Transcriptional Regulation

In general terms, genes encoding AKTl-like K+ channels do not show great differences in expression levels in response to the external supply of K+ as it has been observed for AtAKTl and CaAKTl (Lagarde et al. 1996; Martínez-Cordero et al. 2005; Pilot et al. 2003). However, K+ withdrawal from the growth solution increased TaAKT1 transcript levels (Buschmann et al. 2000), and NaCl treatments or hormonal addition to the external medium produce changes in AtAKT1 expression (Kaddour et al. 2009; Pilot et al. 2003).

Substantial differences are found in the expression pattern of the genes encoding HAKl-type K+ transporters in comparison to that observed in the aforementioned genes for K+ channels. It seems that an important point in the regulation of this type of transporters resides in the control of gene expression.

K+-starvation is a common inducer in the gene expression of the HAKl-type K+ transporters. This induction has been observed in HvHAK1 (Santa-María et al. l997), AtHAK5 (Ahn et al. 2004; Armengaud et al. 2004; Gierth et al. 2005; Qi et al. 2008; Shin and Schachtman 2004), OsHAK1 (Bañuelos et al. 2002), LeHAK5 (Nieves-Cordones et al. 2007; Wang et al. 2002), CaHAK1 (Martínez-Cordero et al. 2004), and ThHAK5 (Alemán et al. 2009b). The reduction in root K+ concentration below a threshold level has been proposed as the stimulus that could trigger the increase in the transcription of these genes (Martínez-Cordero et al. 2005).

Some other factors have been shown to considerably modify gene transcription of the HAK1-type genes. CaHAK1 expression in K+-starved plants was reduced, if half of the nitrogen of the growth solution was supplied as NH4+ (Martínez-Cordero et al. 2005). Similarly, AtHAK5 promoter activity was diminished in K+-starved plants after exposure to NH4 + (Qi et al. 2008). Conversely, HvHAK1 transcript levels were increased by the presence of NH4+ in the growth solution in ^-sufficient plants (Fulgenzi et al. 2008). Intriguingly, LeHAK5 was downregulated when NO3- was supplied again after a withdrawal period (Wang et al. 2001) . Another case in the regulation of HAK transporters was the increase in AtHAK5 mRNA levels after exposing plants to sucrose in the absence of light (Lejay et al. 2008). Similar results were found previously in Arabidopsis when plants were grown in the sucrose-containing media MS (Rubio et al. 2000): It was observed that in K+-sufficient plants AtHAK5 expression was high; in this medium, K+ starvation did not further increase the expression levels, probably because K+ was substituted for NH4+ in these experiments.

Another important factor affecting the expression of these genes seems to be the presence of salinity. This was illustrated, for example, in the decrease of LeHAK5 (Nieves-Cordones et al. 2007), AtHAK5, and ThHAK5 (Alemán et al. 2009b) transcripts when plants were starved of K+ in the presence of Na+ . Importantly, salinity decreased, to a lesser extent, the levels of ThHAK5 mRNA in T. halophila than those of AtHAK5 transcripts in Arabidopsis.

There is not much information about the signal transduction elements involved in the induction of HAK-type K+ transporters expression. In 2004, the important role of reactive oxygen species (ROS) in the signaling events after removing K+ from the growth solution was described for Arabidopsis (Shin and Schachtman 2004) . Recently, it has been proposed that ethylene signaling acts upstream the increase of ROS during K+ deprivation (Jung et al. 2009).

4n tomato plants, LeHAK5 expression levels correlated with steady plasma membrane potentials registered in root cells (Nieves-Cordones et al. 2008) and high-affinity K+ uptake (Nieves-Cordones et al. 2007). Changes in plasma membrane potentials are one of the first signals that root cells sense after a stress is applied (Wang and Wu 2010). In tomato roots, the recorded plasma membrane potentials were importantly affected by long-term changes in the composition of the growth solution. For instance, the presence of NH4+ and K+ starvation hyperpolarized and Na+ depolarized plasma membrane potentials, which produced an increase and a decrease in the LeHAK5 mRNA levels, respectively. Short-term exposure of depolarizing agents such as CCCP or Vanadate to K+-starved roots also decreased LeHAK5 expression. These changes in LeHAK5 expression at both long- and short-term denoted a tight regulation at the transcription level and they also indicated that LeHAK5 contribution to K+ uptake could be limited to some specific conditions even if K+ deficiency was still present (Nieves-Cordones et al. 2008) .

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