5.3.3 Potassium
The uptake and transport of potassium take place via both active and passive
mechanisms. Unlike N, P, and S which have dedicated transporters, potassium has
been shown to be transported from soil to the roots via several families of both
transporters and ion channels. In Arabidopsis thaliana where the first plant potassium ion channels were identified, a total of six different families of ion channels and
transporters are now known. These are Shaker family, TPK family, Kir-like family,
KUP/HAK/KT family, HKT family, and CPA family (Anderson et al. 1992;
Sentenac et al. 1992; Chen et al. 2008a). Subsequently, five families of ion channels
and transporters for potassium have been proposed—Shaker family, TPK/KCO-2P
family, TPK/KCO-1P family, KUP/HAK/KT family, and HKT family. In addition
to these five families, animal cyclic nucleotide-gated channels (CNGCs), glutamate
receptors, LCT1, cation proton antiporters (CPA), and cation chloride co-transporter
family (CCC) are other probable candidates that participate in potassium uptake,
transport, and assimilation (Very and Sentenac 2003). The members of the Shaker
family have been further sub-divided into three subfamilies as inward rectifying
(IR), weakly inward rectifying (WR), and outward rectifying (OR). AKT1/KC1 is an
example of IR-type of the Shaker family, which is upregulated under low
K-conditions through signaling via CIPK, CBK, PP2CA; AKT2 is an example of
the WR-type, whereas SKOR is an example of the OR-type of K-transporter (Wang
and Wu 2013)). Apart from K+ absorption at the root, these are also likely to play
role in loading of K
+ in xylem and phloem, guard cell and stomatal movement,
pollen tube development and stress regulation (Sentenac et al. 1992; Ache et al.
2000; Pilot et al. 2001; Mouline et al. 2002; Gaymard et al. 1998).
The second family, viz. KT/KUP/HAK includes low affinity (KUP2, KUP3,
KUP4), high affinity (HAK5, OsHAK1), and dual affinity transporters (KUP1). In
plants, members of this family are further sub-divided into four groups based on
affinity and active/passive transport ability. Group I is primarily high affinity
transporters, may be involved in active transport, and may not be able to discriminate
between other cations such as K
+
, Rb
+
, and Cs
+ but are less likely to transport Na
+
and NH4
+ . Members of group II are low affinity and able to transport ions such as
K
+
, Rb
+ Na
+
, and Cs
+
. Members of KT/KUP/HAK family have been shown to be
present in nearly all plant parts including roots, shoots, leaf, flowers (Fu and Luan
1998; Gierth et al. 2005; Grabov 2007; Elumalai et al. 2002), and critical for seedling
establishment (Pyo et al. 2010).
The TPK/KCO family includes KCO-2P and KCO-1P subfamilies that are
divided based on number of transmembrane domains (TMS) and pore (P)-domains.
KCO-2P contains four TMS and two P-domains, whereas KCO-1P contains two
TMS and one P-domain (Very and Sentenac 2003). Members of the KCO family are
membrane localized, involved in K+ transport in nearly all plant parts, and exhibit
Ca
2+ dependency (Czempinski et al. 2002; Voelker et al. 2006; Rehman et al. 2017).
In a recent study, TPK/KCO family members have also been shown to be involved in
nodulation in soybean (Rehman et al. 2017).
148
E. Bhardwaj et al.
The uptake and transport of potassium take place via both active and passive
mechanisms. Unlike N, P, and S which have dedicated transporters, potassium has
been shown to be transported from soil to the roots via several families of both
transporters and ion channels. In Arabidopsis thaliana where the first plant potassium ion channels were identified, a total of six different families of ion channels and
transporters are now known. These are Shaker family, TPK family, Kir-like family,
KUP/HAK/KT family, HKT family, and CPA family (Anderson et al. 1992;
Sentenac et al. 1992; Chen et al. 2008a). Subsequently, five families of ion channels
and transporters for potassium have been proposed—Shaker family, TPK/KCO-2P
family, TPK/KCO-1P family, KUP/HAK/KT family, and HKT family. In addition
to these five families, animal cyclic nucleotide-gated channels (CNGCs), glutamate
receptors, LCT1, cation proton antiporters (CPA), and cation chloride co-transporter
family (CCC) are other probable candidates that participate in potassium uptake,
transport, and assimilation (Very and Sentenac 2003). The members of the Shaker
family have been further sub-divided into three subfamilies as inward rectifying
(IR), weakly inward rectifying (WR), and outward rectifying (OR). AKT1/KC1 is an
example of IR-type of the Shaker family, which is upregulated under low
K-conditions through signaling via CIPK, CBK, PP2CA; AKT2 is an example of
the WR-type, whereas SKOR is an example of the OR-type of K-transporter (Wang
and Wu 2013)). Apart from K+ absorption at the root, these are also likely to play
role in loading of K
+ in xylem and phloem, guard cell and stomatal movement,
pollen tube development and stress regulation (Sentenac et al. 1992; Ache et al.
2000; Pilot et al. 2001; Mouline et al. 2002; Gaymard et al. 1998).
The second family, viz. KT/KUP/HAK includes low affinity (KUP2, KUP3,
KUP4), high affinity (HAK5, OsHAK1), and dual affinity transporters (KUP1). In
plants, members of this family are further sub-divided into four groups based on
affinity and active/passive transport ability. Group I is primarily high affinity
transporters, may be involved in active transport, and may not be able to discriminate
between other cations such as K
+
, Rb
+
, and Cs
+ but are less likely to transport Na
+
and NH4
+ . Members of group II are low affinity and able to transport ions such as
K
+
, Rb
+ Na
+
, and Cs
+
. Members of KT/KUP/HAK family have been shown to be
present in nearly all plant parts including roots, shoots, leaf, flowers (Fu and Luan
1998; Gierth et al. 2005; Grabov 2007; Elumalai et al. 2002), and critical for seedling
establishment (Pyo et al. 2010).
The TPK/KCO family includes KCO-2P and KCO-1P subfamilies that are
divided based on number of transmembrane domains (TMS) and pore (P)-domains.
KCO-2P contains four TMS and two P-domains, whereas KCO-1P contains two
TMS and one P-domain (Very and Sentenac 2003). Members of the KCO family are
membrane localized, involved in K+ transport in nearly all plant parts, and exhibit
Ca
2+ dependency (Czempinski et al. 2002; Voelker et al. 2006; Rehman et al. 2017).
In a recent study, TPK/KCO family members have also been shown to be involved in
nodulation in soybean (Rehman et al. 2017).
148
E. Bhardwaj et al.