Heterologous Expression of Saussurea involucrata SiPIP1;5b Enhances Freezing Tolerance in Arabidopsis and Chilling Tolerance in Tomato

Low temperature is a major constraint limiting the productivity and distribution of crops, and aquaporins have emerged as important determinants of plant stress tolerance. Saussurea involucrata, an alpine plant adapted to extreme low-temperature environments, provides a valuable source of cold-tolerance genes, yet the contribution of its PIP1-type plasma membrane aquaporins to freezing and chilling tolerance in heterologous systems has not been established. Here, we isolated SiPIP1;5b, a PIP1-type plasma membrane intrinsic protein from S. involucrata, and constitutively overexpressed it in Arabidopsis thaliana and tomato (Solanum lycopersicum). Under freezing treatment (−4 °C, 4 h), SiPIP1;5b-overexpressing Arabidopsis lines showed markedly higher survival rates (72.0–81.5%) than the wild type (36.2%), together with lower relative electrolyte leakage and malondialdehyde (MDA) content and higher chlorophyll content. Under chilling stress (0 °C, 4 h), transgenic tomato plants likewise exhibited reduced electrolyte leakage and MDA accumulation, elevated activities of superoxide dismutase, catalase, and peroxidase, and increased proline content relative to wild-type plants. These results indicate that SiPIP1;5b enhances freezing and chilling tolerance and is associated with improved membrane stability, reduced accumulation of excessive reactive oxygen species, and enhanced antioxidant and osmoprotective responses. Our findings establish SiPIP1;5b as a promising candidate gene for improving low-temperature tolerance in crops.