Identification of AQP gene family with functional characterization of MaPIP2;1, MaTIP2;2 and MaPIP1;1 under salinity in Melilotus albus

Soil salinity is a major environmental constraint on global agricultural productivity. Elucidating plant responses to salt stress is crucial for developing crops resilient to salinity. Melilotus albus, a highly salt-tolerant legume widely used to improve saline-alkali soils, serves as a suitable species for investigating salt stress responses. In this work, 44 MaAQP genes were identified in the M. albus genome and grouped into five subfamilies: PIP, TIP, NIP, SIP, and XIP. Members of the same subfamily exhibited conserved NPA motifs and ar/R selectivity filters, and gene duplication analyses indicated that the family expanded under purifying selection. Transcriptome and qRT-PCR analyses revealed that MaAQP genes responded to salt stresses, with salt-induced expression occurring predominantly in roots. Overexpression of MaPIP2;1, MaTIP2;2, and MaPIP1;1 was associated with improved yeast growth under salt stress. In transgenic hairy roots exposed to salt treatment, overexpression of these genes coincided with less severe salt-induced growth inhibition, with root length increasing by 14.8-25.0% and fresh weight by 26.2-53.2%, and with reduced salt-induced damage, as reflected by lower MDA content and electrolyte leakage, together with increased soluble sugar and soluble protein levels related to osmotic adjustment. This study presents initial evidence that AQPs are associated with salinity responses in M. albus, offering insights into AQP-related responses to salinity in this species and providing a basis for further functional investigation.