Positive Regulatory Roles of IManihot esculenta/I HAK5 under K[sup.+] Deficiency or High Salt Stress

HAK/KUP/KT family members have been identified as playing key roles in K[sup.+] uptake and salt tolerance in numerous higher plants. However, their functions in cassava (Manihot esculenta Cantz) remain unknown. In this study, a gene encoding for a high-affinity potassium transporter (MeHAK5) was isolated from cassava and its function was investigated. Subcellular localization analysis showed that MeHAK5 is a plasma membrane-localized transporter. RT-PCR and RT-qPCR indicated that MeHAK5 is predominantly expressed in cassava roots, where it is upregulated by low potassium or high salt; in particular, its highest expression levels separately increased by 2.2 and 2.9 times after 50 µM KCl and 150 mM NaCl treatments. When heterologously expressed in yeast, MeHAK5 mediated K[sup.+] uptake within the cells of the yeast strain CY162 and rescued the salt-sensitive phenotype of AXT3K yeast. MeHAK5 overexpression in transgenic Arabidopsis plants exhibited improved growth and increased shoot K[sup.+] content under low potassium conditions. Under salt stress, MeHAK5 transgenic Arabidopsis plants accumulated more K[sup.+] in the shoots and roots and had reduced Na[sup.+] content in the shoots. As a result, MeHAK5 transgenic Arabidopsis demonstrated a more salt-tolerant phenotype. These results suggest that MeHAK5 functions as a high-affinity K[sup.+] transporter under K[sup.+] starvation conditions, improving K[sup.+] /Na[sup.+] homeostasis and thereby functioning as a positive regulator of salt stress tolerance in transgenic Arabidopsis. Therefore, MeHAK5 may be a suitable candidate gene for improving K[sup.+] utilization efficiency and salt tolerance.