Abstract

Tonoplast Intrinsic Proteins (TIP) are one of five subfamilies of aquaporins in higher plants. Plants typically contain a large number of TIP genes, ranging from 6 to 35 compared to humans. The molecular weight of the TIP subfamily members ranges from 25 to 28 kDa. Despite their sequence diversity, all TIP monomers have the same structure, which consists of six transmembrane helices and five inter-helical loops that form an hourglass shape with a central pore. Four monomers form tetramers, which are functional units in the membrane. TIPs form channels in the tonoplast that basically function as regulators of the intracellular water flow, which implies that they have a role in regulating cell turgor. TIPs are responsible for precisely regulating the movement of not only water, but also some small neutral molecules such as glycerol, urea, ammonia, hydrogen peroxide and formamide. The expression of TIPs may be affected by different environmental stresses, including drought, salinity and cold. TIPs expression is also altered by phytohormones and the appropriate cis-regulatory motifs are identified in the promotor region of the genes encoding TIPs in different plant species. It was shown that manipulating TIP-encoding genes expression in plants could have the potential to improve abiotic stress tolerance.

Highlights

  • Aquaporins, which are members of the major intrinsic proteins (MIPs) superfamily, facilitate the bidirectional flux of water and non-aqua substrates across the cell membranes [1–4]

  • The expression of Tonoplast Intrinsic Proteins (TIP) may be affected by different environmental conditions, including drought, salinity and cold and this has been investigated in many plant species

  • Methyl jasmonate treatment caused a decrease in the nitrogen content in barley leaves, which was associated with an increased expression of the four tonoplast aquaporin genes (HvTIP1;2, HvTIP2;2, HvTIP4;1 and HvTIP4;2), which are predicted to transport the nitrogen compounds from the vacuole to the cytosol

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Summary

Introduction

Aquaporins (aqua-pore, AQP), which are members of the major intrinsic proteins (MIPs) superfamily, facilitate the bidirectional flux of water and non-aqua substrates across the cell membranes [1–4]. These transmembrane proteins form pores in the lipid bilayers of archaea, bacteria, fungi, plants, non-mammalian metazoans and mammals including humans. TIP proteins are most abundant in the tonoplast, but some isoforms have been detected in the chloroplast, e.g. on the luminal site of the thylakoid, the chloroplast membranes or the envelope fraction using mass-spectrometrybased proteomics

Structure and diversity of TIPs
Study of the tissue- and developmental-dependent expression of TIPs
Drought, salinity and cold stress
Phytohormone treatment
Genetic manipulation of the TIP-encoding genes
Conclusions

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