Research Advances on the Structure, Function and Regulation of NIP Aquaporins in Plants

Aquaporin protein (AQP) family, as a "carrier" of water, micronutrients, and solute macromolecules in plants, plays an important role in material transport. Unlike other members of the AQP family, nodulin 26-like intrinsic proteins (NIPs) mainly penetrate small molecules, such as water, glycerin, silicic acid, boric acid, and urea. In addition, it is involved in plant cell osmoregulation, seed germination, lateral root production, leaf and flower growth and development, and response to biotic and abiotic (drought, chilling, salinity, metalloid toxicity, hypoxia, etc.) stresses. So far, structures of the NIP family have been partially reported. The NIPs can be divided into three classes based on the pore size of the ar/R selective filter, which determines the absorption and transportation of different substrates. Moreover, the NIPs contain two NPA motifs, which are located in loop B and loop E and have a typical ring-like hourglass structure. Therefore, different structures of the NIP subfamily play different roles in plant membrane solute transport and water balance. The NIP gene family has been identified in Arabidopsis thaliana, Oryza sativa, and other plants, whose function needs to be achieved through a variety of signaling pathways and the synergistic effect of physiological processes. In recent years, more and more reports have shown how the NIP gene participates in regulating gene expression levels under various stress conditions and interacts with a variety of stress proteins to jointly regulate the water and osmotic balance of plants and improve plant resistance and adaptability. The co-expression of NIPs with AtBOR1, AtNodGS, and AtACR3 genes can enhance the transport of solute and aquaporin. Besides, the interaction between NIP and 14-3-3f proteins can stabilize cell structure and substance synthesis in vivo, thus enhancing plant cold resistance. Interestingly, NIP is regulated by WRKY transcription factors and plays an active role in the regulation and distribution of metalloid elements in plants. In addition, the NIP genes are involved in the response of various hormone signaling pathways in plants, and their gating mechanisms and osmotic stress responses are regulated by the C- and N-terminal phosphorylation sites of NIP proteins, but the specific response mechanisms remain to be further explored. NIPs are a large multi-gene family with functional redundancy among members, and the similarities and differences in functions and mechanisms of different subfamilies of NIP genes need to be investigated. One of our main subsequent research directions is to explore the stress resistance network of the NIP genes, especially their response mechanisms to biotic stress. What's more, NIPs can interact with PIP proteins belonging to the AQP family and affect the diffusion of water molecules by changing their positions. Therefore, whether NIPs interact with other members of the AQP family, such as SIPs and TIPs, deserves to be validated further. In summary, the origin and evolution, structure, classification, biological function, and regulation mechanism of the NIP family in plants are reviewed, which provides new insights and references for further research.