The family of HKT transporters are involved in both influx and efflux of K
+ as
well as Na
+ or H
+ (Schachtman and Schroeder 1994; Rodríguez-Navarro 2000;
Waters et al. 2013; Almeida et al. 2013). The affinity and ability to permit influx
and efflux of these ions has been observed to be concentration dependent in some
species such as Triticum aestivum but not in others as in Arabidopsis thaliana
(Gassman et al. 1996; Rubio et al. 1995; Uozomi et al. 2000; Mäser et al. 2002a).
In multiple species where HKT gene families have been characterized, such as
A. thaliana, O. sativa, T. aestivum, H. vulgare their expression has been observed
in roots, in root and stem in A. thaliana and, in root, leaf, and stem in Eucalyptus
(Rus et al. 2001; Uozomi et al. 2000; Fairbairn et al. 2000; Sharif Shohan et al. 2019;
Wang et al. 1998; Horie et al. 2001; Tada 2019). Members of this family are divided
into two classes, among which class 2 is found only in monocots (Platten et al.
2006). Members of HKT family have been shown to be associated with salt tolerance
in both A. thaliana and O. sativa (Sharif Shohan et al. 2019; Tada 2019).
Members of cyclic nucleotide gated channels (CNGCs) (CNGC1, CNGC2,
CNGC4, and CNGC10) are known to work as K+ rectifying inward channel.
Similarly, members of CATION/Hþ EXCHANGERs CHX family (CHX13,
CHX17, CHX20, CHX21, CHX23) (Cellier et al. 2004; Padmanaban et al. 2007)
and GORK family in the root hairs (Ivashikina et al. 2001) are also known to help in
K
+ transport. Long distance transport is proposed to be carried out by K EFFLUX
ANTIPORTERs (KEAs) which play role in K
+ efflux into xylem sap, whereas
KAT2 and AKT2/3 regulate phloem loading and K homeostasis in the phloem
(Aranda-Sicilia et al. 2012; Deeken et al. 2002; Philippar et al. 2004). Intracellular
transport is carried out by TPK1 (K efflux from the vacuole), NHX acts in early
development stage of plant (Liu et al. 2010). HAK10 is also known to mediate K
release from the vacuole to the cytosol in rice (Banuelos et al. 2002).
5.3.4 Sulfur
Sulfur is taken up as sulfate ion (SO 4
2À
) and reduced to sulfide prior to assimilation
into metabolites by plants. There are four types of sulfate transporters found from
bacteria to plants and are exemplified by proton/sulfate co-transporter (SUL, SULTR
family), sodium/sulfate co-transporter, sulfate/anion exchanger family, and
ABC-type transporter complex (Takahashi et al. 2012). Structural feature of sulfate
transporter includes presence of twelve transmembrane domain, and, sulfate transporter and antistigma factor antagonists (STAS) domain which is involved in sulfate
uptake and flux, (Lass and Ullrich-Eberius 1984; Rouached et al. 2005; Baraniecka
and Kopriva 2014). Sulfate is transported from soil to the roots with the help of
proton/sulfate co-transporters which were initially divided into five groups—Sultr1,
Sultr2, Sultr3, Sultr4, and Sultr5, based on their expression pattern and kinetics of
transport mechanism. Expression pattern of these transporter proteins depends on
soil/intracellular sulfate amount, cysteine and glutathione concentration. High affinity transporters are involved in soil-to-root uptake and intracellular transport,
whereas low affinity transporters participate in vascular transport within the plants.
5 Plant Roots and Mineral Nutrition: An Overview of Molecular Basis of Uptake and. . . 149
+ as
well as Na
+ or H
+ (Schachtman and Schroeder 1994; Rodríguez-Navarro 2000;
Waters et al. 2013; Almeida et al. 2013). The affinity and ability to permit influx
and efflux of these ions has been observed to be concentration dependent in some
species such as Triticum aestivum but not in others as in Arabidopsis thaliana
(Gassman et al. 1996; Rubio et al. 1995; Uozomi et al. 2000; Mäser et al. 2002a).
In multiple species where HKT gene families have been characterized, such as
A. thaliana, O. sativa, T. aestivum, H. vulgare their expression has been observed
in roots, in root and stem in A. thaliana and, in root, leaf, and stem in Eucalyptus
(Rus et al. 2001; Uozomi et al. 2000; Fairbairn et al. 2000; Sharif Shohan et al. 2019;
Wang et al. 1998; Horie et al. 2001; Tada 2019). Members of this family are divided
into two classes, among which class 2 is found only in monocots (Platten et al.
2006). Members of HKT family have been shown to be associated with salt tolerance
in both A. thaliana and O. sativa (Sharif Shohan et al. 2019; Tada 2019).
Members of cyclic nucleotide gated channels (CNGCs) (CNGC1, CNGC2,
CNGC4, and CNGC10) are known to work as K+ rectifying inward channel.
Similarly, members of CATION/Hþ EXCHANGERs CHX family (CHX13,
CHX17, CHX20, CHX21, CHX23) (Cellier et al. 2004; Padmanaban et al. 2007)
and GORK family in the root hairs (Ivashikina et al. 2001) are also known to help in
K
+ transport. Long distance transport is proposed to be carried out by K EFFLUX
ANTIPORTERs (KEAs) which play role in K
+ efflux into xylem sap, whereas
KAT2 and AKT2/3 regulate phloem loading and K homeostasis in the phloem
(Aranda-Sicilia et al. 2012; Deeken et al. 2002; Philippar et al. 2004). Intracellular
transport is carried out by TPK1 (K efflux from the vacuole), NHX acts in early
development stage of plant (Liu et al. 2010). HAK10 is also known to mediate K
release from the vacuole to the cytosol in rice (Banuelos et al. 2002).
5.3.4 Sulfur
Sulfur is taken up as sulfate ion (SO 4
2À
) and reduced to sulfide prior to assimilation
into metabolites by plants. There are four types of sulfate transporters found from
bacteria to plants and are exemplified by proton/sulfate co-transporter (SUL, SULTR
family), sodium/sulfate co-transporter, sulfate/anion exchanger family, and
ABC-type transporter complex (Takahashi et al. 2012). Structural feature of sulfate
transporter includes presence of twelve transmembrane domain, and, sulfate transporter and antistigma factor antagonists (STAS) domain which is involved in sulfate
uptake and flux, (Lass and Ullrich-Eberius 1984; Rouached et al. 2005; Baraniecka
and Kopriva 2014). Sulfate is transported from soil to the roots with the help of
proton/sulfate co-transporters which were initially divided into five groups—Sultr1,
Sultr2, Sultr3, Sultr4, and Sultr5, based on their expression pattern and kinetics of
transport mechanism. Expression pattern of these transporter proteins depends on
soil/intracellular sulfate amount, cysteine and glutathione concentration. High affinity transporters are involved in soil-to-root uptake and intracellular transport,
whereas low affinity transporters participate in vascular transport within the plants.
5 Plant Roots and Mineral Nutrition: An Overview of Molecular Basis of Uptake and. . . 149