Heterologous expression of the durum wheat TdHKT1;4–1 partially complements the mutant athkt1 in Arabidopsis thaliana under severe salt stress

High-affinity K⁺ (HKT) transporters which mediate Na⁺-specific transport or Na⁺-K⁺ co-transport play a key role in plant salt tolerance. In our previous functional study in Xenopus oocytes, we demonstrated that the durum wheat TdHKT1;4–1 acts as a Na⁺-selective transporter. Here, we investigated the function of TdHKT1;4–1 and its contribution in salt stress tolerance in the Arabidopsis athkt1 mutant background. Our results revealed that TdHKT1;4–1 partially complements the salt sensitivity phenotype of the athkt1 transgenic lines. Comparative physiological analyses and oxidative stress status under moderate salt stress (50 mM NaCl) showed that both transgenic lines SH3 and SH5 restored the salt stress tolerance comparable to the level observed in Wt plants. Whereas, under severe salt stress treatment (100 mM NaCl), the athkt1 transgenic lines exhibited an intermediate salt stress tolerance between Wt and athkt1 mutant. Moreover, TdHKT1;4–1 was highly expressed in leaves under moderate and severe salt stress, while in roots, it was largely expressed only under severe salt stress. In addition, antioxidant enzymes such as catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were significantly expressed in SH3 and SH5 lines compared to athkt1 and Wt under moderate stress. Therefore, TdHKT1;4–1 seems to differ from its Arabidopsis homologous counterpart, as it contributes to salt stress tolerance up to a specific threshold, above which the TdHKT1;4–1 expression may lead to higher root Na⁺ influx, hence increasing its toxicity during salt stress.