2.2.1. K+ Uptake and Transfer to the Xylem in Roots
As reported above, maintaining sufficient K
+ uptake is critical to reduce the effects of drought stress. K
+ is taken up from a soil solution that is highly variable in composition, with K
+ concentration that can range from a few µM to several mM. In
Arabidopsis, below 10 µM K
+ and in the absence of NH4
+, only HAK5 is able to mediate K
+ uptake, whereas AKT1 intervenes at higher, but still low, K
+ concentrations, due to the high hyperpolarization of root cell membranes in K
+-deficient soils [
88,
89]. Salinity strongly represses
HAK5 expression in roots [
67]. However, HAK5 is critically required for plant growth under low-K and saline conditions, because Na
+ absorption results in depolarization of membranes of external root cells, which promotes an unusual K
+ efflux via AKT1 [
67]. In these experiments, the involvement of the outwardly rectifying GORK channel, also known to mediate K
+ efflux under salt stress [
90] (see below), was not addressed. Closest homologs of HAK5 belong to clade 1a of the KUP family [
66]. They are involved (or thought to be involved) in K
+ uptake from the soil at low K
+ concentrations [
27]. Like
HAK5, homologous genes in other plant species (rice, barley, pepper, tomato,
Thellungiella halophila) are highly upregulated by K
+ shortage; however, for at least three of them, this upregulation is suppressed by Na
+ [
27]. Although AKT1 and HAK5 seem to be the main players for K
+ uptake from the soil even in the low-K
+ range [
88], AtKUP7 in
Arabidopsis (clade V [
66]) was recently found to also contribute to K
+ uptake under low-K
+ conditions [
91]. In parallel, the AtKC1 subunit, which is expressed in the root cortex, epidermis, and root hairs [
44], and interacts with AKT1 to form a functional heterotetrameric channel, prevents K
+ loss through AKT1 under low-K
+ conditions [
33]. Neither AKT1, HAK5, AtKUP7, nor AtKC1 appear to display strong and durable up- or downregulations at the transcriptional level in response to water stress, heat, or ABA in roots [
44,
45]. In rice, OsAKT1 also contributes to a significant extent to K
+ uptake in roots, in the absence of regulation by an AtKC1-like subunit [
46]. It improves resistance to drought stress triggered by polyethylene glycol (PEG) or reduction of soil watering, by increasing K
+ content in roots. Conversely, it has no effect on resistance to salinity [
47].
The Kout channel GORK is also expressed in roots [
65]. In the case of salt stress, loss of K
+ must be avoided to maintain the K
+/Na
+ ratio. However, Na
+ entry depolarizes the plasma membrane and evokes massive loss of K
+ mainly through GORK [
90,
92,
93]. Non-selective cation channels [
93] are also significant players of K
+ efflux. In pea, they assume this function in place of GORK [
94]. In
Arabidopsis, KUP6 and KUP8 (clade II [
66]) are also thought to mediate K
+ efflux from roots [
68].
KUP8 is downregulated under salt stress, but not the
GORK gene [
56]. Conversely,
GORK is upregulated by heat stress in roots [
56]. The reduction of net K
+ influx observed in olive tree roots exposed to heat [
95] might, thus, be attributed to an increase in K
+ efflux, but this has to be confirmed. Although the role of GORK-mediated K
+ efflux is much less understood than in guard cells, it is thought to be involved in repolarization of the root-hair plasma membrane, for instance, after elicitor-induced depolarization [
96] or in response to different stresses [
93]. GORK is activated by ROS (more specifically hydroxide radicals) that are generated following different stresses including salt stress [
90]. Depending on stress intensity, GORK-mediated K
+ efflux could either trigger programmed cell death or prevent activation of anabolic enzymes by K
+, thus releasing energy for adaptation to stress and reparation [
93]. Repolarization of the plasma membrane could have a role in action potentials and propagation of stress and hormone electrical signaling [
93].
Once taken up by epidermal and cortical cells at the root periphery, K
+ is transported radially (via an apoplastic or symplastic pathway) and then delivered to the xylem. KUP6 and KUP8 are expressed in the pericycle, and they enhance ABA response to inhibit lateral root formation [
68]. The SKOR channel drives about 50% of K
+ transported in the xylem sap to the shoot [
63]. The
SKOR gene transcript is considerably reduced in response to ABA treatment [
63], in accordance with the previously observed strong decrease of outward K
+ currents in the maize stele [
22]. This would ensure that roots can keep growing under stress, and is also expected to provide a signal for drought stress to the shoots [
97], since the decrease in K
+ content in the xylem sap is accompanied by an increase in Ca
2+ [
63] that serves as an intermediate for signaling cascades. Alongside its role in K
+ uptake from roots, KUP7 was identified as a transporter allowing K
+ efflux to the xylem especially under K
+-deficient conditions (accounting for about one-third of xylem K
+ [
91]). Systematic studies on whole plants do not reveal transcriptional regulation by drought, ABA, or heat for the
KUP7 gene [
56]. KUP7 could, therefore, ensure a minimal transfer to the xylem when
SKOR expression is repressed by ABA. In rice, at least two transporters of the KUP family (OsHAK1 and OsHAK5), belonging to the same clade Ia as HAK5, mediate not only K
+ uptake from the soil but also translocation to the xylem, possibly (at least for OsHAK5) by allowing K
+ loading of xylem parenchyma cells prior to K
+ efflux via SKOR-like channels [
85,
98]. A recent report [
69] highlighted the role of OsHAK1 from rice in drought stress tolerance. For its part, OsHAK5 promotes salt resistance by increasing the K
+/Na
+ ratio [
70]. OsHAK21 (clade Ia, [
66]), mainly expressed in xylem parenchyma cells, is involved in K
+ uptake and control of K
+ homeostasis under salt stress [
71].
2.2.3. K+ Fluxes in Guard Cells
Most of the literature on drought stress adaptation deals with stomatal regulation. In shoots, water-saving is tightly controlled by ion-driven guard-cell movements. Due to their importance in plant hydric status preservation and their accessibility to (electro)physiological studies, guard cells became a model for the regulation of ion (notably K
+) fluxes in response to drought stress and ABA. Also, the molecular mechanisms of light-induced stomatal opening and closure were extensively studied (reviewed by References [
105,
106,
107,
108,
109]). Comparatively to drought stress-induced stomatal closing and light-induced stomatal opening, little is known about the effect of high temperatures on stomatal movements. Under high temperatures and drought conditions, guard cells have to face the dilemma of saving water while controlling leaf temperature through transpiration [
110].
ABA perception by PYR/PYL/RCAR (pyrabactin resistance, pyrabactin resistance-like, regulatory component of ABA receptor) ABA receptors triggers an increase in cytoplasmic calcium (due to release from internal stores and Ca
2+ uptake, regulated by ROS and nitric oxide [
42,
111,
112,
113]) leading to activation of anion channels (slow- and quick-activating SLAC1 (slow anion channel 1), SLAH3 (SLAC1 homolog), and QUAC1 (quick anion channel 1)) [
114] that depolarize the plasma membrane. The plasma membrane depolarization, in turn, triggers the opening of the K
+ voltage-gated GORK channel that mediates K
+ efflux [
64,
115]. This release of ions drives water out of guard cells, which causes guard-cell deflating and stomatal closure [
108,
109]. In addition to GORK, KUP6 and KUP8 are also involved in the control of stomatal closure.
KUP6 is highly responsive to osmotic stress, especially in shoots [
56,
68]. KUP6 over-expressors exhibit a clear drought resistance phenotype, thus highlighting the importance of the
KUP6 gene in stress adaptation, and close their stomata more efficiently than wild-type plants in the presence of ABA [
68]. Shaker genes
KAT1/
KAT2,
AKT1,
GORK, and, to a much lesser extent.
AKT2 and
AtKC1 are expressed in guard-cell protoplasts [
55,
96,
116].
KAT1 and
KAT2 encode inward-rectifying-forming subunits that, after tetramerization, form the KAT1 and KAT2 channels, which are the key actors of stomatal opening. Stomatal opening mechanism is an inversion of stomatal closure in terms of osmolyte accumulation and direction of fluxes. KAT1 and KAT2 mediate K
+ influx that drives water influx and opens stomata [
61]. Upon ABA exposure,
KAT1 and
KAT2 genes are downregulated at the transcriptional level, as expected for genes involved in stomatal opening [
55,
117]. The
GORK gene transcription, which is clearly upregulated by ABA in most tissues [
65] was unexpectedly found insensitive (or weakly sensitive) to ABA in guard-cell protoplasts [
65,
73], thus revealing a specific type of regulation for these cells. AKT1, previously found to be essential for root K
+ uptake [
118], was only recently identified as a player of stomatal movements. In the
akt1 loss-of-function mutant, the stomatal conductance is lower under drought stress and ABA has a much stronger effect on stomatal closing [
43].
Although most data rely on plasma membrane channel regulation, it should be emphasized that vacuoles, which serve as solute reservoirs, play a significant role in stomatal movements. The voltage-independent, K
+-selective TPK1 channel from
Arabidopsis is essential for the control of K
+ homeostasis and vacuolar K
+ release during ABA-mediated stomatal closure [
41,
119].
2.2.4. K+ Fluxes in Reproductive Organs
Pollen germination on the stigma requires cell elongation maintained by K
+ uptake through the
Arabidopsis SPIK channel [
120]. Data are also emerging regarding the contribution of different K
+ transport systems to the mechanisms of response to stress and their consequences on fruit quality. Most of them were obtained on grape berry, considered as a model for fleshy fruits [
50]. Whereas K
+ is often limiting in plants, especially under hot and dry climates, it is sometimes accumulated in excess. In grape berries, potassium accumulates after the véraison stage that marks the beginning of maturation with change in berry color and firmness, dramatic increase in sugar content, loss of acidity, and development of aromas. This K
+ accumulation increased during the past decades and this was attributed to global warming, since high temperatures favor K
+ accumulation in berries [
121]. Although K
+ seems to be beneficial for enhancing berry resistance to drought [
122], it can be deleterious for wine quality. Indeed, due to the neutralization of organic acids by K
+ ions, high amounts of K
+ in the must are associated with high pH [
122], resulting in wines with loss of aromas and poor aging potential [
122,
123]. After véraison, due to the loss of xylem functionality for solute transport [
124], K
+ enters in the berry mainly via the phloem. The weakly rectifying Shaker channel VvK3.1 which belongs to the AKT2 channel phylogenetic branch, would be involved in the massive K
+ fluxes from the phloem cell cytosol to the berry apoplast during berry K
+ loading [
59]. Once delivered to the apoplastic space, K
+ is taken up by flesh and skin cells [
122] to be stored in vacuoles. Several transport systems related to these functions were identified. Two Kup transporters are involved in K
+ uptake into the skin cells at pre-véraison stages [
125]. VvK1.1, the ortholog of AKT1, is expressed in the phloem and in seeds and is strongly upregulated by water stress and ABA in berries not only before but also after véraison [
50]. However, its expression level in berries is far below that of the other AKT1-type gene, VvK1.2, which bursts after véraison and is highly induced by water stress [
51]. In light of its expression pattern, VvK1.2 is expected to mediate K
+ retrieval by perivascular and flesh cells from the apoplast.