Exogenous boron induced cell expansion and carbohydrate allocation, conferring leaf succulence and salt resistance in...

Boron is imperative for cell division and expansion. Salinity reduces transpiration, thereby inhibiting xylem-dependent boron (B) uptake and leaf growth. Increasing salinity, boron, and abscisic acid (ABA) levels act synergistically to affect water balance in plants. However, exogenous ABA enhances salt stress resistance in many plants. To date, the links between boron-mediated improvements in water relations and leaf succulence in salt-grown sugar beet remain elusive. Therefore, we elucidated the transcriptional expression of membrane ion transporters and cell wall expansion genes, as well as osmotic metabolites and ionic homeostasis, in response to exogenous boron under NaCl stress. Findings indicated that exogenous boron (EB) promoted leaf expansion alongside exogenous ABA (EA) in salt-grown sugar beet. This was substantially increased by boron (¹⁰B) translocation and sucrose accumulation, together with upregulation of sucrose synthesis and transport genes (BvSPS1 and BvSUT1). Consequently, reduced Na⁺ translocation and improved Na⁺/K⁺ homeostasis were accompanied by downregulation of ABA biosynthesis genes BvNCED2 and BvZEP in the leaves. This was associated with lower Na ⁺ influx and increased expression of BvNHX8, BvSOS1 and BvHKT1;3 in the roots. Further, water balance and leaf ion transport are facilitated by upregulation of the membrane ion transporters BvPIP1;1, BvPIP2;2, BvNIP5;1 and BvHKT1;3, as well as by expression of the plasma membrane H⁺-ATPase BvPM4. This aligns with increased mRNA levels of cell wall-loosening and expansion candidates, expansins (BvEXPA12 and BvEXP26), and xyloglucan endotransglucosylases (BvXET8 and BvXET22). Altogether, exogenous boron confers leaf succulence, as does exogenous ABA, and promotes salt resistance in sugar beet leaves.