The specific HbHAK2 promoter from halophytic Hordeum brevisubulatum regulates root development under salt stress

Halophytes face continuous challenges from environmental salt conditions throughout their lifecycle, and adaptive strategies are essential for their growth and survival. Hordeum brevisubulatum, a halophytic forage grass, relies on efficient K⁺ uptake for salt tolerance, as demonstrated by the inducible expression of HbHAK2 (high affinity K⁺ transporter 2). Using genome walking, we isolated a unique 744-bp HbHAK2 promoter, distinct from the upstream sequences of HvHAK2 in barley. Transgenic Arabidopsis plants carrying the GUS gene, driven by the HbHAK2 promoter, exhibited higher activity in tissues and cells exhibiting polar growth, including root hairs, lateral root initiation sites, trichomes, and shoot apical meristems (SAM), particularly under various stresses compared to control conditions. In both Arabidopsis and Brachypodium distachyon, the expression of HbHAK2, driven by its promoter (proHbHAK2) improved K⁺ uptake, promoted a root architecture characterized by increased lateral branching, and enhanced salt tolerance by augmenting the root system (including lateral roots and root-hair development and growth) under salt stress. Therefore, variation in the HbHAK2 promoter represents a specific strategy for halophytic H. brevisubulatum to cope with salt stress, which holds potential for improving salt tolerance in crop breeding programs.