The transcription factor SbbHLH168 enhances salt tolerance by coordinating ion homeostasis and lignin content in sorghum

Abstract

Soil salinization is an escalating global threat to crop productivity. Here, we demonstrate that the sorghum bHLH transcription factor SbbHLH168 is a central regulator of salt-stress tolerance. SbbHLH168 is nuclear-localized, constitutively expressed in all organs and rapidly up-regulated by NaCl. Overexpression of SbbHLH168 in either Arabidopsis or sorghum conferred markedly higher biomass retention, longer roots, lower relative electrical conductivity, less accumulation of superoxide and H₂O₂, and lower Na⁺/K⁺ ratio through decreased Na⁺ accumulation under salt stress, whereas VIGS silent groups displayed the opposite phenotype. qRT-PCR and biochemical assays demonstrated that SbbHLH168 up-regulates key lignin-biosynthetic genes (SbC4H, SbF5H, SbCOMT1) and increases root lignin content under salt stress, thereby reinforcing cell-wall integrity and limiting ion leakage. Furthermore, yeast two-hybrid and BiFC assays confirmed that SbbHLH168 interacts with SbbHLH35 in the nucleus, indicating that SbbHLH168's functional activity depends on heterodimerization with SbbHLH35. Collectively, SbbHLH168 integrates ion homeostasis, ROS scavenging and lignin content to confer salt tolerance, providing a direct gene target for molecular breeding of resilient sorghum cultivars.