Signaling cascades

concentration varies widely in time and space. To

ensure an adequate supply of this element, plants possess

many absorption and transport mechanisms (Ve

´ry and

Sentenac 2003; Ahn et al. 2004). However, despite inten-

sive works, it is still not very clear how plants detect and

respond to changes in K

concentrations in media where

they live (Shin and Schachtman 2004; Li et al. 2006;

Schachtman and Shin 2007; Wang and Wu 2010). Recent

studies suggested the involvement of several compounds

considered as signaling molecules, including hydrogen

1062 Acta Physiol Plant (2014) 36:1055–1070

123

peroxide (H

) (Shin and Schachtman 2004; Shin et al.

2005; Hafsi et al. 2008,2009; Hernandez et al. 2012),

ethylene (Shin and Schachtman 2004), phytohormones

such as auxin and jasmonic acid (Armengaud et al. 2004),

and phosphatidic acid (Hafsi et al. 2008,2009) in response

to K

deprivation. These changes are summarized in

Fig. 4.

Reactive oxygen species (ROS)

Although ROS are conventionally known as toxic mole-

cules for cellular metabolism, recent works suggest that

they can act as messenger molecules. Because of its rela-

tively long life and high permeability across membranes

hydrogen peroxide is considered as a signaling molecule in

plants that mediates responses to abiotic and biotic stresses

(Neill et al. 2002). In roots, H

plays a role in the

signaling cascade involved in the response to K

deﬁ-

ciency. These compounds are necessary for the growth of

root hairs and are implicated in root elongation and gene

expression under low-K

conditions. Shin and Schachtman

(2004) and Shin et al. (2005) demonstrated that H

implicated in the regulation of plant response to K

star-

vation. They showed that H

is rapidly accumulated in

response to short-term K

deprivation, triggering the

expression of some genes. In addition, these authors

demonstrated that changes in the kinetics of K

uptake are

due to H

, whose production is localized in a speciﬁc

region of the roots that has been shown to be active in K

uptake and translocation. More recently, Hernandez et al.

(2012) observed that H

accumulation under short-term

starvation in tomato was located mainly in the epi-

dermal cells of the elongation zone and the meristematic

cells of the root tip and the epidermal cells of the mature

Sensor ?

Ethylene

ROS

Ca2+

CBL 1/9 CPIK23

AKT1

K+

HAK transcript

K+

Ca2+

channel

Ca2+

PLD

Increasedroot

surface area ?

K+deficiency

AtKC1

HAK5

Fig. 4 Generic pathway for plant sensing and signaling to K

deﬁciency. K

deﬁciency is ﬁrst perceived by the membrane

receptors and then activates complex intracellular signaling cascade.

The external K

deﬁciency activates ethylene and calcium signals to

regulate ROS production and the CBL-CIPK23 network, respectively.

CIPK23 phosphorylates and then activates the potassium uptake

channel AKT1. AKT1 activity is also regulated by AtKC1 (a

regulatory K

channel subunit) and SYP121 (a SNARE protein)

leading to the formation of a tripartite complex which plays a pivotal

role in K

uptake. ROS production up-regulates the expression of

potassium transporter HAK5, to enhance K

uptake. Furthermore,

ROS activates phospholipase D (PLD), which hydrolyzes structural

phospholipids to generate phosphatidic acid (PA). PLD may be is also

activated by Ca

. The generated PA can be implicated in the up-

regulation of the expression of HAK5 transporter and/or increased

root hair elongation and primary root growth leading to increased root

surface area and consequently increasing K

uptake

Acta Physiol Plant (2014) 36:1055–1070 1063

123

zones of K

-starved roots. The induction of high-afﬁnity

transporters by K

deﬁciency was accompanied by an

increase (twofold) of H

production. More interestingly,

triggers a high-afﬁnity K

transporter even for

control plants (Shin and Schachtman 2004). The expression

of RHD2 gene was up-regulated in response to K

deﬁ-

ciency, which provides an evidence for the implication of

NADPH oxidase (Foreman et al. 2003)inH

production.

This was conﬁrmed by Shin and Schachtman (2004) using

rhd2 mutant. More recently, Hafsi et al. (2008,2009)

observed an increase in H

production with a maximum

after 6 h of K

deprivation in H. maritimum and H. vulgare

species. The early and transient increase in H

in roots

supports the idea that this molecule could play a signaling

role in response to short-term K

starvation.

Hormones

The accumulation of phytohormones like ethylene, jas-

monic acid, and auxin in response to K

deﬁciency has

been often cited. In Arabidopsis roots, after 6–30 h of K

starvation, the expression of genes coding the enzymes

implicated in the biosynthesis and the production of eth-

ylene increased following K

deprivation (Shin and

Schachtman 2004). The role of ethylene as a messenger

hormone is conﬁrmed by measuring the quantities released

in the atmosphere by plants subjected to K

deﬁciency and

control plants. An increase (about twofold) in ethylene

production was observed which has been implicated in

regulating gene transcription in response to low-K

con-

centrations (Shin and Schachtman 2004). More recently,

Jung et al. (2009) demonstrated that ethylene is important

for changes in both root hair and primary root growth in A.

thaliana and for the expression of AtHAK5, suggesting

that ethylene signaling acts upstream of ROS when plants

are deprived of K

Although auxin can control the expression of K

channels (Philippar et al. 1999), the mechanisms explain-

ing the changes in the content of this hormone in Arabi-

dopsis plants subjected to K

deﬁciency are still unknown.

DR5-GUS gene expression, which is marker of auxin

localization, changes in K

-deprived roots. It has been

reported that auxin is accumulated in central cylinder cells

in the distal elongation zone of Arabidopsis roots after K

deﬁciency (Vicente-Agullo et al. 2004).

The changes in auxin localization, its concentration, and

its sensibility can lead to a restriction of lateral roots

growth under prolonged K

shortage. In Arabidopsis,K

deﬁciency induced a higher expression of genes implicated

in the biosynthesis and signaling of this phytohormone

(Shin and Schachtman 2004). Ma et al. (2012) indicated

that several auxin-related genes responded to K

deﬁ-

ciency in rice.

Jasmonic acid (JA) is considered a member of the signal

transduction pathway in plant responses to environmental

stresses (Kramell et al. 2000; Vigliocco et al. 2002). JA

biosynthesis occurs through the octadecanoid acid path-

way, which starts from the oxidation of polyunsaturated

fatty acids by lipoxygenase and produces a number of

intermediate oxylipins (Stintzi et al. 2001; Howe and

Schilmiller 2002). Several transcripts for proteins involved

in the biosynthesis of JA responded to K

starvation and

resupply indicating that JA levels increase during starva-

tion and rapidly decrease after K

resupply (Armengaud

et al. 2004). A 1.8-fold increase of shoot JA (Troufﬂard

et al. 2010) and a threefold increase (Cao et al. 2006) are

determined in K

-deﬁcient Arabidopsis plants. The regu-

lation of K

channels by JA (Evans 2003) and polyamines

(Liu et al. 2000) was established and may contribute to K

redistribution between the different cellular compartments

(Amtmann et al. 2004).

Calcium

Besides its important role in plant growth and develop-

ment, calcium (Ca

) is one of the most important ubiq-

uitous intracellular secondary messengers involved in

signal transduction in plants (Tuteja and Mahaja 2007;

Kudla et al. 2010). It has been documented that low-K

availability increases H

production, which can play a

role as signaling molecule triggering the expression of

certain genes (Shin and Schachtman 2004). This produc-

tion is concomitant with changes in intracellular Ca

concentration (Foreman et al. 2003). More recently, tran-

scriptome analysis of rice root (Ma et al. (2012) identiﬁed

nine calcium sensor protein genes showing changes in

expression levels during K

starvation; ﬁve were up-reg-

ulated and four were down-regulated.

The increase of Ca

concentration in the cytoplasm can

stimulate NADPH oxidase-mediated ROS production

(Yang and Poovaiah 2002) which then activates some

-permeable ion channels leading to further Ca

inﬂux (Demidchik et al. 2007). Therefore, it was suggested

that Ca

may act as a secondary messenger in response to

deﬁciency. In Arabidopsis, Xu et al. (2006) and Li et al.

(2006) identiﬁed three genes critical for plant growth under

-deﬁcient conditions since encoding two Ca

sensors

including calcineurin B-like protein (CBL1 and CBL9) and

their target CBL-interacting protein kinase (CIPK23). The

same authors showed that the protein kinase CIPK23

phosphorylated a voltage-gated inward K

channel

(AKT1). Activation of the CBL–CIPK pathway by Ca

triggers the activation of the AKT1 channel, enhancing K

uptake by roots. The interaction between the CIPKs and

AKT1 involves the kinase domain of the CIPK component

and the ankyrin repeat domain of the channel (Lee et al.

1064 Acta Physiol Plant (2014) 36:1055–1070

123

2007). An A-type protein phosphatase 2C (PP2C) that

physically interacts and inactivates the AKT1 channel was

also identiﬁed by the same authors. Recently, Lan et al.

(2011) documented that PP2Cs interacts with the PPI motif

and the kinase domain to inhibit AKT1 activation by

CIPK-CBL complex through physical binding and

dephosphorylation. Furthermore, these authors identiﬁed

that speciﬁc CBLs interact with and reduce PP2Cs phos-

phatase activity to recover the CIPK-dependent AKT1

channel activity. More recently, using a transcriptomic

approach on K

-starved rice roots, Ma et al. (2012)

observed that the most genes were identiﬁed as kinases

(68 %) and phosphatases (9 %), suggesting that phos-

phorylation and dephosphorylation may be an important

regulatory mechanisms in plant responses to K

deﬁciency.

Phosphatidic acid

As a selective barrier between cells and their environments,

plasma membrane plays a crucial role in both the perception

and transmission of external information. Recently, it has

been shown that phospholipase D (PLD, EC 3.1.4.4), in

addition to its catabolic function, has critical roles in cell

signaling cascades (Meijer and Munnik 2003; Navari-Izzo

et al. 2006; Russo et al. 2007). This enzyme hydrolyzes

structural phospholipids (PL) at the terminal phosphate

diester bond, leading to the formation of phosphatidic acid

(PA) and a free head group (Zhang et al. 2003). PA is a

secondary messenger in response to multiple environmental

stresses, including hyperosmotic stress (Munnik et al. 2000),

copper excess (Navari-Izzo et al. 2006), and P deﬁciency

(Russo et al. 2007). In vivo and in vitro studies on suspen-

sion-cultured rice cells revealed that PLD is activated by

and that this activation involves a protein tyrosine

kinase (Yamaguchi et al. 2004). It has also been suggested

that H

directly modulates the reactive cysteine residues

of protein tyrosine kinase (Meng et al. 2002) and glycer-

aldehyde-3-phosphate dehydrogenase (G3PDH; EC

1.2.1.12), inhibiting their activities (Schuppe-Koistinen et al.

1994). These authors suggested that in addition to its role in

the inactivation of the dehydrogenase activity of G3PDH on

its catalytic site, H

may also endow G3PDH with the

ability to bind PLD, and the resulting association is involved

in the regulation of PLD activity by H

. In addition, it was

demonstrated that plant PLDs contain a Ca

-dependent

phospholipid-binding C2 domain and require Ca

for

activity (Wang 2000). The Arabidopsis genome contains 12

PLD genes, ten of which have the Ca

/phospholipid-bind-

ing C2 structural fold, whereas the two PLDf proteins con-

tain phosphoinositide interacting pleckstrin homology and

Phox homology domains (Qin and Wang 2002). Knockout of

PLDedecreases root growth and biomass accumulation,

whereas over-expression (OE) of PLDeenhances root hair

elongation and primary root growth and biomass accumu-

lation under severe nitrogen deprivation suggesting that

PLDeand PA play a role in nitrogen signaling (Hong et al.

2009). These authors showed also that under P- and K

deﬁcient conditions, PLDe-OE roots elongated faster than

those of wild type, but the difference was not as great as that

under N-deprived conditions. In two barley species subjected

to a short K

deprivation, a clear relationship among H

production, G3PDH inhibition, and PLD activation was

demonstrated (Hafsi et al. 2008,2009) suggesting a possible

involvement of these three compounds in an early response

to K

starvation. Further works are needed to clarify in more

detail the implication of PLD and PA in plant response to K

deﬁciency.

Conclusion and perspectives

is an important macronutrient and the most abundant

cation in plants. It accomplishes important functions in

plants including photosynthesis, osmoregulation, enzymes

activities, maintaining the plasma membrane potential, and

protein synthesis. K

deﬁciency is an abiotic stress that

leads to plant growth and productivity depressions as a

consequence of a wide of perturbations that trigger in

plants. To cope with this constraint plants develop numer-

ous tolerance strategies involving morphological and

physiological modiﬁcations and regulation of K

transport

systems which is important for K

uptake and homeostasis

in plant cells under low-K

conditions. However, although

several works are conducted to understand these mecha-

nisms, much remains to be known about the physiological

and molecular mechanisms by which plants detect and

respond to changes in K

concentrations in the external

media (Schachtman and Shin 2007; Wang and Wu 2010).

Hence, it is of major importance to deeply investigate the

complex sensing and signaling pathway in response to K

deﬁciency. The identiﬁcation and further examination of the

signaling cascade and valorization of natural variation in

uptake and utilization efﬁciencies between genotypes

will be essential for an eventual breeding program.

Author contribution Chokri HAFSI designed and wrote

the review. Ahmed DEBEZ checked the English. Chedly

ABDELLY provided scientiﬁc advices and supported the

work.

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Integrated transcriptomic and metabolomic analyses reveal key metabolic pathways in response to potassium deficiency in coconut (Cocos nucifera L.) seedlings

Article

Full-text available

Feb 2023

Potassium ions (K+) are important for plant growth and crop yield. However, the effects of K+ deficiency on the biomass of coconut seedlings and the mechanism by which K+ deficiency regulates plant growth remain largely unknown. Therefore, in this study, we compared the physiological, transcriptome, and metabolite profiles of coconut seedling leaves under K+-deficient and K+-sufficient conditions using pot hydroponic experiments, RNA-sequencing, and metabolomics technologies. K+ deficiency stress significantly reduced the plant height, biomass, and soil and plant analyzer development value, as well as K content, soluble protein, crude fat, and soluble sugar contents of coconut seedlings. Under K+ deficiency, the leaf malondialdehyde content of coconut seedlings were significantly increased, whereas the proline (Pro) content was significantly reduced. Superoxide dismutase, peroxidase, and catalase activities were significantly reduced. The contents of endogenous hormones such as auxin, gibberellin, and zeatin were significantly decreased, whereas abscisic acid content was significantly increased. RNA-sequencing revealed that compared to the control, there were 1003 differentially expressed genes (DEGs) in the leaves of coconut seedlings under K+ deficiency. Gene Ontology analysis revealed that these DEGs were mainly related to “integral component of membrane,” “plasma membrane,” “nucleus”, “transcription factor activity,” “sequence-specific DNA binding,” and “protein kinase activity.” Kyoto Encyclopedia of Genes and Genomes pathway analysis indicated that the DEGs were mainly involved in “MAPK signaling pathway-plant,” “plant hormone signal transduction,” “starch and sucrose metabolism,” “plant-pathogen interaction,” “ABC transporters,” and “glycerophospholipid metabolism.” Metabolomic analysis showed that metabolites related to fatty acids, lipidol, amines, organic acids, amino acids, and flavonoids were generally down-regulated in coconut seedlings under K+ deficiency, whereas metabolites related to phenolic acids, nucleic acids, sugars, and alkaloids were mostly up-regulated. Therefore, coconut seedlings respond to K+ deficiency stress by regulating signal transduction pathways, primary and secondary metabolism, and plant-pathogen interaction. These results confirm the importance of K+ for coconut production, and provide a more in-depth understanding of the response of coconut seedlings to K+ deficiency and a basis for improving K+ utilization efficiency in coconut trees.

Transcriptional and metabolic responses of apple to different potassium environments

Article

Full-text available

Mar 2023

Potassium (K) is one of the most important macronutrients for plant development and growth. The influence mechanism of different potassium stresses on the molecular regulation and metabolites of apple remains largely unknown. In this research, physiological, transcriptome, and metabolite analyses were compared under different K conditions in apple seedlings. The results showed that K deficiency and excess conditions influenced apple phenotypic characteristics, soil plant analytical development (SPAD) values, and photosynthesis. Hydrogen peroxide (H2O2) content, peroxidase (POD) activity, catalase (CAT) activity, abscisic acid (ABA) content, and indoleacetic acid (IAA) content were regulated by different K stresses. Transcriptome analysis indicated that there were 2,409 and 778 differentially expressed genes (DEGs) in apple leaves and roots under K deficiency conditions in addition to 1,393 and 1,205 DEGs in apple leaves and roots under potassium excess conditions, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment showed that the DEGs were involved in flavonoid biosynthesis, photosynthesis, and plant hormone signal transduction metabolite biosynthetic processes in response to different K conditions. There were 527 and 166 differential metabolites (DMAs) in leaves and roots under low-K stress as well as 228 and 150 DMAs in apple leaves and roots under high-K stress, respectively. Apple plants regulate carbon metabolism and the flavonoid pathway to respond to low-K and high-K stresses. This study provides a basis for understanding the metabolic processes underlying different K responses and provides a foundation to improve the utilization efficiency of K in apples.

Impact of potassium starvation on the uptake, transportation, photosynthesis, and abiotic stress tolerance

Article

Full-text available

Nov 2022
PLANT GROWTH REGUL

Potassium (K⁺) plays a crucial role in plant homeostasis, and its deficiency significantly impacts photosynthesis, triggering a decrease in growth and crop production. K⁺ starvation induced a significant reduction in the net photosynthetic rate, and the drop is associated with resistance of CO2 diffusion through stomatal conductance, mesophyll conductance, and lowered carboxylase activity of Rubisco, electron transport rate of PSII, and with many biochemical limitations. The complex interaction of all the above factors contributed to limiting photosynthesis under K⁺ starved conditions. Low chlorophyll content and poor chloroplast structure may also define photosynthetic processes and causes a decrease in crop growth and productivity under K⁺ starvation. Under abiotic stresses such as drought, waterlogging, temperature, salinity, and heavy metal, the application of exogenous K⁺ promotes plant tolerance by activating the antioxidant system, which limits the overproduction of reactive oxygen species (ROS), avoiding the associated oxidative damages where other signaling molecules such as nitric oxide (NO) and hydrogen sulfide (H2S) may involve. The review highlights the decisive role of exogenous and endogenous K⁺ in modulating diverse physiological and biochemical processes in a healthy and stressful environment. Furthermore, this review appraises the involvement of K⁺ with another signaling molecule in enhancing abiotic stress tolerance in plants. Therefore, this review provides a comprehensive update on the relevance of K⁺ in higher plants. Its exogenous application should be a potential tool, especially in crops under adverse environmental conditions.

Utilization of Different Concentration Sugarcane Molasses to the Quality of Goat Feces-Chicken Excreta-Coconut Husk Organic Liquid Fertilizer

Article

Full-text available

Feb 2023

Indonesia is an agricultural country that certainly has the potential for agricultural waste and by-products. Some of those in Indonesia are livestock manure, coconut husk, and sugarcane molasses. Utilization of this waste is limited, which is causing various environmental problems such as environmental pollution. The study is aimed to determine the quality of organic liquid fertilizer made by raw material from goat feces-chicken excreta-coconut husk with various sugarcane molasses concentrations (P0= 0%; P1= 5%; P3= 15%; P4= 20%; P5= 25%). The fermentation was carried out anaerobically for 21 days. The parameters of temperature, pH, organoleptic (color and odor), levels of C-organic, nitrogen (N) total, phosphorus (P) total, potassium (K) total, and biological tests were observed. Biological tests were conducted in the growth of mung bean (Vigna radiata) and spinach (Amaranthus tricolor). Data were analyzed using one-way ANOVA. Variations in the addition of molasses sugarcane did not affect the liquid fertilizer's temperature and pH during fermentation. However, the treatments had positive effect on organoleptic test, C-organic, N total, P total, K total, and biological tests. The addition of 15% sugarcane molasses (P3) showed a dominating performance toward most of the parameters tested. The P3 treatment produced liquid fertilizer with a pleasant smell, and the contents of C-organic, total N, total P, and total K were 2.12%, 0.25%, 0.13%, and 0.13%, respectively. and produce spinach as a biological test with plant height, number of leaves, leaf width, stem diameter, and root length were 21.82 cm, 6.66 sheets, 3.59 cm, 4.09 mm, and 14.67 cm, respectively.

Plant Beneficial Bacteria and Their Potential Applications in Vertical Farming Systems

Article

Full-text available

Jan 2023

In this literature review, we discuss the various functions of beneficial plant bacteria in improving plant nutrition, the defense against biotic and abiotic stress, and hormonal regulation. We also review the recent research on rhizophagy, a nutrient scavenging mechanism in which bacteria enter and exit root cells on a cyclical basis. These concepts are covered in the contexts of soil agriculture and controlled environment agriculture, and they are also used in vertical farming systems. Vertical farming-its advantages and disadvantages over soil agriculture, and the various climatic factors in controlled environment agriculture-is also discussed in relation to plant-bacterial relationships. The different factors under grower control, such as choice of substrate, oxygenation rates, temperature, light, and CO 2 supplementation, may influence plant-bacterial interactions in unintended ways. Understanding the specific effects of these environmental factors may inform the best cultural practices and further elucidate the mechanisms by which beneficial bacteria promote plant growth.

Morphophysiological and transcriptome analysis reveal that reprogramming of metabolism, phytohormones and root development pathways governs the potassium (K+) deficiency response in two contrasting chickpea cultivars

Article

Full-text available

Jan 2023

Potassium (K +) is an essential macronutrient for plant growth and development. K + deficiency hampers important plant processes, such as enzyme activation, protein synthesis, photosynthesis and stomata movement. Molecular mechanism of K + deficiency tolerance has been partly understood in model plants Arabidopsis, but its knowledge in legume crop chickpea is missing. Here, morphophysiological analysis revealed that among five high yielding desi chickpea cultivars, PUSA362 shows stunted plant growth, reduced primary root growth and low K + content under K + deficiency. In contrast, PUSA372 had negligible effect on these parameters suggesting that PUSA362 is K + deficiency sensitive and PUSA372 is a K + deficiency tolerant chickpea cultivar. RNA-seq based transcriptome analysis under K + deficiency revealed a total of 820 differential expressed genes (DEG's) in PUSA362 and 682 DEGs in PUSA372. These DEGs belongs to different functional categories, such as plant metabolism, signal transduction components, transcription factors, ion/ nutrient transporters, phytohormone biosynthesis and signalling, and root growth and development. RNA-seq expression of randomly selected 16 DEGs was validated by RT-qPCR. Out of 16 genes, 13 showed expression pattern similar to RNA-seq expression, that verified the RNA-seq expression data. Total 258 and 159 genes were exclusively up-regulated, and 386 and 347 genes were down-regulated, respectively in PUSA362 and PUSA372. 14 DEGs showed contrasting expression pattern as they were up-regulated in PUSA362 and down-regulated in PUSA372. These include somatic embryogenesis receptor-like kinase 1, thaumatin-like protein, ferric reduction oxidase 2 and transcription factor bHLH93. Nine genes which were down-regulated in PUSA362 found to be up-regulated in PUSA372, including glutathione S-transferase like, putative calmodulin-like 19, high affinity nitrate transporter Frontiers in Plant Science

A Simple Reversed Iontophoresis-Based Sensor to Enable In Vivo Multiplexed Measurement of Plant Biomarkers Using Screen-Printed Electrodes

Article

Full-text available

Jan 2023
SENSORS-BASEL

The direct quantification of plant biomarkers in sap is crucial to enhancing crop production. However, current approaches are inaccurate, involving the measurement of non-specific parameters such as colour intensity of leaves, or requiring highly invasive processes for the extraction of sap. In addition, these methods rely on bulky and expensive equipment, and they are time-consuming. The present work reports for the first time a low-cost sensing device that can be used for the simultaneous determination of sap K+ and pH in living plants by means of reverse iontophoresis. A screen-printed electrode was modified by deposition of a K+-selective membrane, achieving a super-Nernstian sensitivity of 70 mV Log[K+]−1 and a limit of detection within the micromolar level. In addition, the cathode material of the reverse iontophoresis device was modified by electrodeposition of RuOx particles. This electrode could be used for the direct extraction of ions from plant leaves and the amperometric determination of pH within the physiological range (pH 3–8), triggered by the selective reaction of RuOx with H+. A portable and low-cost (<£60) microcontroller-based device was additionally designed to enable its use in low-resource settings. The applicability of this system was demonstrated by measuring the changes in concentration of K+ and pH in tomato plants before and after watering with deionised water. These results represent a step forward in the design of affordable and non-invasive devices for the monitoring of key biomarkers in plants, with a plethora of applications in smart farming and precision agriculture among others.

Potassium application enhances drought tolerance in sesame by mitigating oxidative damage and regulating osmotic adjustment

Article

Full-text available

Dec 2022

Potassium (K) is known for alleviating the negative effects of abiotic stresses on plants. To explore the functions of K in controlling reactive oxygen species (ROS), antioxidant activities, and osmoregulation in sesame under drought stress, a pot experiment was conducted with three K levels (0, 60, and 120 kg ha–1, recorded as K0, K1, and K2, respectively) and exposed to well-watered (WW, 75% ± 5% soil relative water content) and drought-stressed (DS, 50% ± 5% soil relative water content) conditions. The results showed that DS stimulated the production of ROS such as increased hydrogen peroxide (H2O2), leading to lipid peroxidation as characterized by higher malondialdehyde (MDA) and, consequently, resulting in the decline in relative water content (RWC) and photosynthetic pigments as compared with WW plants. These adverse effects were exacerbated when drought stress was prolonged. Concurrently, K application alleviated the magnitude of decline in the RWC, chlorophyll a, and chlorophyll b, and plants applied with K exhibited superior growth, with the optimal mitigation observed under K2 treatment. Additionally, DS plants treated with K exhibited lower lipid peroxidation, higher antioxidant activities, and increased osmotic solute accumulation in comparison with plants under K deficiency, which suggested that exogenous K application mitigated the oxidative damages and this was more prominent under K2 treatment. Noteworthily, proline and soluble protein, respectively, dominated in the osmotic regulation at 3 and 6 days of drought stress according to the analysis of the quantitative comparison among different osmotically active solutes. Based on the correlation of the aforementioned traits and the analysis of variance on the interaction effects of drought stress and potassium, we propose that superoxide dismutase (SOD), glutathione reductase (GR), and MDA could be critical indicators in balancing ROS detoxification and reproduction. In summary, our studies suggest that optimized K application keeps a balance between the production of antioxidants and ROS and simultaneously affects osmoregulation to alleviate the damage from drought stress.

An Indirect Colorimetric Method for Potassium Determination in Soil Using a Paper Device and Smartphone

Article

Nov 2022

This study presents a simple method for determination of potassium in microliter scale using a paper device together with a smartphone. The method begins with the ion-pair extraction of dibenzo-18-crown-6-K+ complex into dichloromethane with an excess amount of calmagite. The aqueous phase containing the remaining calmagite is transferred to the paper device, where a smartphone is used to capture the color and convert to RGB value. The linear detection range was found to cover potassium concentrations from 20 mg L-1 to 120 mg L-1. The detection and quantification are 5.41 mg L-1 and 18.03 mg L-1, respectively. Potassium detection was carried out in a variety of actual soil samples, and the results were validated against spectrophotometric results using a paired t-test, which indicated high accuracy. The proposed method is simple, fast, and inexpensive, and it requires no complicated equipment, making it ideally suited for detection of potassium in soil.

Respon Pertumbuhan Bibit Kelapa Sawit (Elaeis guineensis Jacq.)Di Pre Nursery Terhadap Pemberian Limbah Lumpur Kering Crude Palm Oil (CPO)

Article

Dec 2022

Growth and mineral nutrition of field crops, third edition

Book

Oct 2010

By the year 2050, the world’s population is expected to reach nine billion. To feed and sustain this projected population, world food production must increase by at least 50 percent on much of the same land that we farm today. To meet this staggering challenge, scientists must develop the technology required to achieve an “evergreen” revolution-one that increases crop productivity without degrading natural resources. With 30 percent new material, the updated and revised third edition of Growth and Mineral Nutrition of Field Crops covers all aspects of crop growth and mineral nutrition that contribute to sustainable, high-yield agriculture. Bringing together international scientific knowledge of crop production and the impacts of agriculture on the environment, this book: •Includes two new chapters on remediation of heavy-metal contaminated soils and cover crops •Covers theoretical and practical aspects of mineral nutrition of field crops •Provides recommendations for judicious use of fertilizers, which will reduce cost of crop production and enhance high crop yields without risking environmental pollution •Provides growth patterns for annual crops and forages •Includes a handful of color pictures of nutrient deficiencies for easy diagnostic purposes To make the book as practical as possible, each chapter is supported by experimental results and extensive references. A large number of figures and tables are also included to save readers time when researching. The overall emphasis of this reference is on the soil’s ability to sustain high crop yields and a healthy human population.

Principles of Plant Nutrition

Book

Jan 2001

This is the 5th edition of a well-established book Principles of Plant Nutrition which was first published in 1978. The same format is maintained as in previous editions with the primary aim of the authors to consider major processes in soils and plants that are of relevance to plant nutrition.This new edition gives an up-to-date account of the scientific advances of the subject by making reference to about 2000 publications. An outstanding feature of the book, which distinguishes it from others, is its wide approach encompassing not only basic nutrition and physiology, but also practical aspects of plant nutrition involving fertilizer usage and crop production of direct importance to human nutrition. Recognizing the international readership of the book, the authors, as in previous editions, have attempted to write in a clear concise style of English for the benefit of the many readers for whom English is not their mother tongue. The book will be of use to undergraduates and postgraduates in Agriculture, Horticulture, Forestry and Ecology as well as those researching in Plant Nutrition.

Marschner's Mineral Nutrition of Higher Plants

Book

Jan 2012

P. Marschner

An understanding of the mineral nutrition of plants is of fundamental importance in both basic and applied plant sciences. The Second Edition of this book retains the aim of the first in presenting the principles of mineral nutrition in the light of current advances. This volume retains the structure of the first edition, being divided into two parts: Nutritional Physiology and Soil-Plant Relationships. In Part I, more emphasis has been placed on root-shoot interactions, stress physiology, water relations, and functions of micronutrients. In view of the worldwide increasing interest in plant-soil interactions, Part II has been considerably altered and extended, particularly on the effects of external and interal factors on root growth and chapter 15 on the root-soil interface. The second edition will be invaluable to both advanced students and researchers.

Essential nutrient elements for plants: requirement and interaction in plants

Article

Jan 1963
PLANT PHYSIOL

E.J. Hewitt

Potassium status of temperate region soils

Article

Jan 1985

Soil Fertility and Fertilizers- an Introduction to Nutrient Management

Article

Calcium signaling in Plants

Article

Jan 2004

Physical Chemistry of Soil Potassium

Chapter

Jan 1985

The Arabidopsis outward K+ channel GORK is involved in regulation of stomatal movements and plant transpiration (vol 100, pg 5549, 2003)

Article

Jun 2003

Microscopic pores present in the epidermis of plant aerial organs, called stomata, allow gas exchanges between the inner photosyn-thetic tissue and the atmosphere. Regulation of stomatal aperture, preventing excess transpirational vapor loss, relies on turgor changes of two highly differentiated epidermal cells surrounding the pore, the guard cells. Increased guard cell turgor due to increased solute accumulation results in stomatal opening, whereas decreased guard cell turgor due to decreased solute accumulation results in stomatal closing. Here we provide direct evidence, based on reverse genetics approaches, that the Arabi-dopsis GORK Shaker gene encodes the major voltage-gated outwardly rectifying K channel of the guard cell membrane. Expression of GORK dominant negative mutant polypeptides in transgenic Arabidopsis was found to strongly reduce outwardly rectifying K channel activity in the guard cell membrane, and disruption of the GORK gene (T-DNA insertion knockout mutant) fully suppressed this activity. Bioassays on epidermal peels revealed that disruption of GORK activity resulted in impaired stomatal closure in response to darkness or the stress hormone azobenzenearsonate. Transpiration measurements on excised rosettes and intact plants (grown in hydroponic conditions or submitted to water stress) revealed that absence of GORK activity resulted in increased water consumption. The whole set of data indicates that GORK is likely to play a crucial role in adaptation to drought in fluctuating environments.

CLONING AND EXPRESSION IN YEAST OF A PLANT K+ TRANSPORT-SYSTEM

Article

Jan 1993

Potassium deficiency in plants: Effects and signaling cascades

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