Major intrinsic proteins (MIPs) in plants: a complex gene family with major impacts on plant phenotype

Abstract

The ubiquitous cell membrane proteins called aquaporins are now firmly established as channel proteins that control the specific transport of water molecules across cell membranes in all living organisms. The aquaporins are thus likely to be of fundamental significance to all facets of plant growth and development affected by plant-water relations. A majority of plant aquaporins have been found to share essential structural features with the human aquaporin and exhibit water-transporting ability in various functional assays, and some have been shown experimentally to be of critical importance to plant survival. Furthermore, substantial evidence is now available from a number of plant species that shows differential gene expression of aquaporins in response to abiotic stresses such as salinity, drought, or cold and clearly establishes the aquaporins as major players in the response of plants to conditions that affect water availability. This review summarizes the function and regulation of these genes to develop a greater understanding of the response of plants to water insufficiency, and particularly, to identify tolerant genotypes of major crop species including wheat and rice and plants that are important in agroforestry.

AtTIP1;3 and AtTIP5;1, the only highly expressed Arabidopsis pollen‐specific aquaporins, transport water and urea

“…Natural candidates involved in water or solute movement are aquaporins, membrane proteins that specifically regulate the flow of large amounts of water across cellular membranes [2]. Plant aquaporins are classified into four main subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin26-like intrinsic proteins (NIPs) and small and basic membrane intrinsic proteins (SIPs) [3]. Many of the TIPs and PIPs are water channels although some of the isoforms additionally transport solutes such as glycerol, urea or boric acid [3].…”

“…Plant aquaporins are classified into four main subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin26-like intrinsic proteins (NIPs) and small and basic membrane intrinsic proteins (SIPs) [3]. Many of the TIPs and PIPs are water channels although some of the isoforms additionally transport solutes such as glycerol, urea or boric acid [3]. Considering that pollen hydration is the first step for pollination, our hypothesis is that aquaporins might be present in mature pollen grains and potentially involved in water and/or solute uptake from the stigma or/and style.…”

Aquaporin structure–function relationships: Water flow through plant living cells

“…The accumulation of transcripts arising from NgMIP2, NgMIP3 and NgMIP4 diminished dramatically in drought-stressed plants. This down-regulation of MIP gene expression may result in reduced membrane water permeability and may encourage cellular water conservation during periods of dehydration stress [116,122,123].…”

“…There is growing evidence that suggests that aquaporins play different roles throughout plant development [120][121][122][123][124][125]. Therefore, aquaporin genomic information is important because, assigning physiological function via transgenic reduction or removal of gene expression requires sequence information for precise targeting.…”

“…The transcriptional and/or post-translational regulation of aquaporins would determine changes in membrane water permeability. Both phosphorylation and translocation to/from vesicles have been reported as post-translational mechanisms [123][124][125]. However, translocation in plants has not yet been shown.…”