Insights into Populus XIP aquaporins: evolutionary expansion, protein functionality, and environmental regulation

Area of Science:

• Plant molecular biology
• Membrane protein research
• Evolutionary biology

Background:

• A novel class of plant proteins, X (for unrecognized) intrinsic proteins (XIPs), are poorly characterized.
• XIPs share weak similarities with known aquaporins, necessitating further investigation.

Purpose of the Study:

• To conduct a systematic molecular characterization of XIPs in flowering plants.
• To analyze the evolutionary patterns and functional properties of XIPs, particularly in the genus Populus.

Main Methods:

• Phylogenetic analysis of XIP sequences from flowering plant databases.
• Gene expression profiling of Populus XIP (PtXIP) isoforms in various tissues and under stress conditions.
• Functional characterization of PtXIPs by expressing them in Xenopus laevis oocytes to assess water transport.

Main Results:

• XIPs were classified into at least five distinct groups with taxon-specific evolutionary patterns in angiosperms.
• The genus Populus displays significant XIP diversity, with nine PtXIP isoforms identified across three groups.
• Only two PtXIP isoforms, PtXIP2;1 and PtXIP3;2, were transcribed in vegetative tissues, showing differential expression patterns.
• PtXIP2;1 demonstrated differential expression in response to drought, salicylic acid, and wounding.
• Functional assays revealed that PtXIP2;1 and PtXIP3;3 facilitate water transport, while other PtXIPs do not.

Conclusions:

• XIPs represent a unique subfamily of plant proteins with diverse evolutionary histories.
• Populus XIPs exhibit specialized expression patterns and functional roles, particularly in water transport and stress response.
• The functional diversity of XIPs, including their water channel activity, highlights their importance in plant physiology.