Although all cells in an organism contain essentially the same DNA, cell types and cell functions vary because of qualitative and quantitative differences in their gene expression. Overexpression and downregulation of key genes will provide plants with the necessary resources to grow properly in the global warming challenge. The nutrition of K+, one of the most important macronutrients presently found in most commercial fertilizers, could be improved in this way. Thus, enhanced expression of K+ transporters and channels could result in an increase in plant K+ uptake which could lead to high-yield production. At the same time, improved K+ nutrition could lead to enhanced salt tolerance, because of the aforementioned competition between K+ and Na+, or to alleviation of drought stress as K+ is a known osmolite (Amtmann et al. 2004; Romheld and Kirkby 2010).
To our knowledge, only two plasma membrane K+ transporters or channels have been overexpressed. Overexpression of the barley high-affinity K+ transporter HvHAK1 in Arabidopsis transgenic plants has been characterized (Fulgenzi et al. 2008). They showed increased K+ uptake when plants were deprived of K+ but not in K+-sufficient conditions. Also a correlation between HvHAK1 transcript level and K+ uptake was absent. The cation:proton antiporter AtCHX13 has also been overexpressed (Zhao et al. 2008). Driven by the 35S promoter, AtCHX13 increases K+ (Rb+) uptake in intact plants with a Km of 196 mM, although it is not clear whether AtCHX13 localizes to the plasma membrane of the root cortex in native tissue. More K+ transporters and/or channels have to be upregulated to unravel their biotechnological potential. Biotechnological success has been recently reported by cell-specific overexpression of an HKT transporter. Specific expression of AtHKT1 in the mature root stele leads to a reduction of root-to-shoot transfer of Na+ and increased salinity tolerance (Moller et al. 2009). Interestingly, not only Arabidopsis but also rice plants exhibit salinity tolerance with the specific expression of AtHKT1 in root cortex cells (Plett et al. 2010).
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