Characterization and Expression of KT/HAK/KUP Transporter Family Genes in Willow under Potassium Deficiency, Drought, and Salt Stresses

Abstract

The K⁺ transporter/high-affinity K⁺/K⁺ uptake (KT/HAK/KUP) transporters dominate K⁺ uptake, transport, and allocation that play a pivotal role in mineral homeostasis and plant adaptation to adverse abiotic stresses. However, molecular mechanisms towards K⁺ nutrition in forest trees are extremely rare, especially in willow. In this study, we identified 22 KT/HAK/KUP transporter genes in purple osier willow (designated as <i>SpuHAK1</i> to <i>SpuHAK22</i>) and examined their expression under K⁺ deficiency, drought, and salt stress conditions. Both transcriptomic and quantitative real-time PCR (qRT-PCR) analyses demonstrated that <i>SpuHAKs</i> were predominantly expressed in stems, and the expression levels of <i>SpuHAK1</i>, <i>SpuHAK2</i>, <i>SpuHAK3</i>, <i>SpuHAK7</i>, and <i>SpuHAK8</i> were higher at the whole plant level, whereas <i>SpuHAK9</i>, <i>SpuHAK11</i>, <i>SpuHAK20</i>, and <i>SpuHAK22</i> were hardly detected in tested tissues. In addition, both K⁺ deficiency and salt stress decreased the tissue K⁺ content, while drought increased the tissue K⁺ content in purple osier plant. Moreover, <i>SpuHAK</i> genes were differentially responsive to K⁺ deficiency, drought, and salt stresses in roots. K⁺ deficiency and salt stress mainly enhanced the expression level of responsive <i>SpuHAK</i> genes. Fifteen putative <i>cis</i>-acting regulatory elements, including the stress response, hormone response, circadian regulation, and nutrition and development, were identified in the promoter region of <i>SpuHAK</i> genes. Our findings provide a foundation for further functional characterization of KT/HAK/KUP transporters in forest trees and may be useful for breeding willow rootstocks that utilize potassium more efficiently.