Identification of the High-Affinity Potassium Transporter Gene Family in Perennial Ryegrass (Lolium perenne) and Its Potential Role in Salt Stress

<b>Background:</b> Perennial ryegrass (<i>Lolium perenne</i> L.), a widely cultivated turfgrass and forage species in Europe and North America, exhibits rapid growth and notable salt tolerance. The <i>high-affinity potassium transporter</i> (<i>HKT</i>) gene family has been implicated in salt stress responses across multiple plant species. However, whether the salt tolerance of <i>L. perenne</i> is closely associated with its HKT gene family remains unclear. <b>Methods and Results:</b> In this study, we systematically identified HKT family members in the <i>L. perenne</i> genome. Five <i>HKT</i> genes were identified and classified into three subfamilies. Among these, <i>LpHKT1a-c</i> exhibited canonical class I features with a conserved serine (S) residue in the P1 domain, whereas <i>LpHKT2</i> belonged to class II, characterized by a glycine (G) residue in the same domain. Notably, LpHKT3 formed a distinct subfamily with a truncated structure and divergent P1/P2 domains, suggesting potential non-canonical functions. <i>LpHKT1a</i> likely lacked the P4 domain. Promoter analysis revealed that all five <i>LpHKT</i> genes contain multiple stress-related <i>cis</i>-acting elements. Real-time quantitative reverse transcription polymerase chain reaction results showed that <i>LpHKT1b/c</i> and <i>LpHKT2</i> were highly expressed in both roots and leaves. Under low-concentration NaCl stress (25 mM), the expression of these three genes significantly increased by 8- to 12-fold at 6-12 h post-treatment (vs. control). Ion accumulation analysis demonstrated a rapid increase in Na⁺ levels following NaCl treatment, whereas K⁺ concentrations initially remained stable but significantly increased after 24 h. <b>Conclusions:</b> Combined with the cellular localization of <i>LpHKT1c</i> predominantly in the xylem, these findings suggest that <i>LpHKT</i> genes may be involved in Na⁺ and K⁺ transport in roots. This study represents the first genome-wide identification of the <i>HKT</i> gene family in <i>L. perenne</i>, providing critical insights into the molecular mechanisms underlying its salt tolerance and offering valuable genetic resources for molecular breeding aimed at enhancing stress resilience.