A central question is how specificity in cellular responses to the eukaryotic second messenger Ca2+ is achieved. Plant guard cells, that form stomatal pores for gas exchange, provide a powerful system for in depth investigation of Ca2+-signaling specificity in plants. In intact guard cells, abscisic acid (ABA) enhances (primes) the Ca2+-sensitivity of downstream signaling events that result in activation of S-type anion channels during stomatal closure, providing a specificity mechanism in Ca2+-signaling. However, the underlying genetic and biochemical mechanisms remain unknown. Here we show impairment of ABA signal transduction in stomata of calcium-dependent protein kinase quadruple mutant plants. Interestingly, protein phosphatase 2Cs prevent non-specific Ca2+-signaling. Moreover, we demonstrate an unexpected interdependence of the Ca2+-dependent and Ca2+-independent ABA-signaling branches and the in planta requirement of simultaneous phosphorylation at two key phosphorylation sites in SLAC1. We identify novel mechanisms ensuring specificity and robustness within stomatal Ca2+-signaling on a cellular, genetic, and biochemical level.DOI: http://dx.doi.org/10.7554/eLife.03599.001

The relative contribution of the high-affinity K(+) transporter AtHAK5 and the inward rectifier K(+) channel AtAKT1 to K(+) uptake in the high-affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col-0). The results obtained with wild-type lines, with T-DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the NH4+-sensitive and the Ba(2+)-sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high-affinity range of Rb(+) concentrations. Using Rb(+) as a K(+) analogue, it was shown that AtHAK5 mediates absorption at lower Rb(+) concentrations than AtAKT1 and depletes external Rb(+) to values around 1 muM. Factors such as the presence of K(+) or NH4+ during plant growth determine the relative contribution of each system. The presence of NH4+ in the growth solution inhibits the induction of AtHAK5 by K(+) starvation. In K(+)-starved plants grown without NH4+, both systems are operative, but when NH4+ is present in the growth solution, AtAKT1 is probably the only system mediating Rb(+) absorption, and the capacity of the roots to deplete Rb(+) is reduced.

K(+) uptake in the high-affinity range of concentrations and its components have been widely studied. In Arabidposis thaliana, the AtHAK5 transporter and the AtAKT1 channel have been shown to be the main transport proteins involved in this process. Here, we study the role of these two systems under two important stress conditions: low K(+) supply or the presence of salinity. T-DNA insertion lines disrupting AtHAK5 and AtAKT1 are employed for long-term experiments that allow physiological characterization of the mutant lines. We found that AtHAK5 is required for K(+) absorption necessary to sustain plant growth at low K(+) in the absence as well as in the presence of salinity. Salinity greatly reduced AtHAK5 transcript levels and promoted AtAKT1-mediated K(+) efflux, resulting in an important impairment of K(+) nutrition. Although having a limited capacity, AtHAK5 plays a major role for K(+) acquisition from low K(+) concentrations in the presence of salinity.

K(+) is an essential macronutrient required by plants to complete their life cycle. It fulfills important functions and it is widely used as a fertilizer to increase crop production. Thus, the identification of the systems involved in K(+) acquisition by plants has always been a research goal as it may eventually produce molecular tools to enhance crop productivity further. This review is focused on the recent findings on the systems involved in K(+) acquisition. From Epstein's pioneering work >40 years ago, K(+) uptake was considered to consist of a high- and a low-affinity component. The subsequent molecular approaches identified genes encoding K(+) transport systems which could be involved in the first step of K(+) uptake at the plant root. Insights into the regulation of these genes and the proteins that they encode have also been gained in recent studies. A demonstration of the role of the two main K(+) uptake systems at the root, AtHKA5 and AKT1, has been possible with the study of Arabidopsis thaliana T-DNA insertion lines that knock out these genes. AtHAK5 was revealed as the only uptake system at external concentrations <10 μM. Between 10 and 200 μM both AtHAK5 and AKT1 contribute to K(+) acquisition. At external concentrations >500 μM, AtHAK5 is not relevant and AKT1's contribution to K(+) uptake becomes more important. At 10 mM K(+), unidentified systems may provide sufficient K(+) uptake for plant growth.

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