AKT1, HAK5, SKOR, HKT1;5, SOS1 and NHX1 synergistically control Na+ and K+ homeostasis in sugar beet (Beta vulgaris L.) seedlings under saline conditions

Soil salinity is a growing problem for agricultural productivity worldwide. Our previous studies showed that tetraploid cultivar exhibited more tolerant to salinity than diploid one in sugar beet (Beta vulgaris L.) by maintaining higher ratio of K⁺/Na⁺ and stronger capacity of selective absorption for K⁺ over Na⁺. Several kind of K⁺ and Na⁺ transport systems, such as inward-rectifier shaker K⁺ channel (AKT1), high-affinity K⁺ transporter 5 (HAK5), shaker-like K⁺ outward rectifying channel (SKOR), high-affinity K⁺ transporter 1;5 (HKT1;5), tonoplast Na⁺/H⁺ antiporter 1 (NHX1), and plasma membrane Na⁺/H⁺ antiporter 1 (SOS1), have been documented to play key roles in ion homeostasis. However, the precise mechanisms underlying the control of K⁺ and Na⁺ homeostasis in sugar beet still remained unknown. Here, the expression patters of six genes BvAKT1, BvHAK5, BvSKOR, BvHKT1;5, BvSOS1 and BvNHX1 were investigated in both diploid and tetraploid exposed to 50 and 300 mM NaCl, and a hypothetical model for ions transporters regulating K⁺/Na⁺ homeostasis was proposed. Results showed that BvAKT1 was significantly induced in roots of tetraploid at 12 h under 300 mM NaCl. BvHAK5 was observably repressed by salinity in roots of both diploid and tetraploid, whereas its transcript levels were 85.1% and 105.4% higher in tetraploid than those in diploid at 12 and 24 h at 300 mM NaCl, respectively. BvSKOR was increased significantly in roots of tetraploid at 12, 24 and 72 h under 300 mM NaCl. BvHKT1;5 was up-regulated remarkably in roots of both diploid and tetraploid at 72 h under 50 and 300 mM NaCl, and its expression levels were 43.9% lower in tetraploid than in diploid under 50 mM NaCl, but 2.6-fold higher under 300 mM NaCl. The expression levels of BvSOS1 in roots were 88.5% higher in tetraploid than in diploid at 12 h under 300 mM NaCl. BvNHX1 was clearly induced in shoots of tetraploid, while maintained lower level in diploid under 300 mM NaCl. Taken together, these results implied that BvAKT1, BvHAK5, BvSKOR, BvHKT1;5, BvSOS1 and BvNHX1 play vital roles in synergistically controlling K⁺ and Na⁺ homeostasis at the whole plant level. The present study provides molecular insights into understanding salt tolerance of autotetraploid in sugar beet.