Identification of sugar transporter MrSTP20 responding to high-salt stress and hexoses induction in apple rootstock (Malus robusta Rehd).

Background: Sugar distribution regulation, especially in the roots, is an important pathway for plants to cope with abiotic stress. Sugar transport proteins (STPs) mediate the transmembrane transport of hexoses such as glucose and fructose; however, the molecular mechanism underlying the response to salt stress remains unclear.

Results: Here, we identified 22 STP genes in apple (Malus domestica) using genome-wide screening. Under salt stress, 17 genes were detected in the roots of the apple rootstock Malus robusta Rehd; of them, MrSTP3, MrSTP7, MrSTP14, and MrSTP20 showed stronger positive responses to salt stress. Under exogenous sugar induction, MrSTP7, MrSTP17, and MrSTP20 were significantly upregulated for all three hexoses (glucose, fructose, and mannose), one sugar alcohol (mannitol), and sucrose. MrSTP20 exhibited the strongest response to high salt, glucose, and fructose concentrations. Furthermore, we found that MrSTP20 has a conserved STP domain and is expressed in Arabidopsis protoplasts localized to the plasma membrane, supporting its function in transporting hexoses from the extracellular space into the cells. Association analysis showed that MrSTP20 synergized with the salt-responsive glycosyltransferase gene (UGT71), pentatriocopeptide repeat protein (PPR), cytochrome c oxidase synthesis (HCC1), and S-phase kinase-associated protein 1 (SKP1) at the gene expression level.

Conclusions: These findings suggest that MrSTP20 transports hexoses from the extracellular space into cells, positively involving in salt stress tolerance and sugar transport regulation, thereby providing new insights into the salt tolerance mechanism in apple trees.