The Roles of Aquaporins in Plant Stress Responses

Abstract

Aquaporins are membrane channel proteins ubiquitously present in all kingdoms of life. Although aquaporins were originally discovered as water channels, their roles in the transport of small neutral solutes, gasses, and metal ions are now well established. Plants contain the largest number and greatest diversity of aquaporin homologs with diverse subcellular localization patterns, gating properties, and solute specificity. The roles of aquaporins in physiological functions throughout plant growth and development are well known. As an integral regulator of plant-water relations, they are presumed to play an important role in plant defense responses against biotic and abiotic stressors. This review highlights involvement of various aquaporin homologs in plant stress responses against a variety of environmental stresses that disturb plant cell osmotic balance and nutrient homeostasis.

Keywords: abiotic stress; aquaporins; biotic stress; cold stress; drought; nutrient homeostasis; osmotic stress; salinity.

Figure 1

General structure and localization of aquaporins. (A) Major intrinsic protein (MIP)monomer includes six alpha helical transmembrane helices (TM1–TM6), five inter-helical loops (LA-LE), an AEF(Ala-Glu-Phe) or AEFXXT motif in the N-terminal domain and two highly conserved NPA (Asp-Pro-Ala) motifs; (B) General localization of aquaporins. Plasma membrane intrinsic proteins (PIPs), nodulin-26 like intrinsic proteins (NIPs), and uncategorized X intrinsic proteins (XIPs) are generally localized to the plasma membrane, and expressed on the entire cell surface. Small basic intrinsic proteins (SIPs) and some NIPs have been found localizing to endoplasmic reticulum (ER). Tonoplast intrinsic proteins (TIPs) are localized to tonoplast, the membrane of vacuole. Some PIPs and TIPs have been predicted to localized to the inner envelop and thylakoids, while AtTIP5;1 has been found to be located to tonoplast.

Figure 2

Water flow in plant from roots to aerial parts. Leaf cross-section shows flow of water through apoplastic and symplastic water pathways. In apoplastic pathway, water flows across the cells through cell wall. In symplastic pathway, water moves through cytoplasm and cellular membranes. Aquaporins are involved in symplastic path facilitating transmembrane water flow across cells and subcellular compartments.

Figure 3

A generalized summary of plant aquaporins in response to environmental stimuli. Both PIPs and TIPs are more responsive to drought, salt and cold stress that disturb cell osmotic balance, they regulate root hydraulic conductivity (Lp_r) and transpiration rates. TIPs along with NIPs are involved in biotic stress responses and involved in regulating nutrient homeostasis between host and its pathogen. NIPs and TIPs are also found to interact with some pathogen proteins.