ZxAKT1 is essential for K + uptake and K + /Na + homeostasis in the succulent xerophyte Zygophyllum xanthoxylum.

The inward-rectifying K ⁺ channel AKT1 constitutes an important pathway for K ⁺ acquisition in plant roots. In glycophytes, excessive accumulation of Na ⁺ is accompanied by K ⁺ deficiency under salt stress. However, in the succulent xerophyte Zygophyllum xanthoxylum, which exhibits excellent adaptability to adverse environments, K ⁺ concentration remains at a relatively constant level despite increased levels of Na ⁺ under salinity and drought conditions. In this study, the contribution of ZxAKT1 to maintaining K ⁺ and Na ⁺ homeostasis in Z. xanthoxylum was investigated. Expression of ZxAKT1 rescued the K ⁺ -uptake-defective phenotype of yeast strain CY162, suppressed the salt-sensitive phenotype of yeast strain G19, and complemented the low-K ⁺ -sensitive phenotype of Arabidopsis akt1 mutant, indicating that ZxAKT1 functions as an inward-rectifying K ⁺ channel. ZxAKT1 was predominantly expressed in roots, and was induced under high concentrations of either KCl or NaCl. By using RNA interference technique, we found that ZxAKT1-silenced plants exhibited stunted growth compared to wild-type Z. xanthoxylum. Further experiments showed that ZxAKT1-silenced plants exhibited a significant decline in net uptake of K ⁺ and Na ⁺ , resulting in decreased concentrations of K ⁺ and Na ⁺ , as compared to wild-type Z. xanthoxylum grown under 50 mm NaCl. Compared with wild-type, the expression levels of genes encoding several transporters/channels related to K ⁺ /Na ⁺ homeostasis, including ZxSKOR, ZxNHX, ZxSOS1 and ZxHKT1;1, were reduced in various tissues of a ZxAKT1-silenced line. These findings suggest that ZxAKT1 not only plays a crucial role in K ⁺ uptake but also functions in modulating Na ⁺ uptake and transport systems in Z. xanthoxylum, thereby affecting its normal growth.
(© 2016 The Authors The Plant Journal © 2016 John Wiley & Sons Ltd.)