Overexpression of the rice AKT1 potassium channel affects potassium nutrition and rice drought tolerance

Abstract

Potassium (K(+)) is the most important cationic nutrient for all living organisms and has roles in most aspects of plant physiology. To assess the impact of one of the main K(+) uptake components, the K(+) inward rectifying channel AKT1, we characterized both loss of function and overexpression of OsAKT1 in rice. In many conditions, AKT1 expression correlated with K(+) uptake and tissue K(+) levels. No salinity-related growth phenotype was observed for either loss or gain of function mutants. However, a correlation between AKT1 expression and root Na(+) when the external Na/K ratio was high suggests that there may be a role for AKT1 in Na(+) uptake in such conditions. In contrast to findings with Arabidopsis thaliana, we did not detect any change in growth of AKT1 loss of function mutants in the presence of NH4 (+) Nevertheless, NH4 (+)-dependent inhibition was detected during K(+) uptake assays in loss of function and wild type plants, depending on pre-growth conditions. The most prominent result of OsAKT1 overexpression was a reduction in sensitivity to osmotic/drought stress in transgenic plants: the data suggest that AKT1 overexpression improved rice osmotic and drought stress tolerance by increasing tissue levels of K(+), especially in the root.

Keywords: Drought; Oryza sativa; OsAKT1; ion channel; osmotic stress; rice..

Fig. 1.

Relative growth rate and tissue K⁺ for hydroponically grown rice. (a) Relative growth rate (RGR, % day⁻¹) of 5-week-old rice plants exposed to standard medium (SM), and media containing 0mM K⁺, 100 µM K⁺, and 60mM K⁺. (b) Root K⁺ content of plants grown in SM, 0mM K⁺ and 100 µM K⁺ conditions. (c) Shoot K⁺ content of plants grown in the conditions mentioned under (b). *Significant difference by t-test at a probability level of P<0.05 between each genotype and its respective wild type.

Fig. 2.

Net K⁺ uptake in hydroponically grown rice. (a) Net K⁺ uptake for WT (Nipponbare) and two independent AKT1 overexpressing lines from a solution containing 100 µM K⁺. (b) Same as (a) for WT (NN and DJ) and akt1 loss of function mutants.

Fig. 3.

The effect of ammonium on growth rate and K⁺ uptake. (a) Relative growth rate (RGR, % day⁻¹) of 5-week-old rice plants exposed to standard medium (SM), and media containing 0mM K⁺ and zero ammonium (0K + 3NO₃), or 0mM K⁺ plus 3 or 10mM ammonium (0K + 3NH₄ or 0K + 10NH₄). (b–e) Net K⁺ uptake for WT (Nipponbare; b and c) and akt1-2 (d and e) plants from a solution containing 50 µM K⁺, either in the absence or the presence of 3 or 10mM NH₄ ⁺. Plants were pregrown in the absence of NH₄ ⁺ (b and d) or with 1.5mM NH₄ ⁺ in the growth medium (c and e). *Significant difference by t-test at a probability level of P<0.05 between each genotype and its respective wild type.

Fig. 4.

Relative growth rate and K⁺ content of osmotically stressed rice. (a) Relative growth rate (RGR, % day⁻¹) of 5-week-old rice plants exposed to standard medium (SM), and media containing 5, 10 or 12% polyethylene glycol (PEG). (b) Root K⁺ content of plants grown in the standard medium, and 5 and 10% PEG conditions. (c) Shoot K⁺ content of plants grown in the conditions mentioned under (b). (d) Leaf stomatal conductance of the five genotypes for plants grown in standard medium or plants exposed to osmotic stress. *Significant difference by t-test at a probability level of P<0.05 between each genotype and its respective wild type.

Fig. 5.

Growth and tissue and xylem K⁺ content of drought treated rice. (a) RGR of pot grown plants exposed for 6 weeks to full watering (100% field capacity) or limited water supply (~40% field capacity). (b) Root K⁺ content of plants grown in the conditions mentioned under (a). (c) Shoot K⁺ content of plants grown in the conditions mentioned under (a). (d) Xylem sap K⁺ concentration. *Significant difference by t-test at a probability level of P<0.05 between each genotype and its respective wild type.

Fig. 6.

Relative growth rate and tissue Na⁺ for hydroponically grown rice. (a) Relative growth rate (RGR, % day⁻¹) of 5-week-old rice plants exposed to standard medium (SM), and 60mM Na⁺. (b) Root Na⁺ content of plants grown in 0K and 60mM NaCl conditions. (c) Shoot Na⁺ content of plants grown in 0K and 60mM NaCl conditions. *Significant difference by t-test at a probability level of P<0.05 between each genotype and its respective wild type.