The role of HAK/KUP/KT family of transporters and potassium channels in abiotic stress responses in Arabidopsis and rice

Abiotic stress is a worldwide problem that affects the productivity of crops. Salinity stress for example, affects about 950 million hectares of arable lands while about 64% of the global land is affected by water deficit (drought). Drought and salinity are the most deleterious stresses, resulting in nutritional imbalance. Therefore, the main mechanism of improving a plant tolerance to stresses is by improving nutritional acquisition such as K+ uptake.
The Two Pore K+ vacuolar Channel (TPK1) was reported to function in K+ release from guard cell vacuoles, therefore stomatal closure. However, attpk1 showed weak phenotype, suggesting a good level of redundancy by other transporters predicted to localize in the guard cell tonoplast. The K+ high affinity uptake permeases HAK/KUP/KT transporters AtKUP12 and AtHAK8 are among these candidates. We test this hypothesis in the model plant Arabidopsis thaliana. Loss of function of AtKUP12 mutants showed severe inhibition of germination, seedling establishment and growth of plants during K+ deficiency resulted from high NH4+/K+ ratio. Therefore, we suggest a potential role for this transporter in K+ flux during osmotic, salinity and K+ starvation stresses. We also suggest a possible epistasis relationship between AtKUP12 and AtTPK1. Possibly, they both function in K+ efflux from vacuoles to cytoplasm during stress. Our results also suggest no functional relationship between AtTPK1 and AtHAK8. AtKUP12 was able to rescue the growth of K+ deficient yeast strain during K+ starvation.
In rice, we study the role of the Stelar K+ Outwardly Rectifying channel (SKOR) in K+ transport from roots to shoots under different stresses. We aim at improving K+ transport to shoots therefore the overall plant response to abiotic stress, using different overexpressor lines. Overexpressing the TPKb (an isoform of the TPK channel localized in the storage vacuoles) resulted in higher K+ levels in the transgenic plants during K+ starvation. We test the possibility of the involvement of other transporters in this phenomenon. Expression levels of the HAK1 and KAT1 as well as the GORK transporter and channel genes were measured in TPKb overexpressor lines. OsHAK1 showed slight increase in expression while other transporters did not show significant differences.