Optimization of ammonium acquisition and metabolism by potassium in rice (Oryza sativa L. cv. IR-72)
2010
BALKOS, KONSTANTINE D. | BRITTO, DEV T. | KRONZUCKER, HERBERT J.
We present the first characterization of K⁺ optimization of N uptake and metabolism in an NH₄⁺-tolerant species, tropical lowland rice (cv. IR-72). ¹³N radiotracing showed that increased K⁺ supply reduces futile NH₄⁺ cycling at the plasma membrane, diminishing the excessive rates of both unidirectional influx and efflux. Pharmacological testing showed that low-affinity NH₄⁺ influx may be mediated by both K⁺ and non-selective cation channels. Suppression of NH₄⁺ influx by K⁺ occurred within minutes of increasing K⁺ supply. Increased K⁺ reduced free [NH₄⁺] in roots and shoots by 50-75%. Plant biomass was maximized on 10 m m NH₄⁺ and 5 m m K⁺, with growth 160% higher than 10 m m NO₃⁻-grown plants, and 220% higher than plants grown at 10 m m NH₄⁺ and 0.1 m m K⁺. Unlike in NH₄⁺-sensitive barley, growth optimization was not attributed to a reduced energy cost of futile NH₄⁺ cycling at the plasma membrane. Activities of the key enzymes glutamine synthetase and phosphoenolpyruvate carboxylase (PEPC) were strongly stimulated by elevated K⁺, mirroring plant growth and protein content. Improved plant performance through optimization of K⁺ and NH₄⁺ is likely to be of substantial agronomic significance in the world's foremost crop species.
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